Chemically Reacting Turbulent Flow.
1987-04-14
two stages of gen I tubes equipped with P-47 phosphor screens The detector chosen for the camera was a Reticon RL128S* line detectoI- .,hich consists...the Stud’, of Turbulent Mixing," William M. Pitts, Nuclear Engineering Seminar of the Department of Chemical and Nuclear Engineering, University of
Stochastic models for turbulent reacting flows
Kerstein, A. [Sandia National Laboratories, Livermore, CA (United States)
1993-12-01
The goal of this program is to develop and apply stochastic models of various processes occurring within turbulent reacting flows in order to identify the fundamental mechanisms governing these flows, to support experimental studies of these flows, and to further the development of comprehensive turbulent reacting flow models.
Direct numerical simulation of turbulent reacting flows
Chen, J.H. [Sandia National Laboratories, Livermore, CA (United States)
1993-12-01
The development of turbulent combustion models that reflect some of the most important characteristics of turbulent reacting flows requires knowledge about the behavior of key quantities in well defined combustion regimes. In turbulent flames, the coupling between the turbulence and the chemistry is so strong in certain regimes that is is very difficult to isolate the role played by one individual phenomenon. Direct numerical simulation (DNS) is an extremely useful tool to study in detail the turbulence-chemistry interactions in certain well defined regimes. Globally, non-premixed flames are controlled by two limiting cases: the fast chemistry limit, where the turbulent fluctuations. In between these two limits, finite-rate chemical effects are important and the turbulence interacts strongly with the chemical processes. This regime is important because industrial burners operate in regimes in which, locally the flame undergoes extinction, or is at least in some nonequilibrium condition. Furthermore, these nonequilibrium conditions strongly influence the production of pollutants. To quantify the finite-rate chemistry effect, direct numerical simulations are performed to study the interaction between an initially laminar non-premixed flame and a three-dimensional field of homogeneous isotropic decaying turbulence. Emphasis is placed on the dynamics of extinction and on transient effects on the fine scale mixing process. Differential molecular diffusion among species is also examined with this approach, both for nonreacting and reacting situations. To address the problem of large-scale mixing and to examine the effects of mean shear, efforts are underway to perform large eddy simulations of round three-dimensional jets.
PDF approach for compressible turbulent reacting flows
Hsu, A. T.; Tsai, Y.-L. P.; Raju, M. S.
1993-01-01
The objective of the present work is to develop a probability density function (pdf) turbulence model for compressible reacting flows for use with a CFD flow solver. The probability density function of the species mass fraction and enthalpy are obtained by solving a pdf evolution equation using a Monte Carlo scheme. The pdf solution procedure is coupled with a compressible CFD flow solver which provides the velocity and pressure fields. A modeled pdf equation for compressible flows, capable of capturing shock waves and suitable to the present coupling scheme, is proposed and tested. Convergence of the combined finite-volume Monte Carlo solution procedure is discussed, and an averaging procedure is developed to provide smooth Monte-Carlo solutions to ensure convergence. Two supersonic diffusion flames are studied using the proposed pdf model and the results are compared with experimental data; marked improvements over CFD solutions without pdf are observed. Preliminary applications of pdf to 3D flows are also reported.
Quantitative imaging of turbulent and reacting flows
Paul, P.H. [Sandia National Laboratories, Livermore, CA (United States)
1993-12-01
Quantitative digital imaging, using planar laser light scattering techniques is being developed for the analysis of turbulent and reacting flows. Quantitative image data, implying both a direct relation to flowfield variables as well as sufficient signal and spatial dynamic range, can be readily processed to yield two-dimensional distributions of flowfield scalars and in turn two-dimensional images of gradients and turbulence scales. Much of the development of imaging techniques to date has concentrated on understanding the requisite molecular spectroscopy and collision dynamics to be able to determine how flowfield variable information is encoded into the measured signal. From this standpoint the image is seen as a collection of single point measurements. The present effort aims at realizing necessary improvements in signal and spatial dynamic range, signal-to-noise ratio and spatial resolution in the imaging system as well as developing excitation/detection strategies which provide for a quantitative measure of particular flowfield scalars. The standard camera used for the study is an intensified CCD array operated in a conventional video format. The design of the system was based on detailed modeling of signal and image transfer properties of fast UV imaging lenses, image intensifiers and CCD detector arrays. While this system is suitable for direct scalar imaging, derived quantities (e.g. temperature or velocity images) require an exceptionally wide dynamic range imaging detector. To apply these diagnostics to reacting flows also requires a very fast shuttered camera. The authors have developed and successfully tested a new type of gated low-light level detector. This system relies on fast switching of proximity focused image-diode which is direct fiber-optic coupled to a cooled CCD array. Tests on this new detector show significant improvements in detection limit, dynamic range and spatial resolution as compared to microchannel plate intensified arrays.
A PDF closure model for compressible turbulent chemically reacting flows
Kollmann, W.
1992-01-01
The objective of the proposed research project was the analysis of single point closures based on probability density function (pdf) and characteristic functions and the development of a prediction method for the joint velocity-scalar pdf in turbulent reacting flows. Turbulent flows of boundary layer type and stagnation point flows with and without chemical reactions were be calculated as principal applications. Pdf methods for compressible reacting flows were developed and tested in comparison with available experimental data. The research work carried in this project was concentrated on the closure of pdf equations for incompressible and compressible turbulent flows with and without chemical reactions.
Turbulence-chemistry interactions in reacting flows
Barlow, R.S.; Carter, C.D. [Sandia National Laboratories, Livermore, CA (United States)
1993-12-01
Interactions between turbulence and chemistry in nonpremixed flames are investigated through multiscalar measurements. Simultaneous point measurements of major species, NO, OH, temperature, and mixture fraction are obtained by combining spontaneous Raman scattering, Rayleigh scattering, and laser-induced fluorescence (LIF). NO and OH fluorescence signals are converted to quantitative concentrations by applying shot-to-shot corrections for local variations of the Boltzmann fraction and collisional quenching rate. These measurements of instantaneous thermochemical states in turbulent flames provide insights into the fundamental nature of turbulence-chemistry interactions. The measurements also constitute a unique data base for evaluation and refinement of turbulent combustion models. Experimental work during the past year has focused on three areas: (1) investigation of the effects of differential molecular diffusion in turbulent combustion: (2) experiments on the effects of Halon CF{sub 3}Br, a fire retardant, on the structure of turbulent flames of CH{sub 4} and CO/H{sub 2}/N{sub 2}; and (3) experiments on NO formation in turbulent hydrogen jet flames.
Spectral kinetic energy transfer in turbulent premixed reacting flows.
Towery, C A Z; Poludnenko, A Y; Urzay, J; O'Brien, J; Ihme, M; Hamlington, P E
2016-05-01
Spectral kinetic energy transfer by advective processes in turbulent premixed reacting flows is examined using data from a direct numerical simulation of a statistically planar turbulent premixed flame. Two-dimensional turbulence kinetic-energy spectra conditioned on the planar-averaged reactant mass fraction are computed through the flame brush and variations in the spectra are connected to terms in the spectral kinetic energy transport equation. Conditional kinetic energy spectra show that turbulent small-scale motions are suppressed in the burnt combustion products, while the energy content of the mean flow increases. An analysis of spectral kinetic energy transfer further indicates that, contrary to the net down-scale transfer of energy found in the unburnt reactants, advective processes transfer energy from small to large scales in the flame brush close to the products. Triadic interactions calculated through the flame brush show that this net up-scale transfer of energy occurs primarily at spatial scales near the laminar flame thermal width. The present results thus indicate that advective processes in premixed reacting flows contribute to energy backscatter near the scale of the flame.
High speed turbulent reacting flows: DNS and LES
Givi, Peyman
1990-01-01
Work on understanding the mechanisms of mixing and reaction in high speed turbulent reacting flows was continued. Efforts, in particular, were concentrated on taking advantage of modern computational methods to simulate high speed turbulent flows. In doing so, two methodologies were used: large eddy simulations (LES) and direct numerical simulations (DNS). In the work related with LES the objective is to study the behavior of the probability density functions (pdfs) of scalar properties within the subgrid in reacting turbulent flows. The data base obtained by DNS for a detailed study of the pdf characteristics within the subgrid was used. Simulations are performed for flows under various initializations to include the effects of compressibility on mixing and chemical reactions. In the work related with DNS, a two-dimensional temporally developing high speed mixing layer under the influence of a second-order non-equilibrium chemical reaction of the type A + B yields products + heat was considered. Simulations were performed with different magnitudes of the convective Mach numbers and with different chemical kinetic parameters for the purpose of examining the isolated effects of the compressibility and the heat released by the chemical reactions on the structure of the layer. A full compressible code was developed and utilized, so that the coupling between mixing and chemical reactions is captured in a realistic manner.
Numerical Investigation of a Statistically Stationary Turbulent Reacting Flow
Overholt, Matthew R.; Pope, Stephen B.
1997-11-01
Direct numerical simulation (DNS) has been very useful in the study of inert scalar mixing in turbulent flows, and has recently become feasible for studies of reacting scalars. We have formulated an accessible inhomogeneous nonpremixed turbulent reactive flow for investigating the effects of mixing on reaction and testing mixing models. The mixture fraction-progress variable approach is used with a model single-step reversible finite-rate thermochemistry, yielding non-trivial stationary solutions corresponding to stable reaction and allowing local extinction to occur. A mean gradient in the mixture fraction gives rise to stationarity without forcing, as well as a flame brush. A range of reaction zone thicknesses and Damkohler numbers are examined, yielding a broad spectrum of behavior, ranging from thick to thin flames, and from local extinction to near equilibrium. Based on this study results from full probability density function (PDF) simulations using the IEM and EMST mixing models are evaluated. Conditional moment closure (CMC) results are evaluated as well.
Poludnenko, Alexei
2016-11-01
Turbulent reacting flows are pervasive both in our daily lives on Earth and in the Universe. They power modern society being at the heart of many energy generation and propulsion systems, such as gas turbines, internal combustion and jet engines. On astronomical scales, thermonuclear turbulent flames are the driver of some of the most powerful explosions in the Universe, knows as Type Ia supernovae. Despite this ubiquity in Nature, turbulent reacting flows still pose a number of fundamental questions often exhibiting surprising and unexpected behavior. In this talk, we will discuss several such phenomena observed in direct numerical simulations of high-speed, premixed, turbulent flames. We show that turbulent flames in certain regimes are intrinsically unstable even in the absence of the surrounding combustor walls or obstacles, which can support the thermoacoustic feedback. Such instability can fundamentally change the structure and dynamics of the turbulent cascade, resulting in a significant (and anisotropic) redistribution of kinetic energy from small to large scales. In particular, three effects are observed. 1) The turbulent burning velocity can develop pulsations with significant peak-to-peak amplitudes. 2) Unstable burning can result in pressure build-up and the formation of pressure waves or shocks when the flame speed approaches or exceeds the speed of a Chapman-Jouguet deflagration. 3) Coupling of pressure and density gradients across the flame can lead to the anisotropic generation of turbulence inside the flame volume and flame acceleration. We extend our earlier analysis, which relied on a simplified single-step reaction model, by demonstrating existence of these effects in realistic chemical flames (hydrogen and methane) and in thermonuclear flames in degenerate, relativistic plasmas found in stellar interiors. Finally, we discuss the implications of these results for subgrid-scale LES combustion models. This work was supported by the Air Force
DNS, LES and Stochastic Modeling of Turbulent Reacting Flows
1994-03-01
the analytical results derived by Fendell (1965) via the method of matched asymptotic expansions. A typical DNS scatter plot of the product mass...fields. In Buckmaster, J. D., Jackson, T. L., and Kumar, A., editors, Combustion in High-Speed Flows. in press. Fendell , F. E. (1965). Ignition and
Comparison of PDF and Moment Closure Methods in the Modeling of Turbulent Reacting Flows
Norris, Andrew T.; Hsu, Andrew T.
1994-01-01
In modeling turbulent reactive flows, Probability Density Function (PDF) methods have an advantage over the more traditional moment closure schemes in that the PDF formulation treats the chemical reaction source terms exactly, while moment closure methods are required to model the mean reaction rate. The common model used is the laminar chemistry approximation, where the effects of turbulence on the reaction are assumed negligible. For flows with low turbulence levels and fast chemistry, the difference between the two methods can be expected to be small. However for flows with finite rate chemistry and high turbulence levels, significant errors can be expected in the moment closure method. In this paper, the ability of the PDF method and the moment closure scheme to accurately model a turbulent reacting flow is tested. To accomplish this, both schemes were used to model a CO/H2/N2- air piloted diffusion flame near extinction. Identical thermochemistry, turbulence models, initial conditions and boundary conditions are employed to ensure a consistent comparison can be made. The results of the two methods are compared to experimental data as well as to each other. The comparison reveals that the PDF method provides good agreement with the experimental data, while the moment closure scheme incorrectly shows a broad, laminar-like flame structure.
Ensemble Averaged Probability Density Function (APDF) for Compressible Turbulent Reacting Flows
Shih, Tsan-Hsing; Liu, Nan-Suey
2012-01-01
In this paper, we present a concept of the averaged probability density function (APDF) for studying compressible turbulent reacting flows. The APDF is defined as an ensemble average of the fine grained probability density function (FG-PDF) with a mass density weighting. It can be used to exactly deduce the mass density weighted, ensemble averaged turbulent mean variables. The transport equation for APDF can be derived in two ways. One is the traditional way that starts from the transport equation of FG-PDF, in which the compressible Navier- Stokes equations are embedded. The resulting transport equation of APDF is then in a traditional form that contains conditional means of all terms from the right hand side of the Navier-Stokes equations except for the chemical reaction term. These conditional means are new unknown quantities that need to be modeled. Another way of deriving the transport equation of APDF is to start directly from the ensemble averaged Navier-Stokes equations. The resulting transport equation of APDF derived from this approach appears in a closed form without any need for additional modeling. The methodology of ensemble averaging presented in this paper can be extended to other averaging procedures: for example, the Reynolds time averaging for statistically steady flow and the Reynolds spatial averaging for statistically homogeneous flow. It can also be extended to a time or spatial filtering procedure to construct the filtered density function (FDF) for the large eddy simulation (LES) of compressible turbulent reacting flows.
A numerical study of mixing in stationary, nonpremixed, turbulent reacting flows
Overholt, Matthew Ryan
1998-10-01
In this work a detailed numerical study is made of a statistically-stationary, non-premixed, turbulent reacting model flow known as Periodic Reaction Zones. The mixture fraction-progress variable approach is used, with a mean gradient in the mixture fraction and a model, single-step, reversible, finite-rate thermochemistry, yielding both stationary and local extinction behavior. The passive scalar is studied first, using a statistical forcing scheme to achieve stationarity of the velocity field. Multiple independent direct numerical simulations (DNS) are performed for a wide range of Reynolds numbers with a number of results including a bilinear model for scalar mixing jointly conditioned on the scalar and x2-component of velocity, Gaussian scalar probability density function tails which were anticipated to be exponential, and the quantification of the dissipation of scalar flux. A new deterministic forcing scheme for DNS is then developed which yields reduced fluctuations in many quantities and a more natural evolution of the velocity fields. This forcing method is used for the final portion of this work. DNS results for Periodic Reaction Zones are compared with the Conditional Moment Closure (CMC) model, the Quasi-Equilibrium Distributed Reaction (QEDR) model, and full probability density function (PDF) simulations using the Euclidean Minimum Spanning Tree (EMST) and the Interaction by Exchange with the Mean (IEM) mixing models. It is shown that CMC and QEDR results based on the local scalar dissipation match DNS wherever local extinction is not present. However, due to the large spatial variations of scalar dissipation, and hence local Damkohler number, local extinction is present even when the global Damkohler number is twenty-five times the critical value for extinction. Finally, in the PDF simulations the EMST mixing model closely reproduces CMC and DNS results when local extinction is not present, whereas the IEM model results in large error.
Turbulent diffusion of chemically reacting gaseous admixtures
Elperin, T.; Kleeorin, N.; Liberman, M.; Rogachevskii, I.
2014-11-01
We study turbulent diffusion of chemically reacting gaseous admixtures in a developed turbulence. In our previous study [Phys. Rev. Lett. 80, 69 (1998), 10.1103/PhysRevLett.80.69] using a path-integral approach for a delta-correlated in a time random velocity field, we demonstrated a strong modification of turbulent transport in fluid flows with chemical reactions or phase transitions. In the present study we use the spectral τ approximation that is valid for large Reynolds and Peclet numbers and show that turbulent diffusion of the reacting species can be strongly depleted by a large factor that is the ratio of turbulent and chemical times (turbulent Damköhler number). We have demonstrated that the derived theoretical dependence of a turbulent diffusion coefficient versus the turbulent Damköhler number is in good agreement with that obtained previously in the numerical modeling of a reactive front propagating in a turbulent flow and described by the Kolmogorov-Petrovskii-Piskunov-Fisher equation. We have found that turbulent cross-effects, e.g., turbulent mutual diffusion of gaseous admixtures and turbulent Dufour effect of the chemically reacting gaseous admixtures, are less sensitive to the values of stoichiometric coefficients. The mechanisms of the turbulent cross-effects differ from the molecular cross-effects known in irreversible thermodynamics. In a fully developed turbulence and at large Peclet numbers the turbulent cross-effects are much larger than the molecular ones. The obtained results are applicable also to heterogeneous phase transitions.
LES, DNS, and RANS for the Analysis of High-Speed Turbulent Reacting Flows
Colucci, P. J.; Jaberi, F. A.; Givi, P.
1996-01-01
A filtered density function (FDF) method suitable for chemically reactive flows is developed in the context of large eddy simulation. The advantage of the FDF methodology is its inherent ability to resolve subgrid scales (SGS) scalar correlations that otherwise have to be modeled. Because of the lack of robust models to accurately predict these correlations in turbulent reactive flows, simulations involving turbulent combustion are often met with a degree of skepticism. The FDF methodology avoids the closure problem associated with these terms and treats the reaction in an exact manner. The scalar FDF approach is particularly attractive since it can be coupled with existing hydrodynamic computational fluid dynamics (CFD) codes.
Heat and mass transfer for turbulent flow of chemically reacting gas in eccentric annular channels
Besedina, T. V.; Tverkovkin, B. E.; Udot, A. V.; Yakushev, A. P.
1987-08-01
An algorithm is proposed for calculating the velocity, temperature, and concentration fields under conditions of cooling of a cylindrical heat-releasing rod, placed off-center in a circular casing pipe, by a longitudinal flow of chemically reacting gas [N2O4].
Combustion characteristics and turbulence modeling of swirling reacting flow in solid fuel ramjet
Musa, Omer; Xiong, Chen; Changsheng, Zhou
2017-10-01
This paper reviews the historical studies have been done on the solid-fuel ramjet engine and difficulties associated with numerical modeling of swirling flow with combustible gases. A literature survey about works related to numerical and experimental investigations on solid-fuel ramjet as well as using swirling flow and different numerical approaches has been provided. An overview of turbulence modeling of swirling flow and the behavior of turbulence at streamline curvature and system rotation are presented. A new and simple curvature/correction factor is proposed in order to reduce the programming complexity of SST-CC turbulence model. Finally, numerical and experimental investigations on the impact of swirling flow on SFRJ have been carried out. For that regard, a multi-physics coupling code is developed to solve the problems of multi-physics coupling of fluid mechanics, solid pyrolysis, heat transfer, thermodynamics, and chemical kinetics. The connected-pipe test facility is used to carry out the experiments. The results showed a positive impact of swirling flow on SFRJ along with, three correlations are proposed.
Trisjono, Philipp; Kang, Seongwon; Pitsch, Heinz
2016-12-01
The main objective of this study is to present an accurate and consistent numerical framework for turbulent reacting flows based on a high-order finite difference (HOFD) scheme. It was shown previously by Desjardins et al. (2008) [4] that a centered finite difference scheme discretely conserving the kinetic energy and an upwind-biased scheme for the scalar transport can be combined into a useful scheme for turbulent reacting flows. With a high-order spatial accuracy, however, an inconsistency among discretization schemes for different conservation laws is identified, which can disturb a scalar field spuriously under non-uniform density distribution. Various theoretical and numerical analyses are performed on the sources of the unphysical error. From this, the derivative of the mass-conserving velocity and the local Péclet number are identified as the primary factors affecting the error. As a solution, an HOFD stencil for the mass conservation is reformulated into a flux-based form that can be used consistently with an upwind-biased scheme for the scalar transport. The effectiveness of the proposed formulation is verified using two-dimensional laminar flows such as a scalar transport problem and a laminar premixed flame, where unphysical oscillations in the scalar fields are removed. The applicability of the proposed scheme is demonstrated in an LES of a turbulent stratified premixed flame.
Yu, Rixin; Yu, Jiangfei; Bai, Xue-Song
2012-06-01
We present an improved numerical scheme for numerical simulations of low Mach number turbulent reacting flows with detailed chemistry and transport. The method is based on a semi-implicit operator-splitting scheme with a stiff solver for integration of the chemical kinetic rates, developed by Knio et al. [O.M. Knio, H.N. Najm, P.S. Wyckoff, A semi-implicit numerical scheme for reacting flow II. Stiff, operator-split formulation, Journal of Computational Physics 154 (2) (1999) 428-467]. Using the material derivative form of continuity equation, we enhance the scheme to allow for large density ratio in the flow field. The scheme is developed for direct numerical simulation of turbulent reacting flow by employing high-order discretization for the spatial terms. The accuracy of the scheme in space and time is verified by examining the grid/time-step dependency on one-dimensional benchmark cases: a freely propagating premixed flame in an open environment and in an enclosure related to spark-ignition engines. The scheme is then examined in simulations of a two-dimensional laminar flame/vortex-pair interaction. Furthermore, we apply the scheme to direct numerical simulation of a homogeneous charge compression ignition (HCCI) process in an enclosure studied previously in the literature. Satisfactory agreement is found in terms of the overall ignition behavior, local reaction zone structures and statistical quantities. Finally, the scheme is used to study the development of intrinsic flame instabilities in a lean H2/air premixed flame, where it is shown that the spatial and temporary accuracies of numerical schemes can have great impact on the prediction of the sensitive nonlinear evolution process of flame instability.
MacArt, Jonathan F.; Mueller, Michael E.
2016-12-01
Two formally second-order accurate, semi-implicit, iterative methods for the solution of scalar transport-reaction equations are developed for Direct Numerical Simulation (DNS) of low Mach number turbulent reacting flows. The first is a monolithic scheme based on a linearly implicit midpoint method utilizing an approximately factorized exact Jacobian of the transport and reaction operators. The second is an operator splitting scheme based on the Strang splitting approach. The accuracy properties of these schemes, as well as their stability, cost, and the effect of chemical mechanism size on relative performance, are assessed in two one-dimensional test configurations comprising an unsteady premixed flame and an unsteady nonpremixed ignition, which have substantially different Damköhler numbers and relative stiffness of transport to chemistry. All schemes demonstrate their formal order of accuracy in the fully-coupled convergence tests. Compared to a (non-)factorized scheme with a diagonal approximation to the chemical Jacobian, the monolithic, factorized scheme using the exact chemical Jacobian is shown to be both more stable and more economical. This is due to an improved convergence rate of the iterative procedure, and the difference between the two schemes in convergence rate grows as the time step increases. The stability properties of the Strang splitting scheme are demonstrated to outpace those of Lie splitting and monolithic schemes in simulations at high Damköhler number; however, in this regime, the monolithic scheme using the approximately factorized exact Jacobian is found to be the most economical at practical CFL numbers. The performance of the schemes is further evaluated in a simulation of a three-dimensional, spatially evolving, turbulent nonpremixed planar jet flame.
Caughey, David
2010-10-08
A Symposium on Turbulence and Combustion was held at Cornell University on August 3-4, 2009. The overall goal of the Symposium was to promote future advances in the study of turbulence and combustion, through an unique forum intended to foster interactions between leading members of these two research communities. The Symposium program consisted of twelve invited lectures given by world-class experts in these fields, two poster sessions consisting of nearly 50 presentations, an open forum, and other informal activities designed to foster discussion. Topics covered in the lectures included turbulent dispersion, wall-bounded flows, mixing, finite-rate chemistry, and others, using experiment, modeling, and computations, and included perspectives from an international community of leading researchers from academia, national laboratories, and industry.
Shih, Tsan-Hsing; Liu, Nan-Suey
2008-01-01
This paper describes an approach which aims at bridging the gap between the traditional Reynolds-averaged Navier-Stokes (RANS) approach and the traditional large eddy simulation (LES) approach. It has the characteristics of the very large eddy simulation (VLES) and we call this approach the partially-resolved numerical simulation (PRNS). Systematic simulations using the National Combustion Code (NCC) have been carried out for fully developed turbulent pipe flows at different Reynolds numbers to evaluate the PRNS approach. Also presented are the sample results of two demonstration cases: nonreacting flow in a single injector flame tube and reacting flow in a Lean Direct Injection (LDI) hydrogen combustor.
Multiphase reacting flows modelling and simulation
Marchisio, Daniele L
2007-01-01
The papers in this book describe the most widely applicable modeling approaches and are organized in six groups covering from fundamentals to relevant applications. In the first part, some fundamentals of multiphase turbulent reacting flows are covered. In particular the introduction focuses on basic notions of turbulence theory in single-phase and multi-phase systems as well as on the interaction between turbulence and chemistry. In the second part, models for the physical and chemical processes involved are discussed. Among other things, particular emphasis is given to turbulence modeling strategies for multiphase flows based on the kinetic theory for granular flows. Next, the different numerical methods based on Lagrangian and/or Eulerian schemes are presented. In particular the most popular numerical approaches of computational fluid dynamics codes are described (i.e., Direct Numerical Simulation, Large Eddy Simulation, and Reynolds-Averaged Navier-Stokes approach). The book will cover particle-based meth...
Iannetti, Anthony C.; Moder, Jeffery P.
2010-01-01
Developing physics-based tools to aid in reducing harmful combustion emissions, like Nitrogen Oxides (NOx), Carbon Monoxide (CO), Unburnt Hydrocarbons (UHC s), and Sulfur Dioxides (SOx), is an important goal of aeronautics research at NASA. As part of that effort, NASA Glenn Research Center is performing a detailed assessment and validation of an in-house combustion CFD code known as the National Combustion Code (NCC) for turbulent reacting flows. To assess the current capabilities of NCC for simulating turbulent reacting flows with liquid jet fuel injection, a set of Single Swirler Lean Direct Injection (LDI) experiments performed at the University of Cincinnati was chosen as an initial validation data set. This Jet-A/air combustion experiment operates at a lean equivalence ratio of 0.75 at atmospheric pressure and has a 4 percent static pressure drop across the swirler. Detailed comparisons of NCC predictions for gas temperature and gaseous emissions (CO and NOx) against this experiment are considered in a previous work. The current paper is focused on detailed comparisons of the spray characteristics (radial profiles of drop size distribution and at several radial rakes) from NCC simulations against the experimental data. Comparisons against experimental data show that the use of the correlation for primary spray break-up implemented by Raju in the NCC produces most realistic results, but this result needs to be improved. Given the single or ten step chemical kinetics models, use of a spray size correlation gives similar, acceptable results
Wu, Hao; Ihme, Matthias
2016-11-01
High-fidelity turbulent reactive flow simulations are typically associated with small time step sizes (h combustion simulations due to the reduced number of internal iterations and excessive implicitness. In this study, an improved 4th-order Rosenbrock-Krylov (ROK4L) scheme is developed for the system of chemical reactions. This class of schemes replaces the Jacobian matrix by its low-rank Krylov approximation, thus introducing partial implicitness. The scheme is improved in both accuracy and efficiency by fulfilling additional order conditions and reducing the number of function evaluations. The ROK4L scheme is demonstrated to possess superior efficiency in comparison to CVODE due to the minimal degree of implicitness for small time-step sizes and the avoidance of other overhead associated with the start-up process of multi-step methods. Financial support from NASA Transformational Tools and Technologies Project with Award No. NNX15AV04A.
Pdf - Transport equations for chemically reacting flows
Kollmann, W.
1989-01-01
The closure problem for the transport equations for pdf and the characteristic functions of turbulent, chemically reacting flows is addressed. The properties of the linear and closed equations for the characteristic functional for Eulerian and Lagrangian variables are established, and the closure problem for the finite-dimensional case is discussed for pdf and characteristic functions. It is shown that the closure for the scalar dissipation term in the pdf equation developed by Dopazo (1979) and Kollmann et al. (1982) results in a single integral, in contrast to the pdf, where double integration is required. Some recent results using pdf methods obtained for turbulent flows with combustion, including effects of chemical nonequilibrium, are discussed.
Direct simulations of chemically reacting turbulent mixing layers, part 2
Metcalfe, Ralph W.; Mcmurtry, Patrick A.; Jou, Wen-Huei; Riley, James J.; Givi, Peyman
1988-01-01
The results of direct numerical simulations of chemically reacting turbulent mixing layers are presented. This is an extension of earlier work to a more detailed study of previous three dimensional simulations of cold reacting flows plus the development, validation, and use of codes to simulate chemically reacting shear layers with heat release. Additional analysis of earlier simulations showed good agreement with self similarity theory and laboratory data. Simulations with a two dimensional code including the effects of heat release showed that the rate of chemical product formation, the thickness of the mixing layer, and the amount of mass entrained into the layer all decrease with increasing rates of heat release. Subsequent three dimensional simulations showed similar behavior, in agreement with laboratory observations. Baroclinic torques and thermal expansion in the mixing layer were found to produce changes in the flame vortex structure that act to diffuse the pairing vortices, resulting in a net reduction in vorticity. Previously unexplained anomalies observed in the mean velocity profiles of reacting jets and mixing layers were shown to result from vorticity generation by baroclinic torques.
张健; 普勇; 周力行
2006-01-01
This paper presents an experimental investigation of the turbulent reacting flow in a swirl combustor with staged air injection. The air injected into the combustor is composed of the primary swirling jet and the secondary non-swirling jet. A three dimension-laser particle dynamic analyzer (PDA) was employed to measure the instantaneous gas velocity. The probability density functions (PDF) for the instantaneous gas axial and tangential velocities at each measuring location, as well as the radial profiles of the root mean square of fluctuating gas axial and tangential velocities and the second-order moment for the fluctuating gas axial and tangential velocities are obtained. The measured results delineate the turbulence properties of the swirling reacting flow under the conditions of staged combustion.
Chemically reacting supersonic flow calculation using an assumed PDF model
Farshchi, M.
1990-01-01
This work is motivated by the need to develop accurate models for chemically reacting compressible turbulent flow fields that are present in a typical supersonic combustion ramjet (SCRAMJET) engine. In this paper the development of a new assumed probability density function (PDF) reaction model for supersonic turbulent diffusion flames and its implementation into an efficient Navier-Stokes solver are discussed. The application of this model to a supersonic hydrogen-air flame will be considered.
Laser Diagnostics for Reacting Flows
2010-01-11
absorption diagnostic for vapor-phase measurements in an evaporating n-decane aerosol,” Appied Physics B. 97, 215-225, (2009). 30. J.M. Porter, J.B...fluorescence of toluene for time- resolved imaging of gaseous flows,” Appied Physics B, 2010, in press. 35. J.M. Porter, J.B. Jeffries and R.K. Hanson
Algorithm For Computation Of Chemically Reacting Flow
Chen, Yen-Sen; Chen, Chein-Pin; Shang, Huan-Min
1995-01-01
Efficient algorithm developed for use in solving differential equations of transient, chemically reacting flows at all speeds from zero to high mach numbers. Mathematical models represent coupling of thermal, chemical, and dynamical effects. Original intended application is numerical simulation of flows in rocket engines; also applicable to other complex flows affected by finite-rate chemistry - for example, flows in turbines and in internal-combustion engines.
Numerical Methods For Chemically Reacting Flows
Leveque, R. J.; Yee, H. C.
1990-01-01
Issues related to numerical stability, accuracy, and resolution discussed. Technical memorandum presents issues in numerical solution of hyperbolic conservation laws containing "stiff" (relatively large and rapidly changing) source terms. Such equations often used to represent chemically reacting flows. Usually solved by finite-difference numerical methods. Source terms generally necessitate use of small time and/or space steps to obtain sufficient resolution, especially at discontinuities, where incorrect mathematical modeling results in unphysical solutions.
Lee, Insu
Confined non-reacting turbulent jets are ideal for recirculating the hot flue gas back into the furnace from an external exhaust duct. Such jets are also used inside the furnace to internally entrain and recirculate the hot flue gas to preheat and dilute the reactants. Both internal and external implementation of confined turbulent jets increase the furnace thermal efficiency. For external implementation, depending on the circumstances, the exhaust gas flow may be co- or counter-flow relative to the jet flow. Inside the furnaces, fuel and air jets are injected separately. To create a condition which can facilitate near homogeneous combustion, these jets have to first mix with the burned gas inside the furnace and simultaneously being heated and diluted prior to combustion. Clearly, the combustion pattern and emissions from reacting confined turbulent jets are affected by jet interactions, mixing and entrainment of hot flue gas. In this work, the flow and mixing characteristics of a non-reacting and reacting confined turbulent jet are investigated experimentally and numerically. This work consists of two parts: (i) A study of flow and mixing characteristics of non-reacting confined turbulent jets with co- or counter-flowing exhaust/flue gas. Here the axial and radial distributions of temperature, velocity and NO concentration (used as a tracer gas) were measured. FLUENT was used to numerically simulate the experimental results. This work provides the basic understanding of the flow and mixing characteristics of confined turbulent jets and develops some design considerations for recirculating flue gas back into the furnace as expressed by the recirculation zone and the stagnation locations. (ii) Numerical calculations of near homogeneous combustion are performed for the existing furnace. The exact geometry of the furnace in the lab is used and the real dimensional boundary conditions are considered. The parameters such as air nozzle diameter (dair), fuel nozzle
Nonlinear Krylov acceleration of reacting flow codes
Kumar, S.; Rawat, R.; Smith, P.; Pernice, M. [Univ. of Utah, Salt Lake City, UT (United States)
1996-12-31
We are working on computational simulations of three-dimensional reactive flows in applications encompassing a broad range of chemical engineering problems. Examples of such processes are coal (pulverized and fluidized bed) and gas combustion, petroleum processing (cracking), and metallurgical operations such as smelting. These simulations involve an interplay of various physical and chemical factors such as fluid dynamics with turbulence, convective and radiative heat transfer, multiphase effects such as fluid-particle and particle-particle interactions, and chemical reaction. The governing equations resulting from modeling these processes are highly nonlinear and strongly coupled, thereby rendering their solution by traditional iterative methods (such as nonlinear line Gauss-Seidel methods) very difficult and sometimes impossible. Hence we are exploring the use of nonlinear Krylov techniques (such as CMRES and Bi-CGSTAB) to accelerate and stabilize the existing solver. This strategy allows us to take advantage of the problem-definition capabilities of the existing solver. The overall approach amounts to using the SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) method and its variants as nonlinear preconditioners for the nonlinear Krylov method. We have also adapted a backtracking approach for inexact Newton methods to damp the Newton step in the nonlinear Krylov method. This will be a report on work in progress. Preliminary results with nonlinear GMRES have been very encouraging: in many cases the number of line Gauss-Seidel sweeps has been reduced by about a factor of 5, and increased robustness of the underlying solver has also been observed.
Multiple collisions in turbulent flows
kuhle, Michel Voß; Wilkinson, Michael; Pumir, Alain
2013-01-01
In turbulent suspensions, collision rates determine how rapidly particles coalesce or react with each other. To determine the collision rate, many numerical studies rely on the 'Ghost Collision Approximation' (GCA), which simply records how often pairs of point particles come within a threshold distance. In many applications, the suspended particles stick (or in the case of liquid droplets, coalesce) upon collision, and it is the frequency of first contact which is of interest. If a pair of 'ghost' particles undergoes multiple collisions, the GCA may overestimate the true collision rate. Here, using fully resolved Direct Numerical Simulations of turbulent flows at moderate Reynolds number (R_\\lambda = 130), we investigate the prevalence and properties of multiple collisions. We demonstrate that the GCA leads to a systematic overestimate of the collision rate, which is of the order of 15% when the particle inertia is small, and slowly decreases when inertia increases. We investigate the probability P(N) for a ...
Localized turbulence in pipe flow
Kuik, D.J.
2011-01-01
In this thesis the transition to turbulence in pipe flow is investigated. At low Reynolds numbers, the flow returns to the laminar state spontaneously. At high Reynolds number a small perturbation causes the flow to suddenly become turbulent. In the intermediate regime localized turbulence is observ
Hoffie, Andreas Frank
model. The chemical reaction is simulated with a global single-step, second-order equilibrium reaction with an Arrhenius reaction rate. The two benchmark cases of constant density reacting and variable density non-reacting shear layers used to determine ODT parameters yield perfect agreement with regards to first and second-order flow statistics as well as shear layer growth rate. The variable density non-reacting shear layer also serves as a testing case for the LES-ODT model to simulate passive scalar mixing. The variable density, reacting shear layer cases only agree reasonably well and indicate that more work is necessary to improve variable density coupling of ODT and LES. The disagreement is attributed to the fact that the ODT filtered density is kept constant across the Runge-Kutta steps. Furthermore, a more in-depth knowledge of large scale and subgrid turbulent kinetic energy (TKE) spectra at several downstream locations as well as TKE budgets need to be studied to obtain a better understanding about the model as well as about the flow under investigation. The local Reynolds number based on the one-percent thickness at the exit is Redelta ≈ 5300, for the constant density reacting and for the variable density non-reacting case. For the variable density reacting shear layer, the Reynolds number based on the 1% thickness is Redelta ≈ 2370. The variable density reacting shear layers show suppressed growth rates due to density variations caused by heat release. This has also been reported in literature. A Lewis number parameter study is performed to extract non-unity Lewis number effects. An increase in the Lewis number leads to a further suppression of the growth rate, however to an increase spread of second-order flow statistics. Major focus and challenge of this work is to improve and advance the three-dimensional coupling of the one-dimensional ODT domains while keeping the solution correct. This entails major restructuring of the model. The turbulent
Measurements of Non-reacting and Reacting Flow Fields of a Liquid Swirl Flame Burner
CHONG Cheng Tung; HOCHGREB Simone
2015-01-01
The understanding of the liquid fuel spray and flow field characteristics inside a combustor is crucial for designing a fuel efficient and low emission device. Characterisation of the flow field of a model gas turbine liquid swirl burner is performed by using a 2-D particle imaging velocimetry(PIV) system. The flow field pattern of an axial flow burner with a fixed swirl intensity is compared under confined and unconfined conditions, i.e., with and without the combustor wall. The effect of temperature on the main swirling air flow is investigated under open and non-reacting conditions. The result shows that axial and radial velocities increase as a result of decreased flow density and increased flow volume. The flow field of the main swirling flow with liquid fuel spray injection is compared to non-spray swirling flow. Introduction of liquid fuel spray changes the swirl air flow field at the burner outlet, where the radial velocity components increase for both open and confined environment. Under reacting condition, the enclosure generates a corner recirculation zone that intensifies the strength of radial velocity. The reverse flow and corner recirculation zone assists in stabilizing the flame by preheating the reactants. The flow field data can be used as validation target for swirl combustion modelling.
Species Source Term Mapping for Reacting Flow CFD Project
National Aeronautics and Space Administration — Simulations of reacting flow in applications such as scramjet engines are currently limited in their utility or accuracy by the chemistry sub-models employed....
Simultaneous Temperature and Velocity Diagnostic for Reacting Flows Project
National Aeronautics and Space Administration — A diagnostic technique is proposed for measuring temperature and velocity simultaneously in a high temperature reacting flow for aiding research in propulsion. The...
Borg, A.; Revstedt, J.
1996-04-01
The purpose of this work has been to do a preliminary study of how well numerical calculations with different turbulence models can predict the flow and temperature fields of a strongly swirling and combusting flow in an experimental combustion chamber and to see which parameters in the mathematical model are the most important. The combustion chamber on which we have done the calculations is called Validation Rig II and was designed by Volvo Aero Corporation. The main part of the study has been carried out on a non-reacting flow but some work has also been done on reacting flow. In most cases it has not been meaningful to compare the calculations with the measurements because they differ quite a lot from each other. For the non-reacting case the following investigations have been made: * How the solution differs for different turbulence models, * The solutions sensitivity to inlet boundary conditions, * How different types of leakage disturb the flow, and * The difference in results between two different CFD-codes, the commercial code CFDS-Flow3D and a code developed at the department of fluid mechanics. For the reacting cases we have studied the influence of: * one or two reaction steps, * the effects of a change in reaction rate, * the influence of thermal radiation, and * the effects of changing the boundary conditions for temperature on the walls. The results from these calculations show that the inlet turbulence intensity has very little effect on the values of the turbulent quantities as well as the velocity profiles at the outlet. Changing the turbulence model or the outlet boundary conditions gives some change in velocity profiles at the outlet but only marginal effects on the swirl number. 21 refs, 54 figs, 19 tabs
Controllability of flow turbulence.
Guan, Shuguang; Wei, G W; Lai, C-H
2004-06-01
In this paper, we study the controllability of real-world flow turbulence governed by the two-dimensional Navier-Stokes equations, using strategies developed in chaos control. A case of control/synchronization of turbulent dynamics is observed when only one component of the velocity field vector is unidirectionally coupled to a target state, while the other component is uncoupled. Unlike previous results, it is shown that the dynamics of the whole velocity field cannot be completely controlled/synchronized to the target, even in the limit of long time and strong coupling strength. It is further revealed that the controlled component of the velocity field can be fully controlled/synchronized to the target, but the other component, which is not directly coupled to the target, can only be partially controlled/synchronized to the target. By extending an auxiliary method to distributed dynamic systems, the partial synchronization of two turbulent orbits in the present study can be categorized in the domain of generalized synchronization of spatiotemporal dynamics.
Turbulence Measurements in Swirling Flows
V. M. Domkundwar
1981-10-01
Full Text Available Investigation have been conducted to find out the region of high turbulent intensities in a swirling jet passing through a divergent passage. A hot wire anemometer is used to measure the turbulence intensity using a four position method. It has been concluded that the jet spreads with increasing diffuser angle and the region of high turbulent intensity also spreads. The high turbulence intensity region lies around the recirculation zone and it decays rapidly along the main flow direction.
Statistical analysis of the velocity and scalar fields in reacting turbulent wall-jets
Pouransari, Z; Johansson, A V
2015-01-01
The concept of local isotropy in a chemically reacting turbulent wall-jet flow is addressed using direct numerical simulation (DNS) data. Different DNS databases with isothermal and exothermic reactions are examined. The chemical reaction and heat release effects on the turbulent velocity, passive scalar and reactive species fields are studied using their probability density functions (PDF) and higher order moments for velocities and scalar fields, as well as their gradients. With the aid of the anisotropy invariant maps for the Reynolds stress tensor the heat release effects on the anisotropy level at different wall-normal locations are evaluated and found to be most accentuated in the near-wall region. It is observed that the small-scale anisotropies are persistent both in the near-wall region and inside the jet flame. Two exothermic cases with different Damkohler number are examined and the comparison revealed that the Damkohler number effects are most dominant in the near-wall region, where the wall cooli...
Numerical methods for turbulent flow
Turner, James C., Jr.
1988-01-01
It has generally become accepted that the Navier-Strokes equations predict the dynamic behavior of turbulent as well as laminar flows of a fluid at a point in space away form a discontinuity such as a shock wave. Turbulence is also closely related to the phenomena of non-uniqueness of solutions of the Navier-Strokes equations. These second order, nonlinear partial differential equations can be solved analytically for only a few simple flows. Turbulent flow fields are much to complex to lend themselves to these few analytical methods. Numerical methods, therefore, offer the only possibility of achieving a solution of turbulent flow equations. In spite of recent advances in computer technology, the direct solution, by discrete methods, of the Navier-Strokes equations for turbulent flow fields is today, and in the foreseeable future, impossible. Thus the only economically feasible way to solve practical turbulent flow problems numerically is to use statistically averaged equations governing mean-flow quantities. The objective is to study some recent developments relating to the use of numerical methods to study turbulent flow.
Flow Field Effects on Nucleation in a Reacting Mixture Layer.
1984-11-01
chemically reacting flows has been analysed by Fendell (1965) who considered the effect of the straining motion in a stagnation point flow on ignition...stagnation point diffusion flame ( Fendell , 1965, Linan, 1974). In the present study the effect of the strain rate or velocity gradient on nucleation kinetics...Symposium (International) on Corn- bustion, 799-810, Academic Press. Fendell , F. E. (1965). Ignition and extinction in combustion of initially unmixed
Suresha, Suhas; Sujith, R. I.; Emerson, Benjamin; Lieuwen, Tim
2016-10-01
The flame or flow behavior of a turbulent reacting wake is known to be fundamentally different at high and low values of flame density ratio (ρu/ρb ), as the flow transitions from globally stable to unstable. This paper analyzes the nonlinear dynamics present in a bluff-body stabilized flame, and identifies the transition characteristics in the wake as ρu/ρb is varied over a Reynolds number (based on the bluff-body lip velocity) range of 1000-3300. Recurrence quantification analysis (RQA) of the experimentally obtained time series of the flame edge fluctuations reveals that the time series is highly aperiodic at high values of ρu/ρb and transitions to increasingly correlated or nearly periodic behavior at low values. From the RQA of the transverse velocity time series, we observe that periodicity in the flame oscillations are related to periodicity in the flow. Therefore, we hypothesize that this transition from aperiodic to nearly periodic behavior in the flame edge time series is a manifestation of the transition in the flow from globally stable, convective instability to global instability as ρu/ρb decreases. The recurrence analysis further reveals that the transition in periodicity is not a sudden shift; rather it occurs through an intermittent regime present at low and intermediate ρu/ρb . During intermittency, the flow behavior switches between aperiodic oscillations, reminiscent of a globally stable, convective instability, and periodic oscillations, reminiscent of a global instability. Analysis of the distribution of the lengths of the periodic regions in the intermittent time series and the first return map indicate the presence of type-II intermittency.
Compressibility, turbulence and high speed flow
Gatski, Thomas B
2013-01-01
Compressibility, Turbulence and High Speed Flow introduces the reader to the field of compressible turbulence and compressible turbulent flows across a broad speed range, through a unique complimentary treatment of both the theoretical foundations and the measurement and analysis tools currently used. The book provides the reader with the necessary background and current trends in the theoretical and experimental aspects of compressible turbulent flows and compressible turbulence. Detailed derivations of the pertinent equations describing the motion of such turbulent flows is provided and
Numerical Computation of the Chemically Reacting Flow around the National Aero-Space Plane
Tannehill, J. C.
1999-01-01
This final report summarizes the research accomplished. The research performed during the grant period can be divided into the following major areas: (1) Computation of chemically reacting Supersonic combustion ramjet (scramjet) flowfields. (2) Application of a two-equation turbulence model to supersonic combustion flowfields. (3) Computation of the integrated aerodynamic and propulsive flowfields of a generic hypersonic space plane. (4) Computation of hypersonic flows with finite-catalytic walls. (5) Development of an upwind Parabolized Navier-Stokes (PNS) code for thermo-chemical nonequilibrium flows.
Turbulent transport in hydromagnetic flows
Brandenburg, A; Del Sordo, F; Hubbard, A; Käpylä, P J; Rheinhardt, M
2010-01-01
The predictive power of mean-field theory is emphasized by comparing theory with simulations under controlled conditions. The recently developed test-field method is used to extract turbulent transport coefficients both in kinematic as well as nonlinear and quasi-kinematic cases. A striking example of the quasi-kinematic method is provided by magnetic buoyancy-driven flows that produce an alpha effect and turbulent diffusion.
Multiphase integral reacting flow computer code (ICOMFLO): User`s guide
Chang, S.L.; Lottes, S.A.; Petrick, M.
1997-11-01
A copyrighted computational fluid dynamics computer code, ICOMFLO, has been developed for the simulation of multiphase reacting flows. The code solves conservation equations for gaseous species and droplets (or solid particles) of various sizes. General conservation laws, expressed by elliptic type partial differential equations, are used in conjunction with rate equations governing the mass, momentum, enthalpy, species, turbulent kinetic energy, and turbulent dissipation. Associated phenomenological submodels of the code include integral combustion, two parameter turbulence, particle evaporation, and interfacial submodels. A newly developed integral combustion submodel replacing an Arrhenius type differential reaction submodel has been implemented to improve numerical convergence and enhance numerical stability. A two parameter turbulence submodel is modified for both gas and solid phases. An evaporation submodel treats not only droplet evaporation but size dispersion. Interfacial submodels use correlations to model interfacial momentum and energy transfer. The ICOMFLO code solves the governing equations in three steps. First, a staggered grid system is constructed in the flow domain. The staggered grid system defines gas velocity components on the surfaces of a control volume, while the other flow properties are defined at the volume center. A blocked cell technique is used to handle complex geometry. Then, the partial differential equations are integrated over each control volume and transformed into discrete difference equations. Finally, the difference equations are solved iteratively by using a modified SIMPLER algorithm. The results of the solution include gas flow properties (pressure, temperature, density, species concentration, velocity, and turbulence parameters) and particle flow properties (number density, temperature, velocity, and void fraction). The code has been used in many engineering applications, such as coal-fired combustors, air
Microbubble clustering in turbulent flow
Calzavarini, E; Luther, S; Toschi, F; Van den Berg, T H; Berg, Thomas H. van den; Calzavarini, Enrico; Lohse, Detlef; Luther, Stefan; Toschi, Federico
2006-01-01
Single-point hot-wire measurements in the bulk of a turbulent channel have been performed in order to detect and quantify the phenomenon of preferential bubble accumulation. We show that statistical analysis of the bubble-probe colliding-times series can give a robust method for investigation of clustering in the bulk regions of a turbulent flow where, due to the opacity of the flow, no imaging technique can be employed. We demonstrate that micro-bubbles (radius R_0 ~ 0.1 mm) in a developed turbulent flow, where the Kolmogorov length-scale is, eta ~ R_0, display preferential concentration in small scale structures with a typical statistical signature ranging from the dissipative range, O(eta), up to the lower end of inertial range, O(100 eta). A comparison with Eulerian-Lagrangian numerical simulations is also performed and arising similarities and differences are discussed.
Toward parallel, adaptive mesh refinement for chemically reacting flow simulations
Devine, K.D.; Shadid, J.N.; Salinger, A.G. Hutchinson, S.A. [Sandia National Labs., Albuquerque, NM (United States); Hennigan, G.L. [New Mexico State Univ., Las Cruces, NM (United States)
1997-12-01
Adaptive numerical methods offer greater efficiency than traditional numerical methods by concentrating computational effort in regions of the problem domain where the solution is difficult to obtain. In this paper, the authors describe progress toward adding mesh refinement to MPSalsa, a computer program developed at Sandia National laboratories to solve coupled three-dimensional fluid flow and detailed reaction chemistry systems for modeling chemically reacting flow on large-scale parallel computers. Data structures that support refinement and dynamic load-balancing are discussed. Results using uniform refinement with mesh sequencing to improve convergence to steady-state solutions are also presented. Three examples are presented: a lid driven cavity, a thermal convection flow, and a tilted chemical vapor deposition reactor.
Numerical experiments modelling turbulent flows
Trefilík Jiří
2014-03-01
Full Text Available The work aims at investigation of the possibilities of modelling transonic flows mainly in external aerodynamics. New results are presented and compared with reference data and previously achieved results. For the turbulent flow simulations two modifications of the basic k – ω model are employed: SST and TNT. The numerical solution was achieved by using the MacCormack scheme on structured non-ortogonal grids. Artificial dissipation was added to improve the numerical stability.
Suppression of turbulent resistivity in turbulent Couette flow
Si, Jiahe, E-mail: jsi@nmt.edu; Sonnenfeld, Richard G.; Colgate, Arthur S.; Westpfahl, David J.; Romero, Van D.; Martinic, Joe [New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801 (United States); Colgate, Stirling A.; Li, Hui [Los Alamos National Laboratory, Los Alamos, New Mexico 87544 (United States); Nornberg, Mark D. [University of Wisconsin-Madison, Madison, Wisconsin 53706 (United States)
2015-07-15
Turbulent transport in rapidly rotating shear flow very efficiently transports angular momentum, a critical feature of instabilities responsible both for the dynamics of accretion disks and the turbulent power dissipation in a centrifuge. Turbulent mixing can efficiently transport other quantities like heat and even magnetic flux by enhanced diffusion. This enhancement is particularly evident in homogeneous, isotropic turbulent flows of liquid metals. In the New Mexico dynamo experiment, the effective resistivity is measured using both differential rotation and pulsed magnetic field decay to demonstrate that at very high Reynolds number rotating shear flow can be described entirely by mean flow induction with very little contribution from correlated velocity fluctuations.
Turbulence introduction to theory and applications of turbulent flows
Westerweel, Jerry; Nieuwstadt, Frans T M
2016-01-01
This book provides a general introduction to the topic of turbulent flows. Apart from classical topics in turbulence, attention is also paid to modern topics. After studying this work, the reader will have the basic knowledge to follow current topics on turbulence in scientific literature. The theory is illustrated with a number of examples of applications, such as closure models, numerical simulations and turbulent diffusion, and experimental findings. The work also contains a number of illustrative exercises.
3D Reacting Flow Analysis of LANTR Nozzles
Stewart, Mark E. M.; Krivanek, Thomas M.; Hemminger, Joseph A.; Bulman, M. J.
2006-01-01
This paper presents performance predictions for LANTR nozzles and the system implications for their use in a manned Mars mission. The LANTR concept is rocket thrust augmentation by injecting Oxygen into the nozzle to combust the Hydrogen exhaust of a Nuclear Thermal Rocket. The performance predictions are based on three-dimensional reacting flow simulations using VULCAN. These simulations explore a range of O2/H2 mixture ratios, injector configurations, and concepts. These performance predictions are used for a trade analysis within a system study for a manned Mars mission. Results indicate that the greatest benefit of LANTR will occur with In-Situ Resource Utilization (ISRU). However, Hydrogen propellant volume reductions may allow greater margins for fitting tanks within the launch vehicle where packaging issues occur.
Numerical experiments for turbulent flows
Příhoda Jaromír
2013-04-01
Full Text Available The aim of the work is to explore the possibilities of modelling transonic flows in the internal and external aerodynamics. Several configurations were analyzed and calculations were performed using both inviscid and viscous models of flow. Viscous turbulent flows have been simulated using either zero equation algebraic Baldwin-Lomax model and two equation k—ω model in its basic version and improved TNT variant. The numerical solution was obtained using Lax-Wendroff scheme in the MacCormack form on structured non-ortogonal grids. Artificial dissipation was added to improve the numerical stability. Achieved results are compared with experimental data.
Numerical simulation of high speed chemically reacting flows
Schuricht, Scott Richard
. The scramjet case demonstrates the usefulness of the method as a predictive tool for reacting flows. The rocket motor nozzle case demonstrates the capability of the method to handle mixed subsonic/transonic/supersonic flow environments. Five different chemical reaction mechanisms were studied illustrating the flexibility of the method.
Turbulent spots in hypervelocity flow
Jewell, Joseph S.; Leyva, Ivett A.; Shepherd, Joseph E.
2017-04-01
The turbulent spot propagation process in boundary layer flows of air, nitrogen, carbon dioxide, and air/carbon dioxide mixtures in thermochemical nonequilibrium at high enthalpy is investigated. Experiments are performed in a hypervelocity reflected shock tunnel with a 5-degree half-angle axisymmetric cone instrumented with flush-mounted fast-response coaxial thermocouples. Time-resolved and spatially demarcated heat transfer traces are used to track the propagation of turbulent bursts within the mean flow, and convection rates at approximately 91, 74, and 63% of the boundary layer edge velocity, respectively, are observed for the leading edge, peak, and trailing edge of the spots. A simple model constructed with these spot propagation parameters is used to infer spot generation rates from observed transition onset to completion distance. Spot generation rates in air and nitrogen are estimated to be approximately twice the spot generation rates in air/carbon dioxide mixtures.
DEPOSITION OF PARTICLES IN TURBULENT PIPE FLOW
Xiaowei Luo; Suyuan Yu
2006-01-01
The deposition of particles in turbulent pipe flow was investigated in terms of two mechanisms, turbulent and thermophoretic. A general equation incorporating these two mechanisms was formulated to calculate the deposition efficiency of aerosol particles in turbulent pipe flow together with thermophoretic deposition. The validity of the equation was confirmed by good agreement between calculated and measured results.
Dynamic Load Balancing Strategies for Parallel Reacting Flow Simulations
Pisciuneri, Patrick; Meneses, Esteban; Givi, Peyman
2014-11-01
Load balancing in parallel computing aims at distributing the work as evenly as possible among the processors. This is a critical issue in the performance of parallel, time accurate, flow simulators. The constraint of time accuracy requires that all processes must be finished with their calculation for a given time step before any process can begin calculation of the next time step. Thus, an irregularly balanced compute load will result in idle time for many processes for each iteration and thus increased walltimes for calculations. Two existing, dynamic load balancing approaches are applied to the simplified case of a partially stirred reactor for methane combustion. The first is Zoltan, a parallel partitioning, load balancing, and data management library developed at the Sandia National Laboratories. The second is Charm++, which is its own machine independent parallel programming system developed at the University of Illinois at Urbana-Champaign. The performance of these two approaches is compared, and the prospects for their application to full 3D, reacting flow solvers is assessed.
PDF methods for turbulent reactive flows
Hsu, Andrew T.
1995-01-01
Viewgraphs are presented on computation of turbulent combustion, governing equations, closure problem, PDF modeling of turbulent reactive flows, validation cases, current projects, and collaboration with industry and technology transfer.
BOOK REVIEW: Statistical Mechanics of Turbulent Flows
Cambon, C.
2004-10-01
This is a handbook for a computational approach to reacting flows, including background material on statistical mechanics. In this sense, the title is somewhat misleading with respect to other books dedicated to the statistical theory of turbulence (e.g. Monin and Yaglom). In the present book, emphasis is placed on modelling (engineering closures) for computational fluid dynamics. The probabilistic (pdf) approach is applied to the local scalar field, motivated first by the nonlinearity of chemical source terms which appear in the transport equations of reacting species. The probabilistic and stochastic approaches are also used for the velocity field and particle position; nevertheless they are essentially limited to Lagrangian models for a local vector, with only single-point statistics, as for the scalar. Accordingly, conventional techniques, such as single-point closures for RANS (Reynolds-averaged Navier-Stokes) and subgrid-scale models for LES (large-eddy simulations), are described and in some cases reformulated using underlying Langevin models and filtered pdfs. Even if the theoretical approach to turbulence is not discussed in general, the essentials of probabilistic and stochastic-processes methods are described, with a useful reminder concerning statistics at the molecular level. The book comprises 7 chapters. Chapter 1 briefly states the goals and contents, with a very clear synoptic scheme on page 2. Chapter 2 presents definitions and examples of pdfs and related statistical moments. Chapter 3 deals with stochastic processes, pdf transport equations, from Kramer-Moyal to Fokker-Planck (for Markov processes), and moments equations. Stochastic differential equations are introduced and their relationship to pdfs described. This chapter ends with a discussion of stochastic modelling. The equations of fluid mechanics and thermodynamics are addressed in chapter 4. Classical conservation equations (mass, velocity, internal energy) are derived from their
Simulation and modeling of turbulent flows
Gatski, Thomas B; Lumley, John L
1996-01-01
This book provides students and researchers in fluid engineering with an up-to-date overview of turbulent flow research in the areas of simulation and modeling. A key element of the book is the systematic, rational development of turbulence closure models and related aspects of modern turbulent flow theory and prediction. Starting with a review of the spectral dynamics of homogenous and inhomogeneous turbulent flows, succeeding chapters deal with numerical simulation techniques, renormalization group methods and turbulent closure modeling. Each chapter is authored by recognized leaders in their respective fields, and each provides a thorough and cohesive treatment of the subject.
Effect of the nature of vitiated crossflow on the flow-field of a transverse reacting jet
Panda, Pratikash P.; Busari, Oluwatobi; Lucht, Robert P.; Laster, Walter R.
2017-02-01
The effect of the nature of vitiated crossflow on the structure and dynamics of non-reacting/reacting transverse jets is investigated. In this study, the vitiated crossflow is produced either by a low-swirl burner (LSB) that adds a swirling component to the crossflow or a bluff-body burner (BBB) that produces a uniform crossflow. The jet fluid is injected through a contoured injector, which provides a top-hat velocity profile. The swirling crossflow exhibits considerable swirl at the point of injection of the transverse jet. Two component high-repetition-rate PIV measurements demonstrate the influence of a vitiated crossflow generated by a low-swirl/bluff-body burner on the near-wake flow-field of the jet. Measurements at a plane below the injection location of the jet indicate that there is a continuous entrainment of PIV particles in case of swirling crossflow. The time-averaged flow-field shows that the velocity field for reacting/non-reacting jets in the LSB crossflow exhibits higher velocity gradients, in the measurement plane along jet cross section, as compared to BBB crossflow. It is found that the vorticity magnitude is lower in case of jets in the BBB crossflow and there is a delay in the formation of the wake vortex structure. The conditional turbulent statistics of the jet flow-field in the two crossflows shows that there is a higher degree of intermittency related to the wake vortex structure in case of a BBB crossflow, which results in a non-Gaussian distribution of the turbulent statistics. The wake Strouhal number calculation shows the influence of the nature of crossflow on the rate of wake vortex shedding. The wake Strouhal number for the jets in BBB crossflow is found to be lower than for the LSB crossflow. A decrease in the wake Strouhal number is observed with an increase in the nozzle separation distance. There is an increase in the dilatation rate owing to heat release which results in higher wake Strouhal number for reacting jets as compared
Mathematical modeling of turbulent reacting plumes - I. General theory and model formulation
Georgopoulos, P.G.; Seinfeld, J.H.
1986-01-01
A new, comprehensive model for a chemically reacting plume is presented that accounts for the effects of incomplete turbulent macro- and micromixing on chemical reactions between plume and atmospheric constituents. The model is modular in nature, allowing for the use of different levels of approximation of the phenomena involved. The core of the model consists of the evolution equations for reaction progress variables appropriate for evolving spatially varying systems. These equations estimate the interaction of mixing and chemical reaction and require input parameters characterizing internal plume behavior, such as relative dispersion and fine scale plume segregation. The model addresses deficiencies in previous reactive plume models. Part II is devoted to atmospheric application of the model. (authors).
Turbulent fluctuations around Bjorken flow
Floerchinger, Stefan
2011-01-01
We study the evolution of local event-by-event deviations from smooth average fluid dynamic fields, as they can arise in heavy ion collisions from the propagation of fluctuating initial conditions. Local fluctuations around Bjorken flow are found to be governed by non-linear equations whose solutions can be characterized qualitatively in terms of Reynolds numbers. Perturbations at different rapidities decouple quickly, and satisfy (after suitable coordinate transformations) an effectively two-dimensional Navier-Stokes equation of non-relativistic form. We discuss the conditions under which non-linearities in these equations cannot be neglected and turbulent behavior is expected to set in.
Non-reacting Flow Analysis from Combustor Inlet to Outlet using Computational Fluid Dynamics Code
G. Ananda Reddy
2004-10-01
Full Text Available This paper describes non-reacting flow analysis of a gas turbine combustion system. The method is based on the solution of Navier-Strokes equations using generalised non-orthogonal coordinate system. The turbulence effects are modelled through the renormalisation group k-E model. The method has been applied to a practical gas turbine combustor. The combustionsystem includes swirler vane passages, fuel nozzles, rotor bleed, customer bleed, air-blast atomiser, swirl cone, and all holes in primary , dilution , dome, flare, and cooling ring. Thetotal geometry has been created using the pre-processors GAMBIT and CATIA, and the meshing has been done using GAMBIT, and the analysis carried out in a FLUENT solver. The interaction between the diffuser and the combustor external flows plays a key role in controlling the pressure loss, air flow distribution around the combustor liner, durability, and stability. The aero gas turbine combustor designs are generally guided by experimental methods and past experience; however, experimental methods are inherently slow, costly, especially at hightemperature engine-operating conditions. These drawbacks and the growing need to understand the complex flow-field phenomenon involved, have led to the development of a numericalmodel for predicting flow in the gas turbine combustor. These models are used to optimise the design of the combustor and its subcomponents, and reduce cost, time, and the number ofexperiments.
Approximate Model for Turbulent Stagnation Point Flow.
Dechant, Lawrence [Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)
2016-01-01
Here we derive an approximate turbulent self-similar model for a class of favorable pressure gradient wedge-like flows, focusing on the stagnation point limit. While the self-similar model provides a useful gross flow field estimate this approach must be combined with a near wall model is to determine skin friction and by Reynolds analogy the heat transfer coefficient. The combined approach is developed in detail for the stagnation point flow problem where turbulent skin friction and Nusselt number results are obtained. Comparison to the classical Van Driest (1958) result suggests overall reasonable agreement. Though the model is only valid near the stagnation region of cylinders and spheres it nonetheless provides a reasonable model for overall cylinder and sphere heat transfer. The enhancement effect of free stream turbulence upon the laminar flow is used to derive a similar expression which is valid for turbulent flow. Examination of free stream enhanced laminar flow suggests that the rather than enhancement of a laminar flow behavior free stream disturbance results in early transition to turbulent stagnation point behavior. Excellent agreement is shown between enhanced laminar flow and turbulent flow behavior for high levels, e.g. 5% of free stream turbulence. Finally the blunt body turbulent stagnation results are shown to provide realistic heat transfer results for turbulent jet impingement problems.
Turbulent combustion flow through variable cross section channel
Rogov, B.V.; Sokolova, I.A.
1999-07-01
The object of this study is to develop a new evolutionary numerical method for solving direct task of Laval nozzle, which provides non-iterative calculations of chemical reacting turbulent flows with detailed kinetic chemistry. The numerical scheme of fourth order along the normal coordinate and second order along the streamwise one is derived for calculation of difference-differential equations of the second order and the first order. Marching method provides the possibility of computing field flow in subsonic section of nozzle and near an expansion. Critical mass consumption is calculated with controlled accuracy. After critical cross section of nozzle a combined marching method with global iterations over axial pressure (only) makes it possible to overcome ill posedness of mixed supersonic flow and calculate the whole flow field near and after critical cross section. Numerical results are demonstrated on turbulent burning hydrogen-oxygen flow through Laval nozzle with curvature of wall K{sub w} = 0.5.
Aeroacoustic Computations for Turbulent Airfoil Flows
Shen, Wen Zhong; Zhu, Wei Jun; Sørensen, Jens Nørkær
2009-01-01
The How-acoustic splitting technique for aeroacoustic computations is extended to simulate the propagation of acoustic waves generated by three-dimensional turbulent flows. In the flow part, a subgrid-scale turbulence model (the mixed model) is employed for large-eddy simulations. The obtained in...
Computational fluid dynamics incompressible turbulent flows
Kajishima, Takeo
2017-01-01
This textbook presents numerical solution techniques for incompressible turbulent flows that occur in a variety of scientific and engineering settings including aerodynamics of ground-based vehicles and low-speed aircraft, fluid flows in energy systems, atmospheric flows, and biological flows. This book encompasses fluid mechanics, partial differential equations, numerical methods, and turbulence models, and emphasizes the foundation on how the governing partial differential equations for incompressible fluid flow can be solved numerically in an accurate and efficient manner. Extensive discussions on incompressible flow solvers and turbulence modeling are also offered. This text is an ideal instructional resource and reference for students, research scientists, and professional engineers interested in analyzing fluid flows using numerical simulations for fundamental research and industrial applications. • Introduces CFD techniques for incompressible flow and turbulence with a comprehensive approach; • Enr...
Flow-distributed oscillations: Stationary chemical waves in a reacting flow
Kærn, Mads; Menzinger, Michael
1999-10-01
A recent prediction of stationary waves in open, reacting flows is experimentally verified. We show that stationary waves are generated by a mechanism whereby the flow carries a time-oscillating subelement, behaving like a batch reactor, through space while a fixed boundary condition at the inflow locks the phase of the oscillation. This mechanism can generate stationary patterns when all diffusion coefficients are equal. The experimental system is the ferroin-catalyzed Belousov-Zhabotinsky reaction in a tubular reactor, fed by the outflow of a continuous flow stirred tank reactor (CSTR). Parameter conditions are such that the concentrations are constant in the CSTR while they oscillate in the flow tube.
Simulation of Compressible Multi-Phase Turbulent Reacting Flows
2008-09-01
positivity and entropy preservation. The known instability of this solver to odd-even decoupling and carbuncle phenomenon is cured by employing the HLLE...Two-Shock Riemann Solver and the Roe Riemann solver with Harten-Hyman entropy corrections showed very strong sensitiveness to the instability. The...Paper 2005–0314, 2005. [23] B. Fryxell, K. Olson, P. Ricker, F. X. Timmes, M. Zingale, D. Q. Lamb , P. Mac- Neice, R. Rosner, J. W. Truran, and H. Tufo
RECENT ADVANCES IN STUDIES ON MULTIPHASE AND REACTING FLOWS IN CHINA
周力行
2002-01-01
The recent developments and advances of studies on multiphase and reacting flows, including gas-solid, gas-liquid, liquid-solid and reacting flows, in China are reviewed. Special emphasis is laid on the fundamental studies and numerical models. Some important experimental results are also reported. But measurement techniques are not covered.
COARSE-GRID SIMULATION OF REACTING AND NON-REACTING GAS-PARTICLE FLOWS
Sankaran Sundaresan
2004-03-01
The principal goal of this project, funded under the ''DOE Vision 21 Virtual Demonstration Initiative'' is virtual demonstration of circulating fluidized bed performance. We had proposed a ''virtual demonstration tool'', which is based on the open-domain CFD code MFIX. The principal challenge funded through this grant is to devise and implement in this CFD code sound physical models for the rheological characteristics of the gas-particle mixtures. Within the past year, which was the third year of the project, we have made the following specific advances. (a) We have completed a study of the impact of sub-grid models of different levels of detail on the results obtained in coarse-grid simulations of gas-particle flow. (b) We have also completed a study of a model problem to understand the effect of wall friction, which was proved in our earlier work to be very important for stable operation of standpipes in a circulating fluidized bed circuit. These are described in a greater detail in this report.
Modelling of structural effects on chemical reactions in turbulent flows
Gammelsaeter, H.R.
1997-12-31
Turbulence-chemistry interactions are analysed using algebraic moment closure for the chemical reaction term. The coupling between turbulence and chemical length and time scales generate a complex interaction process. This interaction process is called structural effects in this work. The structural effects are shown to take place on all scales between the largest scale of turbulence and the scales of the molecular motions. The set of equations describing turbulent correlations involved in turbulent reacting flows are derived. Interactions are shown schematically using interaction charts. Algebraic equations for the turbulent correlations in the reaction rate are given using the interaction charts to include the most significant couplings. In the frame of fundamental combustion physics, the structural effects appearing on the small scales of turbulence are proposed modelled using a discrete spectrum of turbulent scales. The well-known problem of averaging the Arrhenius law, the specific reaction rate, is proposed solved using a presumed single variable probability density function and a sub scale model for the reaction volume. Although some uncertainties are expected, the principles are addressed. Fast chemistry modelling is shown to be consistent in the frame of algebraic moment closure when the turbulence-chemistry interaction is accounted for in the turbulent diffusion. The modelling proposed in this thesis is compared with experimental data for an laboratory methane flame and advanced probability density function modelling. The results show promising features. Finally it is shown a comparison with full scale measurements for an industrial burner. All features of the burner are captured with the model. 41 refs., 33 figs.
Secondary turbulent flow in an infinte bend
Christensen, H. Bo; Gislason, Kjartan; Fredsøe, Jørgen
1999-01-01
The flow in an infinite circular bend is inverstigated in both the laminar and fully turbulent flow case, by use of laminar flow solver, a k-e turbulence model, and a fully Reynolds stress turbulence model. The topic of the analysis is to investigate whether a counter-rotating secondary flow cell...... is formed near the surface at the outer bank. This cell might help to stabilise the bank and hereby be an important factor for the morphology in a meandering river. In the laminar runs stability criterion related to a Dean number was estabilshed. In the simulations with the k-e model and the Reynolds stress...... model, the influence of the curvature ratio and cross section geometry on the vortex pattern is investigated. Furthermore, it is demonstrated that an-isotropy of turbulence plays an important role for the structure of flow pattern and existence of an extra flow cell....
Turbulent Shear Layers in Supersonic Flow
Smits, Alexander J
2006-01-01
A good understanding of turbulent compressible flows is essential to the design and operation of high-speed vehicles. Such flows occur, for example, in the external flow over the surfaces of supersonic aircraft, and in the internal flow through the engines. Our ability to predict the aerodynamic lift, drag, propulsion and maneuverability of high-speed vehicles is crucially dependent on our knowledge of turbulent shear layers, and our understanding of their behavior in the presence of shock waves and regions of changing pressure. Turbulent Shear Layers in Supersonic Flow provides a comprehensive introduction to the field, and helps provide a basis for future work in this area. Wherever possible we use the available experimental work, and the results from numerical simulations to illustrate and develop a physical understanding of turbulent compressible flows.
Prediction of engine and near-field plume reacting flows in low-thrust chemical rockets
Weiss, Jonathan M.; Merkle, Charles L.
1993-01-01
A computational model is employed to study the reacting flow within the engine and near-field plumes of several small gaseous hydrogen-oxygen thrusters. The model solves the full Navier-Stokes equations coupled with species diffusion equations for a hydrogen-oxygen reaction kinetics system and includes a two-equation q-omega model for turbulence. Predictions of global performance parameters and localized flowfield variables are compared with experimental data in order to assess the accuracy with which these flowfields are modeled and to identify aspects of the model which require improvement. Predicted axial and radial velocities 3 mm downstream of the exit plane show reasonable agreement with the measurements. The predicted peak in axial velocity in the hydrogen film coolant along the nozzle wall shows the best agreement; however, predictions within the core region are roughly 15 percent below measured values, indicating an underprediction of the extent to which the hydrogen diffuses and mixes with the core flow. There is evidence that this is due to three-dimensional mixing processes which are not included in the axisymmetric model.
Identifying turbulent flow in carbonate aquifers
Worthington, Stephen R. H.; Soley, Robert W. N.
2017-09-01
Turbulent flow has a different hydraulic response compared to laminar flow and so it is important to be able to identify its occurrence in an aquifer, and to predict where it is likely to be found. Turbulent flow is associated with large apertures and rapid velocities, and these occur most frequently in carbonate aquifers. Methods for identifying turbulent flow include correlating spring discharge with head variation, calculating Reynolds numbers from spring discharge and tracer velocity, and plotting the spatial variation of head differences between high flow and low flow. The probability of turbulent flow increases as a function of permeability and of spring discharge, and the probability increases in a downgradient direction in an aquifer. Spring discharge is a key parameter for evaluating the presence of turbulent flow, which is likely to occur where a spring with a discharge > 1 L/s is fed by a single channel. Turbulent flow appears to be a major contributing factor to the occurrence of groundwater flooding in carbonate aquifers.
Ultimate Turbulent Taylor-Couette Flow
Huisman, Sander G; Grossmann, Siegfried; Sun, Chao; Lohse, Detlef
2011-01-01
The flow structure of strongly turbulent Taylor-Couette flow with Reynolds numbers up to Re_i = 2*10^6 of the inner cylinder is experimentally examined with high-speed particle image velocimetry (PIV). The wind Reynolds numbers Re_w of the turbulent Taylor-vortex flow is found to scale as Re_w ~ Ta^(1/2), exactly as predicted for the ultimate turbulence regime, in which the boundary layers are turbulent. The dimensionless angular velocity flux has an effective scaling of Nu_{\\omega} ~ Ta^0.38, also in correspondence with turbulence in the ultimate regime. The scaling of Nu_{\\omega} is confirmed by local angular velocity flux measurements extracted from high-speed PIV measurements: though the flux shows huge fluctuations, its spatial and temporal average nicely agrees with the result from the global torque measurements.
Roy, Sukesh [Spectral Energies, LLC, 5100 Springfield Street, Ste. 301, Dayton, OH 45431 (United States); Gord, James R. [Air Force Research Laboratory, Wright-Patterson AFB, OH 45433 (United States); Patnaik, Anil K. [Air Force Research Laboratory, Wright-Patterson AFB, OH 45433 (United States); Department of Physics, Wright State University, Dayton, OH 45435 (United States)
2010-04-15
Coherent anti-Stokes Raman scattering (CARS) spectroscopy is widely used for measuring temperature and species concentration in reacting flows. This paper reviews the advances made over the last twelve years in the development and application of CARS spectroscopy in gas-phase reacting flows. The advent of high-power nanosecond (ns) lasers and off-the-shelf compact picosecond (ps) and femtosecond (fs) lasers is enabling the rapid expansion of the application of single-shot or high-bandwidth CARS spectroscopy in a way that would have been quite unimaginable two decades ago. Furthermore, compact ps lasers are paving the way for the development of a fiber-based CARS system for use in harsh environments. The objective of this paper is to provide an overview of recent progresses in ns-, ps-, and fs-CARS spectroscopy for gas-phase thermometry and species-concentration measurements since the second edition of A.C. Eckbreth's book entitled Laser Diagnostics for Combustion Temperature and Species, which was published in 1996. During the last two decades, four encompassing issues have driven the fundamental development and application of CARS spectroscopy in reacting flows: 1) measurement of temperature and concentration of multiple species with one CARS system, 2) extension of the application of traditional ns-CARS to challenging reacting flow environments, 3) performance of nonresonant background-free and collision-free measurements in high-pressure reacting flows, and 4) measurement of temperature and species concentration at high bandwidth, typically 1 kHz or greater, to address the instability and transient phenomena associated with turbulent reacting flows in the combustors and augmentors of modern propulsion systems. This review is focused on identifying and discussing the recent results of gas-phase CARS spectroscopy related to the four issues mentioned above. The feasibility of performing high-bandwidth CARS spectroscopy with one laser beam as well as the
Adaptive methods in computational fluid dynamics of chemically reacting flows
Rogg, B.
1991-09-01
Possible approaches to fully implicit adaptive algorithms suitable for the numerical simulation of unsteady two-dimensional reactive flows are examined. Emphasis is placed on self-adaptive gridding procedures applicable to time-dependent two-dimensional reactive flows. Pulsating flame propagation, autoignition in a nonpremixed flow, flame propagation in a strained mixing layer, and hot-spot-like self-ignition are considered as examples.
Numerical simulation of turbulent slurry flows
Haghgoo, Mohammad Reza; Spiteri, Reymond J.; Bergstrom, Donlad J.
2016-11-01
Slurry flows, i.e., the flow of an agglomeration of liquid and particles, are widely employed in many industrial applications, such as hydro-transport systems, pharmaceutical batch crystallizers, and wastewater disposal. Although there are numerous studies available in the literature on turbulent gas-particle flows, the hydrodynamics of turbulent liquid-particle flows has received much less attention. In particular, the fluid-phase turbulence modulation due to the particle fluctuating motion is not yet well understood and remains challenging to model. This study reports the results of a numerical simulation of a vertically oriented slurry pipe flow using a two-fluid model based on the kinetic theory of granular flows. The particle stress model also includes the effects of frictional contact. Different turbulence modulation models are considered, and their capability to capture the characteristic features of the turbulent flow is assessed. The model predictions are validated against published experimental data and demonstrate the significant effect of the particles on the fluid-phase turbulence.
Turbulence modelling of thermal plasma flows
Shigeta, Masaya
2016-12-01
This article presents a discussion of the ideas for modelling turbulent thermal plasma flows, reviewing the challenges, efforts, and state-of-the-art simulations. Demonstrative simulations are also performed to present the importance of numerical methods as well as physical models to express turbulent features. A large eddy simulation has been applied to turbulent thermal plasma flows to treat time-dependent and 3D motions of multi-scale eddies. Sub-grid scale models to be used should be able to express not only turbulent but also laminar states because both states co-exist in and around thermal plasmas which have large variations of density as well as transport properties under low Mach-number conditions. Suitable solution algorithms and differencing schemes must be chosen and combined appropriately to capture multi-scale eddies and steep gradients of temperature and chemical species, which are turbulent features of thermal plasma flows with locally variable Reynolds and Mach numbers. Several simulations using different methods under different conditions show commonly that high-temperature plasma regions exhibit less turbulent structures, with only large eddies, whereas low-temperature regions tend to be more turbulent, with numerous small eddies. These numerical results agree with both theoretical insight and photographs that show the characteristics of eddies. Results also show that a turbulence transition of a thermal plasma jet through a generation-breakup process of eddies in a torch is dominated by fluid dynamic instability after ejection rather than non-uniform or unsteady phenomena.
Turbulent pipe flows subjected to temporal decelerations
Jeong, Wongwan; Lee, Jae Hwa
2016-11-01
Direct numerical simulations of temporally decelerating turbulent pipe flows were performed to examine effects of temporal decelerations on turbulence. The simulations were started with a fully developed turbulent pipe flow at a Reynolds number, ReD =24380, based on the pipe radius (R) and the laminar centerline velocity (Uc 0). Three different temporal decelerations were imposed to the initial flow with f= | d Ub / dt | =0.00127, 0.00625 and 0.025, where Ub is the bulk mean velocity. Comparison of Reynolds stresses and turbulent production terms with those for steady flow at a similar Reynolds number showed that turbulence is highly intensified with increasing f due to delay effects. Furthermore, inspection of the Reynolds shear stress profiles showed that strong second- and fourth-quadrant Reynolds shear stresses are greatly increased, while first- and third-quadrant components are also increased. Decomposition of streamwise Reynolds normal stress with streamwise cutoff wavelength (λx) 1 R revealed that the turbulence delay is dominantly originated from delay of strong large-scale turbulent structures in the outer layer, although small-scale motions throughout the wall layer adjusted more rapidly to the temporal decelerations. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2014R1A1A2057031).
Rule-Based Multidisciplinary Tool for Unsteady Reacting Real-Fluid Flows Project
National Aeronautics and Space Administration — A design and analysis computational tool is proposed for simulating unsteady reacting flows in combustor devices used in reusable launch vehicles. Key aspects...
Instabilities of flows and transition to turbulence
Sengupta, Tapan K
2012-01-01
Introduction to Instability and TransitionIntroductionWhat Is Instability?Temporal and Spatial InstabilitySome Instability MechanismsComputing Transitional and Turbulent FlowsFluid Dynamical EquationsSome Equilibrium Solutions of the Basic EquationBoundary Layer TheoryControl Volume Analysis of Boundary LayersNumerical Solution of the Thin Shear Layer (TSL) EquationLaminar Mixing LayerPlane Laminar JetIssues of Computing Space-Time Dependent FlowsWave Interaction: Group Velocity and Energy FluxIssues of Space-Time Scale Resolution of FlowsTemporal Scales in Turbulent FlowsComputing Time-Averag
Turbulent Flow Measurement in Vortex Settling Basin
Jafar Chapokpour
2011-12-01
Full Text Available This paper presents the findings of an experimental study on the three-dimensional turbulent flow field in vortex settling basin. An ADV (Acoustic Doppler Velocity Meter were used to catch 3D velocitycomponents inside the basin. Detailed measurements of time-averaged velocity components, turbulent intensity components and turbulent kinetic energy were determined at different radial sections of chamber. Also the normalized time averaged absolute velocity of 3D components in contour type exhibition were conducted and it was found that the absolute velocity generally is influenced by u component of flow. It trends from high magnitude in basin center to the constant magnitude in basin side wall. The normalized turbulent intensity ofthree components was investigated individually. It was found that intensity of 3D components in vicinity of central air core is higher than other regions, decreasing by moving towards basin sidewall except for the sections that influenced directly by entrance flow jet and sidewall exiting overflow. The results of turbulence kinetic energy also had the same interpretation like turbulence intensity and affected by the same boundary conditions which cover turbulence intensity of 3 velocity components overly.
Turbulence Modulation and Particle Segregation in a Turbulent Channel Flow
Fong, Kee Onn; Toloui, Mostafa; Amili, Omid; Hong, Jiarong; Coletti, Filippo
2016-11-01
Particle-laden flows are ubiquitous in biological, environmental, and engineering flows, but our understanding of the mechanism by which particles modulate turbulence is incomplete. Simulations involve a wide range of scales, and shall be corroborated by measurements that reconstruct the motion of both the continuous and dispersed phases. We present experimental observations on the interaction between inertial particles and turbulent flow through a vertical channel in two-way coupled regime. The working fluid is air laden with size-selected glass particles, which we investigate by planar particle image velocimetry and digital inline holography. Unlike most previous experiments, we focus on a regime in which particle segregation and turbulence modulation are both strong. PIV shows that turbulence modulation is especially pronounced near the wall, where particles accumulate by turbophoresis. The segregation, however, is much weaker than what suggested by one-way coupled simulations. Results from digital holography confirm the trends in particle concentration and velocities, and additionally provide information on the three-dimensional clustering. The findings are compared to previous investigations and discussed in the context of modeling strategies.
Clustering of Aerosols in Atmospheric Turbulent Flow
Elperin, T; L'vov, V; Liberman, M A; Rogachevskii, I
2007-01-01
A mechanism of formation of small-scale inhomogeneities in spatial distributions of aerosols and droplets associated with clustering instability in the atmospheric turbulent flow is discussed. The particle clustering is a consequence of a spontaneous breakdown of their homogeneous space distribution due to the clustering instability, and is caused by a combined effect of the particle inertia and a finite correlation time of the turbulent velocity field. In this paper a theoretical approach proposed in Phys. Rev. E 66, 036302 (2002) is further developed and applied to investigate the mechanisms of formation of small-scale aerosol inhomogeneities in the atmospheric turbulent flow. The theory of the particle clustering instability is extended to the case when the particle Stokes time is larger than the Kolmogorov time scale, but is much smaller than the correlation time at the integral scale of turbulence. We determined the criterion of the clustering instability for the Stokes number larger than 1. We discussed...
DISPERSION OF CYLINDRICAL PARTICLES IN TURBULENT FLOWS
GAO Zhen-yu; LIN Jian-zhong
2004-01-01
With consideration of the Stokes drag and virtual mass force, the equations for mean and fluctuating velocities in rotation and translation were given for rigid cylindrical particles moving in a turbulent flow. Then the rotational and translational dispersion coefficients of particle were derived. The relationships between the dispersion coefficients and flow length scale as well as particle characteristic parameters were analyzed. The resulting dispersion coefficients were proved to decrease as the particle length increases. The conclusions are helpful for the further research on the motion of cylindrical particles in turbulent flows.
Controlling Flow Turbulence Using Local Pinning Feedback
TANG Guo-Ning; HU Gang
2006-01-01
Flow turbulence control in two-dimensional Navier-Stokes equation is considered．By applying local pinning control only to a sjngle component of flow velocity field,the flow turbulence can be controlled to desirable targets．It is found that with certain number of controllers there exist an optimal control strength at which control error takes minimum value,and larger and smaller control strengths give worse control efficiency．The phvsical mechanism underlying these strange control results is analysed based on the interactions between different types of modes.
The pdf approach to turbulent flow
Kollmann, W.
1990-01-01
This paper provides a detailed discussion of the theory and application of probability density function (pdf) methods, which provide a complete statistical description of turbulent flow fields at a single point or a finite number of points. The basic laws governing the flow of Newtonian fluids are set up in the Eulerian and the Lagrangian frame, and the exact and linear equations for the characteristic functionals in those frames are discussed. Pdf equations in both frames are derived as Fourier transforms of the equations of the characteristic functions. Possible formulations for the nonclosed terms in the pdf equation are discussed, their properties are assessed, and closure modes for the molecular-transport and the fluctuating pressure-gradient terms are reviewed. The application of pdf methods to turbulent combustion flows, supersonic flows, and the interaction of turbulence with shock waves is discussed.
Herault, Johann; Fauve, Stephan
2016-01-01
We report the experimental observation of $1/f$ fluctuations in three different turbulent flow configurations: the large scale velocity driven by a two-dimensional turbulent flow, the magnetic field generated by a turbulent swirling flow of liquid sodium and the pressure fluctuations due to vorticity filaments in a swirling flow. For these three systems, $1/f$ noise is shown to result from the dynamics of coherent structures that display transitions between a small number of states. The interevent duration is distributed as a power-law. The exponent of this power-law and the nature of the dynamics (transition between symmetric states or asymmetric ones) select the exponent of the $1/f$ fluctuations.
Liquid infused surfaces in turbulent channel flow
Fu, Matthew; Stone, Howard; Smits, Alexander; Jacobi, Ian; Samaha, Mohamed; Wexler, Jason; Shang, Jessica; Rosenberg, Brian; Hellström, Leo; Fan, Yuyang; Wang, Karen; Lee, Kevin; Hultmark, Marcus
2014-11-01
A turbulent channel flow facility is used to measure the drag reduction capabilities and dynamic behavior of liquid-infused micro-patterned surfaces. Liquid infused surfaces have been proposed as a robust alternative to traditional air-cushion-based superhydrophobic surfaces. The mobile liquid lubricant creates a surface slip with the outer turbulent shear flow as well as an energetic sink to dampen turbulent fluctuations. Micro-manufactured surfaces can be mounted flush in the channel and exposed to turbulent flows. Two configurations are possible, both capable of producing laminar and turbulent flows. The first configuration allows detailed investigation of the infused liquid layer and the other allows well resolved pressure gradient measurements. Both of the configurations have high aspect ratios 15-45:1. Drag reduction for a variety of liquid-infused surface architectures is quantified by measuring pressure drop in the channel. Flow in the oil film is simultaneously visualized using fluorescent dye. Supported under ONR Grants N00014-12-1-0875 and N00014-12-1-0962 (program manager Ki-Han Kim).
Marine particle aggregate breakup in turbulent flows
Rau, Matthew; Ackleson, Steven; Smith, Geoffrey
2016-11-01
The dynamics of marine particle aggregate formation and breakup due to turbulence is studied experimentally. Aggregates of clay particles, initially in a quiescent aggregation tank, are subjected to fully developed turbulent pipe flow at Reynolds numbers of up to 25,000. This flow arrangement simulates the exposure of marine aggregates in coastal waters to a sudden turbulent event. Particle size distributions are measured by in-situ sampling of the small-angle forward volume scattering function and the volume concentration of the suspended particulate matter is quantified through light attenuation measurements. Results are compared to measurements conducted under laminar and turbulent flow conditions. At low shear rates, larger sized particles indicate that aggregation initially governs the particle dynamics. Breakup is observed when large aggregates are exposed to the highest levels of shear in the experiment. Models describing the aggregation and breakup rates of marine particles due to turbulence are evaluated with the population balance equation and results from the simulation and experiment are compared. Additional model development will more accurately describe aggregation dynamics for remote sensing applications in turbulent marine environments.
Tackling turbulent flows in engineering
Dewan, Anupam
2011-01-01
Focusing on the engineering aspects of fluid turbulence, this volume offers solutions to the problem in a number of settings. Emphasizing real-world applications rather than mathematics, it will be a must-read text in both industrial and academic environments.
Liu, Nan-Suey; Shih, Tsan-Hsing; Wey, C. Thomas
2011-01-01
A series of numerical simulations of Jet-A spray reacting flow in a single-element lean direct injection (LDI) combustor have been conducted by using the National Combustion Code (NCC). The simulations have been carried out using the time filtered Navier-Stokes (TFNS) approach ranging from the steady Reynolds-averaged Navier-Stokes (RANS), unsteady RANS (URANS), to the dynamic flow structure simulation (DFS). The sub-grid model employed for turbulent mixing and combustion includes the well-mixed model, the linear eddy mixing (LEM) model, and the filtered mass density function (FDF/PDF) model. The starting condition of the injected liquid spray is specified via empirical droplet size correlation, and a five-species single-step global reduced mechanism is employed for fuel chemistry. All the calculations use the same grid whose resolution is of the RANS type. Comparisons of results from various models are presented.
Non-equilibrium reacting gas flows kinetic theory of transport and relaxation processes
Nagnibeda, Ekaterina; Nagnibeda, Ekaterina
2009-01-01
This volume develops the kinetic theory of transport phenomena and relaxation processes in the flows of reacting gas mixtures. The theory is applied to the modeling of non-equilibrium flows behind strong shock waves, in the boundary layer, and in nozzles.
Comparison of turbulent particle dispersion models in turbulent shear flows
S. Laín
2007-09-01
Full Text Available This work compares the performance of two Lagrangian turbulent particle dispersion models: the standard model (e.g., that presented in Sommerfeld et al. (1993, in which the fluctuating fluid velocity experienced by the particle is composed of two components, one correlated with the previous time step and a second one randomly sampled from a Wiener process, and the model proposed by Minier and Peirano (2001, which is based on the PDF approach and performs closure at the level of acceleration of the fluid experienced by the particle. Formulation of a Langevin equation model for the increments of fluid velocity seen by the particle allows capturing some underlying physics of particle dispersion in general turbulent flows while keeping the mathematical manipulation of the stochastic model simple, thereby avoiding some pitfalls and simplifying the derivation of macroscopic relations. The performance of both dispersion models is tested in the configurations of grid-generated turbulence (Wells and Stock (1983 experiments, simple shear flow (Hyland et al., 1999 and confined axisymmetric jet flow laden with solids (Hishida and Maeda (1987 experiments.
Georgopoulos, P.G.; Seinfeld, J.H.
1986-01-01
Calculations performed with the Turbulent Reacting plume model (TPRM) developed in Part I are compared with the experimental data of Builtjes (1981, Netherlands Organization for Applied Scientific Research, Div. of Technology for Soc., Ref. No. 81-013563) for the reaction between NO in a point source plume and ambient O/sub 3/, taking place in a wind tunnel simulating a neutral atmospheric boundary layer. The comparison shows the TRPM capable of quantitatively predicting the retardation imposed on the evolution of nonlinear plume chemistry by incomplete mixing. (authors).
Large-eddy simulations of turbulent flows in internal combustion engines
Banaeizadeh, Araz
The two-phase compressible scalar filtered mass density function (FMDF) model is further developed and employed for large-eddy simulations (LES) of turbulent spray combustion in internal combustion (IC) engines. In this model, the filtered compressible Navier-Stokes equations are solved in a generalized curvilinear coordinate system with high-order, multi-block, compact differencing schemes for the turbulent velocity and pressure. However, turbulent mixing and combustion are computed with a new two-phase compressible scalar FMDF model. The spray and droplet dispersion/evaporation are modeled with a Lagrangian method. A new Lagrangian-Eulerian-Lagrangian computational method is employed for solving the flow, spray and scalar equation. The pressure effect in the energy equation, as needed in compressible flows, is included in the FMDF formulation. The performance of the new compressible LES/FMDF model is assessed by simulating the flow field and scalar mixing in a rapid compression machine (RCM), in a shock tube and in a supersonic co-axial jet. Consistency of temperatures predicted by the Eulerian finite-difference (FD) and Lagrangian Monte Carlo (MC) parts of the LES/FMDF model are established by including the pressure on the FMDF. It is shown that the LES/FMDF model is able to correctly capture the scalar mixing in both compressible subsonic and supersonic flows. Using the new two-phase LES/FMDF model, fluid dynamics, heat transfer, spray and combustion in the RCM with flat and crevice piston are studied. It is shown that the temperature distribution in the RCM with crevice piston is more uniform than the RCM with flat piston. The fuel spray characteristics and the spray parameters affecting the fuel mixing inside the RCM in reacting and non-reacting flows are also studied. The predicted liquid penetration and flame lift-off lengths for respectively non-reacting and reacting sprays are found to compare well with the available experimental data. Temperatures and
Particle spin in a turbulent shear flow
Mortensen, P.H.; Andersson, H.I.; Gillissen, J.J.J.; Boersma, B.J.
2007-01-01
The translational and rotational motions of small spherical particles dilutely suspended in a turbulent channel flow have been investigated. Three different particle classes were studied in an Eulerian-Lagrangian framework to examine the effect of the response times on the particle statistics. The r
Data-parallel DNS of turbulent flow
Verstappen, R.W.C.P.; Veldman, A.E.P.; Emerson, DR; Ecer, A; Periaux, J; Satofuka, N
1998-01-01
This contribution deals with direct numerical simulation (DNS) of incompressible turbulent flows on parallel computers. We make use of the data-parallel model on shared memory systems as well as on a distributed memory machine. The combination of fast parallel computers and efficient numerical algor
Slip Effects in Compressible Turbulent Channel Flow
Skovorodko, P A
2012-01-01
The direct numerical simulation of compressible fully developed turbulent Couette flow between two parallel plates with equal temperatures moving in opposite directions with some velocity was performed. The algorithm was tested on well known numerical solution for incompressible Poiseuille channel flow and found to provide its well description. The slip effects in studied flow are found to be negligibly small at the values of accommodation coefficients for velocity and temperature of the order of unity. The considerable increase of mean temperature with decreasing the accommodation coefficient for temperature was discovered. The effect may be important in the problems of heat exchange in compressible turbulent boundary layer for some combinations of flowing gas, surface and adsorbing gas.
Slip effects in compressible turbulent channel flow
Skovorodko, P. A.
2012-11-01
The direct numerical simulation of compressible fully developed turbulent Couette flow between two parallel plates with temperature Tw moving with velocities ±Uw was performed. The algorithm was tested on well known numerical solution for incompressible Poiseuille channel flow and found to provide its well description. The slip effects in studied flow are found to be negligibly small at the values of accommodation coefficients αu and αT of the order of unity. The considerable increase of mean temperature with decreasing the accommodation coefficient αT for fixed value of αu = 1 was discovered. The effect may be important in the problems of heat exchange in compressible turbulent boundary layer for some combinations of flowing gas, surface and adsorbing gas.
Optimum Pipe Size Selection for Turbulent Flow
Timothy A. AKINTOLA
2009-07-01
Full Text Available Pipelines are normally designed to deliver fluid at the required head and flow rate in a cost effective manner. Increase in conduit diameter leads to increase in annual capital costs, and decrease in operating costs. Selection of an optimum conduit diameter for a particular fluid flow will therefore be a vital economic decision. This paper presents a computer aided optimisation technique for determination of optimum pipe diameter for a number of idealized turbulent flow. Relationships were formulated connecting theories of turbulent fluid flow with pipeline costing. These were developed into a computer program, written in Microsoft Visual C++ language, for a high-level precision estimate of the optimum pipe diameter, through the least total cost approach. The validity of the program was ascertained through case studies, representative of fluids with different densities and compressibility. The optimum conduit diameter was found to increase linearly with increase in compressibility.
On Turbulent Contribution to Frictional Drag in Wall-Bounded Turbulent Flow
LI Feng-Chen; KAWAGUCHI Yasuo; HISHIDA Koichi; OSHIMA Marie
2006-01-01
@@ We propose a simple model for turbulent contribution to the frictional drag in a wall-bounded turbulent flow based on the characteristic parameters of turbulent bursting events. It is verified on water and drag-reducing surfactant solution flows investigated by particle image velocimetry in experiments. It is obtained that the turbulent contribution to the skin friction factor is linearly proportional to the product of the spatial frequency and strength of turbulent bursts originated from the wall.
Turbulence dynamics in unsteady atmospheric flows
Momen, Mostafa; Bou-Zeid, Elie
2016-11-01
Unsteady pressure-gradient forcing in geophysical flows challenges the quasi-steady state assumption, and can strongly impact the mean wind and higher-order turbulence statistics. Under such conditions, it is essential to understand when turbulence is in quasi-equilibrium, and what are the implications of unsteadiness on flow characteristics. The present study focuses on the unsteady atmospheric boundary layer (ABL) where pressure gradient, Coriolis, buoyancy, and friction forces interact. We perform a suite of LES with variable pressure-gradient. The results indicate that the dynamics are mainly controlled by the relative magnitudes of three time scales: Tinertial, Tturbulence, and Tforcing. It is shown that when Tf Tt , the turbulence is no longer in a quasi-equilibrium state due to highly complex mean-turbulence interactions; consequently, the log-law and turbulence closures are no longer valid in these conditions. However, for longer and, surprisingly, for shorter forcing times, quasi-equilibrium is maintained. Varying the pressure gradient in the presence of surface buoyancy fluxes primarily influences the buoyant destruction in the stable ABLs, while under unstable conditions it mainly influences the transport terms. NSF-PDM under AGS-10266362. Cooperative Institute for Climate Science, NOAA-Princeton University under NA08OAR4320752. Simulations performed at NCAR, and Della server at Princeton University.
王兵; 张会强; 王希麟
2004-01-01
The instantaneous and time-averaged statistic characteristics of the sub-grid scale (SGS) turbulent kinetic energy and SGS dissipation in a backward-facing step turbulent flow have been studied bylarge eddy simulation. The SGS turbulent kinetic energy and SGS turbulent dissipation vary in different flow regions and decrease with the flow developing spatially. The fluid molecular dissipation shares about 14% to 28% of the whole dissipation.
CFD prediction of the reacting flow field inside a subscale scramjet combustor
Chitsomboon, T.; Northam, G. B.; Rogers, R. C.; Diskin, G. S.
1988-01-01
A three-dimensional, Reynolds-averaged Navier-Stokes CFD code has been used to calculate the reacting flowfield inside a hydrogen-fueled, subscale scramjet combustor. Pilot fuel was injected transversely upstream of the combustor and the primary fuel was injected transversely downstream of a backward facing step. A finite rate combustion model with two-step kinetics was used. The CFD code used the explicit MacCormack algorithm with point-implicit treatment of the chemistry source terms. Turbulent mixing of the jets with the airstream was simulated by a simple mixing length scheme, whereas near wall turbulence was accounted for by the Baldwin-Lomax model. Computed results were compared with experimental wall pressure measurements.
Turbulent Poiseuille & Couette flows at high Re
Lee, Myoungkyu; Moser, Robert D.
2016-11-01
We present the results of direct numerical simulation (DNS) of high Re turbulent Poiseuille and Couette flows. Couette flow has been simulated with a streamwise (x) domain that is 100 πδ long at Reynolds number up to Reτ 500 . In addition Poiseuille flow simulations up to Reτ 5200 were performed. In Couette flow, extremely large scale motions, which are approximately 50 πδ long in the x-direction with very strong intensity, have been observed. In this presentation we will focus on a comparison between these two flows in terms of the vorticity-velocity co-spectra, which are interesting because of the relationship between the Reynolds stress and the velocity-vorticity correlation (∂y = - ). Also considered will be the spectra of the turbulent transport term in the evolution equation for the turbulent kinetic energy. In both (co)-spectra it is shown that the difference between the two flows at high Re are primarily at large scales. This work was supported by NSF (OCI-0749223 and PRAC Grant 0832634), and computation resources were provided by the Argonne Leadership Computing Facility through the Early Science, INCITE 2013 and Directors Discretionary Programs.
Patterns of the turbulent Taylor-Couette flow
Prigent, Arnaud; Talioua, Abdessamad; Mutabazi, Innocent
2016-11-01
We are interested in the study of the transition to turbulence in the Taylor-Couette flow, the flow between two independently rotating coaxial cylinders. Once the geometry is fixed, the flow is controlled by the inner and outer Reynolds numbers and present a large variety of flow regimes. In counter-rotation, the transition is characterized by a succession of more or less turbulent flow regimes: intermittency with turbulent spots, spiral turbulence, featureless turbulence. For larger values of the inner Reynolds number, turbulent Taylor roll re-emerge from the featureless turbulence and remain for very large values of the Reynolds numbers. Bifurcations between different turbulent rolls states are even observed in the ultimate turbulence regime. Nevertheless the transition from the featureless turbulence to the turbulent rolls still requires a detailed study and the mechanism which causes and sustains turbulent spots or turbulent spirals remains unknown. In this study we present new experimental information on the organization of the flow for the different regimes with turbulence. The experiments are conducted in a Taylor-Couette flow with η = 0 . 8 . Stereo-Particle Image Velocimetry measurements and visualizations of the different flow regimes are realized and discussed. This work was supported by the ANR TRANSFLOW - ANR-13-BS09-0025.
Turbulent flow in longitudinally finned tubes
Edwards, D.P.; Hirsa, A.; Jensen, M.K. [Rensselaer Polytechnic Inst., Troy, NY (United States). Dept. of Mechanical Engineering, Aeronautical Engineering and Mechanics
1996-09-01
An experimental investigation of fully developed, steady, turbulent flow in longitudinally finned tubes has been performed. A two-channel, four-beam, laser-Doppler velocimeter was used to measure velocity profiles and turbulent statistics of air flow seeded with titanium dioxide particles. Mean velocities in axial, radial, and circumferential directions were measured over the tube cross sections and pressure drop in the tubes was measured at six stations along the test section length in order to calculate the fully developed friction factor. Four experimental tube geometries were studied: one smooth tube; two 8-finned tubes (fin height-to-radius ratios of 0.333 and 0.167), and one 16-finned tube (fin height-to-radius ratio of 0.167); detailed measurements were taken at air flow rates corresponding to Reynolds numbers of approximately 5,000, 25,000, and 50,000. Friction factor data were compared to literature results and showed good agreement for both smooth and finned tubes. The wall shear stress distribution varied significantly with reynolds number, particularly for Reynolds numbers of 25,000 and below. Maximum wall shear stress was found at the fin tip and minimum at the fin root. Four secondary flow cells were detected per fin (one in each interfin spacing and one in each core region for each fin); secondary flows were found to be small in comparison to the mean axial flow and relative magnitudes were unaffected by axial flow rate at Reynolds numbers above 25,000. The fluctuating velocities had a structure similar to that of the smooth tube in the core region while the turbulence in the interfin region was greatly reduced. The principal, primary shear stress distribution differed considerably from that of the smooth tube, particularly in the interfin region, and the orientation was found to be approximately in the same direction as the gradient of the mean axial velocity, supporting the use of an eddy viscosity formulation in turbulence modeling.
DNS of turbulent flows of dense gases
Sciacovelli, L.; Cinnella, P.; Gloerfelt, X.; Grasso, F.
2017-03-01
The influence of dense gas effects on compressible turbulence is investigated by means of numerical simulations of the decay of compressible homogeneous isotropic turbulence (CHIT) and of supersonic turbulent flows through a plane channel (TCF). For both configurations, a parametric study on the Mach and Reynolds numbers is carried out. The dense gas considered in these parametric studies is PP11, a heavy fluorocarbon. The results are systematically compared to those obtained for a diatomic perfect gas (air). In our computations, the thermodynamic behaviour of the dense gases is modelled by means of the Martin-Hou equation of state. For CHIT cases, initial turbulent Mach numbers up to 1 are analyzed using mesh resolutions up to 5123. For TCF, bulk Mach numbers up to 3 and bulk Reynolds numbers up to 12000 are investigated. Average profiles of the thermodynamic quantities exhibit significant differences with respect to perfect-gas solutions for both of the configurations. For high-Mach CHIT, compressible structures are modified with respect to air, with weaker eddy shocklets and stronger expansions. In TCF, the velocity profiles of dense gas flows are much less sensitive to the Mach number and collapse reasonably well in the logarithmic region without any special need for compressible scalings, unlike the case of air, and the overall flow behaviour is midway between that of a variable-property liquid and that of a gas.
Liquid Infused Surfaces in Turbulent Channel Flow
Fu, Matthew; Liu, Ying; Stone, Howard; Hultmark, Marcus
2016-11-01
Liquid infused surfaces have been proposed as a robust method for turbulent drag reduction. These surfaces consist of functionalized roughness elements wetted with a liquid lubricant that is immiscible with external fluids. The presence of the lubricant creates mobile, fluid-fluid interfaces, each of which can support a localized slip. Collectively, these interfaces yield a finite slip velocity at the effective surface, which has been demonstrated to reduce skin friction drag in turbulent flows. Retention of the lubricant layer is critical to maintaining the drag reduction effect. A turbulent channel-flow facility is used to characterize the drag reduction and robustness of various liquid infused surfaces. Micro-manufactured surfaces are mounted flush in the channel and exposed to turbulent flows. The retention of fluorescent lubricants and pressure drop are monitored to characterize the effects of surface geometry and lubricant properties. Supported under ONR Grants N00014-12-1-0875 and N00014-12-1-0962 (program manager Ki-Han Kim) and by the Department of Defense (DoD) through the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program.
Shih, Tsan-Hsing; Liu, Nan-Suey; Moder, Jeffrey P.
2015-01-01
This paper presents the numerical simulations of confined three-dimensional coaxial water jets. The objectives are to validate the newly proposed nonlinear turbulence models of momentum and scalar transport, and to evaluate the newly introduced scalar APDF and DWFDF equation along with its Eulerian implementation in the National Combustion Code (NCC). Simulations conducted include the steady RANS, the unsteady RANS (URANS), and the time-filtered Navier-Stokes (TFNS); both without and with invoking the APDF or DWFDF equation. When the APDF (ensemble averaged probability density function) or DWFDF (density weighted filtered density function) equation is invoked, the simulations are of a hybrid nature, i.e., the transport equations of energy and species are replaced by the APDF or DWFDF equation. Results of simulations are compared with the available experimental data. Some positive impacts of the nonlinear turbulence models and the Eulerian scalar APDF and DWFDF approach are observed.
Superhydrophobic surfaces in turbulent channel flow
Li, Yixuan; Alame, Karim; Mahesh, Krishnan
2016-11-01
The drag reduction effect of superhydrophobic surfaces in turbulent channel flow is studied using direct numerical simulation. The volume of fluid (VOF) methodology is used to resolve the dynamics of the interface. Laminar flow simulations show good agreement with experiment, and illustrate the relative importance of geometry and interface boundary condition. An analytical solution for the multi-phase problem is obtained that shows good agreement with simulation. Turbulent simulations over a longitudinally grooved surface show drag reduction even in the fully wetted regime. The statistics show that geometry alone can cause an apparent slip to the external flow. Instantaneous plots indicate that the grooves prevent the penetration of near wall vorticity, yielding overall drag reduction. Results for spectra, wall pressure fluctuations and correlations will be presented. Unsteady effects on the air-vapor interface will be discussed. Results for random roughness surfaces will be presented. Supported by Office of Naval Research.
Refinement of turbulent flow velocity characteristics
Y.V. Bryanskaya
2013-10-01
Full Text Available The basic laws of Prandtl semi-empirical turbulence theory were analyzed in the article. It was shown, that the Prandtl – Nikuradse logarithmic distribution of velocities are not strictly universal. The change of the first and second turbulence constants was analyzed on the basis of experimental data of I. Nikuradse. The logarithmic velocity profiles for smooth and rough pipes have been transformed. A united velocity logarithmic profile for flows in pipes, appropriate for any rate of hydraulic resistance was received. A more precise, consistent with the resistance laws, description of the kinematic structure of the flow with varying parameters of the velocity profiles was set. It was shown that the position of the average velocity point for the flow in pipe remained constant when the parameters of the velocity profile changed.
Stochastic chaos in a turbulent swirling flow
Faranda, Davide; Saint-Michel, Brice; Wiertel, Cecile; Padilla, Vincent; Dubrulle, Berengere; Daviaud, Francois
2016-01-01
We report the experimental evidence of the existence of a random attractor in a fully developed turbulent swirling flow. By defining a global observable which tracks the asymmetry in the flux of angular momentum imparted to the flow, we can first reconstruct the associated turbulent attractor and then follow its route towards chaos. We further show that the experimental attractor can be modeled by stochastic Duffing equations, that match the quantitative properties of the experimental flow, namely the number of quasi-stationary states and transition rates among them, the effective dimensions, and the continuity of the first Lyapunov exponents. Such properties can neither be recovered using deterministic models nor using stochastic differential equations based on effective potentials obtained by inverting the probability distributions of the experimental global observables. Our findings open the way to low dimensional modeling of systems featuring a large number of degrees of freedom and multiple quasi-station...
Relaminarisation of fully turbulent flow in pipes
Kuehnen, Jakob; Hof, Bjoern
2014-11-01
Drag reduction still remains one of the most alluring applications of turbulence control. We will show that flattening the streamwise velocity profile in pipes can force turbulent flow to decay and become laminar. Two different experimental control schemes are presented: one with a local modification of the flow profile by means of a stationary obstacle and one with a moving wall, where a part of the pipe is shifted in the streamwise direction. Both control schemes act on the flow such that the streamwise velocity profile becomes more flat and turbulence gradually grows faint and disappears. Since, in a smooth straight pipe, the flow remains laminar from that position a reduction in skin friction by a factor of 5 can be accomplished. We will present measurements with high-speed particle image velocimetry, measurements of the pressure drop and videos of the development of the flow during relaminarisation. The guiding fundamental principle behind our approach to control the velocity profile will be explained and discussed.
On stability and turbulence of fluid flows
Heisenberg, Werner
1951-01-01
This investigation is divided into two parts, the treatment of the stability problem of fluid flows on the one hand, and that of the turbulent motion on the other. The first part summarizes all previous investigations under a unified point of view, that is, sets up as generally as possible the conditions under which a profile possesses unstable or stable characteristics, and indicates the methods for solution of the stability equation for any arbitrary velocity profile and for calculation of the critical Reynolds number for unstable profiles. In the second part, under certain greatly idealizing assumptions, differential equations for the turbulent motions are derived and from them qualitative information about several properties of the turbulent velocity distribution is obtained.
Modeling turbulence structure. Chemical kinetics interaction in turbulent reactive flows
Magnussen, B.F. [The Norwegian Univ. of Science and Technology, Trondheim (Norway)
1997-12-31
The challenge of the mathematical modelling is to transfer basic physical knowledge into a mathematical formulation such that this knowledge can be utilized in computational simulation of practical problems. The combustion phenomena can be subdivided into a large set of interconnected phenomena like flow, turbulence, thermodynamics, chemical kinetics, radiation, extinction, ignition etc. Combustion in one application differs from combustion in another area by the relative importance of the various phenomena. The difference in fuel, geometry and operational conditions often causes the differences. The computer offers the opportunity to treat the individual phenomena and their interactions by models with wide operational domains. The relative magnitude of the various phenomena therefore becomes the consequence of operational conditions and geometry and need not to be specified on the basis of experience for the given problem. In mathematical modelling of turbulent combustion, one of the big challenges is how to treat the interaction between the chemical reactions and the fluid flow i.e. the turbulence. Different scientists adhere to different concepts like the laminar flamelet approach, the pdf approach of the Eddy Dissipation Concept. Each of these approaches offers different opportunities and problems. All these models are based on a sound physical basis, however none of these have general validity in taking into consideration all detail of the physical chemical interaction. The merits of the models can only be judged by their ability to reproduce physical reality and consequences of operational and geometric conditions in a combustion system. The presentation demonstrates and discusses the development of a coherent combustion technology for energy conversion and safety based on the Eddy Dissipation Concept by Magnussen. (author) 30 refs.
Spatial Convergence of Three Dimensional Turbulent Flows
Park, Michael A.; Anderson, W. Kyle
2016-01-01
Finite-volume and finite-element schemes, both implemented within the FUN3D flow solver, are evaluated for several test cases described on the Turbulence-Modeling Resource (TMR) web site. The cases include subsonic flow over a hemisphere cylinder, subsonic flow over a swept bump configuration, and supersonic flow in a square duct. The finite- volume and finite-element schemes are both used to obtain solutions for the first two cases, whereas only the finite-volume scheme is used for the supersonic duct. For the hemisphere cylinder, finite-element solutions obtained on tetrahedral meshes are compared with finite- volume solutions on mixed-element meshes. For the swept bump, finite-volume solutions have been obtained for both hexahedral and tetrahedral meshes and are compared with finite-element solutions obtained on tetrahedral meshes. For the hemisphere cylinder and the swept bump, solutions are obtained on a series of meshes with varying grid density and comparisons are made between drag coefficients, pressure distributions, velocity profiles, and profiles of the turbulence working variable. The square duct shows small variation due to element type or the spatial accuracy of turbulence model convection. It is demonstrated that the finite-element scheme on tetrahedral meshes yields similar accuracy as the finite- volume scheme on mixed-element and hexahedral grids, and demonstrates less sensitivity to the mesh topology (biased tetrahedral grids) than the finite-volume scheme.
Mathematical and Numerical Modeling of Turbulent Flows
João M. Vedovoto
2015-06-01
Full Text Available The present work is devoted to the development and implementation of a computational framework to perform numerical simulations of low Mach number turbulent flows over complex geometries. The algorithm under consideration is based on a classical predictor-corrector time integration scheme that employs a projection method for the momentum equations. The domain decomposition strategy is adopted for distributed computing, displaying very satisfactory levels of speed-up and efficiency. The Immersed Boundary Methodology is used to characterize the presence of a complex geometry. Such method demands two separate grids: An Eulerian, where the transport equations are solved with a Finite Volume, second order discretization and a Lagrangian domain, represented by a non-structured shell grid representing the immersed geometry. The in-house code developed was fully verified by the Method of Manufactured Solu- tions, in both Eulerian and Lagrangian domains. The capabilities of the resulting computational framework are illustrated on four distinct cases: a turbulent jet, the Poiseuille flow, as a matter of validation of the implemented Immersed Boundary methodology, the flow over a sphere covering a wide range of Reynolds numbers, and finally, with the intention of demonstrating the applicability of Large Eddy Simulations - LES - in an industrial problem, the turbulent flow inside an industrial fan.
Model Polyelectrolytes in Turbulent Couette Flow
Price, Brian; Hoagland, David A.
1997-03-01
Isolated polymer chains in strong flow are deformed significantly from their equilibrium conformations, imparting a pronounced change in the local velocity field. Turbulent drag reduction by dilute polymer solutions is an important example. The onset of drag reduction appears dependent on a characteristic shear stress at the wall τw for a given polymer. (Virk, P.S. AIChE Journal 21 1975) Length and time scales formed from τw and solvent kinematic viscosity provide different scalings of the onset with chain length. It is likely that length polydispersity could be responsible for the disparity among the previously reported results concerning the correct onset condition. We have employed preparative gel electrophoresis to produce samples of very low polydispersity to determine the onset scaling of drag reduction in turbulent couette flow. The same technique provides information about chain scission in turburlence, yielding an indirect indication of chain conformation.
Oblique Laminar-Turbulent Interfaces in Plane Shear Flows
Duguet, Yohann; Schlatter, Philipp
2013-01-01
Localized structures such as turbulent stripes and turbulent spots are typical features of transitional wall-bounded flows in the subcritical regime. Based on an assumption for scale separation between large and small scales, we show analytically that the corresponding laminar-turbulent interfaces are always oblique with respect to the mean direction of the flow. In the case of plane Couette flow, the mismatch between the streamwise flow rates near the boundaries of the turbulence patch generates a large-scale flow with a nonzero spanwise component. Advection of the small-scale turbulent fluctuations (streaks) by the corresponding large-scale flow distorts the shape of the turbulence patch and is responsible for its oblique growth. This mechanism can be easily extended to other subcritical flows such as plane Poiseuille flow or Taylor-Couette flow.
Bulk flow scaling for turbulent channel and pipe flows
Chen, Xi; She, Zhen-Su
2016-01-01
We report a theory deriving bulk flow scaling for canonical wall-bounded flows. The theory accounts for the symmetries of boundary geometry (flat plate channel versus circular pipe) by a variational calculation for a large-scale energy length, which characterizes its bulk flow scaling by a simple exponent, i.e. $m=4$ for channel and 5 for pipe. The predicted mean velocity shows excellent agreement with several dozen sets of quality empirical data for a wide range of the Reynolds number (Re), with a universal bulk flow constant $\\kappa\\approx0.45$. Predictions for dissipation and turbulent transport in the bulk flow are also given, awaiting data verification.
Statistical theory of turbulent incompressible multimaterial flow
Kashiwa, B.
1987-10-01
Interpenetrating motion of incompressible materials is considered. ''Turbulence'' is defined as any deviation from the mean motion. Accordingly a nominally stationary fluid will exhibit turbulent fluctuations due to a single, slowly moving sphere. Mean conservation equations for interpenetrating materials in arbitrary proportions are derived using an ensemble averaging procedure, beginning with the exact equations of motion. The result is a set of conservation equations for the mean mass, momentum and fluctuational kinetic energy of each material. The equation system is at first unclosed due to integral terms involving unknown one-point and two-point probability distribution functions. In the mean momentum equation, the unclosed terms are clearly identified as representing two physical processes. One is transport of momentum by multimaterial Reynolds stresses, and the other is momentum exchange due to pressure fluctuations and viscous stress at material interfaces. Closure is approached by combining careful examination of multipoint statistical correlations with the traditional physical technique of kappa-epsilon modeling for single-material turbulence. This involves representing the multimaterial Reynolds stress for each material as a turbulent viscosity times the rate of strain based on the mean velocity of that material. The multimaterial turbulent viscosity is related to the fluctuational kinetic energy kappa, and the rate of fluctuational energy dissipation epsilon, for each material. Hence a set of kappa and epsilon equations must be solved, together with mean mass and momentum conservation equations, for each material. Both kappa and the turbulent viscosities enter into the momentum exchange force. The theory is applied to (a) calculation of the drag force on a sphere fixed in a uniform flow, (b) calculation of the settling rate in a suspension and (c) calculation of velocity profiles in the pneumatic transport of solid particles in a
Fractal flow design how to design bespoke turbulence and why
Vassilicos, Christos
2016-01-01
This book focuses on turbulent flows generated and/or influenced by multiscale/fractal structures. It consists of six chapters which demonstrate, each one in its own way, how such structures and objects can be used to design bespoke turbulence for particular applications and also how they can be used for fundamental studies of turbulent flows.
Driver gas flow with fluctuations. [shock tube turbulent bursts
Johnson, J. A., III; Jones, W. R.; Santiago, J.
1980-01-01
A shock tube's driver gas can apparently provide flow with turbulent bursts. The fluctuations are interpreted using a boundary layer model of contact surface flow and results form a kinetic theory of turbulence. With this, a lower limit of 4 on the ratio of maximum to minimum turbulent intensities in contact surface instabilities has been estimated.
QIU Xiang
2006-01-01
Turbulence structures and turbulent Counter-Gradient Transport(CGT) properties in the stratified flows with a sharp temperature interface are investigated by experimental measurements using LIF and PIV, by LES and by correlation analysis.
El Khoury, George K; Schlatter, Philipp; Brethouwer, Geert; Johansson, Arne V
2014-01-01
Direct numerical simulation data of fully developed turbulent pipe flow are extensively compared with those of turbulent channel flow and zero-pressure-gradient boundary layer flow for Re-tau up to 1 000...
Anisotropic electrostatic turbulence and zonal flow generation
Balescu, R [Physique Statistique-Plasmas, Association Euratom-Etat Belge, Universite Libre de Bruxelles, Campus Plaine, Bd. du Triomphe, 1050 Bruxelles (Belgium); Petrisor, I [Department of Physics, Association Euratom-MEdC, Romania, University of Craiova, 13 A.I.Cuza Str., 200585 Craiova (Romania); Negrea, M [Department of Physics, Association Euratom-MEdC, Romania, University of Craiova, 13 A.I.Cuza Str., 200585 Craiova (Romania)
2005-12-15
In this paper we analyse the running and asymptotic diffusion coefficients of a plasma in the case of zonal flow generation by an anisotropic stochastic electrostatic potential. Both the weak and relatively strong turbulence regimes were analysed. The analysis of the diffusion coefficients in wave vector space provides an illustration of the fragmentation of drift wave structures in the radial direction and the generation of long-wavelength structures in the poloidal direction that are identified as zonal flows. We have shown that the fragmentation of drift wave structures is strongly influenced by the anisotropy parameter, the electrostatic Kubo number and by the initial values of the wave vector.
Turbulent unsteady flow profiles over an adverse slope
Bose, Sujit K; Dey, Subhasish
2013-01-01
.... The time dependent turbulent flow is treated here by appropriately reducing the two-dimensional Reynolds averaged Navier-Stokes equation along with the equation of continuity considering turbulence closure...
Two-Dimensional Turbulent Separated Flow. Volume 1
1985-06-01
of detached turbulent boundary layers, even when the sign of U is changed to account for mean backflows. Thus, earlier researchers, such as Kuhn and...Turbulent Shear Layer," Third Symposium on Turbulent Shear Flows, pp. 16.23-16.29. Hillier, R., Latour , M.E.M.P., and Cherry, N.J. (1983), "Unsteady...344. Kuhn , G.D. and Nielsen, J.N. (1971), "An Analytical Method for Calculating Turbulent Separated Flows Due to Adverse Pressure Gradients
Recent progress of laser metrology in chemically reacting flows at onera
Mohamed, A.; Dorval, N.; Vilmart, G.; Orain, M.; George, R.; Scherman, M.; Nafa, M.; Bresson, A.; Attal-Tretout, B.; Lefebvre, M.
2017-06-01
This paper presents some of the development actions performed these last years at ONERA using laser spectroscopic techniques to probe chemically reacting flows. Techniques like laser absorption, laser induced fluorescence (LIF), and Raman scattering will be described with focus on present drawbacks as well as expectations from new laser technologies (Interband Cascade Lasers (ICL) diodes, Optical Parametrical Oscillators (OPO), frequency comb, and femto/picosecond lasers) before showing some results of recent applications in ground facilities.
Sensitivity to draught in turbulent air flows
Todde, V.
1998-09-01
Even though the ventilation system is designed to supply air flows at constant low velocity and controlled temperature, the resulting air movement in rooms is strongly characterised by random fluctuations. When an air flow is supplied from an inlet, a shear layer forms between the incoming and the standstill air in the room, and large scale vortices develops by coalescence of the vorticity shed at the inlet of the air supply. After a characteristically downstream distance, large scale vortices loose their identity because of the development of cascading eddies and transition to turbulence. The interaction of these vortical structures will rise a complicated three dimensional air movement affected by fluctuations whose frequencies could vary from fractions of Hz to several KHz. The perception and sensitivity to the cooling effect enhanced by these air movements depend on a number of factors interacting with each other: physical properties of the air flow, part and extension of the skin surface exposed to the air flow, exposure duration, global thermal condition, gender and posture of the person. Earlier studies were concerned with the percentage of dissatisfied subjects as a function of air velocity and temperature. Recently, experimental observations have shown that also the fluctuations, the turbulence intensity and the direction of air velocity have an important impact on draught discomfort. Two experimental investigations have been developed to observe the human reaction to horizontal air movements on bared skin surfaces, hands and neck. Attention was concentrated on the effects of relative turbulence intensity of air velocity and exposure duration on perception and sensitivity to the air movement. The air jet flows, adopted for the draught experiment in the neck, were also the object of an experimental study. This experiment was designed to observe the centre-line velocity of an isothermal circular air jet, as a function of the velocity properties at the outlet
Elastically bound particle in a turbulent flow
Gudmundsson, Kristjan; Prosperetti, Andrea
2011-11-01
The results of a direct numerical simulation of the behavior of a finite-size spherical particle subject to a linear elastic force in a turbulent flow are described. The turbulence is obtained by a physical space linear forcing due to Lundgren (see also Rosales and Meneveau, PoF 2005). The fluid-particle interaction is simulated by means of the Physalis method which permits the accurate calculation of hydrodynamic forces and couples acting on the particle using a fixed Cartesian grid. We vary the particle size with respect to the integral length scale along with the spring constant and therefore the natural frequency of the oscillator. Some results of a similar calculation with torsional springs and a fixed particle center will also be described. Funding provided by the IMPACT institute, the Netherlands.
Adaptive LES Methodology for Turbulent Flow Simulations
Oleg V. Vasilyev
2008-06-12
Although turbulent flows are common in the world around us, a solution to the fundamental equations that govern turbulence still eludes the scientific community. Turbulence has often been called one of the last unsolved problem in classical physics, yet it is clear that the need to accurately predict the effect of turbulent flows impacts virtually every field of science and engineering. As an example, a critical step in making modern computational tools useful in designing aircraft is to be able to accurately predict the lift, drag, and other aerodynamic characteristics in numerical simulations in a reasonable amount of time. Simulations that take months to years to complete are much less useful to the design cycle. Much work has been done toward this goal (Lee-Rausch et al. 2003, Jameson 2003) and as cost effective accurate tools for simulating turbulent flows evolve, we will all benefit from new scientific and engineering breakthroughs. The problem of simulating high Reynolds number (Re) turbulent flows of engineering and scientific interest would have been solved with the advent of Direct Numerical Simulation (DNS) techniques if unlimited computing power, memory, and time could be applied to each particular problem. Yet, given the current and near future computational resources that exist and a reasonable limit on the amount of time an engineer or scientist can wait for a result, the DNS technique will not be useful for more than 'unit' problems for the foreseeable future (Moin & Kim 1997, Jimenez & Moin 1991). The high computational cost for the DNS of three dimensional turbulent flows results from the fact that they have eddies of significant energy in a range of scales from the characteristic length scale of the flow all the way down to the Kolmogorov length scale. The actual cost of doing a three dimensional DNS scales as Re{sup 9/4} due to the large disparity in scales that need to be fully resolved. State-of-the-art DNS calculations of isotropic
Modeling of turbulent chemical reaction
Chen, J.-Y.
1995-01-01
Viewgraphs are presented on modeling turbulent reacting flows, regimes of turbulent combustion, regimes of premixed and regimes of non-premixed turbulent combustion, chemical closure models, flamelet model, conditional moment closure (CMC), NO(x) emissions from turbulent H2 jet flames, probability density function (PDF), departures from chemical equilibrium, mixing models for PDF methods, comparison of predicted and measured H2O mass fractions in turbulent nonpremixed jet flames, experimental evidence of preferential diffusion in turbulent jet flames, and computation of turbulent reacting flows.
Incompressible Turbulent Wing-Body Junction Flow
Krishnamurthy, R.; Cagle, Corey D.; Chandra, S.
1998-01-01
-stream flow. The lateral curvature of the wing/strat causes the oncoming turbulent layer to skew about am axis (x-axis) parallel to the plane (xz-plane) of the mean shear. This is the principle mechanism for the generation of secondary flow. Such skew-induced secondary flows are slow to be attenuated by Reynolds stresses. Additional contribution to the generation of secondary flow comes from anisotropies in Reynolds stresses. Upstream of the strut, the mean-vorticity is directed span wise (along the y-direction). The presence of secondary flow in the vicinity of the strut causes the vorticity to stretch around the obstacle in a horse-shoe shape, with each leg having a vorticity of the opposite sense. The blockage effect of the strut imposes a severe adverse pressure gradient on the oncoming turbulent shear layer, causing boundary layer separation ahead of the leading edge, resulting in a vortex that rolls up and flows downstream into the juncture region. The separation vortices trailing in the wake of the wing can alter the lift or drag characteristics of the surfaces downstream of the wing-body juncture. Likewise, on submarines, the wake flow behind the appendage can degrade the performance of the propeller located downstream. The complex nature of this flow is caused by the presence of all six components of Reynolds stresses. Devenport and Simpson report that in the vicinity of the horse-shoe vortex there is intense recirculation with turbulent stresses being much larger than those normally observed in turbulent flows. These features contribute to making this flow a challenge to predict numerically. Some of the past studies provide useful insights into this flow that would guide our numerical efforts. In measurements reported by Shabaka and Bradshaw, the eddy viscosity tensor is seen to be non-isotropic and has negative components in certain regions. In an effort to evaluate the closure assumptions of various turbulence models, Devenport and Simpson used their own extensive
Dynamic Multiscale Averaging (DMA) of Turbulent Flow
Richard W. Johnson
2012-09-01
A new approach called dynamic multiscale averaging (DMA) for computing the effects of turbulent flow is described. The new method encompasses multiple applications of temporal and spatial averaging, that is, multiscale operations. Initially, a direct numerical simulation (DNS) is performed for a relatively short time; it is envisioned that this short time should be long enough to capture several fluctuating time periods of the smallest scales. The flow field variables are subject to running time averaging during the DNS. After the relatively short time, the time-averaged variables are volume averaged onto a coarser grid. Both time and volume averaging of the describing equations generate correlations in the averaged equations. These correlations are computed from the flow field and added as source terms to the computation on the next coarser mesh. They represent coupling between the two adjacent scales. Since they are computed directly from first principles, there is no modeling involved. However, there is approximation involved in the coupling correlations as the flow field has been computed for only a relatively short time. After the time and spatial averaging operations are applied at a given stage, new computations are performed on the next coarser mesh using a larger time step. The process continues until the coarsest scale needed is reached. New correlations are created for each averaging procedure. The number of averaging operations needed is expected to be problem dependent. The new DMA approach is applied to a relatively low Reynolds number flow in a square duct segment. Time-averaged stream-wise velocity and vorticity contours from the DMA approach appear to be very similar to a full DNS for a similar flow reported in the literature. Expected symmetry for the final results is produced for the DMA method. The results obtained indicate that DMA holds significant potential in being able to accurately compute turbulent flow without modeling for practical
Turbulent statistics and flow structures in spanwise-rotating turbulent plane Couette flows
Gai, Jie; Xia, Zhenhua; Cai, Qingdong; Chen, Shiyi
2016-09-01
A series of direct numerical simulations of spanwise-rotating turbulent plane Couette flows at a Reynolds number of 1300 with rotation numbers Ro between 0 and 0.9 is carried out to investigate the effects of anticyclonic rotation on turbulent statistics and flow structures. Several typical turbulent statistics are presented, including the mean shear rate at the centerline, the wall-friction Reynolds number, and volume-averaged kinetic energies with respect to the secondary flow field, turbulent field, and total fluctuation field. Our results show that the rotation changes these quantities in different manners. Volume-averaged balance equations for kinetic energy are analyzed and it turns out that the interaction term acts as a kinetic energy bridge that transfers energy from the secondary flow to the turbulent fluctuations. Several typical flow regimes are identified based on the correlation functions across the whole channel and flow visualizations. The two-dimensional roll cells are observed at weak rotation Ro=0.01 , where alternant clustering of vortices appears. Three-dimensional roll cells emerge around Ro≈0.02 , where the clustering of vortices shows the meandering and bifurcating behavior. For moderate rotation 0.07 ≲Ro≲0.36 , well-organized structures are observed, where the herringbonelike vortices are clustered between streaks from the top view of three-dimensional flow visualization and form annuluses. More importantly, the vortices are rather confined to one side of the walls when Ro≤0.02 and are inclined from the bottom to upper walls when Ro≥0.07 .
Multiple states in highly turbulent Taylor-Couette flow
Huisman, Sander G; Sun, Chao; Lohse, Detlef
2016-01-01
The ubiquity of turbulent flows in nature and technology makes it of utmost importance to fundamentally understand turbulence. Kolmogorov's 1941 paradigm suggests that for strongly turbulent flows with many degrees of freedom and its large fluctuations, there would only be \\emph{one} turbulent state as the large fluctuations would explore the entire higher-dimensional phase space. Here we report the first conclusive evidence of multiple turbulent states for large Reynolds number $\\text{Re}=\\mathcal{O}(10^6)$ (Taylor number $\\text{Ta}=\\mathcal{O}(10^{12})$) Taylor-Couette flow in the regime of ultimate turbulence, by probing the phase space spanned by the rotation rates of the inner and outer cylinder. The manifestation of multiple turbulent states is exemplified by providing combined global torque and local velocity measurements. This result verifies the notion that bifurcations can occur in high-dimensional flows i.e. very large $\\text{Re}$) and questions Kolmogorov's paradigm.
Blade manipulators in turbulent channel flow
Vasudevan, B.; Prabhu, A.; Narasimha, R.
1992-01-01
We report here the results of a series of careful experiments in turbulent channel flow, using various configurations of blade manipulators suggested as optimal in earlier boundary layer studies. The mass flow in the channel could be held constant to better than 0.1%, and the uncertainties in pressure loss measurements were less than 0.1 mm of water; it was therefore possible to make accurate estimates of the global effects of blade manipulation of a kind that are difficult in boundary layer flows. The flow was fully developed at the station where the blades were mounted, and always relaxed to the same state sufficiently far downstream. It is found that, for a given mass flow, the pressure drop to any station downstream is always higher in the manipulated than in the unmanipulated flow, demonstrating that none of the blade manipulators tried reduces net duct losses. However the net increase in duct losses is less than the drag of the blade even in laminar flow, showing that there is a net reduction in the total skin friction drag experienced by the duct, but this relief is only about 20% of the manipulator drag at most.
4. Large-Eddy Simulation of Turbulent Channel Flow
Yasuaki, DOI; Tsukasa, KIMURA; Hiroshima University; Mitsubishi Precision
1989-01-01
Turbulent channel flow is studied numerically by using Large-Eddy Simulation (LES). Finite difference method is employed in the LES. The simulation is stably executed by using the 3rd order upwind difference scheme which dissipate numerical errors. Several pilot tests are performed in order to investigate the effect of numerical dissipation and the wall damping function on the calculated results. Time dependent feature and turbulent flow structures in a turbulent channel flow are numerically ...
High Resolution Simulation of Turbulent Flow in a Channel.
1987-09-25
chosen to maintain the original Poiseuille flow . The introduction of highly unstable disturbances causes transition to turbulence so that the wall...for Turbulent Channel Flow ," Phys. Rev. Lett, Vol. 47, 832-835 (1981). 2. S.A. Orszag and L.C. Kells, "Transition to turbulence in plane Poiseuille and...plane Couette Flow ," J. Fluid Mech., Vol. 96, pp. 159-205. 3. Kreplin, H.-P. and Eckelmann, H., "Behavior of the Three Fluctucting Velocity
CISM-IUTAM School on Advanced Turbulent Flow Computations
Krause, Egon
2000-01-01
This book collects the lecture notes concerning the IUTAM School on Advanced Turbulent Flow Computations held at CISM in Udine September 7–11, 1998. The course was intended for scientists, engineers and post-graduate students interested in the application of advanced numerical techniques for simulating turbulent flows. The topic comprises two closely connected main subjects: modelling and computation, mesh pionts necessary to simulate complex turbulent flow.
Shih, Tsan-Hsing; Liu, Nan-Suey
2012-01-01
This paper presents the numerical simulations of the Jet-A spray reacting flow in a single element lean direct injection (LDI) injector by using the National Combustion Code (NCC) with and without invoking the Eulerian scalar probability density function (PDF) method. The flow field is calculated by using the Reynolds averaged Navier-Stokes equations (RANS and URANS) with nonlinear turbulence models, and when the scalar PDF method is invoked, the energy and compositions or species mass fractions are calculated by solving the equation of an ensemble averaged density-weighted fine-grained probability density function that is referred to here as the averaged probability density function (APDF). A nonlinear model for closing the convection term of the scalar APDF equation is used in the presented simulations and will be briefly described. Detailed comparisons between the results and available experimental data are carried out. Some positive findings of invoking the Eulerian scalar PDF method in both improving the simulation quality and reducing the computing cost are observed.
Instantaneous aerosol dynamics in a turbulent flow
Zhou, Kun
2012-01-01
Dibutyl phthalate aerosol particles evolution dynamics in a turbulent mixing layer is simulated by means of direct numerical simulation for the flow field and the direct quadrature method of moments for the aerosol evolution. Most par-ticles are nucleated in a thin layer region corresponding to a specific narrow temperature range near the cool stream side. However, particles undergo high growth rate on the hot stream side due to condensation. Coagulation decreases the total particle number density at a rate which is highly correlated to the in-stantaneous number density.
Current status of droplet evaporation in turbulent flows
Birouk, Madjid [Department of Mechanical and Manufacturing Engineering, University of Manitoba, Winnipeg, MB (Canada); Goekalp, Iskender [Laboratoire de Combustion et Systemes Reactifs, Centre National de la Recherche Scientifique, 45071 Orleans Cedex 2 (France)
2006-07-01
This article reviews the available literature results concerning the effects of turbulence on the transport (heat and mass transfer) rates from a droplet. The survey emphasizes recent findings related specifically to physical models and correlations for predicting turbulence effects on the vaporization rate of a droplet. In addition, several research challenges on the vaporization of fuel droplets in turbulent flow environments are outlined. (author)
ON THE EDDY VISCOSITY MODEL OF PERIODIC TURBULENT SHEAR FLOWS
王新军; 罗纪生; 周恒
2003-01-01
Physical argument shows that eddy viscosity is essentially different from molecular viscosity. By direct numerical simulation, it was shown that for periodic turbulent flows, there is phase difference between Reynolds stress and rate of strain. This finding posed great challenge to turbulence modeling, because most turbulence modeling, which use the idea of eddy viscosity, do not take this effect into account.
Turbulent bands in a planar shear flow without walls
Chantry, Matthew; Barkley, Dwight
2015-01-01
Turbulent bands are a ubiquitous feature of transition in wall-bounded shear flows. We show that these are also a robust feature of Waleffe flow -- a shear flow driven by a sinusoidal body force between stress-free boundaries -- thus demonstrating that rigid walls are not a prerequisite for band formation. Exploiting the Fourier dependence of Waleffe forcing, we construct a model flow that uses only four wavenumbers in the shear direction and yet captures uniform turbulence, turbulent bands, and spot expansion. The model is simultaneously a reduction of the full Navier-Stokes equations and an extension of minimal models of the self-sustaining process of shear turbulence.
Bailly, Christophe
2015-01-01
This book covers the major problems of turbulence and turbulent processes, including physical phenomena, their modeling and their simulation. After a general introduction in Chapter 1 illustrating many aspects dealing with turbulent flows, averaged equations and kinetic energy budgets are provided in Chapter 2. The concept of turbulent viscosity as a closure of the Reynolds stress is also introduced. Wall-bounded flows are presented in Chapter 3, and aspects specific to boundary layers and channel or pipe flows are also pointed out. Free shear flows, namely free jets and wakes, are considered in Chapter 4. Chapter 5 deals with vortex dynamics. Homogeneous turbulence, isotropy, and dynamics of isotropic turbulence are presented in Chapters 6 and 7. Turbulence is then described both in the physical space and in the wave number space. Time dependent numerical simulations are presented in Chapter 8, where an introduction to large eddy simulation is offered. The last three chapters of the book summarize remarka...
LARGE EDDY SIMULATION OF PULSATING TURBULENT OPEN CHANNEL FLOW
ZOU Li-yong; LIU Nan-sheng; LU Xi-yun
2004-01-01
Pulsating turbulent open channel flow has been investigated by the use of Large Eddy Simulation (LES) technique coupled with dynamic Sub-Grid-Scale (SGS) model for turbulent SGS stress to closure the governing equations. Three-dimensional filtered Navier-Stokes equations are numerically solved by a fractional-step method. The objective of this study is to deal with the behavior of the pulsating turbulent open channel flow and to examine the reliability of the LES approach for predicting the pulsating turbulent flow. In this study, the Reynolds number (Reτ ) is chosen as 180 based on the friction velocity and the channel depth. The frequency of the driving pressure gradient for the pulsating turbulent flow ranges low, medium and high value. Statistical turbulence quantities as well as the flow structures are analyzed.
The pressure distribution in thermally bistable turbulent flows
2005-01-01
We present a systematic numerical study of the effect of turbulent velocity fluctuations on the thermal pressure distribution in thermally bistable flows. The simulations employ a random turbulent driving generated in Fourier space rather than star-like heating. The turbulent fluctuations are characterized by their rms Mach number M and the energy injection wavenumber, k_for. Our results are consistent with the picture that as either of these parameters is increased, the local ratio of turbul...
Turbulent Flow past High Temperature Surfaces
Mehmedagic, Igbal; Thangam, Siva; Carlucci, Pasquale; Buckley, Liam; Carlucci, Donald
2014-11-01
Flow over high-temperature surfaces subject to wall heating is analyzed with applications to projectile design. In this study, computations are performed using an anisotropic Reynolds-stress model to study flow past surfaces that are subject to radiative flux. The model utilizes a phenomenological treatment of the energy spectrum and diffusivities of momentum and heat to include the effects of wall heat transfer and radiative exchange. The radiative transport is modeled using Eddington approximation including the weighted effect of nongrayness of the fluid. The time-averaged equations of motion and energy are solved using the modeled form of transport equations for the turbulence kinetic energy and the scalar form of turbulence dissipation with an efficient finite-volume algorithm. The model is applied for available test cases to validate its predictive capabilities for capturing the effects of wall heat transfer. Computational results are compared with experimental data available in the literature. Applications involving the design of projectiles are summarized. Funded in part by U.S. Army, ARDEC.
Flow Structure and Turbulence in Wind Farms
Stevens, Richard J. A. M.; Meneveau, Charles
2017-01-01
Similar to other renewable energy sources, wind energy is characterized by a low power density. Hence, for wind energy to make considerable contributions to the world's overall energy supply, large wind farms (on- and offshore) consisting of arrays of ever larger wind turbines are being envisioned and built. From a fluid mechanics perspective, wind farms encompass turbulent flow phenomena occurring at many spatial and temporal scales. Of particular interest to understanding mean power extraction and fluctuations in wind farms are the scales ranging from 1 to 10 m that comprise the wakes behind individual wind turbines, to motions reaching 100 m to kilometers in scale, inherently associated with the atmospheric boundary layer. In this review, we summarize current understanding of these flow phenomena (particularly mean and second-order statistics) through field studies, wind tunnel experiments, large-eddy simulations, and analytical modeling, emphasizing the most relevant features for wind farm design and operation.
Drag reduction in turbulent MHD pipe flows
Orlandi, P.
1996-01-01
This is a preliminary study devoted to verifying whether or not direct simulations of turbulent Magneto-Hydro-Dynamic (MHD) flows in liquid metals reproduce experimental observations of drag reduction. Two different cases have been simulated by a finite difference scheme which is second order accurate in space and time. In the first case, an external azimuthal magnetic field is imposed. In this case, the magnetic field acts on the mean axial velocity and complete laminarization of the flow at N(sub a) = 30 has been achieved. In the second case, an axial magnetic field is imposed which affects only fluctuating velocities, and thus the action is less efficient. This second case is more practical, but comparison between numerical and experimental results is only qualitative.
TWO MODIFICATORY K-ε TURBULENCE MODELS FOR TURBULENT SWIRLING FLOWS
Wang Ze; Liu Wei-ming
2003-01-01
Since the standard K-ε model used to predict the strongly swirling flow leads to a large deviation from experimental results, it is necessary to introduce modification to the standard K-ε model. Based on the algebraic Reynolds stress model and Bradshaw's turbulent length scale modification conception, we present two modified K-ε models. To investigate the behaviour of the modified turbulence models, they are used to predict two representative turbulent swirling flows. The computational results, after compared with the experimental data, show that the modified K-ε models substantially improve the prediction of the standard K-ε model for the turbulent swirling flows.
Applications of URANS on predicting unsteady turbulent separated flows
Jinglei Xu; Huiyang Ma
2009-01-01
Accurate prediction of unsteady separated turbu-lent flows remains one of the toughest tasks and a practi-cal challenge for turbulence modeling. In this paper, a 2D flow past a circular cylinder at Reynolds number 3,900 is numerically investigated by using the technique of unsteady RANS (URANS). Some typical linear and nonlinear eddy viscosity turbulence models (LEVM and NLEVM) and a quadratic explicit algebraic stress model (EASM) are evalu-ated. Numerical results have shown that a high-performance cubic NLEVM, such as CLS, are superior to the others in simulating turbulent separated flows with unsteady vortex shedding.
Applications of URANS on predicting unsteady turbulent separated flows
Xu, Jinglei; Ma, Huiyang
2009-06-01
Accurate prediction of unsteady separated turbulent flows remains one of the toughest tasks and a practical challenge for turbulence modeling. In this paper, a 2D flow past a circular cylinder at Reynolds number 3,900 is numerically investigated by using the technique of unsteady RANS (URANS). Some typical linear and nonlinear eddy viscosity turbulence models (LEVM and NLEVM) and a quadratic explicit algebraic stress model (EASM) are evaluated. Numerical results have shown that a high-performance cubic NLEVM, such as CLS, are superior to the others in simulating turbulent separated flows with unsteady vortex shedding.
Experimental studies of occupation times in turbulent flows
Mann, J.; Ott, Søren; Pécseli, H.L.;
2003-01-01
The motion of passively convected particles in turbulent flows is studied experimentally in approximately homogeneous and isotropic turbulent flows, generated in water by two moving grids. The simultaneous trajectories of many small passively convected, neutrally buoyant, polystyrene particles ar....... In the present formulation, the results of the analysis are relevant for understanding details in the feeding rate of micro-organisms in turbulent waters, for instance....
Turbulence measurements in shock induced flow using hot wire anemometry
Hartung, Lin C.; Duffy, Robert E.; Trolier, James W.
1988-01-01
Heat transfer measurements over various geometric shapes have been made by immersing models in shock-induced flows. The heat transfer to a body is strongly dependent on the turbulence level of the stream. The interpretation of such heat transfer measurements requires a knowledge of the turbulence intensity. Turbulence intensity measurements, using hot-wire anemometry, have been successfully carried out in shock-induced flows. The experimental procedures for making such measurements and the techniques required are discussed.
On laminar-turbulent transition in nanofluid flows
Rudyak, V. Ya.; Minakov, A. V.; Guzey, D. V.; Zhigarev, V. A.; Pryazhnikov, M. I.
2016-09-01
The paper presents experimental data on the laminar-turbulent transition in the nanofluid flow in the pipe. The transition in the flows of such fluids is shown to have lower Reynolds numbers than in the base fluid. The degree of the flow destabilization increases with an increase in concentration of nanoparticles and a decrease in their size. On the other hand, in the turbulent flow regime, the presence of particles in the flow leads to the suppression of smallscale turbulent fluctuations. The correlation of the measured viscosity coefficient of considered nanofluids is presented.
Combined LAURA-UPS solution procedure for chemically-reacting flows. M.S. Thesis
Wood, William A.
1994-01-01
A new procedure seeks to combine the thin-layer Navier-Stokes solver LAURA with the parabolized Navier-Stokes solver UPS for the aerothermodynamic solution of chemically-reacting air flowfields. The interface protocol is presented and the method is applied to two slender, blunted shapes. Both axisymmetric and three dimensional solutions are included with surface pressure and heat transfer comparisons between the present method and previously published results. The case of Mach 25 flow over an axisymmetric six degree sphere-cone with a noncatalytic wall is considered to 100 nose radii. A stability bound on the marching step size was observed with this case and is attributed to chemistry effects resulting from the noncatalytic wall boundary condition. A second case with Mach 28 flow over a sphere-cone-cylinder-flare configuration is computed at both two and five degree angles of attack with a fully-catalytic wall. Surface pressures are seen to be within five percent with the present method compared to the baseline LAURA solution and heat transfers are within 10 percent. The effect of grid resolution is investigated and the nonequilibrium results are compared with a perfect gas solution, showing that while the surface pressure is relatively unchanged by the inclusion of reacting chemistry the nonequilibrium heating is 25 percent higher. The procedure demonstrates significant, order of magnitude reductions in solution time and required memory for the three dimensional case over an all thin-layer Navier-Stokes solution.
Mechanics of dense suspensions in turbulent channel flows
Picano, F.; Costa, P.; Breugem, W.P.; Brandt, L.
2015-01-01
Dense suspensions are usually investigated in the laminar limit where inertial effects are insignificant. When the flow rate is high enough, i.e. at high Reynolds number, the flow may become turbulent and the interaction between solid and liquid phases modifies the turbulence we know in single-phase
Methods for Prediction of High-Speed Reacting Flows in Aerospace Propulsion
Drummond, J. Philip
2014-01-01
Research to develop high-speed airbreathing aerospace propulsion systems was underway in the late 1950s. A major part of the effort involved the supersonic combustion ramjet, or scramjet, engine. Work had also begun to develop computational techniques for solving the equations governing the flow through a scramjet engine. However, scramjet technology and the computational methods to assist in its evolution would remain apart for another decade. The principal barrier was that the computational methods needed for engine evolution lacked the computer technology required for solving the discrete equations resulting from the numerical methods. Even today, computer resources remain a major pacing item in overcoming this barrier. Significant advances have been made over the past 35 years, however, in modeling the supersonic chemically reacting flow in a scramjet combustor. To see how scramjet development and the required computational tools finally merged, we briefly trace the evolution of the technology in both areas.
Five layers in a turbulent pipe flow
Lee, Jinyoung; Ahn, Junsun; Sung, Hyung Jin
2016-11-01
The scaling laws governing the five layers of the mean velocity distribution of a turbulent pipe flow were characterized using the available DNS data (Reτ = 544 , 934, 3008). Excluding the very near-wall and core regions, the buffer, meso- and log layers were identified by examining the streamwise mean momentum equation and the net force spectra. The (outer) log layer was located in the overlap region where the viscous force was negligible. Another (inner) log layer was observed in the buffer layer, in which the viscous force was directly counterbalanced by the turbulent inertia. A meso-layer between the buffer and outer log layers was found to feature viscous effects. The acceleration force of the large-scale motions (LSMs) penetrated the outer log layer at higher Reynolds numbers, as observed in the net force spectra. The acceleration force of the LSMs became strong and was counterbalanced by the deceleration force of the small-scale motions (SSMs), indicating that the inner and outer length scales contributed equally to the meso-layer. The outer log layer was established by forming an extended connection link between the meso- and outer layers. This work was supported by the Creative Research Initiatives (No. 2016-004749) program of the National Research Foundation of Korea (MSIP) and partially supported by KISTI under the Strategic Supercomputing Support Program.
Evaporation of polydispersed droplets in a highly turbulent channel flow
Cochet, M.; Bazile, Rudy; Ferret, B.; Cazin, S.
2009-09-01
A model experiment for the study of evaporating turbulent two-phase flows is presented here. The study focuses on a situation where pre-atomized and dispersed droplets vaporize and mix in a heated turbulent flow. The test bench consists in a channel flow with characteristics of homogeneous and isotropic turbulence where fluctuations levels reach very high values (25% in the established zone). An ultrasonic atomizer allows the injection of a mist of small droplets of acetone in the carrier flow. The large range diameters ensure that every kind of droplet behavior with regards to turbulence is possible. Instantaneous concentration fields of the vaporized phase are extracted from fluorescent images (PLIF) of the two phase flow. The evolution of the mixing of the acetone vapor is analyzed for two different liquid mass loadings. Despite the high turbulence levels, concentration fluctuations remain significant, indicating that air and acetone vapor are not fully mixed far from the injector.
Evaporation of polydispersed droplets in a highly turbulent channel flow
Cochet, M.; Bazile, Rudy; Ferret, B.; Cazin, S. [INPT, UPS, IMFT (Institut de Mecanique des Fluides de Toulouse), Universite de Toulouse (France)
2009-09-15
A model experiment for the study of evaporating turbulent two-phase flows is presented here. The study focuses on a situation where pre-atomized and dispersed droplets vaporize and mix in a heated turbulent flow. The test bench consists in a channel flow with characteristics of homogeneous and isotropic turbulence where fluctuations levels reach very high values (25% in the established zone). An ultrasonic atomizer allows the injection of a mist of small droplets of acetone in the carrier flow. The large range diameters ensure that every kind of droplet behavior with regards to turbulence is possible. Instantaneous concentration fields of the vaporized phase are extracted from fluorescent images (PLIF) of the two phase flow. The evolution of the mixing of the acetone vapor is analyzed for two different liquid mass loadings. Despite the high turbulence levels, concentration fluctuations remain significant, indicating that air and acetone vapor are not fully mixed far from the injector. (orig.)
Turbulent flows and intermittency in laboratory experiments
Anselmet, F.; Antonia, R. A.; Danaila, L.
2001-10-01
In turbulent flows, the transfer of energy from large to small scales is strongly intermittent, in contradiction with Kolmogorov's (Dokl. Akad. Nauk. SSSR 30 (1941) 299; hereafter K41) assumptions. The statistical properties associated with these energy transfer fluctuations at a given scale r have been widely studied theoretically, experimentally and numerically over the last 30 years or so. Such fluctuations are also encountered in various Planetary and Space Science domains. The present paper presents a review of laboratory experiments which clearly display the fractal nature of the (spatial or temporal) energy distribution at scale r, the departures from the K41 predictions being generally quantified through high-order moments of velocity increments.
Liu, Xiao-Feng; Thomas, Flint O.; Nelson, Robert C.
2001-01-01
Turbulence kinetic energy (TKE) is a very important quantity for turbulence modeling and the budget of this quantity in its transport equation can provide insight into the flow physics. Turbulence kinetic energy budget measurements were conducted for a symmetric turbulent wake flow subjected to constant zero, favorable and adverse pressure gradients in year-three of research effort. The purpose of this study is to clarify the flow physics issues underlying the demonstrated influence of pressure gradient on wake development and provide experimental support for turbulence modeling. To ensure the reliability of these notoriously difficult measurements, the experimental procedure was carefully designed on the basis of an uncertainty analysis. Four different approaches, based on an isotropic turbulence assumption, a locally axisymmetric homogeneous turbulence assumption, a semi-isotropy assumption and a forced balance of the TKE equation, were applied for the estimate of the dissipation term. The pressure transport term is obtained from a forced balance of the turbulence kinetic energy equation. This report will present the results of the turbulence kinetic energy budget measurement and discuss their implication on the development of strained turbulent wakes.
Optimal transient growth in turbulent pipe flow
Yang SONG; Chunxiao XU; Weixi HUANG; Guixiang CUI
2015-01-01
The optimal transient growth process of perturbations driven by the pressure gradient is studied in a turbulent pipe flow. A new computational method is proposed, based on the projection operators which project the governing equations onto the sub-space spanned by the radial vorticity and radial velocity. The method is validated by comparing with the previous studies. Two peaks of the maximum transient growth am-plification curve are found at different Reynolds numbers ranging from 20 000 to 250 000. The optimal flow structures are obtained and compared with the experiments and DNS results. The location of the outer peak is at the azimuthal wave number n=1, while the location of the inner peak is varying with the Reynolds number. It is observed that the velocity streaks in the buffer layer with a spacing of 100δv are the most amplified flow structures. Finally, we consider the optimal transient growth time and its dependence on the azimuthal wave length. It shows a self-similar behavior for perturbations of different scales in the optimal transient growth process.
Hydrodynamic turbulence in quasi-Keplerian rotating flows
Shi, Liang; Hof, Björn; Rampp, Markus; Avila, Marc
2017-04-01
We report a direct-numerical-simulation study of the Taylor-Couette flow in the quasi-Keplerian regime at shear Reynolds numbers up to O (105) . Quasi-Keplerian rotating flow has been investigated for decades as a simplified model system to study the origin of turbulence in accretion disks that is not fully understood. The flow in this study is axially periodic and thus the experimental end-wall effects on the stability of the flow are avoided. Using optimal linear perturbations as initial conditions, our simulations find no sustained turbulence: the strong initial perturbations distort the velocity profile and trigger turbulence that eventually decays.
PDF methods for combustion in high-speed turbulent flows
Pope, Stephen B.
1995-01-01
This report describes the research performed during the second year of this three-year project. The ultimate objective of the project is extend the applicability of probability density function (pdf) methods from incompressible to compressible turbulent reactive flows. As described in subsequent sections, progress has been made on: (1) formulation and modelling of pdf equations for compressible turbulence, in both homogeneous and inhomogeneous inert flows; and (2) implementation of the compressible model in various flow configurations, namely decaying isotropic turbulence, homogeneous shear flow and plane mixing layer.
Criterion of Turbulent Transition in Pressure Driven Flows
Dou, Hua-Shu; Khoo, Boo Cheong
2012-11-01
It has been found from numerical simulations and experiments that velocity inflection could result in turbulent transition in viscous parallel flows. However, there are exceptions, for example, in the plane Poiseuille-Couette flow. Thus, whether velocity inflection necessarily leads to turbulent transition is still not clear. To-date, there is still no consensus on the physics of turbulence transition in the scientific community. In this study, the mechanism of turbulent transition is investigated using the energy gradient method. It is found that the transition to turbulence from a laminar flow depends on the magnitudes of the energy gradient function and the energy of the disturbance imposed (including both the amplitude and the frequency). Our study further reveals that the criterion of turbulent transition is different in pressure and shear driven flows. In pressure driven parallel flows, it is found that the necessary and sufficient condition of turbulent transition is the existence of an inflection point on the velocity profile. This criterion is found to be consistent with the available experimental data and numerical simulation results. On contrast, velocity inflection in shear driven flows does not necessarily lead to turbulent transition.
Turbulence characteristics in skimming flows on stepped spillways
Carosi, G.; Chanson, H. [Queensland Univ., Brisbane (Australia). Div. of Civil Engineering
2008-09-15
Stepped spillways are used to increase the rate of energy dissipation of reinforced cement concrete (RCC) dams. Modern stepped spillways are often designed for large discharge capacities that correspond to skimming flow regimes. Skimming flows are non-aerated at the upstream end of the chute, while free-surface aeration occurs when turbulent shear next to the free surface is larger than the bubble resistance created by surface tension and buoyancy. This study investigated the air-water flow properties in skimming flows related to turbulent characteristics. Experiments were conducted at a hydraulics laboratory using a broad-crested weir with a stepped chute. Measurements were conducted using phase-detection intrusive probes. Air-water flow properties were recorded for several flow rates in order to determine the distributions of turbulence intensity and integral length scales. Air-water interfacial velocities were obtained using a basic correlation analysis between the 2 sensors of a double-tip probe. Turbulence levels were derived from the relative width of a cross-correlation function. Probability distribution functions of the air bubbles and water droplet chords were analyzed in terms of bubble chords in the bubbly flow. The study demonstrated that some turbulent energy was dissipated in the form of large vortices in the bull of the flow, while the stepped cavities contributed to turbulence production. It was concluded that the rate of energy dissipation on stepped spillways is related to high turbulence levels and large-scale vortical structures. 43 refs., 3 tabs., 9 figs.
Small scale aspects of flows in proximity of the turbulent/non-turbulent interface
Holzner, M; Nikitin, N; Kinzelbach, W; Tsinober, A
2007-01-01
The work reported below is a first of its kind study of the properties of turbulent flow without strong mean shear in a Newtonian fluid in proximity of the turbulent/non-turbulent interface, with emphasis on the small scale aspects. The main tools used are a three-dimensional particle tracking system (3D-PTV) allowing to measure and follow in a Lagrangian manner the field of velocity derivatives and direct numerical simulations (DNS). The comparison of flow properties in the turbulent (A), intermediate (B) and non-turbulent (C) regions in the proximity of the interface allows for direct observation of the key physical processes underlying the entrainment phenomenon. The differences between small scale strain and enstrophy are striking and point to the definite scenario of turbulent entrainment via the viscous forces originating in strain.
On the interaction of turbulence and flows in toroidal plasmas
Stroth, U; Manz, P; Ramisch, M [Institut fuer Plasmaforschung, Universitaet Stuttgart, 70569 Stuttgart (Germany)
2011-02-15
In toroidally confined plasmas, background E x B flows, microturbulence and zonal flows constitute a tightly coupled dynamic system and the description of confinement transitions needs a self-consistent treatment of these players. The background radial electric field, linked to neoclassical ambipolar transport, has an impact on the interaction between zonal flows and turbulence by tilting and anisotropization of turbulent eddies. Zonal-flow drive is shown to be non-local in wavenumber space and is described as a straining-out process instead as a local inverse cascade. The straining-out process is also discussed as an option to explain turbulence suppression in sheared flows and could be the cause of predator-prey oscillations in the turbulence zonal-flow system.
Generation of Turbulent Inflow Conditions for Pipe Flow via an Annular Ribbed Turbulator
Moallemi, Nima; Brinkerhoff, Joshua
2016-11-01
The generation of turbulent inflow conditions adds significant computational expense to direct numerical simulations (DNS) of turbulent pipe flows. Typical approaches involve introducing boxes of isotropic turbulence to the velocity field at the inlet of the pipe. In the present study, an alternative method is proposed that incurs a lower computational cost and allows the anisotropy observed in pipe turbulence to be physically captured. The method is based on a periodic DNS of a ribbed turbulator upstream of the inlet boundary of the pipe. The Reynolds number based on the bulk velocity and pipe diameter is 5300 and the blockage ratio (BR) is 0.06 based on the rib height and pipe diameter. The pitch ratio is defined as the ratio of rib streamwise spacing to rib height and is varied between 1.7 and 5.0. The generation of turbulent flow structures downstream of the ribbed turbulator are identified and discussed. Suitability of this method for accurate representation of turbulent inflow conditions is assessed through comparison of the turbulent mean properties, fluctuations, Reynolds stress profiles, and spectra with published pipe flow DNS studies. The DNS results achieve excellent agreement with the numerical and experimental data available in the literature.
THE DENSITY DISTRIBUTION IN TURBULENT BISTABLE FLOWS
Gazol, Adriana [Centro de Radioastronomia y Astrofisica, UNAM, A. P. 3-72, c.p. 58089 Morelia, Michoacan (Mexico); Kim, Jongsoo, E-mail: a.gazol@crya.unam.mx, E-mail: jskim@kasi.re.kr [Korea Astronomy and Space Science Institute, 61-1, Hwaam-Dong, Yuseong-Ku, Daejeon 305-348 (Korea, Republic of)
2013-03-01
We numerically study the volume density probability distribution function (n-PDF) and the column density probability distribution function ({Sigma}-PDF) resulting from thermally bistable turbulent flows. We analyze three-dimensional hydrodynamic models in periodic boxes of 100 pc by side, where turbulence is driven in the Fourier space at a wavenumber corresponding to 50 pc. At low densities (n {approx}< 0.6 cm{sup -3}), the n-PDF is well described by a lognormal distribution for an average local Mach number ranging from {approx}0.2 to {approx}5.5. As a consequence of the nonlinear development of thermal instability (TI), the logarithmic variance of the distribution of the diffuse gas increases with M faster than in the well-known isothermal case. The average local Mach number for the dense gas (n {approx}> 7.1 cm{sup -3}) goes from {approx}1.1 to {approx}16.9 and the shape of the high-density zone of the n-PDF changes from a power law at low Mach numbers to a lognormal at high M values. In the latter case, the width of the distribution is smaller than in the isothermal case and grows slower with M. At high column densities, the {Sigma}-PDF is well described by a lognormal for all of the Mach numbers we consider and, due to the presence of TI, the width of the distribution is systematically larger than in the isothermal case but follows a qualitatively similar behavior as M increases. Although a relationship between the width of the distribution and M can be found for each one of the cases mentioned above, these relations are different from those of the isothermal case.
The wall shear rate in non-Newtonian turbulent pipe flow
Trinh, K T
2010-01-01
This paper presents a method for calculating the wall shear rate in pipe turbulent flow. It collapses adequately the data measured in laminar flow and turbulent flow into a single flow curve and gives the basis for the design of turbulent flow viscometers. Key words: non-Newtonian, wall shear rate, turbulent, rheometer
Shadid, J.N.; Moffat, H.K.; Hutchinson, S.A.; Hennigan, G.L.; Devine, K.D.; Salinger, A.G.
1996-05-01
The theoretical background for the finite element computer program, MPSalsa, is presented in detail. MPSalsa is designed to solve laminar, low Mach number, two- or three-dimensional incompressible and variable density reacting fluid flows on massively parallel computers, using a Petrov-Galerkin finite element formulation. The code has the capability to solve coupled fluid flow, heat transport, multicomponent species transport, and finite-rate chemical reactions, and to solver coupled multiple Poisson or advection-diffusion- reaction equations. The program employs the CHEMKIN library to provide a rigorous treatment of multicomponent ideal gas kinetics and transport. Chemical reactions occurring in the gas phase and on surfaces are treated by calls to CHEMKIN and SURFACE CHEMKIN, respectively. The code employs unstructured meshes, using the EXODUS II finite element data base suite of programs for its input and output files. MPSalsa solves both transient and steady flows by using fully implicit time integration, an inexact Newton method and iterative solvers based on preconditioned Krylov methods as implemented in the Aztec solver library.
MPSalsa a finite element computer program for reacting flow problems. Part 2 - user`s guide
Salinger, A.; Devine, K.; Hennigan, G.; Moffat, H. [and others
1996-09-01
This manual describes the use of MPSalsa, an unstructured finite element (FE) code for solving chemically reacting flow problems on massively parallel computers. MPSalsa has been written to enable the rigorous modeling of the complex geometry and physics found in engineering systems that exhibit coupled fluid flow, heat transfer, mass transfer, and detailed reactions. In addition, considerable effort has been made to ensure that the code makes efficient use of the computational resources of massively parallel (MP), distributed memory architectures in a way that is nearly transparent to the user. The result is the ability to simultaneously model both three-dimensional geometries and flow as well as detailed reaction chemistry in a timely manner on MT computers, an ability we believe to be unique. MPSalsa has been designed to allow the experienced researcher considerable flexibility in modeling a system. Any combination of the momentum equations, energy balance, and an arbitrary number of species mass balances can be solved. The physical and transport properties can be specified as constants, as functions, or taken from the Chemkin library and associated database. Any of the standard set of boundary conditions and source terms can be adapted by writing user functions, for which templates and examples exist.
MPSalsa a finite element computer program for reacting flow problems. Part 2 - user`s guide
Salinger, A.; Devine, K.; Hennigan, G.; Moffat, H. [and others
1996-09-01
This manual describes the use of MPSalsa, an unstructured finite element (FE) code for solving chemically reacting flow problems on massively parallel computers. MPSalsa has been written to enable the rigorous modeling of the complex geometry and physics found in engineering systems that exhibit coupled fluid flow, heat transfer, mass transfer, and detailed reactions. In addition, considerable effort has been made to ensure that the code makes efficient use of the computational resources of massively parallel (MP), distributed memory architectures in a way that is nearly transparent to the user. The result is the ability to simultaneously model both three-dimensional geometries and flow as well as detailed reaction chemistry in a timely manner on MT computers, an ability we believe to be unique. MPSalsa has been designed to allow the experienced researcher considerable flexibility in modeling a system. Any combination of the momentum equations, energy balance, and an arbitrary number of species mass balances can be solved. The physical and transport properties can be specified as constants, as functions, or taken from the Chemkin library and associated database. Any of the standard set of boundary conditions and source terms can be adapted by writing user functions, for which templates and examples exist.
Simulation of the Partially Ionized Reacting Plasma Flow in a Negative Hydrogen Ion Source
Gatsonis, Nikolaos; Averkin, Sergey; Olson, Lynn
2012-10-01
A High Pressure Discharge Negative Ion Source (HPDNIS) operating on hydrogen is been under investigation. The Negative Ion Production (NIP) section of the HPDNIS attaches to the 10-100 Torr RF-discharge chamber with a micronozzle and ends with a grid that extracts the negative ion beam. The partially ionized and reacting plasma flow in the NIP section is simulated using an unstructured three-dimensional Direct Simulation Monte Carlo (U3DSMC) code. The NIP section contains a low-pressure plasma that includes H2, vibrationally-rotationally excited H2^*, negative hydrogen atoms H^-, and electrons. Primary reactions in the NIP section are dissociate attachment, H2^*+e->H^0+H^-and electron collisional detachment, e+H^-->H+2e. The U3DSMC computational domain includes the entrance to the NIP nozzle and the extraction grid at the exit. The flow parameters at the entrance are based on conditions in the RF-discharge chamber and are implemented in U3DSMC using a Kinetic-Moment subsonic boundary conditions method. The rotational and vibrational degrees of freedom in U3DSMC are implemented using the Larsen-Borgnakke model. Chemical reactions are implemented in U3DSMC using the Quantum-Kinetic model. Simulations cover the regime of operation of the HPDNIS and examine the flow characteristics inside the NIP section.
Shih, Tsan-Hsing; Liu, Nan-Suey
2013-01-01
This paper presents the very large eddy simulations (VLES) of a Jet-A spray reacting flow in a single element lean direct injection (LDI) injector by using the National Combustion Code (NCC) with and without invoking the Eulerian scalar DWFDF method, in which DWFDF is defined as the density weighted time filtered fine grained probability density function. The flow field is calculated by using the time filtered compressible Navier-Stokes equations (TFNS) with nonlinear subscale turbulence models, and when the Eulerian scalar DWFDF method is invoked, the energy and species mass fractions are calculated by solving the equation of DWFDF. A nonlinear subscale model for closing the convection term of the Eulerian scalar DWFDF equation is used and will be briefly described in this paper. Detailed comparisons between the results and available experimental data are carried out. Some positive findings of invoking the Eulerian scalar DWFDF method in both improving the simulation quality and maintaining economic computing cost are observed.
Speed and structure of turbulent fronts in pipe flow
Song, Baofang; Hof, Björn; Avila, Marc
2016-01-01
The dynamics of laminar-turbulent fronts in pipe flow is investigated for Reynolds numbers between Re=1900 and Re=5500 using extensive direct numerical simulations. In this range the flow undergoes a continuous transition from localised puffs to weakly expanding and ultimately to strongly expanding turbulent slugs (Barkley et al. 2015). We here investigate the physical distinction between these two types of slug by analysing time-resolved statistics of their downstream fronts in the frame moving at the bulk turbulent advection speed. While weak fronts travel slower than the bulk turbulent advection speed, implying local relaminarisation, strong fronts travel faster and so feed on the laminar flow ahead. At Re$\\approx$2900 the downstream front speed becomes faster than the advection speed, marking the onset of strong fronts. We argue that large temporal fluctuations of production and dissipation at the laminar-turbulent interface drive the dynamical switches between the two types of front observed up to Re$\\si...
Numerical simulation of wall-bounded turbulent shear flows
Moin, P.
1982-01-01
Developments in three dimensional, time dependent numerical simulation of turbulent flows bounded by a wall are reviewed. Both direct and large eddy simulation techniques are considered within the same computational framework. The computational spatial grid requirements as dictated by the known structure of turbulent boundary layers are presented. The numerical methods currently in use are reviewed and some of the features of these algorithms, including spatial differencing and accuracy, time advancement, and data management are discussed. A selection of the results of the recent calculations of turbulent channel flow, including the effects of system rotation and transpiration on the flow are included.
Interaction between mean flow and turbulence in two dimensions
Falkovich, Gregory
2016-07-01
This short note is written to call attention to an analytic approach to the interaction of developed turbulence with mean flows of simple geometry (jets and vortices). It is instructive to compare cases in two and three dimensions and see why the former are solvable and the latter are not (yet). We present the analytical solutions for two-dimensional mean flows generated by an inverse turbulent cascade on a sphere and in planar domains of different aspect ratios. These solutions are obtained in the limit of small friction when the flow is strong while turbulence can be considered weak and treated perturbatively. I then discuss when these simple solutions can be realized and when more complicated flows may appear instead. The next step of describing turbulence statistics inside a flow and directions of possible future progress are briefly discussed at the end.
Advances in the analysis and prediction of turbulent viscoelastic flows
Gatski, T. B.; Thais, L.; Mompean, G.
2014-08-01
It has been well-known for over six decades that the addition of minute amounts of long polymer chains to organic solvents, or water, can lead to significant turbulent drag reduction. This discovery has had many practical applications such as in pipeline fluid transport, oil well operations, vehicle design and submersible vehicle projectiles, and more recently arteriosclerosis treatment. However, it has only been the last twenty-five years that the full utilization of direct numerical simulation of such turbulent viscoelastic flows has been achieved. The unique characteristics of viscoelastic fluid flow are dictated by the nonlinear differential relationship between the flow strain rate field and the extra-stress induced by the additive polymer. A primary motivation for the analysis of these turbulent fluid flows is the understanding of the effect on the dynamic transfer of energy in the turbulent flow due to the presence of the extra-stress field induced by the presence of the viscoelastic polymer chain. Such analyses now utilize direct numerical simulation data of fully developed channel flow for the FENE-P (Finite Extendable Nonlinear Elastic - Peterlin) fluid model. Such multi-scale dynamics suggests an analysis of the transfer of energy between the various component motions that include the turbulent kinetic energy, and the mean polymeric and elastic potential energies. It is shown that the primary effect of the interaction between the turbulent and polymeric fields is to transfer energy from the turbulence to the polymer.
Two-equation turbulence modeling for 3-D hypersonic flows
Bardina, J. E.; Coakley, T. J.; Marvin, J. G.
1992-01-01
An investigation to verify, incorporate and develop two-equation turbulence models for three-dimensional high speed flows is presented. The current design effort of hypersonic vehicles has led to an intensive study of turbulence models for compressible hypersonic flows. This research complements an extensive review of experimental data and the current development of 2D turbulence models. The review of experimental data on 2D and 3D flows includes complex hypersonic flows with pressure profiles, skin friction, wall heat transfer, and turbulence statistics data. In a parallel effort, turbulence models for high speed flows have been tested against flat plate boundary layers, and are being tested against the 2D database. In the present paper, we present the results of 3D Navier-Stokes numerical simulations with an improved k-omega two-equation turbulence model against experimental data and empirical correlations of an adiabatic flat plate boundary layer, a cold wall flat plate boundary layer, and a 3D database flow, the interaction of an oblique shock wave and a thick turbulent boundary layer with a free stream Mach number = 8.18 and Reynolds number = 5 x 10 to the 6th.
Comment on "Flow-distributed oscillations: Stationary chemical waves in a reacting flow"
Andresen, Peter Ragnar; Mosekilde, Erik; Dewel, G.;
2000-01-01
In a recent paper by Kern and Menzinger [Phys. Rev. E 60, R3471 (1999)] a successful verification of the stationary space-periodic structures predicted by Andresen et al. [Phys. Rev. E 60, 297 (1999)] was reported. Kaern and Menzinger suggest a mechanism for the formation of such structures...... that yields a linear relationship between the selected wavelength and the flow rate. We find this mechanism too simple and produce numerical simulations that support the original interpretation of these structures....
Satish, G.; Vashista, G. A.; Majumdar, Sekhar
2017-04-01
Most of the widely used popular mathematical models of turbulence use a judicious combination of intuition, empiricism and the governing equations of instantaneous and mean motion-valid strictly for fully developed turbulence without any laminar region. In reality however, any wall bounded or free shear flow may consist of some laminar flow patches which eventually undergo transition over a finite length to grow into fully turbulent flows. Most of the turbulence models used in commercial CFD codes, are unable to predict the dynamics of turbulent flows with laminar patches. However, accurate prediction of transitional flows is often essential to estimate the pressure losses and/or heat transfer in industrial applications. The present paper implements two different transition models in an existing finite volume URANS-based code RANS3D, developed in house and validated against reliable measurement data for flow past flat plates with different free stream turbulence levels and flow past SD7003 aerofoil at a chord-based Reynolds number of 60,000.
Gilmore, Mark A. [University of New Mexico
2013-06-27
Final Report for grant DE-FG02-06ER54898. The dynamics and generation of intermittent plasma turbulent structures, widely known as "blobs" have been studied in the presence of sheared plasma flows in a controlled laboratory experiment.
Parallelization of Lower-Upper Symmetric Gauss-Seidel Method for Chemically Reacting Flow
Yoon, Seokkwan; Jost, Gabriele; Chang, Sherry
2005-01-01
Development of technologies for exploration of the solar system has revived an interest in computational simulation of chemically reacting flows since planetary probe vehicles exhibit non-equilibrium phenomena during the atmospheric entry of a planet or a moon as well as the reentry to the Earth. Stability in combustion is essential for new propulsion systems. Numerical solution of real-gas flows often increases computational work by an order-of-magnitude compared to perfect gas flow partly because of the increased complexity of equations to solve. Recently, as part of Project Columbia, NASA has integrated a cluster of interconnected SGI Altix systems to provide a ten-fold increase in current supercomputing capacity that includes an SGI Origin system. Both the new and existing machines are based on cache coherent non-uniform memory access architecture. Lower-Upper Symmetric Gauss-Seidel (LU-SGS) relaxation method has been implemented into both perfect and real gas flow codes including Real-Gas Aerodynamic Simulator (RGAS). However, the vectorized RGAS code runs inefficiently on cache-based shared-memory machines such as SGI system. Parallelization of a Gauss-Seidel method is nontrivial due to its sequential nature. The LU-SGS method has been vectorized on an oblique plane in INS3D-LU code that has been one of the base codes for NAS Parallel benchmarks. The oblique plane has been called a hyperplane by computer scientists. It is straightforward to parallelize a Gauss-Seidel method by partitioning the hyperplanes once they are formed. Another way of parallelization is to schedule processors like a pipeline using software. Both hyperplane and pipeline methods have been implemented using openMP directives. The present paper reports the performance of the parallelized RGAS code on SGI Origin and Altix systems.
Tomographic imaging of reacting flows in 3D by laser absorption spectroscopy
Foo, J.; Martin, P. A.
2017-05-01
This paper describes the development of an infrared laser absorption tomography system for the 3D volumetric imaging of chemical species and temperature in reacting flows. The system is based on high-resolution near-infrared tunable diode laser absorption spectroscopy (TDLAS) for the measurement of water vapour above twin, mixed fuel gas burners arranged with an asymmetrical output. Four parallel laser beams pass through the sample region and are rotated rapidly in a plane to produce a wide range of projection angles. A rotation of 180° with 0.5° sampling was achieved in 3.6 s. The effects of changes to the burner fuel flow were monitored in real time for the 2D distributions. The monitoring plane was then moved vertically relative to the burners enabling a stack of 2D images to be produced which were then interpolated to form a 3D volumetric image of the temperature and water concentrations above the burners. The optical transmission of each beam was rapidly scanned around 1392 nm and the spectrum was fitted to find the integrated absorbance of the water transitions and although several are probed in each scan, two of these transitions possess opposite temperature dependencies. The projections of the integrated absorbances at each angle form the sinogram from which the 2D image of integrated absorbance of each line can be reconstructed by the direct Fourier reconstruction based on the Fourier slice theorem. The ratio of the integrated absorbances of the two lines can then be related to temperature alone in a method termed, two-line thermometry. The 2D temperature distribution obtained was validated for pattern and accuracy by thermocouple measurements. With the reconstructed temperature distribution, the temperature-dependent line strengths could be determined and subsequently the concentration distribution of water across the 2D plane whilst variations in burner condition were carried out. These results show that the measurement system based on TDLAS can be
Houle, Amanda
2006-01-01
This article describes the author's experiences as a student participating in a general education program called "Reacting to the Past," in which college students play elaborate games set at pivotal moments in the past, their roles informed by great texts. She found that the experience of reenacting pivotal historical moments produced an intensely…
Structures and scaling laws of turbulent Couette flow
Oberlack, Martin; Avsarkisov, Victor; Hoyas, Sergio; Rosteck, Andreas; Garcia-Galache, Jose P.; Frank, Andy
2014-11-01
We conducted a set of large scale DNS of turbulent Couette flow with the two key objectives: (i) to better understand large scale coherent structures and (ii) to validate new Lie symmetry based turbulent scaling laws for the mean velocity and higher order moments. Though frequently reported in the literature large scale structures pose a serious constraint on our ability to conduct DNS of turbulent Couette flow as the largest structures grow with increasing Re#, while at the same time Kolmogorov scale decreases. Other than for the turbulent Poiseuille flow a too small box is immediately visible in low order statistics such as the mean and limited our DNS to Reτ = 550 . At the same time we observed that scaling of the mean is peculiar as it involves a certain statistical symmetry which has never been observed for any other parallel wall-bounded turbulent shear flow. Symmetries such as Galilean group lie at the heart of fluid dynamics, while for turbulence statistics due to the multi-point correlation equations (MPCE) additional statistical symmetries are admitted. Most important, symmetries are the essential to construct exact solutions to the MPCE, which with the new above-mentioned special statistical symmetry led to a new turbulent scaling law for the Couette flow. DFG Grant No; KH 257/2-1.
Zonal Flows and Turbulence in Fluids and Plasmas
Parker, Jeffrey B
2015-01-01
In geophysical and plasma contexts, zonal flows are well known to arise out of turbulence. We elucidate the transition from statistically homogeneous turbulence without zonal flows to statistically inhomogeneous turbulence with steady zonal flows. Starting from the Hasegawa--Mima equation, we employ both the quasilinear approximation and a statistical average, which retains a great deal of the qualitative behavior of the full system. Within the resulting framework known as CE2, we extend recent understanding of the symmetry-breaking `zonostrophic instability'. Zonostrophic instability can be understood in a very general way as the instability of some turbulent background spectrum to a zonally symmetric coherent mode. As a special case, the background spectrum can consist of only a single mode. We find that in this case the dispersion relation of zonostrophic instability from the CE2 formalism reduces exactly to that of the 4-mode truncation of generalized modulational instability. We then show that zonal flow...
Elastic Turbulence in Channel Flows at Low Reynolds number
Qin, Boyang
2016-01-01
We experimentally demonstrate the existence of elastic turbulence in straight channel flow at low Reynolds numbers. Velocimetry measurements show non-periodic fluctuations in the wake of curved cylinders as well as in a parallel shear flow region. The flow in these two locations of the channel is excited over a broad range of frequencies and wavelengths, consistent with the main features of elastic turbulence. However, the decay of the initial elastic turbulence around the cylinders is followed by a growth downstream in the straight region. The emergence of distinct flow characteristics both in time and space suggests a new type of elastic turbulence, markedly different from that near the curved cylinders. We propose a self-sustaining mechanism to explain the sustained fluctuations in the parallel shear region.
Three Kinds of Velocity Structure Function in Turbulent Flows
LIU Wei; JIANG Nan
2004-01-01
Based on the local multi-scale eddy structures in turbulent flows, we elucidate the essential difference between the real turbulent field with a finite Reynolds number and the Kolmogorov fully developed random field. The motion of fluid particles in the real turbulent field is not fully random. There exist multi-scale structures due to the effect of viscosity. Actually the movements of fluid particles in the turbulent field are restricted by such eddy structures. Furthermore, concept of the locally averaged velocity structure function is put forward to describe the relative strain distortion of two adjacent turbulent eddy structures at a certain scale. The time sequence of the longitudinal velocity component at different vertical locations in turbulent boundary layer has been elaborately measured by the constant temperature anemometry of model IFA-300 in a wind tunnel. The experiment proves that the locally averaged velocity structure function is in agreement with the wavelet-coefficient structure function.
无
2000-01-01
Based on the standark κ-ε turbulence model,a new compressible κ-ε model considering the pressure expansion influence due to the compressibility of fluid is developed and aplied to the simulation of 3D transonic turbulent flows in a nozzle and a cascade.The Reynolds averaged N-S equations in generalized curvilinear coordinates are solved with implementation of the new model,the high resolution TVD scheme is used to discretize the convective terms.The numerical results show that the compressible κ-ε odel behaves well in the simulation of transonic internal turbulent flows.
CONTINUOUS WAVELET TRANSFORM OF TURBULENT BOUNDARY LAYER FLOW
LIU Ying-zheng; KE Feng; CHEN Han-ping
2005-01-01
The spatio-temporal characteristics of the velocity fluctuations in a fully-developed turbulent boundary layer flow was investigated using hotwire. A low-speed wind tunnel was established. The experimental data was extensively analyzed in terms of continuous wavelet transform coefficients and their auto-correlation. The results yielded a potential wealth of information on inherent characteristics of coherent structures embedded in turbulent boundary layer flow. Spatial and temporal variations of the low- and high- frequency motions were revealed.
Flow Separation and Turbulence in Jet Pumps for Thermoacoustic Applications
Oosterhuis, Joris P.; Verbeek, Anton A.; Bühler, Simon; Wilcox, Douglas; Meer, van der Theo H.
2016-01-01
The effect of flow separation and turbulence on the performance of a jet pump in oscillatory flows is investigated. A jet pump is a static device whose shape induces asymmetric hydrodynamic end effects when placed in an oscillatory flow. This will result in a time-averaged pressure drop which can be
Modeling of turbulent bubbly flows; Modelisation des ecoulements turbulents a bulles
Bellakhal, Ghazi
2005-03-15
The two-phase flows involve interfacial interactions which modify significantly the structure of the mean and fluctuating flow fields. The design of the two-fluid models adapted to industrial flows requires the taking into account of the effect of these interactions in the closure relations adopted. The work developed in this thesis concerns the development of first order two-fluid models deduced by reduction of second order closures. The adopted reasoning, based on the principle of decomposition of the Reynolds stress tensor into two statistically independent contributions turbulent and pseudo-turbulent parts, allows to preserve the physical contents of the second order relations closure. Analysis of the turbulence structure in two basic flows: homogeneous bubbly flows uniform and with a constant shear allows to deduce a formulation of the two-phase turbulent viscosity involving the characteristic scales of bubbly turbulence, as well as an analytical description of modification of the homogeneous turbulence structure induced by the bubbles presence. The Eulerian two-fluid model was then generalized with the case of the inhomogeneous flows with low void fractions. The numerical results obtained by the application of this model integrated in the computer code MELODIF in the case of free sheared turbulent bubbly flow of wake showed a satisfactory agreement with the experimental data and made it possible to analyze the modification of the characteristic scales of such flow by the interfacial interactions. The two-fluid first order model is generalized finally with the case of high void fractions bubbly flows where the hydrodynamic interactions between the bubbles are not negligible any more. (author)
Fully developed turbulence in slugs of pipe flows
Cerbus, Rory; Liu, Chien-Chia; Sakakibara, Jun; Gioia, Gustavo; Chakraborty, Pinaki
2015-11-01
Despite over a century of research, transition to turbulence in pipe flows remains a mystery. In theory the flow remains laminar for arbitrarily large Reynolds number, Re. In practice, however, the flow transitions to turbulence at a finite Re whose value depends on the disturbance, natural or artificial, in the experimental setup. The flow remains in the transition state for a range of Re ~ 0 (1000) ; for larger Re the flow becomes fully developed. The transition state for Re > 3000 consists of axially segregated regions of laminar and turbulent patches. These turbulent patches, known as slugs, grow as they move downstream. Their lengths span anywhere between a few pipe diameters to the whole length of the pipe. Here we report Stereo Particle Image Velocimetry measurements in the cross-section of the slugs. Notwithstanding the continuous growth of the slugs, we find that the mean velocity and stress profiles in the slugs are indistinguishable from that of statistically-stationary fully-developed turbulent flows. Our results are independent of the length of the slugs. We contrast our results with the well-known work of Wygnanski & Champagne (1973), whose measurements, we argue, are insufficient to draw a clear conclusion regarding fully developed turbulence in slugs.
Impact of large scale flows on turbulent transport
Sarazin, Y.; Grandgirard, V.; Dif-Pradalier, G.; Fleurence, E.; Garbet, X.; Ghendrih, Ph; Bertrand, P.; Besse, N.; Crouseilles, N.; Sonnendrücker, E.; Latu, G.; Violard, E.
2006-12-01
The impact of large scale flows on turbulent transport in magnetized plasmas is explored by means of various kinetic models. Zonal flows are found to lead to a non-linear upshift of turbulent transport in a 3D kinetic model for interchange turbulence. Such a transition is absent from fluid simulations, performed with the same numerical tool, which also predict a much larger transport. The discrepancy cannot be explained by zonal flows only, despite they being overdamped in fluids. Indeed, some difference remains, although reduced, when they are artificially suppressed. Zonal flows are also reported to trigger transport barriers in a 4D drift-kinetic model for slab ion temperature gradient (ITG) turbulence. The density gradient acts as a source drive for zonal flows, while their curvature back stabilizes the turbulence. Finally, 5D simulations of toroidal ITG modes with the global and full-f GYSELA code require the equilibrium density function to depend on the motion invariants only. If not, the generated strong mean flows can completely quench turbulent transport.
Impact of large scale flows on turbulent transport
Sarazin, Y [Association Euratom-CEA, CEA/DSM/DRFC centre de Cadarache, 13108 St-Paul-Lez-Durance (France); Grandgirard, V [Association Euratom-CEA, CEA/DSM/DRFC centre de Cadarache, 13108 St-Paul-Lez-Durance (France); Dif-Pradalier, G [Association Euratom-CEA, CEA/DSM/DRFC centre de Cadarache, 13108 St-Paul-Lez-Durance (France); Fleurence, E [Association Euratom-CEA, CEA/DSM/DRFC centre de Cadarache, 13108 St-Paul-Lez-Durance (France); Garbet, X [Association Euratom-CEA, CEA/DSM/DRFC centre de Cadarache, 13108 St-Paul-Lez-Durance (France); Ghendrih, Ph [Association Euratom-CEA, CEA/DSM/DRFC centre de Cadarache, 13108 St-Paul-Lez-Durance (France); Bertrand, P [LPMIA-Universite Henri Poincare Nancy I, Boulevard des Aiguillettes BP239, 54506 Vandoe uvre-les-Nancy (France); Besse, N [LPMIA-Universite Henri Poincare Nancy I, Boulevard des Aiguillettes BP239, 54506 Vandoe uvre-les-Nancy (France); Crouseilles, N [IRMA, UMR 7501 CNRS/Universite Louis Pasteur, 7 rue Rene Descartes, 67084 Strasbourg (France); Sonnendruecker, E [IRMA, UMR 7501 CNRS/Universite Louis Pasteur, 7 rue Rene Descartes, 67084 Strasbourg (France); Latu, G [LSIIT, UMR 7005 CNRS/Universite Louis Pasteur, Bd Sebastien Brant BP10413, 67412 Illkirch (France); Violard, E [LSIIT, UMR 7005 CNRS/Universite Louis Pasteur, Bd Sebastien Brant BP10413, 67412 Illkirch (France)
2006-12-15
The impact of large scale flows on turbulent transport in magnetized plasmas is explored by means of various kinetic models. Zonal flows are found to lead to a non-linear upshift of turbulent transport in a 3D kinetic model for interchange turbulence. Such a transition is absent from fluid simulations, performed with the same numerical tool, which also predict a much larger transport. The discrepancy cannot be explained by zonal flows only, despite they being overdamped in fluids. Indeed, some difference remains, although reduced, when they are artificially suppressed. Zonal flows are also reported to trigger transport barriers in a 4D drift-kinetic model for slab ion temperature gradient (ITG) turbulence. The density gradient acts as a source drive for zonal flows, while their curvature back stabilizes the turbulence. Finally, 5D simulations of toroidal ITG modes with the global and full-f GYSELA code require the equilibrium density function to depend on the motion invariants only. If not, the generated strong mean flows can completely quench turbulent transport.
Behaviour of organised disturbances in fully developed turbulent channel flow
P K Sen; Srinivas V Veeravali
2000-10-01
In our earlier work we have shown the relevance of stability theory in understanding the sustenance of turbulence in turbulent boundary layers. Here we adopt the same model to study the evolution of organised disturbances in turbulent channel flow. Since the dominant modes are wall modes we find that the stability characteristics in the two flows are nearly identical although the boundary conditions (at the edge of the boundary layer and at the centre of the channel) are different. Comparisons with the experiments of Hussain and Reynolds are also presented.
A streamwise constant model of turbulence in plane Couette flow
Gayme, D. F.; McKeon, B. J.; Papachristodoulou, A.; Bamieh, B; Doyle, J. C.
2010-01-01
Streamwise and quasi-streamwise elongated structures have been shown to play a significant role in turbulent shear flows. We model the mean behaviour of fully turbulent plane Couette flow using a streamwise constant projection of the Navier–Stokes equations. This results in a two-dimensional three-velocity-component (2D/3C) model. We first use a steady-state version of the model to demonstrate that its nonlinear coupling provides the mathematical mechanism that shapes the turbulent velocity p...
The turbulent flow generated by inhomogeneous multiscale grids
Zheng, Shaokai; Bruce, Paul J. K.; Graham, J. Michael R.; Vassilicos, John Christos
2015-11-01
A group of inhomogeneous multiscale grids have been designed and tested in a low speed wind tunnel in an attempt to generate bespoke turbulent shear flows. Cross-wire anemometry measurements were performed in different planes parallel to the grid and at various streamwise locations to study turbulence development behind each of the different geometry grids. Two spatially separated single hot wires were also used to measure transverse integral length scale at selected locations. Results are compared to previous studies of shearless mixing layer grids and fractal grids, including mean flow profiles and turbulence statistics.
Effect of particle clustering on radiative transfer in turbulent flows
Liberman, M; Rogachevskii, I; Haugen, N E L
2016-01-01
The effect of particle clustering on the radiation penetration length in particle laden turbulent flows is studied using a mean-field approach. Particle clustering in temperature stratified turbulence implies the formation of small-scale clusters with a high concentration of particles, exceeding the mean concentration by a few orders of magnitude. We show that the radiative penetration length increases by several orders of magnitude due to the particle clustering in a turbulent flow. Such strong radiative clearing effect plays a key role in a number of atmospheric and astrophysical phenomena, and can be of fundamental importance for understanding the origin of dust explosions.
MODELLING AND COMPUTATION OF UNSTEADY TURBULENT CAVITATION FLOWS
CHEN Ying; LU Chuan-jing; WU Lei
2006-01-01
Unsteady turbulent cavitation flows in a Venturi-type section and around a NACA0012 hydrofoil were simulated by two-dimensional computations of viscous compressible turbulent flow model.The Venturi-type section flow proved numerical precision and reliability of the physical model and the code, and further the cavitation around NACA0012 foil was investigated.These flows were calculated with a code of SIMPLE-type finite volume scheme, associated with a barotropic vapor/liquid state law which strongly links density and pressure variation.To simulate turbulent flows, modified RNG k- ε model was used.Numerical results obtained in the Venturi-type flow simulated periodic shedding of sheet cavity and was compared with experiment data, and the results of the NACA0012 foil show quasi-periodic vortex cavitation phenomenon.Results obtained concerning cavity shape and unsteady behavior, void ratio, and velocity field were found in good agreement with experiment ones.
Stability and suppression of turbulence in relaxing molecular gas flows
Grigoryev, Yurii N
2017-01-01
This book presents an in-depth systematic investigation of a dissipative effect which manifests itself as the growth of hydrodynamic stability and suppression of turbulence in relaxing molecular gas flows. The work describes the theoretical foundations of a new way to control stability and laminar turbulent transitions in aerodynamic flows. It develops hydrodynamic models for describing thermal nonequilibrium gas flows which allow the consideration of suppression of inviscid acoustic waves in 2D shear flows. Then, nonlinear evolution of large-scale vortices and Kelvin-Helmholtz waves in relaxing shear flows are studied. Critical Reynolds numbers in supersonic Couette flows are calculated analytically and numerically within the framework of both linear and nonlinear classical energy hydrodynamic stability theories. The calculations clearly show that the relaxation process can appreciably delay the laminar-turbulent transition. The aim of the book is to show the new dissipative effect, which can be used for flo...
Zonal flow generation and its feedback on turbulence production in drift wave turbulence
Pushkarev, Andrey V; Nazarenko, Sergey V
2012-01-01
Plasma turbulence described by the Hasegawa-Wakatani equations has been simulated numerically for different models and values of the adiabaticity parameter C. It is found that for low values of C turbulence remains isotropic, zonal flows are not generated and there is no suppression of the meridional drift waves and of the particle transport. For high values of C, turbulence evolves toward highly anisotropic states with a dominant contribution of the zonal sector to the kinetic energy. This anisotropic flow leads to a decrease of a turbulence production in the meridional sector and limits the particle transport across the mean isopycnal surfaces. This behavior allows to consider the Hasegawa-Wakatani equations a minimal PDE model which contains the drift-wave/zonal-flow feedback loop prototypical of the LH transition in plasma devices.
Papapostolou, Vassilios
2017-09-11
Enstrophy is an intrinsic feature of turbulent flows, and its transport properties are essential for the understanding of premixed flame-turbulence interaction. The interrelation between the enstrophy transport and flow topologies, which can be assigned to eight categories based on the three invariants of the velocity-gradient tensor, has been analysed here. The enstrophy transport conditional on flow topologies in turbulent premixed flames has been analysed using a Direct Numerical Simulation database representing the corrugated flamelets (CF), thin reaction zones (TRZ) and broken reaction zones (BRZ) combustion regimes. The flame in the CF regime exhibits considerable flame-generated enstrophy, and the dilatation rate and baroclinic torque contributions to the enstrophy transport act as leading order sink and source terms, respectively. Consequently, flow topologies associated with positive dilatation rate values, contribute significantly to the enstrophy transport in the CF regime. By contrast, enstrophy decreases from the unburned to the burned gas side for the cases representing the TRZ and BRZ regimes, with diminishing influences of dilatation rate and baroclinic torque. The enstrophy transport in the TRZ and BRZ regimes is governed by the vortex-stretching and viscous dissipation contributions, similar to non-reacting flows, and topologies existing for all values of dilatation rate remain significant contributors.
Multigrid solution of incompressible turbulent flows by using two-equation turbulence models
Zheng, X.; Liu, C. [Front Range Scientific Computations, Inc., Denver, CO (United States); Sung, C.H. [David Taylor Model Basin, Bethesda, MD (United States)
1996-12-31
Most of practical flows are turbulent. From the interest of engineering applications, simulation of realistic flows is usually done through solution of Reynolds-averaged Navier-Stokes equations and turbulence model equations. It has been widely accepted that turbulence modeling plays a very important role in numerical simulation of practical flow problem, particularly when the accuracy is of great concern. Among the most used turbulence models today, two-equation models appear to be favored for the reason that they are more general than algebraic models and affordable with current available computer resources. However, investigators using two-equation models seem to have been more concerned with the solution of N-S equations. Less attention is paid to the solution method for the turbulence model equations. In most cases, the turbulence model equations are loosely coupled with N-S equations, multigrid acceleration is only applied to the solution of N-S equations due to perhaps the fact the turbulence model equations are source-term dominant and very stiff in sublayer region.
A numerical scheme for modelling reacting flow with detailed chemistry and transport.
Knio, Omar M. (The Johns Hopkins University, Baltimore, MD); Najm, Habib N.; Paul, Phillip H. (Eksigent Technologies LLC, Livermore, CA)
2003-09-01
An efficient projection scheme is developed for the simulation of reacting flow with detailed kinetics and transport. The scheme is based on a zero-Mach-number formulation of the compressible conservation equations for an ideal gas mixture. It is a modified version of the stiff operator-split scheme developed by Knio, Najm & Wyckoff (1999, J. Comput. Phys. 154, 428). Similar to its predecessor, the new scheme relies on Strang splitting of the discrete evolution equations, where diffusion is integrated in two half steps that are symmetrically distributed around a single stiff step for the reaction source terms. The diffusive half-step is integrated using an explicit single-step, multistage, Runge-Kutta-Chebyshev (RKC) method, which replaces the explicit, multi-step, fractional sub-step approach used in the previous formulation. This modification maintains the overall second-order convergence properties of the scheme and enhances the efficiency of the computations by taking advantage of the extended real-stability region of the RKC scheme. Two additional efficiency-enhancements are also explored, based on an extrapolation procedure for the transport coefficients and on the use of approximate Jacobian data evaluated on a coarse mesh. By including these enhancement schemes, performance tests using 2D computations with a detailed C{sub 1}C{sub 2} methane-air mechanism and a detailed mixture-averaged transport model indicate that speedup factors of about 15 are achieved over the previous split-stiff scheme.
A new energy transfer model for turbulent free shear flow
Liou, William W.-W.
1992-01-01
A new model for the energy transfer mechanism in the large-scale turbulent kinetic energy equation is proposed. An estimate of the characteristic length scale of the energy containing large structures is obtained from the wavelength associated with the structures predicted by a weakly nonlinear analysis for turbulent free shear flows. With the inclusion of the proposed energy transfer model, the weakly nonlinear wave models for the turbulent large-scale structures are self-contained and are likely to be independent flow geometries. The model is tested against a plane mixing layer. Reasonably good agreement is achieved. Finally, it is shown by using the Liapunov function method, the balance between the production and the drainage of the kinetic energy of the turbulent large-scale structures is asymptotically stable as their amplitude saturates. The saturation of the wave amplitude provides an alternative indicator for flow self-similarity.
Studies of compressible shear flows and turbulent drag reduction
Orszag, S. A.
1981-04-01
Compressible shear flows and drag reduction were examined and three methods are addressed: (1) the analytical and numerical aspects of conformal mapping were summarized and a new method for computation of these maps is presented; (2) the computer code SPECFD for solution of the three dimensional time dependent Navier-Stokes equations for compressible flow on the CYBER 203 computer is described; (3) results of two equation turbulence modeling of turbulent flow over wavy walls are presented. A modified Jones-Launder model is used in two dimensional spectral code for flow in general wavy geometries.
Turbulence characteristics in a supersonic cascade wake flow
Andrew, P.L.; Ng, W.F. (Virginia Polytechnic Inst. and State Univ., Blacksburg, VA (United States))
1994-10-01
The turbulent character of the supersonic wake of a linear cascade of fan airfoils has been studied using a two-component laser-doppler anemometer. The cascade was tested in the Virginia Polytechnic Institute and State University intermittent wind tunnel facility, where the Mach and Reynolds numbers were 2.36 and 4.8 [times] 10[sup 6], respectively. In addition to mean flow measurements, Reynolds normal and shear stresses were measured as functions of cascade incidence angle and streamwise locations spanning the near-wake and the far-wake. The extremities of profiles of both the mean and turbulent wake properties were found to be strongly influenced by upstream shock-boundary-layer interactions, the strength of which varied with cascade incidence. In contrast, the peak levels of turbulence properties within the shear layer were found to be largely independent of incidence, and could be characterized in terms of the streamwise position only. The velocity defect turbulence level was found to be 23%, and the generally accepted value of the turbulence structural coefficient of 0.30 was found to be valid for this flow. The degree of similarity of the mean flow wake profiles was established, and those profiles demonstrating the most similarity were found to approach a state of equilibrium between the mean and turbulent properties. In general, this wake flow may be described as a classical free shear flow, upon which the influence of upstream shock-boundary-layer interactions has been superimposed.
DNSLab: A gateway to turbulent flow simulation in Matlab
Vuorinen, V.; Keskinen, K.
2016-06-01
Computational fluid dynamics (CFD) research is increasingly much focused towards computationally intensive, eddy resolving simulation techniques of turbulent flows such as large-eddy simulation (LES) and direct numerical simulation (DNS). Here, we present a compact educational software package called DNSLab, tailored for learning partial differential equations of turbulence from the perspective of DNS in Matlab environment. Based on educational experiences and course feedback from tens of engineering post-graduate students and industrial engineers, DNSLab can offer a major gateway to turbulence simulation with minimal prerequisites. Matlab implementation of two common fractional step projection methods is considered: the 2d Fourier pseudo-spectral method, and the 3d finite difference method with 2nd order spatial accuracy. Both methods are based on vectorization in Matlab and the slow for-loops are thus avoided. DNSLab is tested on two basic problems which we have noted to be of high educational value: 2d periodic array of decaying vortices, and 3d turbulent channel flow at Reτ = 180. To the best of our knowledge, the present study is possibly the first to investigate efficiency of a 3d turbulent, wall bounded flow in Matlab. The accuracy and efficiency of DNSLab is compared with a customized OpenFOAM solver called rk4projectionFoam. Based on our experiences and course feedback, the main contribution of DNSLab consists of the following features. (i) The very compact Matlab implementation of present Navier-Stokes solvers provides a gateway to efficient learning of both, physics of turbulent flows, and simulation of turbulence. (ii) Only relatively minor prerequisites on fluid dynamics and numerical methods are required for using DNSLab. (iii) In 2d, interactive results for turbulent flow cases can be obtained. Even for a 3d channel flow, the solver is fast enough for nearly interactive educational use. (iv) DNSLab is made openly available and thus contributing to
V. Ashok
2014-01-01
Full Text Available A hybrid solution methodology has been developed to solve chemically reacting laminar hypersonic flow in chemical Non-equilibrium and thermal equilibrium, by a Cartesian mesh based hybrid solution methodology, which uses an unstructured prism layer solution near the wall and a Cartesian mesh solution away from the wall. The unstructured prism layer for near wall solution is obtained from the normal projection of wall panels of the Cartesian mesh and are stitched with the outer Cartesian mesh. The solver, developed based on this approach when compared with other chemically reacting CFD codes and limited experimental results show good comparison. This procedure has a good potential to handle near-wall resolution for chemically reacting flows with a Cartesian mesh for complex geometries as well.
Turbulence Modeling of Flows with Extensive Crossflow Separation
Argyris G. Panaras
2015-07-01
Full Text Available The reasons for the difficulty in simulating accurately strong 3-D shock wave/turbulent boundary layer interactions (SBLIs and high-alpha flows with classical turbulence models are investigated. These flows are characterized by the appearance of strong crossflow separation. In view of recent additional evidence, a previously published flow analysis, which attributes the poor performance of classical turbulence models to the observed laminarization of the separation domain, is reexamined. According to this analysis, the longitudinal vortices into which the separated boundary layer rolls up in this type of separated flow, transfer external inviscid air into the part of the separation adjacent to the wall, decreasing its turbulence. It is demonstrated that linear models based on the Boussinesq equation provide solutions of moderate accuracy, while non-linear ones and others that consider the particular structure of the flow are more efficient. Published and new Reynolds Averaged Navier–Stokes (RANS simulations are reviewed, as well as results from a recent Large Eddy Simulation (LES study, which indicate that in calculations characterized by sufficient accuracy the turbulent kinetic energy of the reverse flow inside the separation vortices is very low, i.e., the flow is almost laminar there.
Subcritical transition to turbulence in plane channel flows
Orszag, S. A.; Patera, A. T.
1980-01-01
A linear three dimensional mechanism for the transition of plane Poiseuille flows to turbulence is presented which provides good agreement with experimental observations. The mechanism is based on the evolution of states within a band of quasi-equilibria which slowly approach the stable upper branch solutions for the evolution of flow energy but which are strongly unstable to infinitesimal three-dimensional disturbances. Numerical simulation has shown that if two-dimensional flow persists long enough for the three-dimensional perturbations to attain finite amplitude, the resulting three dimensional flow quickly develops a turbulent character with nonperiodic behavior, and thus transition can be predicted from knowledge of the initial two- and three-dimensional energies and time scales. The mechanism predicts transition to turbulence at Reynolds numbers greater than 1000, as observed in experiments, and implies higher threshold three-dimensional energies in plane Couette flow.
Multiscale modeling of turbulent channel flow over porous walls
Yogaraj, Sudhakar; Lacis, Ugis; Bagheri, Shervin
2016-11-01
We perform direct numerical simulations of fully developed turbulent flow through a channel coated with a porous material. The Navier-stokes equations governing the fluid domain and the Darcy equations of the porous medium are coupled using an iterative partitioned scheme. At the interface between the two media, boundary conditions derived using a multiscale homogenization approach are enforced. The main feature of this approach is that the anisotropic micro-structural pore features are directly taken into consideration to derive the constitutive coefficients of the porous media as well as of the interface. The focus of the present work is to study the influence of micro-structure pore geometry on the dynamics of turbulent flows. Detailed turbulence statistics and instantaneous flow field are presented. For comparison, flow through impermeable channel flows are included. Supported by the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant agreement No 708281.
Controlling a Linear Process in Turbulent Channel Flow
Lim, Junwoo; Kim, John
1999-11-01
Recent studies have shown that controllers developed based on a linear system theory work surprisingly well in reducing the viscous drag in turbulent boundary layers, suggesting that the essential dynamics of near-wall turbulence may well be approximated by the linearized model. Of particular interest is the linear process due to the coupling term between the wall-normal velocity and wall-normal vorticity terms in the linearized Navier-Stokes (N-S) equations, which enhances non-normality of the linearized system. This linear process is investigated through numerical simulations of a turbulent channel flow. It is shown that the linear coupling term plays an important role in fully turbulent -- and hence, nonlinear -- flows. Near-wall turbulence is shown to decay in the absence of the linear coupling term. The fact that the coupling term plays an essential role in maintaining near-wall turbulence suggests that an effective control algorithm for the drag reduction in turbulent flows should be aimed at reducing the effect of the coupling term in the wall region. Designing a control algorithm that directly accounts for the coupling term in a cost to be minimized will be discussed.
Space-Time Correlations and Dynamic Coupling in Turbulent Flows
He, Guowei; Jin, Guodong; Yang, Yue
2017-01-01
Space-time correlation is a staple method for investigating the dynamic coupling of spatial and temporal scales of motion in turbulent flows. In this article, we review the space-time correlation models in both the Eulerian and Lagrangian frames of reference, which include the random sweeping and local straining models for isotropic and homogeneous turbulence, Taylor's frozen-flow model and the elliptic approximation model for turbulent shear flows, and the linear-wave propagation model and swept-wave model for compressible turbulence. We then focus on how space-time correlations are used to develop time-accurate turbulence models for the large-eddy simulation of turbulence-generated noise and particle-laden turbulence. We briefly discuss their applications to two-point closures for Kolmogorov's universal scaling of energy spectra and to the reconstruction of space-time energy spectra from a subset of spatial and temporal signals in experimental measurements. Finally, we summarize the current understanding of space-time correlations and conclude with future issues for the field.
Givi, Peyman; Madnia, Cyrus K.; Steinberger, C. J.; Frankel, S. H.
1992-01-01
The principal objective is to extend the boundaries within which large eddy simulations (LES) and direct numerical simulations (DNS) can be applied in computational analyses of high speed reacting flows. A summary of work accomplished during the last six months is presented.
An improved turbulence model for rotating shear flows*
Nagano, Yasutaka; Hattori, Hirofumi
2002-01-01
In the present study, we construct a turbulence model based on a low-Reynolds-number non-linear k e model for turbulent flows in a rotating channel. Two-equation models, in particular the non-linear k e model, are very effective for solving various flow problems encountered in technological applications. In channel flows with rotation, however, the explicit effects of rotation only appear in the Reynolds stress components. The exact equations for k and e do not have any explicit terms concerned with the rotation effects. Moreover, the Coriolis force vanishes in the momentum equation for a fully developed channel flow with spanwise rotation. Consequently, in order to predict rotating channel flows, after proper revision the Reynolds stress equation model or the non-linear eddy viscosity model should be used. In this study, we improve the non-linear k e model so as to predict rotating channel flows. In the modelling, the wall-limiting behaviour of turbulence is also considered. First, we evaluated the non-linear k e model using the direct numerical simulation (DNS) database for a fully developed rotating turbulent channel flow. Next, we assessed the non-linear k e model at various rotation numbers. Finally, on the basis of these assessments, we reconstruct the non-linear k e model to calculate rotating shear flows, and the proposed model is tested on various rotation number channel flows. The agreement with DNS and experiment data is quite satisfactory.
Compressible Turbulent Channel Flows: DNS Results and Modeling
Huang, P. G.; Coleman, G. N.; Bradshaw, P.; Rai, Man Mohan (Technical Monitor)
1994-01-01
The present paper addresses some topical issues in modeling compressible turbulent shear flows. The work is based on direct numerical simulation of two supersonic fully developed channel flows between very cold isothermal walls. Detailed decomposition and analysis of terms appearing in the momentum and energy equations are presented. The simulation results are used to provide insights into differences between conventional time-and Favre-averaging of the mean-flow and turbulent quantities. Study of the turbulence energy budget for the two cases shows that the compressibility effects due to turbulent density and pressure fluctuations are insignificant. In particular, the dilatational dissipation and the mean product of the pressure and dilatation fluctuations are very small, contrary to the results of simulations for sheared homogeneous compressible turbulence and to recent proposals for models for general compressible turbulent flows. This provides a possible explanation of why the Van Driest density-weighted transformation is so successful in correlating compressible boundary layer data. Finally, it is found that the DNS data do not support the strong Reynolds analogy. A more general representation of the analogy is analysed and shown to match the DNS data very well.
Macroscopic effects of the spectral structure in turbulent flows
Tran, T.; Chakraborty, P.; Guttenberg, N.; Prescott, A.; Kellay, H.; Goldburg, W.; Goldenfeld, N.; Gioia, G.
2010-11-01
There is a missing link between macroscopic properties of turbulent flows, such as the frictional drag of a wall-bounded flow, and the turbulent spectrum. To seek the missing link we carry out unprecedented experimental measurements of the frictional drag in turbulent soap-film flows over smooth walls. These flows are effectively two-dimensional, and we are able to create soap-film flows with the two types of turbulent spectrum that are theoretically possible in two dimensions: the "enstrophy cascade," for which the spectral exponent α= 3, and the "inverse energy cascade," for which the spectral exponent α= 5/3. We find that the functional relation between the frictional drag f and the Reynolds number Re depends on the spectral exponent: where α= 3, f ˜Re-1/2; where α= 5/3, f ˜Re-1/4. Each of these scalings may be predicted from the attendant value of α by using a recently proposed spectral theory of the frictional drag. In this theory the frictional drag of turbulent flows on smooth walls is predicted to be f ˜Re^(1-α)/(1+α).
Nicolleau, FCGA; Redondo, J-M
2012-01-01
This book contains a collection of the main contributions from the first five workshops held by Ercoftac Special Interest Group on Synthetic Turbulence Models (SIG42. It is intended as an illustration of the sig's activities and of the latest developments in the field. This volume investigates the use of Kinematic Simulation (KS) and other synthetic turbulence models for the particular application to environmental flows. This volume offers the best syntheses on the research status in KS, which is widely used in various domains, including Lagrangian aspects in turbulence mixing/stirring, partic
Investigation of Turbulent Laminar Patterns in Poiseuille-Couette flow
Nguyen, Quoc; Papavassiliou, Dimitrios
2014-11-01
Laminar-turbulent intermittency has recently been observed in the transitional regime of pipe ... and plane Couette flow .... While many works focus on behavior of these patterns in plane Couette flow, little attention has been paid to Poiseuille flow and transition from Couette to Poiseuille flow. In this study, we investigate behavior of turbulent laminar patterns in Poiseuille-Couette flow, including pure Poiseuille and Couette flows at two limits. Direct Numerical Simulation (DNS) is used to simulate a Poiseuille-Couette channel at a size of 16 πh × 2h × 2 πh (corresponding to a resolution of 512 × 129 × 128 in x, y and z directions), with periodic boundary condition applied in the x and z directions (h is half of the channel height). The Reynolds number is 300, and the flow is at transitional regime in all simulations. Behavior of laminar turbulent patterns as the flow goes from Couette to Poiseuille flow will be presented in details. This would shed some light on the effect of different types of flow on these patterns, as well as how these patterns vary from fully Poiseuille flow to fully Couette flow.
Least Squares Shadowing for Sensitivity Analysis of Turbulent Fluid Flows
Blonigan, Patrick; Wang, Qiqi
2014-01-01
Computational methods for sensitivity analysis are invaluable tools for aerodynamics research and engineering design. However, traditional sensitivity analysis methods break down when applied to long-time averaged quantities in turbulent fluid flow fields, specifically those obtained using high-fidelity turbulence simulations. This is because of a number of dynamical properties of turbulent and chaotic fluid flows, most importantly high sensitivity of the initial value problem, popularly known as the "butterfly effect". The recently developed least squares shadowing (LSS) method avoids the issues encountered by traditional sensitivity analysis methods by approximating the "shadow trajectory" in phase space, avoiding the high sensitivity of the initial value problem. The following paper discusses how the least squares problem associated with LSS is solved. Two methods are presented and are demonstrated on a simulation of homogeneous isotropic turbulence and the Kuramoto-Sivashinsky (KS) equation, a 4th order c...
Turbulent Boyant Jets and Plumes in Flowing Ambient Environments
Chen, Hai-Bo
Turbulent buoyant jets and plumes in flowing ambient environments have been studied theoretically and experimentally. The mechanics of turbulent buoyant jets and plumes in flowing ambients have been discussed. Dimensional analysis was employed to investigate the mean behaviour of the turbulent...... and the stage of plume. The stability criteria for the upstream wedge created by the submerged turbulent buoyant jet were established by applying the Bernoulli equations for a two-dimensional problem and by considering the front velocity driven by the buoyancy force for a three-dimensional problem...... in a crossflowing environment, have been presented and successfully correlated using momentum and buoyancy fluxes and length scales. The analysis demonstrates that the experimental data on the jet trajectories and dilutions can be well correlated using the momentum or buoyancy fluxes and length scales, depending...
Parallel Simulation of 3-D Turbulent Flow Through Hydraulic Machinery
徐宇; 吴玉林
2003-01-01
Parallel calculational methods were used to analyze incompressible turbulent flow through hydraulic machinery. Two parallel methods were used to simulate the complex flow field. The space decomposition method divides the computational domain into several sub-ranges. Parallel discrete event simulation divides the whole task into several parts according to their functions. The simulation results were compared with the serial simulation results and particle image velocimetry (PIV) experimental results. The results give the distribution and configuration of the complex vortices and illustrate the effectiveness of the parallel algorithms for numerical simulation of turbulent flows.
Modeling Rotating Turbulent Flows with the Body Force Potential Model.
Bhattacharya, Amitabh; Perot, Blair
2000-11-01
Like a Reynolds Stress Transport equation model, the turbulent potential model has an explicit Coriolis acceleration term that appears in the model that accounts for rotation effects. In this work the additional secondary effects that system rotation has on the dissipation rate, return-to-isotropy, and fast pressure strain terms are also included in the model. The resulting model is tested in the context of rotating isotropic turbulence, rotating homogeneous shear flow, rotating channel flow, and swirling pipe flow. Many of the model changes are applicable to Reynolds stress transport equation models. All model modifications are frame indifferent.
Shear flow generation and energetics in electromagnetic turbulence
Naulin, V.; Kendl, A.; Garcia, O.E.;
2005-01-01
acoustic mode (GAM) transfer in drift-Alfvén turbulence is investigated. By means of numerical computations the energy transfer into zonal flows owing to each of these effects is quantified. The importance of the three driving ingredients in electrostatic and electromagnetic turbulence for conditions...... relevant to the edge of fusion devices is revealed for a broad range of parameters. The Reynolds stress is found to provide a flow drive, while the electromagnetic Maxwell stress is in the cases considered a sink for the flow energy. In the limit of high plasma β, where electromagnetic effects and Alfvén...
Turbulence, flow and transport: hints from reversed field pinch
Vianello, N.; Antoni, V.; Spada, E.; Spolaore, M.; Serianni, G.; Cavazzana, R.; Bergsåker, H.; Cecconello, M.; Drake, J. R.
2006-04-01
The interplay between sheared E × B flows and turbulence has been experimentally investigated in the edge region of the Extrap-T2R reversed field pinch experiment. Electrostatic fluctuations are found to rule the momentum balance equation representing the main driving term for sheared flows which counterbalances anomalous viscous damping. The driving role of electrostatic fluctuations is proved by the spatial structure of the Reynolds stress and by the time behaviour of the mean energy production term which supports the existence of an energy exchange from the small scales of turbulence to the larger scales of the mean flow.
Numerical prediction of flow, heat transfer, turbulence and combustion
Spalding, D Brian; Pollard, Andrew; Singhal, Ashok K
1983-01-01
Numerical Prediction of Flow, Heat Transfer, Turbulence and Combustion: Selected Works of Professor D. Brian Spalding focuses on the many contributions of Professor Spalding on thermodynamics. This compilation of his works is done to honor the professor on the occasion of his 60th birthday. Relatively, the works contained in this book are selected to highlight the genius of Professor Spalding in this field of interest. The book presents various research on combustion, heat transfer, turbulence, and flows. His thinking on separated flows paved the way for the multi-dimensional modeling of turbu
Cavitation Inception in Turbulent Flows Around a Hydrofoil
ZHANG Min-di; WANG Guo-yu; ZHANG Zhen; GAO Yuan-yin
2006-01-01
The phenomenon of cavitation inception around a hydrofoil is studied experimentally. The flow velocities around the foil are measured by a laser doppler velocimetry (LDV). The inception cavitation aspects are observed by using a high-speed video camera. In the experiment, the Reynolds number is fixed at a value of 7 .0×105. The boundary layer around the foil undergoes turbulent flow under the experiment condition. The LDV measurement results show that the flow in the boundary layer around the foil doesn't separate from the surface. It is found that the cavitation inception in non-separated turbulent flow is related to the coherent structures in the boundary layer. It is clear that the turbulent bursting and the hairpin-shaped vortex structure accompany the incipient cavitation.
Numerical simulation of the characteristics of turbulent Taylor vortex flow
ZHOU Xiantao; PAN Jiazhen; CHEN Liqing; SHI Yan; CHEN Wenmei; CHU Liangyin
2007-01-01
Turbulent Taylor vortex flow,which is contained between a rotating inner cylinder and a coaxial fixed outer cylinder with fixed ends,is simulated by applying the development in Reynolds stress equations mold (RSM) of the micro-perturbation.This resulted from the truncation error between the numerical solution and exact solution of the Reynolds stress equations.Based on the numerical simulation results of the turbulent Taylor vortex flow,its characteristics such as the fluctuation of the flow field,the precipitous drop of azimuthal velocity,the jet flow of radial velocity,the periodicity of axial velocity,the wave periodicity of pressure distribution,the polarization of shear stress on the walls,and the turbulence intensity in the jet region,are discussed.Comparing the pilot results measured by previous methods,the relative error of the characteristics predicted by simulation is less than 30%.
Statistical parameters of thermally driven turbulent anabatic flow
Hilel, Roni; Liberzon, Dan
2016-11-01
Field measurements of thermally driven turbulent anabatic flow over a moderate slope are reported. A collocated hot-films-sonic anemometer (Combo) obtained the finer scales of the flow by implementing a Neural Networks based in-situ calibration technique. Eight days of continuous measurements of the wind and temperature fluctuations reviled a diurnal pattern of unstable stratification that forced development of highly turbulent unidirectional up slope flow. Empirical fits of important turbulence statistics were obtained from velocity fluctuations' time series alongside fully resolved spectra of velocity field components and characteristic length scales. TKE and TI showed linear dependence on Re, while velocity derivative skewness and dissipation rates indicated the anisotropic nature of the flow. Empirical fits of normalized velocity fluctuations power density spectra were derived as spectral shapes exhibited high level of similarity. Bursting phenomenon was detected at 15% of the total time. Frequency of occurrence, spectral characteristics and possible generation mechanism are discussed. BSF Grant #2014075.
Dynamical-systems approach to localised turbulence in pipe flow
Ritter, Paul; Avila, Marc
2015-01-01
Turbulent-laminar patterns are ubiquitous near transition in wall-bounded shear flows. Despite recent progress in describing their dynamics in analogy to nonequilibrium phase transitions, there is no theory explaining their emergence. Dynamical-system approaches suggest that invariant solutions to the Navier-Stokes equations, such as traveling waves and relative periodic orbits in pipe flow, act as building blocks of the disordered dynamics. While recent studies have shown how transient chaos arises from such solutions, the ensuing dynamics lacks the strong fluctuations in size, shape and speed of the turbulent spots observed in experiments. We here show that chaotic spots with distinct dynamical and kinematic properties merge in phase space and give rise to the enhanced spatiotemporal patterns observed in pipe flow. This paves the way for a dynamical-system foundation to the phenomenogloy of turbulent-laminar patterns in wall-bounded extended shear flows.
Experimental Studies on Turbulence Kinetic Energy in Confined Vortex Flows
L.Yan; G.H.Vatistas; 等
2000-01-01
Turbulence kinetic energies in confined vortex flows have been studied.The studies were based on the experiments performed in a vortex chamber,In the experiments,a Laser Doppler Anemometry(LDA) was used to perform flow measurements inside the vortex chamber,which provided the data for the kinetic energy analysis.The studies concentrated on the influences of the contraction ratio and the inlet air flow rate on the kinetic energy,and analyzed the characteristics of the kinetic energy in the confined vortex flows,including the distributions of the tangential component,radial component and total turbulence kinetic energy,In the paper,both the experimental techniques and the experimental results were presented.Based on a similarity analyis and the experimental data,an empirical scaling formula was proposed so that the tangential component of the turbulence kinetic energy was dependent only on the parameter of the contraction ratio.
Effects of spanwise rotation on turbulent channel flow
Brethouwer, Geert
2016-01-01
A study of fully developed plane turbulent channel flow subject to spanwise system rotation through direct numerical simulations is presented. In order to study both the influence of the Reynolds number and spanwise rotation on channel flow, the Reynolds number $Re = U_b h/\
Turbulent oscillating channel flow subjected to wind stress
Kramer, W.; Clercx, H.J.H.; Armenio, V.; Armenio, Vincenzo; Geurts, Bernard; Fröhlich, Jochen
2010-01-01
The channel flow subjected to a wind stress at the free surface and an oscillating pressure gradient is investigated using large-eddy simulations (LES). a slowly pulsating mean flow occurs with the turbulent mechanics essentially being quasi steady. Logarithmic boundary layers are present at both th
Compressibility Corrections to Closure Approximations for Turbulent Flow Simulations
Cloutman, L D
2003-02-01
We summarize some modifications to the usual closure approximations for statistical models of turbulence that are necessary for use with compressible fluids at all Mach numbers. We concentrate here on the gradient-flu approximation for the turbulent heat flux, on the buoyancy production of turbulence kinetic energy, and on a modification of the Smagorinsky model to include buoyancy. In all cases, there are pressure gradient terms that do not appear in the incompressible models and are usually omitted in compressible-flow models. Omission of these terms allows unphysical rates of entropy change.
Surface waves propagation on a turbulent flow forced electromagnetically
Gutiérrez, Pablo
2015-01-01
We study the propagation of monochromatic surface waves on a turbulent flow. The flow is generated in a layer of liquid metal by an electromagnetic forcing. This forcing creates a quasi two-dimensional (2D) turbulence with strong vertical vorticity. The turbulent flow contains much more energy than the surface waves. In order to focus on the surface wave, the deformations induced by the turbulent flow are removed. This is done by performing a coherent phase averaging. For wavelengths smaller than the forcing lengthscale, we observe a significant increase of the wavelength of the propagating wave that has not been reported before. We suggest that it can be explained by the random deflection of the wave induced by the velocity gradient of the turbulent flow. Under this assumption, the wavelength shift is an estimate of the fluctuations of deflection angle. The local measurements of the wave frequency far from the wavemaker do not reveal such systematic behavior, although a small shift is measured. Finally we qu...
Rpt Analysis of Turbulent Flows With Stable Stratification
Sukoriansky, S.; Galperin, B.
The Renormalized Perturbation Technique of successive small scales elimination is applied to turbulent flows with stable stratification. This procedure results in derivation of scale-dependent anisotropic viscosities and diffusivities that naturally incorporate the combined effect of turbulence and internal waves. In addition, this procedure ren- ders means to analyze the fundamentals of the turbulence-internal waves interaction, derive the criterion of the threshold of the internal waves generation that fully accounts for the spectral anisotropy, and derive the dispersion relation for internal waves with turbulence. A closure assumption is then introduced relating the renormalized param- eters to global flow characteristics such as the gradient Richardson number (Ri) or local Froude number. Anisotropic turbulent Prandtl numbers are compared with lab- oratory and numerical experiments. The agreement is good in the entire range of Ri, from very small to large indicating that our model captures the physics of strongly stratified turbulent flows. This result has immediate application value for modeling of atmospheric and oceanic boundary layers.
Turbulent mixing in nonreactive and reactive flows
1975-01-01
Turbulence, mixing and the mutual interaction of turbulence and chemistry continue to remain perplexing and impregnable in the fron tiers of fluid mechanics. The past ten years have brought enormous advances in computers and computational techniques on the one hand and in measurements and data processing on the other. The impact of such capabilities has led to a revolution both in the understanding of the structure of turbulence as well as in the predictive methods for application in technology. The early ideas on turbulence being an array of complicated phenomena and having some form of reasonably strong coherent struc ture have become well substantiated in recent experimental work. We are still at the very beginning of understanding all of the aspects of such coherence and of the possibilities of incorporating such structure into the analytical models for even those cases where the thin shear layer approximation may be valid. Nevertheless a distinguished body of "eddy chasers" has come into existence. T...
Turbulent Buoyant Jets in Flowing Ambients
Chen, Hai-Bo; Larsen, Torben; Petersen, Ole
1991-01-01
and dilution can be successfully correlated by use of momentum or buoyancy length scales. In addition the centerline density deficit and velocity decay are well predicted by using an integral and a k - ∈turbulence model. The results shown here help us to understand in a better way the physical phenornenon...
Chemical Reactions in Turbulent Mixing Flows
1991-09-14
signal amplifier incorporated a three-pole But- water tagged by a fluorescent laser dye (sodium fluores - terworth filter, with a cutoff frequency set...effects and large structure in turbulent mixing la vers. J, Fluid Mech. 64. 775-816. CHiA PMAN N. D. R. 1979 Computational aerodynamics development and
Sound attenuation in rectangular and circular cross-section ducts with flow and bulk-reacting liner
Bies, D. A.; Hansen, C. H.; Bridges, G. E.
1991-04-01
A generalized theory is presented for sound propagation in lined ducts of arbitrary cross-section where acoustic wave propagation in the lining is also taken into account. The effects of a mean fluid flow in the duct airway, an anisotropic bulk reacting liner and a limp, impervious membrane covering the liner are all taken into account. Simple extension of the formalism to include the effect of a perforated facing is also provided. Bulk reacting and locally reacting liners are treated as limiting cases. The general analysis is applied to ducts of both rectangular and circular cross-section, taking into account higher order modes as well as plane wave sound propagation. Design charts for duct attenuation in octave frequency band averages and in terms of dimensionless parameters are presented.
New DNS and modeling results for turbulent pipe flow
Johansson, Arne; El Khoury, George; Grundestam, Olof; Schlatter, Philipp; Brethouwer, Geert; Linne Flow Centre Team
2013-11-01
The near-wall region of turbulent pipe and channel flows (as well as zero-pressure gradient boundary layers) have been shown to exhibit a very high degree of similarity in terms of all statistical moments and many other features, while even the mean velocity profile in the two cases exhibits significant differences between in the outer region. The wake part of the profile, i.e. the deviation from the log-law, in the outer region is of substantially larger amplitude in pipe flow as compared to channel flow (although weaker than in boundary layer flow). This intriguing feature has been well known but has no simple explanation. Model predictions typically give identical results for the two flows. We have analyzed a new set of DNS for pipe and channel flows (el Khoury et al. 2013, Flow, Turbulence and Combustion) for friction Reynolds numbers up to 1000 and made comparing calculations with differential Reynolds stress models (DRSM). We have strong indications that the key factor behind the difference in mean velocity in the outer region can be coupled to differences in the turbulent diffusion in this region. This is also supported by DRSM results, where interesting differences are seen depending on the sophistication of modeling the turbulent diffusion coefficient.
Statistical descriptions of polydisperse turbulent two-phase flows
Minier, Jean-Pierre
2016-12-01
Disperse two-phase flows are flows containing two non-miscible phases where one phase is present as a set of discrete elements dispersed in the second one. These discrete elements, or 'particles', can be droplets, bubbles or solid particles having different sizes. This situation encompasses a wide range of phenomena, from nano-particles and colloids sensitive to the molecular fluctuations of the carrier fluid to inertia particles transported by the large-scale motions of turbulent flows and, depending on the phenomenon studied, a broad spectrum of approaches have been developed. The aim of the present article is to analyze statistical models of particles in turbulent flows by addressing this issue as the extension of the classical formulations operating at a molecular or meso-molecular level of description. It has a three-fold purpose: (1) to bring out the thread of continuity between models for discrete particles in turbulent flows (above the hydrodynamical level of description) and classical mesoscopic formulations of statistical physics (below the hydrodynamical level); (2) to reveal the specific challenges met by statistical models in turbulence; (3) to establish a methodology for modeling particle dynamics in random media with non-zero space and time correlations. The presentation is therefore centered on organizing the different approaches, establishing links and clarifying physical foundations. The analysis of disperse two-phase flow models is developed by discussing: first, approaches of classical statistical physics; then, by considering models for single-phase turbulent flows; and, finally, by addressing current formulations for discrete particles in turbulent flows. This brings out that particle-based models do not cease to exist above the hydrodynamical level and offer great interest when combined with proper stochastic formulations to account for the lack of equilibrium distributions and scale separation. In the course of this study, general results
Velocity Measurements of Turbulent Wake Flow Over a Circular Cylinder
Shih, Chang-Lung; Chen, Wei-Cheng; Chang, Keh-Chin; Wang, Muh-Rong
2016-06-01
There are two general concerns in the velocity measurements of turbulence. One is the temporal characteristics which governs the turbulent mixing process. Turbulence is rotational and is characterized by high levels of fluctuating vorticity. In order to obtain the information of vorticity dynamics, the spatial characteristics is the other concern. These varying needs can be satisfied by using a variety of diagnostic techniques such as invasive physical probes and non-invasive optical instruments. Probe techniques for the turbulent measurements are inherently simple and less expensive than optical methods. However, the presence of a physical probe may alter the flow field, and velocity measurements usually become questionable when probing recirculation zones. The non-invasive optical methods are mostly made of the foreign particles (or seeding) instead of the fluid flow and are, thus, of indirect method. The difference between the velocities of fluid and foreign particles is always an issue to be discussed particularly in the measurements of complicated turbulent flows. Velocity measurements of the turbulent wake flow over a circular cylinder will be made by using two invasive instruments, namely, a cross-type hot-wire anemometry (HWA) and a split-fiber hot-film anemometry (HFA), and a non-invasive optical instrument, namely, particle image velocimetry (PIV) in this study. Comparison results show that all three employed diagnostic techniques yield similar measurements in the mean velocity while somewhat deviated results in the root-mean-squared velocity, particularly for the PIV measurements. It is demonstrated that HFA possesses more capability than HWA in the flow measurements of wake flow. Wake width is determined in terms of either the flatness factor or shear-induced vorticity. It is demonstrated that flow data obtained with the three employed diagnostic techniques are capable of yielding accurate determination of wake width.
Curvature effects on the velocity profile in turbulent pipe flow.
Grossmann, Siegfried; Lohse, Detlef
2017-02-01
Prandtl and von Kármán have developed the famous log-law for the mean velocity profile for turbulent flow over a plate. The log-law has also been applied to turbulent pipe flow, though the wall surface is curved (in span-wise direction) and has finite diameter. Here we discuss the theoretical framework, based on the Navier-Stokes equations, with which one can describe curvature effects and also the well-known finite-size effects in the turbulent mean-velocity profile. When comparing with experimental data we confirm that the turbulent eddy viscosity must contain both curvature and finite-size contributions and that the usual ansatz for the turbulent eddy viscosity as being linear in the wall distance is insufficient, both for small and large wall distances. We analyze the experimental velocity profile in terms of an r-dependent generalized turbulent viscosity [Formula: see text] (with [Formula: see text] being the wall distance, a pipe radius, u * shear stress velocity, and g([Formula: see text]/a) the nondimensionalized viscosity), which reflects the radially strongly varying radial eddy transport of the axial velocity. After the near wall linear viscous sublayer, which soon sees the pipe wall's curvature, a strong transport (eddy) activity steepens the profile considerably, leading to a maximum in g([Formula: see text]/a) at about half radius, then decreasing again towards the pipe center. This reflects the smaller eddy transport effect near the pipe's center, where even in strongly turbulent flow (the so-called "ultimate state") the profile remains parabolic. The turbulent viscous transport is strongest were the deviations of the profile from parabolic are strongest, and this happens in the range around half radius.
Large-scale stationary and turbulent flow over topography
Vallis, G. K.; Roads, J. O.
1984-01-01
The contributions made to the formation of stationary features of flow over topography by linear and nonlinear dynamics were examined with an integrated quasi-geostrophic model with idealized topographic forcing. The simulation was run out to several months and generated time-averaged values which were compared with those obtained with linear theory. Linear predictions were converted to turbulent features through the addition of stationary, nonlinear thermodynamic and transient vorticity fluxes. The turbulence features matched atmospheric data on energy spectra, the direction and magnitude of energy transfers, and the spatial magnitudes involved. Transient flow transferred the majority of energy absorbed by the upscale flow and, by absorbing energy, reduced the energy of stationary flow while retaining resonance signatures. Instability was a pervasive feature of the topographically forced flow except at high wavenumbers. The results confirm that transient eddies are interactive with both asymmetric and zonal flow and cannot be adequately described by linear theory.
Lyapunov Exponents and Covariant Vectors for Turbulent Flow Simulations
Blonigan, Patrick; Murman, Scott; Fernandez, Pablo; Wang, Qiqi
2016-11-01
As computational power increases, engineers are beginning to use scale-resolving turbulent flow simulations for applications in which jets, wakes, and separation dominate. However, the chaotic dynamics exhibited by scale-resolving simulations poses problems for the conventional sensitivity analysis and stability analysis approaches that are vital for design and control. Lyapunov analysis is used to study the chaotic behavior of dynamical systems, including flow simulations. Lyapunov exponents are the growth or a decay rate of specific flow field perturbations called the Lyapunov covariant vectors. Recently, the authors have used Lyapunov analysis to study the breakdown in conventional sensitivity analysis and the cost of new shadowing-based sensitivity analysis. The current work reviews Lyapunov analysis and presents new results for a DNS of turbulent channel flow, wall-modeled channel flow, and a DNS of a low pressure turbine blade. Additionally, the implications of these Lyapunov analyses for computing sensitivities of these flow simulations will be discussed.
Turbulence modelling of flow fields in thrust chambers
Chen, C. P.; Kim, Y. M.; Shang, H. M.
1993-01-01
Following the consensus of a workshop in Turbulence Modelling for Liquid Rocket Thrust Chambers, the current effort was undertaken to study the effects of second-order closure on the predictions of thermochemical flow fields. To reduce the instability and computational intensity of the full second-order Reynolds Stress Model, an Algebraic Stress Model (ASM) coupled with a two-layer near wall treatment was developed. Various test problems, including the compressible boundary layer with adiabatic and cooled walls, recirculating flows, swirling flows, and the entire SSME nozzle flow were studied to assess the performance of the current model. Detailed calculations for the SSME exit wall flow around the nozzle manifold were executed. As to the overall flow predictions, the ASM removes another assumption for appropriate comparison with experimental data to account for the non-isotropic turbulence effects.
DNS and scaling law analysis of compressible turbulent channel flow
无
2001-01-01
Fully developed compressible turbulent channel flow (Ma=0.8,Re=3300) is numerically simulated, and the data base of turbulence is established. The s tatistics such as density_weighted mean velocity and RMS velocity fluctuations i n semi_local coordinates agree well with those from other DNS data. High order s tatistics (skewness and flatness factors) of velocity fluctuations of compressib le turbulence are reported for the first time. Compressibility effects are also discussed. Pressure_dilatation absorbs part of the kinetic energy and makes the streaks of compressible channel flow more smooth. The scaling laws of compressible channel flow are also discussed. The conclusi ons are: (a) Scaling law is found in the center area of the channel. (b) In this area, ESS is also found. (c) When Mach number is not ve ry high, compressibility has little effect on scaling exponents.
Turbulent patterns in wall-bounded flows: a Turing instability?
Manneville, Paul
2012-01-01
In their way to/from turbulence, plane wall-bounded flows display an interesting transitional regime where laminar and turbulent oblique bands alternate, the origin of which is still mysterious. In line with Barkley's recent work about the pipe flow transition involving reaction-diffusion concepts, we consider plane Couette flow in the same perspective and transform Waleffe's classical four-variable model of self-sustaining process into a reaction-diffusion model. We show that, upon fulfillment of a condition on the relative diffusivities of its variables, the featureless turbulent regime becomes unstable against patterning as the result of a Turing instability. A reduced two-variable model helps us to delineate the appropriate region of parameter space. An {\\it intrinsic} status is therefore given to the pattern's wavelength for the first time. Virtues and limitations of the model are discussed, calling for a microscopic support of the phenomenological approach.
Review and assessment of turbulence models for hypersonic flows
Roy, Christopher J.; Blottner, Frederick G.
2006-10-01
Accurate aerodynamic prediction is critical for the design and optimization of hypersonic vehicles. Turbulence modeling remains a major source of uncertainty in the computational prediction of aerodynamic forces and heating for these systems. The first goal of this article is to update the previous comprehensive review of hypersonic shock/turbulent boundary-layer interaction experiments published in 1991 by Settles and Dodson (Hypersonic shock/boundary-layer interaction database. NASA CR 177577, 1991). In their review, Settles and Dodson developed a methodology for assessing experiments appropriate for turbulence model validation and critically surveyed the existing hypersonic experiments. We limit the scope of our current effort by considering only two-dimensional (2D)/axisymmetric flows in the hypersonic flow regime where calorically perfect gas models are appropriate. We extend the prior database of recommended hypersonic experiments (on four 2D and two 3D shock-interaction geometries) by adding three new geometries. The first two geometries, the flat plate/cylinder and the sharp cone, are canonical, zero-pressure gradient flows which are amenable to theory-based correlations, and these correlations are discussed in detail. The third geometry added is the 2D shock impinging on a turbulent flat plate boundary layer. The current 2D hypersonic database for shock-interaction flows thus consists of nine experiments on five different geometries. The second goal of this study is to review and assess the validation usage of various turbulence models on the existing experimental database. Here we limit the scope to one- and two-equation turbulence models where integration to the wall is used (i.e., we omit studies involving wall functions). A methodology for validating turbulence models is given, followed by an extensive evaluation of the turbulence models on the current hypersonic experimental database. A total of 18 one- and two-equation turbulence models are reviewed
Subcritical Transition to Turbulence in Couette-Poiseuille flow
Wesfreid, Jose Eduardo; Klotz, Lukasz
2016-11-01
We study the subcritical transition to turbulence in the plane Couette-Poiseuille shear flow with zero mean advection velocity. Our experimental configuration consists on one moving wall of the test section (the second one remains stationary), which acts like a driving force for the flow, imposing linear streamwise velocity profile (Couette) and adverse pressure gradient in the streamwise direction (Poiseuille) at the same time. This flow, which had only been studied theoretically up to now, is always linearly stable. The transition to turbulence is forced by a very well controlled finite-size perturbation by injection, into the test section, of a water jet during a very short time. Using PIV technique, we characterized quantitatively the initial development of the triggered turbulent spot and compared its energy evolution with the theoretical predictions of the transient growth theory. In addition, we show results concerning the importance of nonlinearities, when waviness of streaks in streamwise direction induced self-sustained process in the turbulent spot. We also measured precisely the large-scale flow which is generated around the turbulent spot and studied its strength as a function of the Reynolds number.
Turbulent characteristics of shear-thinning fluids in recirculating flows
Pereira, A.S. [Inst. Superior de Engenharia do Porto (Portugal). Dept. de Engenharia Quimica; Pinho, F.T. [Centro de Estudos de Fenomenos de Transporte, Departamento de Engenharia Mecanica e Gestao Industrial, Faculdade de Engenharia da Universidade do Porto, Rua dos Bragas, 4050-123 Porto (Portugal)
2000-03-01
A miniaturised fibre optic laser-Doppler anemometer was used to carry out a detailed hydrodynamic investigation of the flow downstream of a sudden expansion with 0.1-0.2% by weight shear-thinning aqueous solutions of xanthan gum. Upstream of the sudden expansion the pipe flow was fully-developed and the xanthan gum solutions exhibited drag reduction with corresponding lower radial and tangential normal Reynolds stresses, but higher axial Reynolds stress near the wall and a flatter axial mean velocity profile in comparison with Newtonian flow. The recirculation bubble length was reduced by more than 20% relative to the high Reynolds number Newtonian flow, and this was attributed to the occurrence further upstream of high turbulence for the non-Newtonian solutions, because of advection of turbulence and earlier high turbulence production in the shear layer. Comparisons with the measurements of Escudier and Smith (1999) with similar fluids emphasized the dominating role of inlet turbulence. The present was less anisotropic, and had lower maximum axial Reynolds stresses (by 16%) but higher radial turbulence (20%) than theirs. They reported considerably longer recirculating bubble lengths than we do for similar non-Newtonian fluids and Reynolds numbers. (orig.)
Turbulent Mixing and Flow Resistance over Dunes and Scours
Dorrell, R. M.; Arfaie, A.; Burns, A. D.; Eggenhuisen, J. T.; Ingham, D. B.; McCaffrey, W. D.
2014-12-01
Flows in both submarine and fluvial channels are subject to lower boundary roughness. Lower boundary roughness occurs as frictional roughness suffered by the flow as it moves over the bed (skin friction) or drag suffered by the flow as it moves past a large obstacle (form drag). Critically, to overcome such roughness the flow must expend (lose) energy and momentum. However, whilst overcoming bed roughness the degree of turbulent mixing in the flow may be enhanced increasing the potential energy of the flow. This is of key importance to density driven flows as the balance between kinetic energy lost and potential energy gained (through turbulent diffusion of suspended particulate material) may critically affect the criterion for autosuspension. Moreover, this effect of lower boundary roughness may go as far as helping to explain why, even on shallow slopes, channelized submarine density currents can run out over ultra long distances. Such effects are also important in fluvial systems, where they will be responsible for maximizing or minimizing sediment capacity and competence in different flow environments. Numerical simulations are performed at a high Reynolds number (O (106)) for a series of crestal length to height ratio (c/h) at a fixed width to height ratio (w/h). Here, we present key findings of shear flow over a range of idealized bedform shapes. We show how the total basal shear stress is split into skin friction and form drag and identify how the respective magnitudes vary as a function of bedform shape and scale. Moreover we demonstrate how said bedforms affect the balance of energy lost (frictional) and energy gained (turbulent mixing). Overall, results demonstrate a slow reduction in turbulent mixing and flow resistance with decreasing bedform side slope angle. This suggests that both capacity and competence of the flow may be reduced through decrease in of the potential energy of the flow as a result of change in slope angles.
Simulations of Turbulent Flows with Strong Shocks and Density Variations
Zhong, Xiaolin
2012-12-13
In this report, we present the research efforts made by our group at UCLA in the SciDAC project Simulations of turbulent flows with strong shocks and density variations. We use shock-fitting methodologies as an alternative to shock-capturing schemes for the problems where a well defined shock is present. In past five years, we have focused on development of high-order shock-fitting Navier-Stokes solvers for perfect gas flow and thermochemical non-equilibrium flow and simulation of shock-turbulence interaction physics for very strong shocks. Such simulation has not been possible before because the limitation of conventional shock capturing methods. The limitation of shock Mach number is removed by using our high-order shock-fitting scheme. With the help of DOE and TeraGrid/XSEDE super computing resources, we have obtained new results which show new trends of turbulence statistics behind the shock which were not known before. Moreover, we are also developing tools to consider multi-species non-equilibrium flows. The main results are in three areas: (1) development of high-order shock-fitting scheme for perfect gas flow, (2) Direct Numerical Simulation (DNS) of interaction of realistic turbulence with moderate to very strong shocks using super computing resources, and (3) development and implementation of models for computation of mutli-species non-quilibrium flows with shock-fitting codes.
Interaction of monopoles, dipoles, and turbulence with a shear flow
Marques Rosas Fernandes, V. H.; Kamp, L. P. J.; van Heijst, G. J. F.; Clercx, H. J. H.
2016-09-01
Direct numerical simulations have been conducted to examine the evolution of eddies in the presence of large-scale shear flows. The numerical experiments consist of initial-value-problems in which monopolar and dipolar vortices as well as driven turbulence are superposed on a plane Couette or Poiseuille flow in a periodic two-dimensional channel. The evolution of the flow has been examined for different shear rates of the background flow and different widths of the channel. Results found for retro-grade and pro-grade monopolar vortices are consistent with those found in the literature. Boundary layer vorticity, however, can significantly modify the straining and erosion of monopolar vortices normally seen for unbounded domains. Dipolar vortices are shown to be much more robust coherent structures in a large-scale shear flow than monopolar eddies. An analytical model for their trajectories, which are determined by self-advection and advection and rotation by the shear flow, is presented. Turbulent kinetic energy is effectively suppressed by the shearing action of the background flow provided that the shear is linear (Couette flow) and of sufficient strength. Nonlinear shear as present in the Poiseuille flow seems to even increase the turbulence strength especially for high shear rates.
Three-dimensional flow and turbulence structure in electrostatic precipitator
Ullum, Thorvald Uhrskov; Larsen, Poul Scheel; Özcan, Oktay
2002-01-01
Stereo PIV is employed to study the three-dimensional velocity and turbulence fields in a laboratory model of a negative corona, barbed-wire, smooth-plate, electrostatic precipitator (figure 1). The study is focused on determining the parametric effects of axial development, mean current density Jm...... and bulk velocity U0 on secondary flows and turbulence levels and structures due to the action of the three-dimensional electrostatic field on the charged gas. At constant bulk velocity (U0 = 1 m/s) and current density (Jm = 0.4 mA/m2), secondary flows in the form of rolls of axial vorticity with swirl...
A crossed hot-wire technique for complex turbulent flows
Cutler, A. D.; Bradshaw, P.
1991-01-01
This paper describes a crossed hot-wire technique for the measurement of all components of mean velocity, Reynolds stresses, and triple products in a complex turbulent flow. The accuracy of various assumptions usually implicit in the use of crossed hot-wire anemometers is examined. It is shown that significant errors can result in flow with gradients in mean velocity or Reynolds stress, but that a first-order correction for these errors can be made using available data. It is also shown how corrections can be made for high turbulence levels using available data.
Unsteady turbulent shear flow in shock tube discontinuities
Johnson, J. A., III; Ramaiah, R.; Lin, I.
1981-01-01
A pressure-ruptured shock tube and an arc driven shock tube, have been used to study the evolution of turbulent fluctuations at contact surfaces with N2O4-2NO2 mixtures and at ionizing shock fronts in argon. The study has focused on point density diagnostics derived from crossed light beam correlations and electric probes. Turbulent bursts are found for which dynamical and spectral analyses suggest a particle-like evolution of fluctuation segments with a unique and characteristic frequency, independent of flow history and overall flow conditions.
How does turbulence spread in plane Couette flows?
Couliou, Marie
2015-01-01
We investigate the growth in the spanwise direction of turbulent spots invading a laminar flow in a plane Couette flow. Direct Numerical Simulation is used to track the nucleation of streaks during the spot growth. Experiment and direct numerical simulation allow us to study the velocity of the spot fronts and of the vortices observed at the spots' edges. All these results show that two mechanisms are involved when turbulent spots grow: a formerly proposed local growth occurring at the spot spanwise tips but also in comparable proportion a global growth induced by large-scale advection identified in the present work.
Turbulence-radiation interactions in a particle-laden flow
Frankel, Ari; Pouransari, Hadi; Iaccarino, Gianluca; Mani, Ali
2014-11-01
Turbulent fluctuations in a radiatively participating medium can significantly alter the mean heat transfer characteristics in a manner that current RANS models cannot accurately capture. While turbulence-radiation interaction has been studied extensively in traditional combustion systems, such interactions have not yet been studied in the context of particle-laden flows. This work is motivated by applications in particle-based solar receivers in which external radiation is primarily absorbed by a dispersed phase and conductively exchanged with the carrier fluid. Direct numerical simulations of turbulence with Lagrangian particles subject to a collimated radiation source are performed with a flux-limited diffusion approximation to radiative transfer. The dependence of the turbulence-radiation interaction statistics on the particle Stokes number will be demonstrated. Supported by PSAAP II.
WEAKLY SWIRLING TURBULENT FLOW IN TURBID WATER HYDRAULIC SEPARATION DEVICE
LI Lin; QIU Xiu-yun; JIN Sheng; XIAO Jun; GONG Shou-yuan
2008-01-01
This article deals with the characteristics of weakly swirling turbulent flow field in a Turbid Water Hydraulic Separation Device (TWHSD) through experimental and numerical researches. The flow field was measured by PIV, which provided streamlines, vortex structure, vorticity and velocity distribution in different test planes in the TWHSD. On the basis of the experimental results, the tangential and radial velocity distributions of the swirling flow field were obtained. Meanwhile, the numerical simulations were conducted with the RNG and RSM turbulence models, respectively. According to the experimental and numerical results, the characteristics of the clear water flow field inside the TWHSD were determined. In view of simulation accuracy and time consumption, it is suggested to apply the RNG model instead of the RSM model, which is more time consuming, to make further study on two-phases flow fields in the device.
DNS of turbulent wall bounded flows with a passive scalar
Araya, Juan Guillermo
In this thesis, Direct Numerical Simulations (DNS) of the velocity and temperature fields are performed for incompressible turbulent flows in plane channels and spatially-developing boundary layers. The main goal is to numerically analyze the behavior of the momentum and thermal boundary layers subjected to different external and upstream conditions, the main focus is given to: (i) local flow perturbations, (ii) different Reynolds numbers, and, (iii) external pressure gradient. Two types of turbulent wall-bounded flows are examined in this investigation. One of them consists of the fully developed turbulent channel. Furthermore, after the developing section, the boundary layers generated by the lower and upper walls collapse. From this point to downstream, periodic boundary conditions are applicable due to the existent homogeneity. The second type of wall bounded flow explored possesses no restriction in the upper zone; consequently, the boundary layer may grow infinitely downstream. This streamwise non-homogeneous state does not allow to prescribe periodic boundary conditions along the direction of the flow. Therefore, time-dependent turbulent information must be assigned at the domain inlet, turning the numerical problem into a very challenging one. The spatially-developing turbulent boundary layer in a flat plate is a typical example of non-homogeneous flow. In the first part of this thesis, the influence of local forcing on an incompressible turbulent channel flow is numerically investigated. The extensive information provided by the DNS enable us to have a better understanding of the physical mechanism responsible for local heat transfer enhancement and drag reduction. Time-periodic blowing/suction is applied by means of thin spanwise slots located at the lower and upper walls of the channel at several forcing frequencies. It was found in Araya et al. (2008-a) the existence of a characteristic frequency, i.e. of f = 0.64 or f* = 0.044, at which maximum local
Langevin and diffusion equation of turbulent fluid flow
Brouwers, J. J. H.
2010-08-01
A derivation of the Langevin and diffusion equations describing the statistics of fluid particle displacement and passive admixture in turbulent flow is presented. Use is made of perturbation expansions. The small parameter is the inverse of the Kolmogorov constant C 0 , which arises from Lagrangian similarity theory. The value of C 0 in high Reynolds number turbulence is 5-6. To achieve sufficient accuracy, formulations are not limited to terms of leading order in C0 - 1 including terms next to leading order in C0 - 1 as well. Results of turbulence theory and statistical mechanics are invoked to arrive at the descriptions of the Langevin and diffusion equations, which are unique up to truncated terms of O ( C0 - 2 ) in displacement statistics. Errors due to truncation are indicated to amount to a few percent. The coefficients of the presented Langevin and diffusion equations are specified by fixed-point averages of the Eulerian velocity field. The equations apply to general turbulent flow in which fixed-point Eulerian velocity statistics are non-Gaussian to a degree of O ( C0 - 1 ) . The equations provide the means to calculate and analyze turbulent dispersion of passive or almost passive admixture such as fumes, smoke, and aerosols in areas ranging from atmospheric fluid motion to flows in engineering devices.
Transition to turbulence in pipe flow as a phase transition
Vasudevan, Mukund; Hof, Björn
2015-11-01
In pipe flow, turbulence first arises in the form of localized turbulent patches called puffs. The flow undergoes a transition to sustained turbulence via spatio-temporal intermittency, with puffs splitting, decaying and merging in the background laminar flow. However, the due to mean advection of the puffs and the long timescales involved (~107 advective time units), it is not possible to study the transition in typical laboratory set-ups. So far, it has only been possible to indirectly estimate the critical point for the transition. Here, we exploit the stochastic memoryless nature of the puff decay and splitting processes to construct a pipe flow set-up, that is periodic in a statistical sense. It then becomes possible to study the flow for sufficiently long times and characterize the transition in detail. We present measurements of the turbulent fraction as a function of Reynolds number which in turn allows a direct estimate of the critical point. We present evidence that the transition has features of a phase transition of second order.
Structure of the velocity gradient tensor in turbulent shear flows
Pumir, Alain
2017-07-01
The expected universality of small-scale properties of turbulent flows implies isotropic properties of the velocity gradient tensor in the very large Reynolds number limit. Using direct numerical simulations, we determine the tensors formed by n =2 and 3 velocity gradients at a single point in turbulent homogeneous shear flows and in the log-layer of a turbulent channel flow, and we characterize the departure of these tensors from the corresponding isotropic prediction. Specifically, we separate the even components of the tensors, invariant under reflexion with respect to all axes, from the odd ones, which identically vanish in the absence of shear. Our results indicate that the largest deviation from isotropy comes from the odd component of the third velocity gradient correlation function, especially from the third moment of the derivative along the normal direction of the streamwise velocity component. At the Reynolds numbers considered (Reλ≈140 ), we observe that these second- and third-order correlation functions are significantly larger in turbulent channel flows than in homogeneous shear flow. Overall, our work demonstrates that a mean shear leads to relatively simple structure of the velocity gradient tensor. How isotropy is restored in the very large Reynolds limit remains to be understood.
Reference Solutions for Benchmark Turbulent Flows in Three Dimensions
Diskin, Boris; Thomas, James L.; Pandya, Mohagna J.; Rumsey, Christopher L.
2016-01-01
A grid convergence study is performed to establish benchmark solutions for turbulent flows in three dimensions (3D) in support of turbulence-model verification campaign at the Turbulence Modeling Resource (TMR) website. The three benchmark cases are subsonic flows around a 3D bump and a hemisphere-cylinder configuration and a supersonic internal flow through a square duct. Reference solutions are computed for Reynolds Averaged Navier Stokes equations with the Spalart-Allmaras turbulence model using a linear eddy-viscosity model for the external flows and a nonlinear eddy-viscosity model based on a quadratic constitutive relation for the internal flow. The study involves three widely-used practical computational fluid dynamics codes developed and supported at NASA Langley Research Center: FUN3D, USM3D, and CFL3D. Reference steady-state solutions computed with these three codes on families of consistently refined grids are presented. Grid-to-grid and code-to-code variations are described in detail.
2017-01-09
other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a ...Conference Paper with Briefing Charts 3. DATES COVERED (From - To) 01 December 2016 – 11 January 2017 4. TITLE AND SUBTITLE A Study of Acoustic...in collaboration with Sierra Lobo , Inc., UCLA; Conference Paper with Briefing Charts 14. ABSTRACT The reacting flow from a single gas-centered
Effects of roughness on density-weighted particle statistics in turbulent channel flows
Milici, Barbara [Faculty of Engineering and Architecture, Cittadella Universitaria - 94100 - Enna (Italy)
2015-12-31
The distribution of inertial particles in turbulent flows is strongly influenced by the characteristics of the coherent turbulent structures which develop in the carrier flow field. In wall-bounded flows, these turbulent structures, which control the turbulent regeneration cycles, are strongly affected by the roughness of the wall, nevertheless its effects on the particle transport in two-phase turbulent flows has been still poorly investigated. The issue is discussed here by addressing DNS combined with LPT to obtain statistics of velocity and preferential accumulation of a dilute dispersion of heavy particles in a turbulent channel flow, bounded by irregular two-dimensional rough surfaces, in the one-way coupling regime.
Feedback Control of Turbulent Shear Flows by Genetic Programming
Duriez, Thomas; von Krbek, Kai; Bonnet, Jean-Paul; Cordier, Laurent; Noack, Bernd R; Segond, Marc; Abel, Markus; Gautier, Nicolas; Aider, Jean-Luc; Raibaudo, Cedric; Cuvier, Christophe; Stanislas, Michel; Debien, Antoine; Mazellier, Nicolas; Kourta, Azeddine; Brunton, Steven L
2015-01-01
Turbulent shear flows have triggered fundamental research in nonlinear dynamics, like transition scenarios, pattern formation and dynamical modeling. In particular, the control of nonlinear dynamics is subject of research since decades. In this publication, actuated turbulent shear flows serve as test-bed for a nonlinear feedback control strategy which can optimize an arbitrary cost function in an automatic self-learning manner. This is facilitated by genetic programming providing an analytically treatable control law. Unlike control based on PID laws or neural networks, no structure of the control law needs to be specified in advance. The strategy is first applied to low-dimensional dynamical systems featuring aspects of turbulence and for which linear control methods fail. This includes stabilizing an unstable fixed point of a nonlinearly coupled oscillator model and maximizing mixing, i.e.\\ the Lyapunov exponent, for forced Lorenz equations. For the first time, we demonstrate the applicability of genetic p...
Turbulent flow computation in a circular U-Bend
Miloud Abdelkrim
2014-03-01
Full Text Available Turbulent flows through a circular 180° curved bend with a curvature ratio of 3.375, defined as the the bend mean radius to pipe diameter is investigated numerically for a Reynolds number of 4.45×104. The computation is performed for a U-Bend with full long pipes at the entrance and at the exit. The commercial ANSYS FLUENT is used to solve the steady Reynolds–Averaged Navier–Stokes (RANS equations. The performances of standard k-ε and the second moment closure RSM models are evaluated by comparing their numerical results against experimental data and testing their capabilities to capture the formation and extend this turbulence driven vortex. It is found that the secondary flows occur in the cross-stream half-plane of such configurations and primarily induced by high anisotropy of the cross-stream turbulent normal stresses near the outer bend.
RANS turbulence model form uncertainty quantification for wind engineering flows
Gorle, Catherine; Zeoli, Stephanie; Bricteux, Laurent
2016-11-01
Reynolds-averaged Navier-Stokes simulations with linear eddy-viscosity turbulence models are commonly used for modeling wind engineering flows, but the use of the results for critical design decisions is hindered by the limited capability of the models to correctly predict bluff body flows. A turbulence model form uncertainty quantification (UQ) method to define confidence intervals for the results could remove this limitation, and promising results were obtained in a previous study of the flow in downtown Oklahoma City. The objective of the present study is to further investigate the validity of these results by considering the simplified test case of the flow around a wall-mounted cube. DNS data is used to determine: 1. whether the marker, which identifies regions that deviate from parallel shear flow, is a good indicator for the regions where the turbulence model fails, and 2. which Reynolds stress perturbations, in terms of the tensor magnitude and the eigenvalues and eigenvectors of the normalized anisotropy tensor, can capture the uncertainty in the flow field. A comparison of confidence intervals obtained with the UQ method and the DNS solution indicates that the uncertainty in the velocity field can be captured correctly in a large portion of the flow field.
Temporal and spatial intermittencies within channel flow turbulence near transition
Kushwaha, Anubhav; Park, Jae Sung; Graham, Michael D.
2017-02-01
Direct numerical simulations (DNS) of plane Poiseuille flow are performed in an extended domain at friction Reynolds numbers ranging from 70 to 100. In minimal domains, turbulence in this Reynolds number range displays substantial intermittency that is associated with chaotic movement of turbulent trajectories between lower and upper branch invariant solutions known as exact coherent states (ECS). The present work aims to address the relationship between temporal dynamics in minimal channels and spatiotemporal dynamics in extended domains. Both temporal and spatial analyses of the turbulent velocity fields are performed, the latter using image analysis methods. These analyses partition the flow characteristics into low-, intermediate- and high-drag classes; we present the differences between flows fields in these classes in terms of simple quantities like mean velocity, wall shear stress, and flow structures. The temporal and spatial analysis methods, although completely independent of one another, yield very similar results for both low- and high-drag regions. In particular, the conditional mean profiles in regions of low drag closely resemble those found in low-drag temporal intervals in the minimal channel. Finally, we address the possibility of similarities between turbulence and exact coherent states in two ways: (1) comparing wall shear stress in localized patches the size of minimal channels in large domains with those in actual minimal channel and (2) comparing conditional mean velocity profiles during low-drag events with mean profiles from lower branch ECS. These analyses show that both the local near-wall flow structure in the low-drag patches of the large domain and the conditional mean profiles in the region y+≲30 resemble those of a lower branch minimal domain ECS. In summary, the results presented here suggest that spatiotemporal intermittency in transitional channel flow turbulence is related to temporal intermittency, and by extension to the
Velocity profiles in strongly turbulent Taylor-Couette flow
Grossmann, S.; Lohse, D.; Sun, C.
2014-01-01
We derive the velocity profiles in strongly turbulent Taylor-Couette flow for the general case of independently rotating cylinders. The theory is based on the Navier-Stokes equations in the appropriate (cylinder) geometry. In particular, we derive the axial and the angular velocity profiles as funct
Numerical simulation of particle-laden turbulent channel flow
Li, Y.; McLaughlin, J.B.; Kontomaris, K.; Portela, L.
2001-01-01
This paper presents results for the behavior of particle-laden gases in a small Reynolds number vertical channel down flow. Results will be presented for the effects of particle feedback on the gas-phase turbulence and for the concentration profile of the particles. The effects of density ratio,
Numerical Simulation of Turbulent Flow in Concentric Annuli
Boersma, B.J.; Breugem, W.-P.
2010-01-01
In this paper we consider a fully developed turbulent flow in a round pipe with a small inner annulus. The diameter of the inner annulus is less than 10% of the diameter of the outer pipe. As a consequence, the surface area of the inner pipe compared to the outer pipe is small. The friction exerted
DNS of 2D turbulent flow around a square cylinder
Wissink, JG
1997-01-01
Two-dimensional 'turbulent' flow around a rectangular cylinder has been simulated at Re = 10,000 using a sixth-order-accurate finite volume method for the discretization of convection and diffusion. The spatial discretization consists of a combination of a seventh-order upwind-biased method for the
Smooth and rough boundaries in turbulent Taylor-Couette flow
Berg, van den Thomas H.; Doering, Charles R.; Lohse, Detlef; Lathrop, Daniel P.
2003-01-01
We examine the torque required to drive the smooth or rough cylinders in turbulent Taylor-Couette flow. With rough inner and outer walls the scaling of the dimensionless torque G is found to be consistent with pure Kolmogorov scaling G~Re2. The results are interpreted within the Grossmann-Lohse theo
Numerical Simulation of Turbulent Flow in Concentric Annuli
Boersma, B.J.; Breugem, W.-P.
2010-01-01
In this paper we consider a fully developed turbulent flow in a round pipe with a small inner annulus. The diameter of the inner annulus is less than 10% of the diameter of the outer pipe. As a consequence, the surface area of the inner pipe compared to the outer pipe is small. The friction exerted
Universal intermittent properties of particle trajectories in highly turbulent flows
Arnèodo, A.; Benzi, R.; Berg, Jacob
2008-01-01
We present a collection of eight data sets from state-of-the-art experiments and numerical simulations on turbulent velocity statistics along particle trajectories obtained in different flows with Reynolds numbers in the range R-lambda is an element of [120740]. Lagrangian structure functions fro...
On the Nature of Magnetic Turbulence in Rotating, Shearing Flows
Walker, Justin; Boldyrev, Stanislav
2015-01-01
The local properties of turbulence driven by the magnetorotational instability (MRI) in rotating, shearing flows are studied in the framework of a shearing-box model. Based on numerical simulations, we propose that the MRI-driven turbulence comprises two components: the large-scale shear-aligned strong magnetic field and the small-scale fluctuations resembling magnetohydrodynamic (MHD) turbulence. The energy spectrum of the large-scale component is close to $k^{-2}$, whereas the spectrum of the small-scale component agrees with the spectrum of strong MHD turbulence $k^{-3/2}$. While the spectrum of the fluctuations is universal, the outer-scale characteristics of the turbulence are not; they depend on the parameters of the system, such as the net magnetic flux. However, there is remarkable universality among the allowed turbulent states -- their intensity $v_0$ and their outer scale $\\lambda_0$ satisfy the balance condition $v_0/\\lambda_0\\sim \\mathrm d\\Omega/\\mathrm d\\ln r$, where $\\mathrm d\\Omega/\\mathrm d\\l...
Shock-induced turbulent flow in baffle systems
Kuhl, A.L. [Lawrence Livermore National Lab., CA (United States); Reichenbach, H. [Fraunhofer-Institut fuer Kurzzeitdynamik - Ernst-Mach-Institut (EMI), Freiburg im Breisgau (Germany)
1993-07-01
Experiments are described on shock propagation through 2-D aligned and staggered baffle systems. Flow visualization was provided by shadow and schlieren photography, recorded by the Cranz-Schardin camera. Also single-frame, infinite-fringe, color interferograms were used. Intuition suggests that this is a rather simple 2-D shock diffraction problem. However, flow visualization reveals that the flow rapidly evolved into a complex 3-D turbulent mixing problem. Mushroom-shaped mixing regions blocked the flow into the next baffle orifice. Thus energy was transferred from the directed kinetic energy (induced by the shock) to rotational energy of turbulent mixing, and then dissipated by molecular effects. These processes dramatically dissipate the strength of the shock wave. The experiments provide an excellent test case that could be used to assess the accuracy of computer code calculations of such problems.
Performance of turbulence models for transonic flows in a diffuser
Liu, Yangwei; Wu, Jianuo; Lu, Lipeng
2016-09-01
Eight turbulence models frequently used in aerodynamics have been employed in the detailed numerical investigations for transonic flows in the Sajben diffuser, to assess the predictive capabilities of the turbulence models for shock wave/turbulent boundary layer interactions (SWTBLI) in internal flows. The eight turbulence models include: the Spalart-Allmaras model, the standard k - 𝜀 model, the RNG k - 𝜀 model, the realizable k - 𝜀 model, the standard k - ω model, the SST k - ω model, the v2¯ - f model and the Reynolds stress model. The performance of the different turbulence models adopted has been systematically assessed by comparing the numerical results with the available experimental data. The comparisons show that the predictive performance becomes worse as the shock wave becomes stronger. The v2¯ - f model and the SST k - ω model perform much better than other models, and the SST k - ω model predicts a little better than the v2¯ - f model for pressure on walls and velocity profile, whereas the v2¯ - f model predicts a little better than the SST k - ω model for separation location, reattachment location and separation length for strong shock case.
EXPERIMENTAL STUDY OF 3-D TURBULENT BEND FLOWS IN OPEN CHANNEL
LIU Yue-qin; ZHENG Shao-wen; WU Qiang
2005-01-01
A generalized bend flow model, treating a 90° single bend and 60° continuous bends, was designed to quantitatively describe 3-D turbulence mechanism of circulating not-fully-developed flow in open channels with bends.The 3-D fluctuating velocities of turbulent flow were measured and analyzed with a 3-D acoustic-Doppler velocimeter.Formula for 3-D turbulent intensity was derived using the dimension analysis approach.Expressions of vertical turbulent-intensity distributions were obtained with the multivariant-regression theory, which agree with experiment data.Distributions of turbulent intensity and turbulent stress were characterized, and their relationships were concluded.In the bend-turbulent-flow core region, longitudinal and lateral turbulent-intensity distributions are coincident with linear distribution, but in near-wall region are coincident with the Gamma distribution.Vertical turbulent intensity distributions are coincident with the Rayleigh distribution.Herein, it is concluded that the bend turbulence is anisotropic.
Laser Doppler velocimetry measurement of turbulent bubbly channel flow
So, S.; Takagi, S.; Matsumoto, Y. [Department of Mechanical Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656 (Japan); Morikita, H. [Morikita Shuppan Co. Ltd, 1-4-11,Fujimi Chiyoda-ku, Tokyo 102-0071 (Japan)
2002-07-01
Measurements of the turbulence properties of gas-liquid bubbly flows with mono-dispersed 1-mm-diameter bubbles are reported for upward flow in a rectangular channel. Bubble size and liquid-phase velocity were measured using image-processing and laser Doppler velocimetry (LDV), respectively. A description is given of the special arrangements for two-dimensional LDV needed to obtain reliable bubbly flow data, in particular the configuration of the optical system, the distinction of signals from the bubbles and liquid phase. To create the mono-dispersed bubbles, a small amount of surfactant (3-pentanol of 20 ppm) was added to the flow. Whilst this caused a drastic change in bubble size distribution and flow field, it did not affect the turbulence properties of the single-phase flow. In this study, experiments with three different bulk Reynolds numbers (1,350, 4,100, 8,200) were conducted with void fractions less than 1.2%. In all three cases, there was a very high accumulation of bubbles near the wall with bubble slip at the wall. The mean velocity profile of the liquid phase was steeper near the wall owing to the driving force of buoyant bubbles, and the streamwise turbulent intensity in the vicinity of the wall was enhanced. Furthermore the mean velocity profiles of the liquid phase were flattened in the wide region around the channel center. This region was lifted up by the bubble sheet near the wall, giving it a plug-like flow structure. In addition, the turbulent fluctuation and Reynolds stress in the liquid phase are very much suppressed in this region. This strong preferential accumulation near the wall produces the dramatic change of the whole flow structure. (orig.)
Laser Doppler velocimetry measurement of turbulent bubbly channel flow
So, S.; Morikita, H.; Takagi, S.; Matsumoto, Y.
2002-05-01
Measurements of the turbulence properties of gas-liquid bubbly flows with mono-dispersed 1-mm-diameter bubbles are reported for upward flow in a rectangular channel. Bubble size and liquid-phase velocity were measured using image-processing and laser Doppler velocimetry (LDV), respectively. A description is given of the special arrangements for two-dimensional LDV needed to obtain reliable bubbly flow data, in particular the configuration of the optical system, the distinction of signals from the bubbles and liquid phase. To create the mono-dispersed bubbles, a small amount of surfactant (3-pentanol of 20 ppm) was added to the flow. Whilst this caused a drastic change in bubble size distribution and flow field, it did not affect the turbulence properties of the single-phase flow. In this study, experiments with three different bulk Reynolds numbers (1,350, 4,100, 8,200) were conducted with void fractions less than 1.2%. In all three cases, there was a very high accumulation of bubbles near the wall with bubble slip at the wall. The mean velocity profile of the liquid phase was steeper near the wall owing to the driving force of buoyant bubbles, and the streamwise turbulent intensity in the vicinity of the wall was enhanced. Furthermore the mean velocity profiles of the liquid phase were flattened in the wide region around the channel center. This region was lifted up by the bubble sheet near the wall, giving it a plug-like flow structure. In addition, the turbulent fluctuation and Reynolds stress in the liquid phase are very much suppressed in this region. This strong preferential accumulation near the wall produces the dramatic change of the whole flow structure.
Turbulent transport measurements in a cold model of GT-burner at realistic flow rates
Gobyzov Oleg
2016-01-01
Full Text Available In the present work simultaneous velocity field and passive admixture concentration field measurements at realistic flow-rates conditions in a non-reacting flow in a model of combustion chamber with an industrial mixing device are reported. In the experiments for safety reasons the real fuel (natural gas was replaced with neon gas to simulate stratification in a strongly swirling flow. Measurements were performed by means of planar laser-induced fluorescence (PLIF and particle image velocimetry technique (PIV at Reynolds number, based on the mean flow rate and nozzle diameter, ≈300 000. Details on experimental technique, features of the experimental setup, images and data preprocessing procedures and results of performed measurements are given in the paper. In addition to the raw velocity and admixture concentration data in-depth evaluation approaches aimed for estimation of turbulent kinetic energy (TKE components, assessment of turbulent Schmidt number and analysis of the gradient closure hypothesis from experimental data are presented in the paper.
Wang Li-bing; Liu Yu-lu; Qiu Xiang
2003-01-01
In this paper, the turbulence characteristics were numerically investigated in an asymmetric turbulent channel flow and the computational results were compared with the relevant experimental data. It shows that the results are consistent with the experiments and there exist Counter-Gradient Momentum Transport(CGMT) phenomena in the central region near the smooth wall, and this region is as large as 6 percent of the channel width. In addition, a region, in which Counter-Gradient-Transport (CGT) phenomena occur more evidently, is found close to the rough wall. These results can help to gain a deeper insight into the mechanism of CGT phenomena.
Lagrangian statistics and flow topology in forced 2-D turbulence
Kadoch, B. [Universite d' Aix-Marseille; Del-Castillo-Negrete, Diego B [ORNL; Bos, W.J.T. [CNRS, Ecole Centrale de Lyon, Universite Claude Bernard Lyon; Schneider, Kai [Universite d' Aix-Marseille
2011-01-01
A study of the relationship between Lagrangian statistics and flow topology in fluid turbulence is presented. The topology is characterized using the Weiss criterion, which provides a conceptually simple tool to partition the flow into topologically different regions: elliptic (vortex dominated), hyperbolic (deformation dominated), and intermediate (turbulent background). The flow corresponds to forced two-dimensional Navier-Stokes turbulence in doubly periodic and circular bounded domains, the latter with no-slip boundary conditions. In the double periodic domain, the probability density function (pdf) of the Weiss field exhibits a negative skewness consistent with the fact that in periodic domains the flow is dominated by coherent vortex structures. On the other hand, in the circular domain, the elliptic and hyperbolic regions seem to be statistically similar. We follow a Lagrangian approach and obtain the statistics by tracking large ensembles of passively advected tracers. The pdfs of residence time in the topologically different regions are computed introducing the Lagrangian Weiss field, i.e., the Weiss field computed along the particles' trajectories. In elliptic and hyperbolic regions, the pdfs of the residence time have self-similar algebraic decaying tails. In contrast, in the intermediate regions the pdf has exponential decaying tails. The conditional pdfs (with respect to the flow topology) of the Lagrangian velocity exhibit Gaussian-like behavior in the periodic and in the bounded domains. In contrast to the freely decaying turbulence case, the conditional pdfs of the Lagrangian acceleration in forced turbulence show a comparable level of intermittency in both the periodic and the bounded domains. The conditional pdfs of the Lagrangian curvature are characterized, in all cases, by self-similar power-law behavior with a decay exponent of order - 2.
Modelling of pressure-strain correlation in compressible turbulent flow
Siyuan Huang; Song Fu
2008-01-01
Previous studies carried out in the early 1990s conjectured that the main compressible effects could be associated with the dilatational effects of velocity fluctuation.Later,it was shown that the main compressibility effect came from the reduced pressure-strain term due to reduced pressure fluctuations.Although better understanding of the compressible turbulence is generally achieved with the increased DNS and experimental research effort,there are still some discrepancies among these recent findings.Analysis of the DNS and experimental data suggests that some of the discrepancies are apparent if the compressible effect is related to the turbulent Mach number,Mt.From the comparison of two classes of compressible flow,homogenous shear flow and inhomogeneous shear flow(mixing layer),we found that the effect of compressibility on both classes of shear flow can be characterized in three categories corresponding to three regions of turbulent Mach numbers:the low-Mt,the moderate-Mt and high-Mt regions.In these three regions the effect of compressibility on the growth rate of the turbulent mixing layer thickness is rather different.A simple approach to the reduced pressure-strain effect may not necessarily reduce the mixing-layer growth rate,and may even cause an increase in the growth rate.The present work develops a new second-moment model for the compressible turbulence through the introduction of some blending functions of Mt to account for the compressibility effects on the flow.The model has been successfully applied to the compressible mixing layers.
Modification and Improvement of Software for Modeling Multidimensional Reacting Fuel Flow.
1986-11-01
block Gauss- Seidel method for F(u) = 0. NLGMR provides a means of accelerating the TEACH iterations as follows (see [16]). - . -Throughout we denote...properties and thermodynamic coefficients. and the velocity-dependent turbulent transport properties. A multistage variation of the block Gauss- Seidel ... method is used to solve this nonlinear ;vstem. The outermost stage consists of cycling between a Poisson-like pressure correction J (pilation (derived
Contaminant dispersal in bounded turbulent shear flow
Wallace, J.M.; Bernard, P.S.; Chiang, K.F.; Ong, L. [Univ. of Maryland, College Park, MD (United States)
1995-12-31
The dispersion of smoke downstream of a line source at the wall and at y{sup +} = 30 in a turbulent boundary layer has been predicted with a non-local model of the scalar fluxes {bar u}c and {bar v}c. The predicted plume from the wall source has been compared to high Schmidt number experimental measurements using a combination of hot-wire anemometry to obtain velocity component data synchronously with concentration data obtained optically. The predicted plumes from the source at y{sup +} = 30 and at the wall also have been compared to a low Schmidt number direct numerical simulation. Near the source, the non-local flux models give considerably better predictions than models which account solely for mean gradient transport. At a sufficient distance downstream the gradient models gives reasonably good predictions.
Tsuji, Takuya; Yokomine, Takehiko; Shimizu, Akihiko
2002-11-01
We have been engaged in the development of multi-scale adaptive simulation technique for incompressible turbulent flow. This is designed as that important scale components in the flow field are detected automatically by lifting wavelet and solved selectively. In conventional incompressible scheme, it is very common to solve Poisson equation of pressure to meet the divergence free constraints of incompressible flow. It may be not impossible to solve the Poisson eq. in the adaptive way, but this is very troublesome because it requires generation of control volume at each time step. We gave an eye on weakly compressible model proposed by Bao(2001). This model was derived from zero Mach limit asymptotic analysis of compressible Navier-Stokes eq. and does not need to solve the Poisson eq. at all. But it is relatively new and it requires demonstration study before the combination with the adaptation by wavelet. In present study, 2-D and 3-D Backstep flow were selected as test problems and applicability to turbulent flow is verified in detail. Besides, combination of adaptation by wavelet with weakly compressible model towards the adaptive turbulence simulation is discussed.
The PDF method for turbulent combustion
Pope, S. B.
1991-01-01
Probability Density Function (PDF) methods provide a means of calculating the properties of turbulent reacting flows. They have been successfully applied to many turbulent flames, including some with finite rate kinetic effects. Here the methods are reviewed with an emphasis on computational issues and their application to turbulent combustion.
Heat Transfer Enhancement in Turbulent Flows by Blocked Surfaces
Onur YEMENİCİ
2013-04-01
Full Text Available In this study, the heat transfer analyses over flat and blocked surfaces were carried out in turbulent flow under the influence of the block height. A constant-temperature hot wire anemometer was used to the velocity and turbulent intensity measurements, while temperature values were measured by copper-constantan thermocouples. The average Stanton numbers for block heights of 15 and 25 mm were higher than those of flat surface by %38 and %84, respectively. The results showed that the presence of the blocks increased the heat transfer and the enhancement rose with block heights
A correction to collision rates of droplets in turbulent flows
Zhang, Huang
2016-01-01
This paper makes a correction to the collision rates of small droplets in turbulent fluid derived by Saffman and Turner(1956). Not only the distortion but also the rotation of the fluid is taken into account between two close droplets. A rotation reference is fixed on one drop, and the fluxes of the other drops moving towards the fixed one are carried out based on this new reference. The behaviors of turbulent flow are analyzed within the smallest eddies under the rotation reference, and a correction is made to the collision rates by multiplying a factor sqrt(2).
FLOW CHARACTERISTICS OF TURBULENT JETS INJECTED OBLIQUELY IN CROSSFLOW
GUO Ting-ting; LI Shao-hua; XU Zhong
2004-01-01
Numerical simulation has been performed to investigate the characteristics of the turbulent jet issuing obliquely into a crossflow, at the injection angles of 90° and 60°. The SIMPLEC algorithm in the body-fitted coordinates and the wall-function method using the RNG k-ε turbulence model have been adopted to simulate this flow at two jet-to-crossflow velocity ratios, 2 and 4. The numerical results are good agreement with the experimental measurements, the jet trajectories have been given, the separation events in the lee of the jet exit have been found, and the forming mechanism has been analyzed.
Shadid, J.N.; Hutchinson, S.A.; Moffat, H.K.; Hendrickson, B.; Leland, R.W. [Sandia National Labs., Albuquerque, NM (United States); Hennigan, G.L. [New Mexico State Univ., Las Cruces, NM (United States)
1994-12-31
Many scientific and engineering applications require a detailed analysis of complex systems with strongly coupled fluid flow, thermal energy transfer, mass transfer and non-equilibrium chemical reactions. Here they describe the performance of a newly developed application code, SALSA, designed to simulate these complex flows on large-scale parallel machines such as the Intel Paragon. SALSA uses 3D unstructure finite element methods to model geometrically complex flow systems. Fully implicit time integration multicomponent mass transport and general gas phase and surface species non-equilibrium chemical kinetics are employed. Using these techniques they have obtained over 65 Gflop/s on a 3D chemically reacting flow CVD problem for Silicon Carbide (SiC) deposition. This represents 46% of the peak performance of the 1904 node Intel Paragon, an outstanding computational rate in view of the required unstructured data communication.
Direction of scalar transport in turbulent channel flow
Srinivasan, Chiranth; Papavassiliou, Dimitrios V.
2011-11-01
The concept of reverse diffusion, introduced by Corrsin to describe the motion of particles as they move towards a location in the flow field, is fundamental to the understanding of mixing. In this work, direct numerical simulations in conjunction with the tracking of scalar markers are utilized in infinitely long channels to study the principal direction of transport of heat (or mass) for both forwards and backwards single particle dispersion. The viscous sub-layer, the transition region (between the viscous sub-layer and the logarithmic region), and the logarithmic region of a Poiseuille flow and a plane Couette flow channel are studied. Fluctuating velocities of scalar markers captured in these regions are used to obtain the full autocorrelation coefficient tensor forwards and backwards with time. The highest eigenvalue of the velocity correlation coefficient tensor quantifies the highest amount of turbulent heat transport, while the corresponding eigenvector points to the main direction of transport. Different Prandtl number, Pr, fluids are simulated for the two types of flow. It is found that the highest eigenvalues are higher in the case of backwards dispersion compared to the case of forwards dispersion for any Pr, in both flow cases. The principal direction for backwards and forwards dispersion is different than for forwards dispersion, for all Pr, and in all flow regions for both flows. Fluids with lower Pr behave different than the higher Pr fluids because of increased molecular diffusion effects. The current study also establishes an interesting analogy of turbulent dispersion to optics defining the turbulent dispersive ratio, a parameter that can be used to identify the differences in the direction of turbulent heat transport between forwards and backwards dispersion. A spectral analysis of the auto-correlation coefficient for both forwards and backwards dispersion shows a universal behavior with slope of -1 at intermediate frequencies.
On the turbulence-particles interaction in turbulent two-phase flows
Mostafa, A. A.; Mongia, H. C.
1986-01-01
A mathematically simple two-equation turbulence model for two-phase flows has been developed to take into account the extra energy dissipation due to the presence of the particles with the carrier phase. The transport equations of mass, momentum, and kinetic energy and its dissipation rate of the carrier phase using an Eulerian formulation are presented. The Lagrangian approach is used to solve for the particles using the Monte Carlo technique. These equations are solved numerically using a finite difference technique to predict a turbulent round gaseous jet laden with solid particles. The predicted mean and turbulence quantities of the carrier and dispersed phases are in good agreement with the recent experimental data.
Turbulent-laminar patterns in plane Poiseuille flow
Tuckerman, Laurette S; Schrobsdorff, Hecke; Schneider, Tobias M; Gibson, John F
2014-01-01
Turbulent-laminar banded patterns in plane Poiseuille flow are studied via direct numerical simulations in a tilted and translating computational domain using a parallel version of the pseudospectral code Channelflow. 3D visualizations via the streamwise vorticity of an instantaneous and a time-averaged pattern are presented, as well as 2D visualizations of the average velocity field and the turbulent kinetic energy. Simulations for Reynolds numbers descending from 2300 to 700 show the gradual development from uniform turbulence to a pattern with wavelength 20 half-gaps near Re=1900, to a pattern with wavelength 40 near Re=1300 and finally to laminar flow near Re=800. These transitions are tracked quantitatively via diagnostics using the amplitude and phase of the Fourier transform and its probability distribution. The propagation velocity of the pattern is approximately that of the mean flux and is a decreasing function of Reynolds number. Examination of the time-averaged flow shows that a turbulent band is ...
Interaction of turbulent deflagrations with representative flow obstacles
Durst, B.; Ardey, N.; Mayinger, F. [Lehrstul a fuer Thermodynamik, Technische Universitat Munchen (Germany)
1997-03-01
In the case of a gradual release of hydrogen in the course of an assumed, severe accident in a light water reactor, the combustion will normally start out as a slow deflagration. Acceleration of an initially slow flame due to interactions of chemical kinetics and turbulent heat and mass transfer can result in very high flame speeds. Therefore, in order to assess hydrogen mitigation techniques, detailed knowledge about flame acceleration and interaction of flames with obstacles is required. The reported investigations are aimed at the investigation of the mechanisms responsible for turbulent flame acceleration and improving present correlations for estimates and models for numerical simulations of hydrogen combustion processes. A medium-scale square cross-section setup is employed, using flow obstacles with shapes representative for reactor containments. The global flame speed is deduced from measurements using thermocouples, pressure transducers and photodiodes. Measurements using a two-component LDA-system are being carried through in order to correlate global flame spread and local turbulence parameters. Results indicate that low blockage-ratio obstacles only marginally influence the flame, as disturbances which are induced remain local to the vicinity of the obstacle and die out very quickly downstream thereof. Flow visualizations by means of a Schlieren setup indicate very complex flow structures in the vicinity of obstacles. The results are being used to validate turbulent reaction models. A model based on probability density functions (pdf) of assumed shape has been developed and initial calculations are presented. (author)
Turbulent Recirculating Flows in Isothermal Combustor Geometries
Lilley, D.; Rhode, D.
1985-01-01
Computer program developed that provides mathematical solution to design and construction of combustion chambers for jet engines. Improved results in areas of combustor flow fields accomplished by this computerprogram solution, cheaper and quicker than experiments involving real systems for models.
Flow reversals in thermally driven turbulence
Sugiyama, Kazuyasu; Stevens, Richard J A M; Chan, Tak Shing; Zhou, Sheng-Qi; Xi, Heng-Dong; Sun, Chao; Grossmann, Siegfried; Xia, Ke-Qing; Lohse, Detlef; 10.1103/PhysRevLett.105.034503
2011-01-01
We analyze the reversals of the large scale flow in Rayleigh-B\\'enard convection both through particle image velocimetry flow visualization and direct numerical simulations (DNS) of the underlying Boussinesq equations in a (quasi) two-dimensional, rectangular geometry of aspect ratio 1. For medium Prandtl number there is a diagonal large scale convection roll and two smaller secondary rolls in the two remaining corners diagonally opposing each other. These corner flow rolls play a crucial role for the large scale wind reversal: They grow in kinetic energy and thus also in size thanks to plume detachments from the boundary layers up to the time that they take over the main, large scale diagonal flow, thus leading to reversal. Based on this mechanism we identify a typical time scale for the reversals. We map out the Rayleigh number vs Prandtl number phase space and find that the occurrence of reversals very sensitively depends on these parameters.
Control of fully turbulent pipe flow
Kuehnen, Jakob; Hof, Bjoern
2013-11-01
We present a novel, very simple passive control technique, where a local modification of the flow profile by means of a stationary obstacle leads to full relaminarisation downstream. Relaminarisation is achieved about 50 diameters downstream of the control point. Since, in a smooth straight pipe, the flow remains laminar from that position significant reduction in skin friction can be accomplished. High-speed stereoscopic particle image velocimetry (S-PIV) has been used to investigate and capture the development of the transitional flow downstream the obstacle. We will present S-PIV measurements as well as pressure drop measurements and videos of the development of the flow during relaminarisation. The guiding fundamental principle behind our approach to control the velocity profile will be explained and discussed.
National Aeronautics and Space Administration — ZONA proposes a Unified Gas Kinetic Scheme (UGKS) to cover the full Knudsen number range from the continuum flow to free molecular flow that can simultaneously exist...
Lee, Hyunchang; Böhm, Benjamin; Sadiki, Amsini; Dreizler, Andreas
2016-07-01
Turbulent mixing is highly important in flows that involve heat and mass transfer. Information on turbulent heat flux is needed to validate the mixing models implemented in numerical simulations. The calculation of turbulent heat fluxes requires instantaneous information on temperature and velocity. Even using minimally intrusive laser optical methods, simultaneous measurement of temperature and velocity is still a challenge. In this study, thermographic phosphor particles are used for simultaneous thermometry and velocimetry: conventional particle image velocimetry is combined with temperature-dependent spectral shifts of BAM:Eu2+ phosphor particles upon UV excitation. The novelty of this approach is the analysis of systematic errors and verification using the well-known properties of a heated turbulent jet issuing into a low velocity, cold coflow. The analysis showed that systematic errors caused by laser fluence, multiple scattering, or preferential signal absorption can be reduced such that reliable measurement of scalar fluxes becomes feasible, which is a prerequisite for applying the method to more complex heat transfer problems.
Magnetic flux concentration and zonal flows in magnetorotational instability turbulence
Bai, Xue-Ning [Institute for Theory and Computation, Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-51, Cambridge, MA 02138 (United States); Stone, James M., E-mail: xbai@cfa.harvard.edu [Department of Astrophysical Sciences, Peyton Hall, Princeton University, Princeton, NJ 08544 (United States)
2014-11-20
Accretion disks are likely threaded by external vertical magnetic flux, which enhances the level of turbulence via the magnetorotational instability (MRI). Using shearing-box simulations, we find that such external magnetic flux also strongly enhances the amplitude of banded radial density variations known as zonal flows. Moreover, we report that vertical magnetic flux is strongly concentrated toward low-density regions of the zonal flow. Mean vertical magnetic field can be more than doubled in low-density regions, and reduced to nearly zero in high-density regions in some cases. In ideal MHD, the scale on which magnetic flux concentrates can reach a few disk scale heights. In the non-ideal MHD regime with strong ambipolar diffusion, magnetic flux is concentrated into thin axisymmetric shells at some enhanced level, whose size is typically less than half a scale height. We show that magnetic flux concentration is closely related to the fact that the turbulent diffusivity of the MRI turbulence is anisotropic. In addition to a conventional Ohmic-like turbulent resistivity, we find that there is a correlation between the vertical velocity and horizontal magnetic field fluctuations that produces a mean electric field that acts to anti-diffuse the vertical magnetic flux. The anisotropic turbulent diffusivity has analogies to the Hall effect, and may have important implications for magnetic flux transport in accretion disks. The physical origin of magnetic flux concentration may be related to the development of channel flows followed by magnetic reconnection, which acts to decrease the mass-to-flux ratio in localized regions. The association of enhanced zonal flows with magnetic flux concentration may lead to global pressure bumps in protoplanetary disks that helps trap dust particles and facilitates planet formation.
Large-eddy simulation of turbulent circular jet flows
Jones, S. C. [Georgia Inst. of Technology, Atlanta, GA (United States); Sotiropoulos, F. [Georgia Inst. of Technology, Atlanta, GA (United States); Sale, M. J. [Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
2002-07-01
This report presents a numerical method for carrying out large-eddy simulations (LES) of turbulent free shear flows and an application of a method to simulate the flow generated by a nozzle discharging into a stagnant reservoir. The objective of the study was to elucidate the complex features of the instantaneous flow field to help interpret the results of recent biological experiments in which live fish were exposed to the jet shear zone. The fish-jet experiments were conducted at the Pacific Northwest National Laboratory (PNNL) under the auspices of the U.S. Department of Energy’s Advanced Hydropower Turbine Systems program. The experiments were designed to establish critical thresholds of shear and turbulence-induced loads to guide the development of innovative, fish-friendly hydropower turbine designs.
Streamline segment scaling behavior in a turbulent wavy channel flow
Rubbert, A.; Hennig, F.; Klaas, M.; Pitsch, H.; Schröder, W.; Peters, N.
2017-02-01
A turbulent flow in a wavy channel was investigated by tomographic particle-image velocimetry measurements and direct numerical simulations. To analyze the turbulent structures and their scaling behavior in a flow undergoing favorable and adverse pressure gradients, the streamline segmentation method proposed by Wang (J Fluid Mech 648:183-203, 2010) was employed. This method yields joint statistical information about velocity fluctuations and length scale distributions of non-overlapping structures within the flow. In particular, the joint statistical properties are notably influenced by the pressure distribution. Previous findings from flat channel flows and synthetic turbulence simulations concerning the normalized segment length distribution could be reproduced and therefore appear to be largely universal. However, the mean streamline segment length of accelerating and decelerating segments varies within one wavelength typically elongating segments of the type which corresponds to the local mean flow. Furthermore, the local pressure gradient was found to significantly impact local joint streamline segmentation statistics as a main influence on their inherent asymmetry.
INVESTIGATION OF THE MEAN-FLOW SCALING AND TRIPPING EFFECT ON FULLY DEVELOPED TURBULENT PIPE FLOW
无
2003-01-01
Fully developed turbulence measurements in pipe flow were made in the Reynolds number range from 10×103 to 350×103 with hot-wire anemometer and a Pitot tube. Comparisons were made with the experimental results of previous researchers. The mean velocity profile and the turbulent intensity in the experiments indicate that for the mean velocity profile, in the fully developed turbulent pipe flow, von Kármán's constant κ is a function of the Reynolds number, I.e. Κ increases slowly with the Reynolds number. For turbulent pipe flow, the outer limit depends on whether the Kármán number R+ is greater or less than 850 in the centerline volocity profile: a log law exists for 850＜R+＜1750 in the experiment, and von Kármán's constant κ is shown to be 0.408. Under the effects of the test trip at the inlet, fully developed turbulence was obtained in pipe flow at lower Reynolds number when the entrance length (xD) was larger than 40. In the experiment it was also found that turbulence quantities in pipe flow remain independent of the upstream conditions when the trip blockage ratio is higher than 20%, and the comparison with channel water flow was also performed.
Numerical modelling of a turbulent bluff-body flow with Reynolds stress turbulent models
LI Guoxiu; Dirk ROEKAERTS
2005-01-01
Numerical modelling of a turbulent bluff-body flow has been performed using differential Reynolds stress models (DRSMs). To clarify the applicability of the existing DRSMs in this complex flow, several typical DRSMs, including LRR-IP model, JM model, SSG model, as well as a modified LRR-IP model, have been validated and evaluated. The performance difference between various DRSMs is quite significant. Most of the above mentioned DRSMs cannot provide overall satisfactory predictions for this challenging test case. Motivated by the deficiency of the existing approaches, a new modification of LRR-IP model has been proposed. A very significant improvement of the prediction of flow field is obtained.
Strange attractors in weakly turbulent Couette-Taylor flow
Brandstater, A.; Swinney, Harry L.
1987-01-01
An experiment is conducted on the transition from quasi-periodic to weakly turbulent flow of a fluid contained between concentric cylinders with the inner cylinder rotating and the outer cylinder at rest. Power spectra, phase-space portraits, and circle maps obtained from velocity time-series data indicate that the nonperiodic behavior observed is deterministic, that is, it is described by strange attractors. Various problems that arise in computing the dimension of strange attractors constructed from experimental data are discussed and it is shown that these problems impose severe requirements on the quantity and accuracy of data necessary for determining dimensions greater than about 5. In the present experiment the attractor dimension increases from 2 at the onset of turbulence to about 4 at a Reynolds number 50-percent above the onset of turbulence.
Wall pressure signatures of turbulent flow over longitudinal
Abdulbari Hayder A.
2016-01-01
Full Text Available Five triangular riblets longitudinal in the streamwise direction have been studied experimentally. The riblets have pick to pick spaced (s equal to 1000 μm and with groove height to space ratio (h/s 0.4, 0.6, 0.8 and 1. The tests were conducted in a full turbulence water channel on a flat plate for Reynolds numbers 13000 to 53000 based on channel hydraulic diameter. Pressure drop was measured using pressure transmitter gauge with pressure tap points of 12.7 mm in diameter were provided at the bottom of the channel. The main purpose of the present study is to investigate the response of turbulent flow to longitudinal grooves of triangular shaped riblets and compare the effect of the turbulence structure over smoothed and grooved surfaces with pressure drop measurements. 10.20 was the maximum drag reduction appear at h/s equal to (1.
SIMULATION OF TURBULENT FLOW AND HEAT TRANSFER OVER A BACKWARD -FACING STEP WITH RIBS TURBULATORS
Khudheyer S Mushatet
2011-01-01
Full Text Available Simulation is presented for a backward facing step flow and heat transfer inside a channel with ribs turbulators. The problem was investigated for Reynolds numbers up to 32000. The effect of a step height, the number of ribs and the rib thickness on the flow and thermal field were investigated. The computed results are presented as streamlines counters, velocity vectors and graphs of Nusselt number and turbulent kinetic energy variation. A control volume method employing a staggered grid techniques was imposed to discretize the governing continuity, full Navier Stockes and energy equations. A computer program using a SIMPLE algorithm was developed to handle the considered problem. The effect of turbulence was modeled by using a k-є model with its wall function formulas. The obtained results show that the strength and size of the re-circulation zones behind the step are increased with the increase of contraction ratio(i.e. with the increase of a step height. The size of recirculation regions and the reattachment length after the ribs are decreased with increasing of the contraction ratio. Also the results show that the Reynolds number and contraction ratio have a significant effect on the variation of turbulent kinetic energy and Nusselt number
Experiments on supersonic turbulent flow development in a square duct
Gessner, F. B.; Ferguson, S. D.; Lo, C. H.
1986-01-01
The nature of supersonic, turbulent, adiabatic-wall flow in a square duct is investigated experimentally over a development length of x/D between 0 and 20 for a uniform flow, Mach 3.9 condition at the duct inlet. Initial discussion centers on the duct configuration itself, which was designed specifically to minimize wave effects and nozzle-induced distortion in the flow. Total pressure contours and local skin friction coefficient distributions are presented which show that the flow develops in a manner similar to that observed for the incompressible case. In particular, undulations exist in total pressure contours within the cross plane and in transverse skin friction coefficient distributions, which are indicative of the presence of a well-defined secondary flow superimposed upon the primary flow. The results are analyzed to show that local law-of-the-wall behavior extends well into the corner region, which implies that wall functions conventionally applied in two-equation type turbulence models, when suitably defined for compressible flow, may also be applied to supersonic streamwise corner flows.
Transition to turbulence for flows without linear criticality
Nagata, Masato
2010-12-01
It is well known that plane Couette flow (PCF) and pipe flow (PF) are linearly stable against arbitrary three-dimensional perturbations at any finite Reynolds number, so that transitions from the basic laminar states, if they exist, must be abrupt. Due to this lack of linear criticality, weakly nonlinear analysis does not work in general and numerical approaches must be resorted to. It is only recently that non-trivial nonlinear states for these flows have been discovered numerically at finite Reynolds number as solutions bifurcating from infinity. The onset of turbulence in a subcritical transition is believed to be related to the appearance of steady/travelling wave states caused by disturbances of finite amplitude that take the flows out of the basin of attraction of the laminar state in phase space. In this paper, we introduce other flows that, in a similar way to PCF and PF, exhibit no linear critical point for the laminar states, namely flow in a square duct and sliding Couette flow in an annulus with a certain range of gap ratio. We shall show our recent numerical investigations on these flows where nonlinear travelling wave states are found for the first time by a homotopy approach. We believe that these states constitute the skeleton around which a time-dependent trajectory in the phase space is organized and help in understanding non-equilibrium turbulent processes.
Modelling and simulation of turbulence and heat transfer in wall-bounded flows
Popovac, M.
2006-01-01
At present it is widely accepted that there is no universal turbulence model, i.e. no turbulence model can give acceptably good predictions for all turbulent flows that are found in nature or engineering. Every turbulence model is based on certain assumptions, and hence it is aimed at certain type o
Particle dispersion models and drag coefficients for particles in turbulent flows
Crowe, C. T.; Chung, J. N.; Troutt, T. R.
1988-01-01
Some of the concepts underlying particle dispersion due to turbulence are reviewed. The traditional approaches to particle dispersion in homogeneous, stationary turbulent fields are addressed, and recent work on particle dispersion in large scale turbulent structures is reviewed. The state of knowledge of particle drag coefficients in turbulent gas-particle flows is also reviewed.
Mixing efficiency of turbulent patches in stably stratified flows
Garanaik, Amrapalli; Venayagamoorthy, Subhas Karan
2016-11-01
A key quantity that is essential for estimating the turbulent diapycnal (irreversible) mixing in stably stratified flow is the mixing efficiency Rf*, which is a measure of the amount of turbulent kinetic energy that is irreversibly converted into background potential energy. In particular, there is an ongoing debate in the oceanographic mixing community regarding the utility of the buoyancy Reynolds number (Reb) , particularly with regard to how mixing efficiency and diapycnal diffusivity vary with Reb . Specifically, is there a universal relationship between the intensity of turbulence and the strength of the stratification that supports an unambiguous description of mixing efficiency based on Reb ? The focus of the present study is to investigate the variability of Rf* by considering oceanic turbulence data obtained from microstructure profiles in conjunction with data from laboratory experiments and DNS. Field data analysis has done by identifying turbulent patches using Thorpe sorting method for potential density. The analysis clearly shows that high mixing efficiencies can persist at high buoyancy Reynolds numbers. This is contradiction to previous studies which predict that mixing efficiency should decrease universally for Reb greater than O (100) . Funded by NSF and ONR.
Sanghyeon Kim
2017-06-01
Full Text Available In this study, cavitation flow of hydrofoils is numerically investigated to characterize the effects of turbulence models on cavitation-flow patterns and the corresponding radiated sound waves. The two distinct flow conditions are considered by varying the mean flow velocity and angle of attack, which are categorized under the experimentally observed unstable or stable cavitation flows. To consider the phase interchanges between the vapor and the liquid, the flow fields around the hydrofoil are analyzed by solving the unsteady compressible Reynolds-averaged Navier–Stokes equations coupled with a mass-transfer model, also referred to as the cavitation model. In the numerical solver, a preconditioning algorithm with dual-time stepping techniques is employed in generalized curvilinear coordinates. The following three types of turbulence models are employed: the laminar-flow model, standard k − ε turbulent model, and filter-based model. Hydro-acoustic field formed by the cavitation flow of the hydrofoil is predicted by applying the Ffowcs Williams and Hawkings equation to the predicted flow field. From the predicted results, the effects of the turbulences on the cavitation flow pattern and radiated flow noise are quantitatively assessed in terms of the void fraction, sound-pressure-propagation directivities, and spectrum.
Kim, Sanghyeon; Cheong, Cheolung; Park, Warn-Gyu
2017-06-01
In this study, cavitation flow of hydrofoils is numerically investigated to characterize the effects of turbulence models on cavitation-flow patterns and the corresponding radiated sound waves. The two distinct flow conditions are considered by varying the mean flow velocity and angle of attack, which are categorized under the experimentally observed unstable or stable cavitation flows. To consider the phase interchanges between the vapor and the liquid, the flow fields around the hydrofoil are analyzed by solving the unsteady compressible Reynolds-averaged Navier-Stokes equations coupled with a mass-transfer model, also referred to as the cavitation model. In the numerical solver, a preconditioning algorithm with dual-time stepping techniques is employed in generalized curvilinear coordinates. The following three types of turbulence models are employed: the laminar-flow model, standard k - ɛ turbulent model, and filter-based model. Hydro-acoustic field formed by the cavitation flow of the hydrofoil is predicted by applying the Ffowcs Williams and Hawkings equation to the predicted flow field. From the predicted results, the effects of the turbulences on the cavitation flow pattern and radiated flow noise are quantitatively assessed in terms of the void fraction, sound-pressure-propagation directivities, and spectrum.
Bridges, Craig; Rajagopal, K R
2010-01-01
We study the flow of a shear-thinning, chemically-reacting fluid that could be used to model the flow of the synovial fluid. The actual geometry where the flow of the synovial fluid takes place is very complicated, and therefore the governing equations are not amenable to simple mathematical analysis. In order to understand the response of the model, we choose to study the flow in a simple geometry. While the flow domain is not a geometry relevant to the flow of the synovial fluid in the human body it yet provides a flow which can be used to assess the efficacy of different models that have been proposed to describe synovial fluids. We study the flow in the annular region between two cylinders, one of which is undergoing unsteady oscillations about their common axis, in order to understand the quintessential behavioral characteristics of the synovial fluid. We use the three models suggested by Hron et al. [ J. Hron, J. M\\'{a}lek, P. Pust\\v{e}jovsk\\'{a}, K. R. Rajagopal, On concentration dependent shear-thinni...
Numerical Simulation of Turbulent Flow in Concentric Annuli
Boersma, B.J.; Breugem, W.-P.
2010-01-01
In this paper we consider a fully developed turbulent flow in a round pipe with a small inner annulus. The diameter of the inner annulus is less than 10% of the diameter of the outer pipe. As a consequence, the surface area of the inner pipe compared to the outer pipe is small. The friction exerted by the wall on the flow is proportional to the surface area and the wall shear stress. Due to the small surface area of the inner annulus the additional stress on the flow due to the presence of th...
An alternative view on dissipation in turbulent flows
Zalczer, Gilbert
2016-01-01
An original experimental setup has been elaborated in order to get a better view of turbulent flows in a von Karman geometry. The availability of a very fast camera allowed to follow in time the evolution of the flows. A surprising finding is that the development of smaller whorls ceases earlier than expected and the aspect of the flows remains the same above Reynolds number of a few thousand. This fact provides an explanation of the constancy of the reduced dissipation in the same range without the need of singularity. Its cause could be in relation with the same type of behavior observed in a rotating frame.
Turbulent Flow over Small Amplitude Solid Waves
1984-01-01
7. Annubar flow meter 8. Butterfly throttling valve 9. Removable blanking plate 10. Diaphragm valve 11. Small pump 12. By pass diaphragm valve...monitored by using an annubar connected to either a mercury or mirriam oil filled manometer. (b) Test Section 2 a A wave with a ratio of 0.014
Closed Loop Control and Turbulent Flows
2005-10-01
first described in some detail by Ingard [8], but re-discovered, developed, and applied to problems in flow control by Glezer and co-workers [19, 20... Ingard . On the theory and design of acoustic resonators. J. Acoustical Soc. of America, 25(6):1037-1060, 1953. [9] J. Kim, P. Moin, and R. Moser
Numerical simulation of turbulent flow in corrugated pipes
Azevedo, Henrique S. de; Morales, Rigoberto E.M.; Franco, Admilson T.; Junqueira, Silvio L.M.; Erthal, Raul H. [Universidade Tecnologica Federal do Parana (UTFPR), Curitiba, PR (Brazil). Dept. Academico de Mecanica (DAMEC)]. E-mails: rique.stel@gmail.com; rmorales@utfpr.edu.br; admilson@utfpr.edu.br; silvio@utfpr.edu.br; rherthal@utfpr.edu.br; Goncalves, Marcelo de Albuquerque Lima [PETROBRAS S.A., Rio de Janeiro, RJ (Brazil). Centro de Pesquisas (CENPES)]. E-mail: marcelog@petrobras.com.br
2008-07-01
Corrugated pipes are used in various engineering applications such heat exchangers and oil transport. In most cases these pipes consist of periodically distributed grooves at the duct inner wall. Numerical and experimental works reported the influence of grooves height and length in the turbulent flow by inspection of several turbulent properties such as velocity fluctuations and Reynolds stress. The present article aims to investigate the influence of grooves height and length in the global friction factor of turbulent flow through periodically corrugated pipes. Mass and momentum conservation equations are revised and specific boundary conditions are set to characterize a periodic fully developed regime in a single axisymmetric bidimensional module which represents the periodically corrugated duct geometry. The set of algebraic equations is discretized through the Finite Volume Method, with the Hybrid interpolation scheme applied to the convective terms, and solved using the commercial software PHOENICS CFD. The simulation of turbulent, incompressible, isothermal and single-phase flow is considered. The algebraic turbulence model LVEL is used. Four geometric configurations are assumed, including grooves height and length variations, in order to compare their influence on the friction factor. The obtained numerical friction factors show good agreement with previous experimental results, specially for Reynolds numbers over 20000. Numerical results for corrugated pipes compared to the Blasius smooth pipe correlation shows that the friction factor increases compared to smooth pipes, and such increase is more significant for higher Reynolds numbers and for larger grooves as well. These trends appear to be related to an enhancement of the momentum transport over the corrugated wall due to the recirculating pattern inside the grooves, in accordance with previous experimental works (author)
Turbulent scales of dilute particle-laden flows in microgravity
Groszmann, Daniel E.; Rogers, Chris B.
2004-12-01
The work described in this paper attempts to characterize the effects of inertia, isolated from gravity, on the dispersion of solid particles in a turbulent air flow. The experiment consisted of releasing particles of various sizes in an enclosed box of fan-generated, near-homogeneous, isotropic, and stationary turbulent airflow and examining the particle behavior in a microgravity environment. The turbulence box was characterized in ground-based experiments using laser Doppler velocimetry techniques. Microgravity was established by free floating the experiment apparatus during the parabolic trajectory of NASA's KC-135 reduced-gravity aircraft. The microgravity generally lasted about 20 s, with about 50 parabolas per flight and one flight per day over a testing period of four days. To cover a broad range of flow regimes of interest, particles with Stokes numbers St of about 1-100 were released in the turbulence box. The three-dimensional measurements of particle motion were made with a particle-tracking algorithm using a three-camera stereo imaging system. Digital photogrammetric techniques were used to determine the particle locations from the calibrated camera images. The epipolar geometry constraint identified matching particles from the three different camera views and a direct spatial intersection scheme determined the coordinates of particles in three-dimensional space. Since particle loadings were light, velocity and acceleration constraints allowed particles in a sequence of frames to be matched, resulting in particle tracks and dispersion measurements. The goal was to compare the dispersion of different Stokes number particles in zero gravity and thereby decouple the effects of gravity from inertia on the dispersion. Results show that higher inertia particles disperse less in isotropic, nondecaying turbulent flows under zero gravity, in agreement with current models. Measurements show that particles with St≈1 dispersed about ten times more than the St
Fractally Fourier decimated homogeneous turbulent shear flow in noninteger dimensions
Fathali, Mani; Khoei, Saber
2017-02-01
Time evolution of the fully resolved incompressible homogeneous turbulent shear flow in noninteger Fourier dimensions is numerically investigated. The Fourier dimension of the flow field is extended from the integer value 3 to the noninteger values by projecting the Navier-Stokes equation on the fractal set of the active Fourier modes with dimensions 2.7 ≤d ≤3.0 . The results of this study revealed that the dynamics of both large and small scale structures are nontrivially influenced by changing the Fourier dimension d . While both turbulent production and dissipation are significantly hampered as d decreases, the evolution of their ratio is almost independent of the Fourier dimension. The mechanism of the energy distribution among different spatial directions is also impeded by decreasing d . Due to this deficient energy distribution, turbulent field shows a higher level of the large-scale anisotropy in lower Fourier dimensions. In addition, the persistence of the vortex stretching mechanism and the forward spectral energy transfer, which are three-dimensional turbulence characteristics, are examined at changing d , from the standard case d =3.0 to the strongly decimated flow field for d =2.7 . As the Fourier dimension decreases, these forward energy transfer mechanisms are strongly suppressed, which in turn reduces both the small-scale intermittency and the deviation from Gaussianity. Besides the energy exchange intensity, the variations of d considerably modify the relative weights of local to nonlocal triadic interactions. It is found that the contribution of the nonlocal triads to the total turbulent kinetic energy exchange increases as the Fourier dimension increases.
Fractally Fourier decimated homogeneous turbulent shear flow in noninteger dimensions.
Fathali, Mani; Khoei, Saber
2017-02-01
Time evolution of the fully resolved incompressible homogeneous turbulent shear flow in noninteger Fourier dimensions is numerically investigated. The Fourier dimension of the flow field is extended from the integer value 3 to the noninteger values by projecting the Navier-Stokes equation on the fractal set of the active Fourier modes with dimensions 2.7≤d≤3.0. The results of this study revealed that the dynamics of both large and small scale structures are nontrivially influenced by changing the Fourier dimension d. While both turbulent production and dissipation are significantly hampered as d decreases, the evolution of their ratio is almost independent of the Fourier dimension. The mechanism of the energy distribution among different spatial directions is also impeded by decreasing d. Due to this deficient energy distribution, turbulent field shows a higher level of the large-scale anisotropy in lower Fourier dimensions. In addition, the persistence of the vortex stretching mechanism and the forward spectral energy transfer, which are three-dimensional turbulence characteristics, are examined at changing d, from the standard case d=3.0 to the strongly decimated flow field for d=2.7. As the Fourier dimension decreases, these forward energy transfer mechanisms are strongly suppressed, which in turn reduces both the small-scale intermittency and the deviation from Gaussianity. Besides the energy exchange intensity, the variations of d considerably modify the relative weights of local to nonlocal triadic interactions. It is found that the contribution of the nonlocal triads to the total turbulent kinetic energy exchange increases as the Fourier dimension increases.
Phase relations of triadic scale interactions in turbulent flows
Duvvuri, Subrahmanyam; McKeon, Beverley
2014-11-01
The quadratic nature of non-linearity in the Navier-Stokes (NS) equations dictates the coupling between scales in a turbulent flow to be of triadic form. An understanding of the triadic coupling affords good insights into the dynamics of turbulence, as demonstrated by Sharma & McKeon (J. Fluid Mech., 2013) through analysis of the NS resolvent operator; a set of three triadically consistent spatio-temporal modes was shown to produce complex structures such as modulating packets of hairpin vortices observed in wall-bounded turbulent flows. Here we interpret Skewness (Sk) of velocity fluctuations and the Amplitude Modulation coefficient (Ram), proposed by Mathis, Hutchins & Marusic (J. Fluid Mech., 2009), to be a measure of the large- and small-scale phase relationship. Through a simple decomposition of scales, both Sk and Ram are shown to be amplitude weighted (and normalized) measures of phase between scales that have direct triadic coupling. An analytical relationship is established between the two quantities and the result is demonstrated using experimental data from canonical and dynamically forced turbulent boundary layers presented in Duvvuri and McKeon (AIAA 2014-2883). The support of AFOSR (Grant No. FA 9550-12-1-0469) and Resnick Institute Graduate Research Fellowship (S.D.) is gratefully acknowledged.
DRAG REDUCTION EFFECT OF COUPLING FLEXIBLE TUBES WITH TURBULENT FLOW
CAI Shu-peng; JIN Guo-yu; LI Da-mei; Yang Lin
2008-01-01
To analyze the mechanism of drag reducing effect by coupling flexible tubes with turbulent flow, based on experimental examination of more obvious turbulent drag reduction effect in flexible tubes than in rigid tubes, experimental investigation was performed on the effect of turbulent drag reduction, fluctuating vibration characteristics of flexible tube and the correlations by using a double-tube system and laser displacement sensor. The results are as follows: with the decrease of the thickness of the flexible tubes, the root mean square of fluctuating amplitude of the outer wall of the tubes increases, and the non-dimensional burst period increases, resulting in the increase of the reduction rate of drag coefficient by coupling flexible tubes with turbulent flow. At applied pressure-balanced air on the outer wall and the Reynolds number of about 1.75 104, the non-dimensional burst periods of the flexible tubes with the thickness of 2 mm, 3 mm, 4 mm are 141, 126, 105, respectively.
Turbulence Modification Structures in an Upward Bubbly Pipe Flow
Tanaka, Tomohiko; Hishida, Koichi; Eaton, John
2002-11-01
The objective of this study is to investigate the mechanism of modification of turbulence in gas-liquid bubbly flow. We especially focused on the effect of void fraction and bubble diameter, which are important factors in turbulence modification. Fluid velocity was measured by applying PIV with fluorescent tracer particles, and bubble shapes and positions were obtained by the shadow-image technique. The experiment consisted of a fully developed vertical upward pipe flow with void fraction 0.5diameter is 2R=44mm and the Re=9700. In order to compare the effect of the bubble diameter at fixed void fraction, nearly 60ppm of 3-Pentanol (C5H11OH) surfactant was added as the surfactant. Bubbles accelerated the mean streamwise velocity near the wall. Thus the mean streamwise velocity profile was flatted. Moreover, the streamwise fluctuation velocity was suppressed at the middle pipe region. It is suggested that the highly concentrated bubbles in the vicinity of the wall disturb the transport of turbulence energy produced by the wall shear layer toward the middle of pipe. Thus the fluctuation velocity is remarkably reduced at the wide region of the pipe center. Moreover, in the middle of pipe, the turbulence structure is governed by the presence of bubbles.
Large-scale structures in turbulent Couette flow
Kim, Jung Hoon; Lee, Jae Hwa
2016-11-01
Direct numerical simulation of fully developed turbulent Couette flow is performed with a large computational domain in the streamwise and spanwise directions (40 πh and 6 πh) to investigate streamwise-scale growth mechanism of the streamwise velocity fluctuating structures in the core region, where h is the channel half height. It is shown that long streamwise-scale structures (> 3 h) are highly energetic and they contribute to more than 80% of the turbulent kinetic energy and Reynolds shear stress, compared to previous studies in canonical Poiseuille flows. Instantaneous and statistical analysis show that negative-u' structures on the bottom wall in the Couette flow continuously grow in the streamwise direction due to mean shear, and they penetrate to the opposite moving wall. The geometric center of the log layer is observed in the centerline with a dominant outer peak in streamwise spectrum, and the maximum streamwise extent for structure is found in the centerline, similar to previous observation in turbulent Poiseuille flows at high Reynolds number. Further inspection of time-evolving instantaneous fields clearly exhibits that adjacent long structures combine to form a longer structure in the centerline. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2014R1A1A2057031).
Lemoult, Grégoire; Aider, Jean-Luc; Wesfreid, José Eduardo
2013-01-01
We present new experimental results on the development of turbulent spots in channel flow. The internal structure of a turbulent spot is measured, with Time Resolved Stereoscopic Particle Image Velocimetry. We report the observation of travelling-wave-like structures at the trailing edge of the turbulent spot. Special attention is paid to the large-scale flow surrounding the spot. We show that this large-scale flow is an asymmetric quadrupole centred on the spot. We measure the time evolution of the turbulent fluctuations and the mean flow distortions and compare these with the predictions of a nonlinear reduced order model predicting the main features of subcritical transition to turbulence.
Bi-stability in turbulent, rotating spherical Couette flow
Zimmerman, Daniel S; Lathrop, Daniel P; 10.1063/1.3593465
2011-01-01
Flow between concentric spheres of radius ratio $\\eta = r_\\mathrm{i}/r_\\mathrm{o} = 0.35$ is studied in a 3 m outer diameter experiment. We have measured the torques required to maintain constant boundary speeds as well as localized wall shear stress, velocity, and pressure. At low Ekman number $E = 2.1\\times10^{-7}$ and modest Rossby number $0.07 < Ro < 3.4$, the resulting flow is highly turbulent, with a Reynolds number ($Re=Ro/E$) exceeding fifteen million. Several turbulent flow regimes are evident as $Ro$ is varied for fixed $E$. We focus our attention on one flow transition in particular, between $Ro = 1.8$ and $Ro = 2.6$, where the flow shows bistable behavior. For $Ro$ within this range, the flow undergoes intermittent transitions between the states observed alone at adjacent $Ro$ outside the switching range. The two states are clearly distinguished in all measured flow quantities, including a striking reduction in torque demanded from the inner sphere by the state lying at higher $Ro$. The redu...
Direct numerical simulation of laminar-turbulent flow over a flat plate at hypersonic flow speeds
Egorov, I. V.; Novikov, A. V.
2016-06-01
A method for direct numerical simulation of a laminar-turbulent flow around bodies at hypersonic flow speeds is proposed. The simulation is performed by solving the full three-dimensional unsteady Navier-Stokes equations. The method of calculation is oriented to application of supercomputers and is based on implicit monotonic approximation schemes and a modified Newton-Raphson method for solving nonlinear difference equations. By this method, the development of three-dimensional perturbations in the boundary layer over a flat plate and in a near-wall flow in a compression corner is studied at the Mach numbers of the free-stream of M = 5.37. In addition to pulsation characteristic, distributions of the mean coefficients of the viscous flow in the transient section of the streamlined surface are obtained, which enables one to determine the beginning of the laminar-turbulent transition and estimate the characteristics of the turbulent flow in the boundary layer.
Chemical Reactions in Turbulent Mixing Flows
1993-07-15
5.0, for AL), especially considering that it is estimated from flow properties on the jet axis. An appropriate inner viscous scale, A, can be estimated...strategy. provided in the computation of complicated wave inter- actions and its robustness have made this scheme especially valuable for the...begins Figure 17 shos the evoluion of as overdrive factor is defined by unsteady detonation propapgtig in un ds"iurbd reptou It is the well-known piston
MEASUREMENTS AND COMPUTATIONS OF FUEL DROPLET TRANSPORT IN TURBULENT FLOWS
Joseph Katz and Omar Knio
2007-01-10
The objective of this project is to study the dynamics of fuel droplets in turbulent water flows. The results are essential for development of models capable of predicting the dispersion of slightly light/heavy droplets in isotropic turbulence. Since we presently do not have any experimental data on turbulent diffusion of droplets, existing mixing models have no physical foundations. Such fundamental knowledge is essential for understanding/modeling the environmental problems associated with water-fuel mixing, and/or industrial processes involving mixing of immiscible fluids. The project has had experimental and numerical components: 1. The experimental part of the project has had two components. The first involves measurements of the lift and drag forces acting on a droplet being entrained by a vortex. The experiments and data analysis associated with this phase are still in progress, and the facility, constructed specifically for this project is described in Section 3. In the second and main part, measurements of fuel droplet dispersion rates have been performed in a special facility with controlled isotropic turbulence. As discussed in detail in Section 2, quantifying and modeling the of droplet dispersion rate requires measurements of their three dimensional trajectories in turbulent flows. To obtain the required data, we have introduced a new technique - high-speed, digital Holographic Particle Image Velocimetry (HPIV). The technique, experimental setup and results are presented in Section 2. Further information is available in Gopalan et al. (2005, 2006). 2. The objectives of the numerical part are: (1) to develop a computational code that combines DNS of isotropic turbulence with Lagrangian tracking of particles based on integration of a dynamical equation of motion that accounts for pressure, added mass, lift and drag forces, (2) to perform extensive computations of both buoyant (bubbles) and slightly buoyant (droplets) particles in turbulence conditions
Stochastic Simulation of Lagrangian Particle Transport in Turbulent Flows
Sun, Guangyuan
This dissertation presents the development and validation of the One Dimensional Turbulence (ODT) multiphase model in the Lagrangian reference frame. ODT is a stochastic model that captures the full range of length and time scales and provides statistical information on fine-scale turbulent-particle mixing and transport at low computational cost. The flow evolution is governed by a deterministic solution of the viscous processes and a stochastic representation of advection through stochastic domain mapping processes. The three algorithms for Lagrangian particle transport are presented within the context of the ODT approach. The Type-I and -C models consider the particle-eddy interaction as instantaneous and continuous change of the particle position and velocity, respectively. The Type-IC model combines the features of the Type-I and -C models. The models are applied to the multi-phase flows in the homogeneous decaying turbulence and turbulent round jet. Particle dispersion, dispersion coefficients, and velocity statistics are predicted and compared with experimental data. The models accurately reproduces the experimental data sets and capture particle inertial effects and trajectory crossing effect. A new adjustable particle parameter is introduced into the ODT model, and sensitivity analysis is performed to facilitate parameter estimation and selection. A novel algorithm of the two-way momentum coupling between the particle and carrier phases is developed in the ODT multiphase model. Momentum exchange between the phases is accounted for through particle source terms in the viscous diffusion. The source term is implemented in eddy events through a new kernel transformation and an iterative procedure is required for eddy selection. This model is applied to a particle-laden turbulent jet flow, and simulation results are compared with experimental measurements. The effect of particle addition on the velocities of the gas phase is investigated. The development of
Theory to predict shear stress on cells in turbulent blood flow.
Morshed, Khandakar Niaz; Bark, David; Forleo, Marcio; Dasi, Lakshmi Prasad
2014-01-01
Shear stress on blood cells and platelets transported in a turbulent flow dictates the fate and biological activity of these cells. We present a theoretical link between energy dissipation in turbulent flows to the shear stress that cells experience and show that for the case of physiological turbulent blood flow: (a) the Newtonian assumption is valid, (b) turbulent eddies are universal for the most complex of blood flow problems, and (c) shear stress distribution on turbulent blood flows is possibly universal. Further we resolve a long standing inconsistency in hemolysis between laminar and turbulent flow using the theoretical framework. This work demonstrates that energy dissipation as opposed to bulk shear stress in laminar or turbulent blood flow dictates local mechanical environment of blood cells and platelets universally.
Large Eddy Simulation of Turbulent Flows in Wind Energy
Chivaee, Hamid Sarlak
This research is devoted to the Large Eddy Simulation (LES), and to lesser extent, wind tunnel measurements of turbulent flows in wind energy. It starts with an introduction to the LES technique associated with the solution of the incompressible Navier-Stokes equations, discretized using a finite...... Reynolds numbers, and thereafter, the fully-developed infinite wind farm boundary later simulations are performed. Sources of inaccuracy in the simulations are investigated and it is found that high Reynolds number flows are more sensitive to the choice of the SGS model than their low Reynolds number...... of attack. Laminar-turbulent transition, generation of laminar boundary layer separation, and formation of stall cells are investigated. The simulated airfoil characteristics are validated against measurements. It is concluded that the LES computations and wind tunnel measurements are in good agreement...
Gyrotactic trapping in laminar and turbulent Kolmogorov flow
Santamaria, Francesco; Cencini, Massimo; Boffetta, Guido
2014-01-01
Phytoplankton patchiness, namely the heterogeneous distribution of microalgae over multiple spatial scales, dramatically impacts marine ecology. A spectacular example of such heterogeneity occurs in thin phytoplankton layers (TPLs), where large numbers of photosynthetic microorganisms are found within a small depth interval. Some species of motile phytoplankton can form TPLs by gyrotactic trapping due to the interplay of their particular swimming style (directed motion biased against gravity) and the transport by a flow with shear along the direction of gravity. Here we consider gyrotactic swimmers in numerical simulations of the Kolmogorov shear flow, both in laminar and turbulent regimes. In the laminar case, we show that the swimmer motion is integrable and the formation of TPLs can be fully characterized by means of dynamical systems tools. We then study the effects of rotational Brownian motion or turbulent fluctuations (appearing when the Reynolds number is large enough) on TPLs. In both cases we show t...
Understanding Nonlinear Saturation in Zonal-Flow-Dominated ITG Turbulence
Plunk, G G; Jenko, F
2014-01-01
We propose a quantitative model of the mode saturation, zonal flow (ZF) regulation and energy cascade that governs ion temperature gradient (ITG) turbulence in magnetized plasmas. Our model is formulated in terms of observable quantities, and tested in detail against numerical simulations to confirm that both its assumptions and predictions are satisfied. Key results include (1) a sensitivity of the nonlinear zonal flow response to the energy content of the ITG mode, (2) a persistence of ZF-controlled saturation at very high temperature gradients, (3) a physical explanation in terms of secondary and tertiary instabilities, and (4) dependence of heat flux in terms of dimensionless parameters at high drive strength. Our findings reconcile ostensibly divergent views on the basic theory of ITG turbulence.
Calculation of turbulent reactive flows in general orthogonal coordinates
Lai, M. K. Y.
1992-02-01
The mathematical and numerical methodology for an extended and enhanced version of the TURCOM computer code, called TURCOM-BFC, is presented. This code solves the conservation equations of multi-component chemically reactive and turbulent flows in general curvilinear orthogonal coordinates. The k-epsilon turbulence submodel is used. Flame chemistry assumes a number of species and chemical reactions. The latter are subdivided into finite-rate reaction steps and a one-step irreversible reaction, whose rate is controlled by a combination of mixing and global kinetics. Both the SIMPLE and PISO algorithms are implemented to solve the system of equations. The capability of TURCOM-BFC is tested and demonstrated by predicting 3-dimensional combustion flow inside a reaction furnace, where both polar-cylindrical and bipolar coordinates are used.
Fundamental Structure of High-Speed Reacting Flows: Supersonic Combustion and Detonation
2016-04-30
supersonic. Oblique Shock Interface Inert Reactants β θ P1 P2e P3eUCJ P1 UCJ P2i Detonation Figure 3. Idealized flow model of a detonation wave with an...Propagation With No Confinement But With Transvers Flow A consistent cross-flow was established by calibrating the height of the gases in time relative...to the controller commands, and then staggering the triggering of the gases such that each species – hydrogen, helium, and oxygen – independently
Chemical Reactions in Turbulent Mixing Flows
1989-10-15
example, Levenspiel (1962). Eq. 27 would be necessary. A first guess is that it might scale with 6/z as it does for subsonic flow. i.e. -(r, s; M., -0 ) -(r...France), 45-63. KELLER. J. 0. and DAILY. J. W. (1985] "The Effect of Highly Exothermic Chemical Reaction on a Two-Dimensional Mixing Layer", LEVENSPIEL ...0. [19621 Chemical Reaction Engineering. An Introduc- ALAA J. 23(12), 1937-1945. tion to the Design of Chemical Reactors . (John Wiley). KERSTEIN. A
Processes of Turbulent Liquid Flows in Pipelines and Channels
R. I. Yesman
2011-01-01
Full Text Available The paper proposes a methodology for an analysis and calculation of processes pertaining to turbulent liquid flows in pipes and channels. Various modes of liquid motion in pipelines of thermal power devices and equipment have been considered in the paper.The presented dependences can be used while making practical calculations of losses due to friction in case of transportation of various energy carriers.
Modeling and Measurement of Turbulent Swirling Flows Through Abrupt Expansions.
1987-03-01
developed Nusselt number for turbulent pipe flow represented by Dittus-Boelter or Sieder - Tate equations Nun Maximum or peak Nusaelt number q Local...temperature differences were moderate-to-large, and hence property variations appreciable, the Sieder -Tate correlation (Kern, 1950) was used to evaluate...For example, if the present results had been normalized using the Dittus-Boelter relation rather than the Sieder -Tate correlation, then peak values
The role of pair dispersion in turbulent flow
Bourgoin, M.; Ouellette, N.T.; Xu, H.T.;
2006-01-01
Mixing and transport in turbulent flows - which have strong local concentration fluctuations - essential in many natural and industrial systems including reactions in chemical mixers, combustion in engines and burners, droplet formation in warm clouds, and biological odor detection and chemotaxis......, that the initial separation of the pair plays an important rote in the subsequent spreading of the fluid elements. These results have surprising consequences for the decay of concentration fluctuations and have applications to biological and chemical systems....
Renormalization Group Analysis of Weakly Rotating Turbulent Flows
王晓宏; 周全
2011-01-01
Dynamic renormalization group (RNG) analysis is applied to the investigation of the behavior of the infrared limits of weakly rotating turbulence. For turbulent How subject to weak rotation, the anisotropic part in the renormalized propagation is considered to be a perturbation of the isotropic part. Then, with a low-order approximation, the coarsening procedure of RNG transformation is performed. After implementing the coarsening and rescaling procedures, the RNG analysis suggests that the spherically averaged energy spectrum has the scaling behavior E(k) ∝ k11/5 for weakly rotating turbulence. It is also shown that the Coriolis force will disturb the stability of the Kolmogorov -5/3 energy spectrum and will change the scaling behavior even in the case of weak rotation.%Dynamic renormalization group(RNG)analysis is applied to the investigation of the behavior of the infrared limits of weakly rotating turbulence.For turbulent flow subject to weak rotation,the anisotropic part in the renormalized propagation is considered to be a perturbation of the isotropic part.Then,with a low-order approximation,the coarsening procedure of RNG transformation is performed.After implementing the coarsening and rescaling procedures,the RNG analysis suggests that the spherically averaged energy spectrum has the scaling behavior E(k)∝ k-11/5 for weakly rotating turbulence.It is also shown that the Coriolis force will disturb the stability of the Kolmogorov-5/3 energy spectrum and will change the scaling behavior even in the case of weak rotation.
Turbulent Flow Simulations in Complex Multilouvered Fins
Tafti, Danesh
2000-11-01
Air-side resistance makes up roughly 80resistance in compact heat exchangers. Multilouvered fins find widespread use in the automotive and HVAC industry for heat transfer augmentation. We will describe the computational methodology for simulating the complex three-dimensional geometry and present results at a Reynolds number of 1100 based on louver pitch and the average flow velocity. The three-dimensionality in the louver geometry occurs along the height of the fin, where the angled louver transitions to the flat landing and joins with the tube surface. The transition region is characterized by a swept leading edge and decreasing flow area between louvers. Results show the formation of spanwise vortices at the leading edge of the angled portion of the louver which convect downstream in the vicinity of the louver surface. Further there is evidence of a separate louver wake instability which interacts with the vortices shed from the leading edge. In the transition region, a high energy streamwise vortex jet is formed. The jet forms in the vicinity of the louver junction with the flat landing and is drawn under the louver in the transition region. The passage of the jet in the vicinity of the louver surface produces a high pressure stagnant zone directly under the jet with a net effect of reducing heat transfer. On the other hand, the top surface of the louver in the transition region experiences high velocities in the vicinity of the surface and exhibits much higher heat transfer coefficients than the bottom surface.
Reacting Flow of Hydrogen Chloride and Ammonia in Experimental and Numerical Modelling
Dariusz Kardas; Katarzyna Falkowska
2003-01-01
The experimental and numerical investigations of the flow with reaction of two gases: hydrogen chloride HCl and ammonia NH_3 were performed.The article contains description of the visualisation method of the formation and flow of particles of ammonia chloride NH_4Cl.Analyses of mean concentration and variance of concentration fluctuations of dispersed phase were performed for different outputs of gases.Numerical calculations were performed for analysed phenomenon. Both numerical and visualisation results were matched and compared.
Direct numerical simulation of turbulent plane Couette flow
Lee, Moon Joo
1991-01-01
Turbulent plane Couette flow was numerically simulated at a Reynolds number (U(sub w)h/nu) of 6000, where U(sub w) is the relative wall speed and h is half the channel-height. Unlike in Poiseuille flow, where the mean shear rate changes its sign at the centerline, the sign of mean shear rate in plane Couette flow remains the same across the whole channel. This difference is expected to yield several differences between the two flows, especially in the core region. The most significant and dramatic difference observed was the existence of large-scale structures in the core region of the plane Couette flow. The large eddies are extremely long in the flow direction and fill the entire channel (i.e., their vertical extent is 2h). The large-scale structures have the largest contribution from the wavenumber (k(sub x)h,k(sub z)h) = (0, plus or minus 1.5), corresponding to a wavelength lambda(sub z)/h is approximately equal to 4. The secondary motion associated with the k(sub x)h = 0 mode consists of the large-scale vortices. The large eddies contribute about 30 percent of turbulent kinetic energy.
Bypass transition and subcritical turbulence in plane Poiseuille flow
Zammert, Stefan
2015-01-01
Plane Poiseuille flow shows turbulence at a Reynolds number that is lower than the critical one for the onset of Tollmien-Schlichting waves. The transition to turbulence follows the same route as the by-pass transition in boundary layers, i.e. finite amplitude perturbations are required and the flow is dominated by downstream vortices and streaks in the transitional regime. In order to relate the phenomenology in plane Poiseuille flow to our previous studies of plane Couette flow (Kreilos & Eckhardt, 2012), we study a symmetric subspace of plane Poiseuille flow in which the bifurcation cascade stands out clearly. By tracing the edge state, which in this system is a travelling wave, and its bifurcations, we can trace the formation of a chaotic attractor, the interior crisis that increase the phase space volume affected by the flow, and the ultimate transition into a chaotic saddle in a crisis bifurcation. After the boundary crisis we can observe transient chaos with exponentially distributed lifetimes.
Near-wall modelling of compressible turbulent flows
So, Ronald M. C.
1990-01-01
Work was carried out to formulate near-wall models for the equations governing the transport of the temperature-variance and its dissipation rate. With these equations properly modeled, a foundation is laid for their extension together with the heat-flux equations to compressible flows. This extension is carried out in a manner similar to that used to extend the incompressible near-wall Reynolds-stress models to compressible flows. The methodology used to accomplish the extension of the near-wall Reynolds-stress models is examined and the actual extension of the models for the Reynolds-stress equations and the near-wall dissipation-rate equation to compressible flows is given. Then the formulation of the near-wall models for the equations governing the transport of the temperature variance and its dissipation rate is discussed. Finally, a sample calculation of a flat plate compressible turbulent boundary-layer flow with adiabatic wall boundary condition and a free-stream Mach number of 2.5 using a two-equation near-wall closure is presented. The results show that the near-wall two-equation closure formulated for compressible flows is quite valid and the calculated properties are in good agreement with measurements. Furthermore, the near-wall behavior of the turbulence statistics and structure parameters is consistent with that found in incompressible flows.
Nakabayashi, K.; Kito, O.; Kato, Y. [Nagoya Institute of Technology, Nagoya (Japan)
1998-10-25
Turbulence intensities in Couette Poiseuille flow, developed between stationary and moving walls, have been measured by I- and X-type hot wires. The intensities in the wall region are affected by non-dimensional shear stress gradient parameter {mu} ({identical_to} u*{sup 3}/{alpha}{nu}), but not by Reynolds number Re* ({identical_to} hu*/{nu}). As |{mu}| decreases, distributions of streamwise and wall-normal turbulence intensities shift upward or downward from those of plane-Couette flow depending on the sign of {mu}. In the turbulent core region, turbulence intensities of Poiseuille-type flow distribute quite differently from that of Couette-type flow. The effective parameter in this region is 13, but the effect of 13 on the turbulence intensities is obscured by the low Reynolds number effect. 13 refs., 12 figs., 2 tabs.
Turbulence comes in bursts in stably stratified flows
Rorai, C; Pouquet, A
2013-01-01
There is a clear distinction between simple laminar and complex turbulent fluids. But in some cases, as for the nocturnal planetary boundary layer, a stable and well-ordered flow can develop intense and sporadic bursts of turbulent activity which disappear slowly in time. This phenomenon is ill-understood and poorly modeled; and yet, it is central to our understanding of weather and climate dynamics. We present here a simple model which shows that in stably stratified turbulence, the stronger bursts can occur when the flow is expected to be more stable. The bursts are generated by a rapid non-linear amplification of energy stored in waves, and are associated with energetic interchanges between vertical velocity and temperature (or density) fluctuations. Direct numerical simulations on grids of 2048^3 points confirm this somewhat paradoxical result of measurably stronger events for more stable flows, displayed not only in the temperature and vertical velocity derivatives, but also in the amplitude of the field...
A new framework for simulating forced homogeneous buoyant turbulent flows
Carroll, Phares L.; Blanquart, Guillaume
2015-06-01
This work proposes a new simulation methodology to study variable density turbulent buoyant flows. The mathematical framework, referred to as homogeneous buoyant turbulence, relies on a triply periodic domain and incorporates numerical forcing methods commonly used in simulation studies of homogeneous, isotropic flows. In order to separate the effects due to buoyancy from those due to large-scale gradients, the linear scalar forcing technique is used to maintain the scalar variance at a constant value. Two sources of kinetic energy production are considered in the momentum equation, namely shear via an isotropic forcing term and buoyancy via the gravity term. The simulation framework is designed such that the four dimensionless parameters of importance in buoyant mixing, namely the Reynolds, Richardson, Atwood, and Schmidt numbers, can be independently varied and controlled. The framework is used to interrogate fully non-buoyant, fully buoyant, and partially buoyant turbulent flows. The results show that the statistics of the scalar fields (mixture fraction and density) are not influenced by the energy production mechanism (shear vs. buoyancy). On the other hand, the velocity field exhibits anisotropy, namely a larger variance in the direction of gravity which is associated with a statistical dependence of the velocity component on the local fluid density.
SIMULATION OF TURBULENT FLOW THROUGH TARBELA DAM TUNNEL 3
Muhammad Abid
2010-11-01
Full Text Available Tarbela dam is one of the largest earth filled dam in the world. The sediments inflow in the Tarbela reservoir has resulted in reduction in water storage capacity. During the recent years, a reasonable increase of sediment particles in the tunnel is observed. This is damaging tunnels, power generating units and is a severe threat to the plant equipment. To the authors knowledge, to-date no comprehensive simulation studies are performed for flooding in the reservoir or turbulent flows in the tunnels. In this paper, turbulent flow using Reynolds Stress Model in Tunnel 3 of the Tarbela Dam is analyzed with and without considering the effect of sediments particle. Results are presented for three different water heads in the reservoir i.e. considering summer, winter and average seasons and for one-way and two-way/full coupling for sediments particle tracking/deposition. The effect of cavitation erosion and damage to the tunnels due to erosion is investigated and results are compared with the experimental erosion results for similar geometries and are found in good agreement. Sediments particulate analysis is also performed for the validation of the samples collected from WAPDA. Moreover, pressure, velocity and erosion rate results are discussed to get complete behavior of the turbulent flow of water in the tunnel.
Extraction of coherent structures in a rotating turbulent flow experiment
Ruppert-Felsot, J E; Sharon, E; Swinney, H L; Ruppert-Felsot, Jori E.; Praud, Olivier; Sharon, Eran; Swinney, Harry L.
2004-01-01
The discrete wavelet packet transform (DWPT) and discrete wavelet transform (DWT) are used to extract and study the dynamics of coherent structures in a turbulent rotating fluid. Three-dimensional (3D) turbulence is generated by strong pumping through tubes at the bottom of a rotating tank (48.4 cm high, 39.4 cm diameter). This flow evolves toward two-dimensional (2D) turbulence with increasing height in the tank. Particle Image Velocimetry (PIV) measurements on the quasi-2D flow reveal many long-lived coherent vortices with a wide range of sizes. The vorticity fields exhibit vortex birth, merger, scattering, and destruction. We separate the flow into a low-entropy ``coherent'' and a high-entropy ``incoherent'' component by thresholding the coefficients of the DWPT and DWT of the vorticity fields. Similar thresholdings using the Fourier transform and JPEG compression together with the Okubo-Weiss criterion are also tested for comparison. We find that the DWPT and DWT yield similar results and are much more ef...
DNS of turbulent flow over longitudinally ridged walls.
Castagna, Jony; Yao, Yufeng
2007-11-01
An in-house DNS code has been developed over the years and validations have been carried out on various flow problems, including boundary-layer, plain channel, bump flow, and many other cases. This finite difference code solves full three-dimensional compressible Navier-Stokes equations using high-order (4th-order) for spatial derivatives and multi-stage Runge-Kutta explicit scheme for time advancement. The code parallelization has also been carried out using the latest MPI library and is portable for various HPC platforms. The unique feature of the code is that it applies the entropy splitting concept for improve the numerical stability, which is one of common problem for DNS code requiring extremely longer run time to get the statistically converged results. In this study, the code has been further extended to include the capability of treating the geometry variation in the spanwise direction by using full 3D grid transformation, similar that done by other researchers. As demonstration, we follow an existing DNS study of turbulent flow over longitudinally ridged walls at the Reynolds number (Reτ) 140, based on the friction velocity, as the first step to validate the new capability of the code. Results from present study have been compared fairly well with available DNS data. In the full version of the paper, detailed analysis will be provided, focusing on three parts: mean propriety, turbulence intensity and turbulent coherent structures.
TURBULENCE TRANSPORT OF SURFACTANT SOLUTION FLOW DURING DRAG REDUCTION DEGENERATION
GU Wei-guo; WANG De-zhong
2012-01-01
Turbulence transport of surfactant solution flow during drag reduction degeneration is investigated experimentally in a two-dimensional channel.Particle Image Velocimetry (P1V) system is used to take two-dimensional velocity frames in the streamwise and wall-normal plane.The additive of surfactant is cetyltrimethyl ammonium chloride (CTAC) with the mass concentration of 25 ppm.Drag reduction degeneration happens in the CTAC solution flow,exhibiting the maximal drag reduction at Re =25000and losing drag reduction completely at Re =40 000.The velocity frames are statistically analyzed in four quadrants which are divided by the u -axis and v-axis.It is found that the phenomenon of“Zero Reynolds shear stress” is caused by the decrease of wallnormal fluctuations and its symmetrical distribution in quadrants.The increase of Reynolds number leads to the enhancement of turbulence burst phenomenon.During thc drag reduction degeneration,the CTAC solution flow contains both high turbulence intensity and drag reduction states.
The first-digit frequencies in data of turbulent flows
Biau, Damien
2015-01-01
Considering the first significant digits (noted d) in data sets of dissipation for turbulent flows, the probability to find a given number (d=1 or 2 or... 9) would be 1/9 for an uniform distribution. Instead the probability closely follows Newcomb-Benford's law, namely P(d)=log(1+1/d). The discrepancies between Newcomb-Benford's law and first-digits frequencies in turbulent data are analysed through Shannon's entropy. The data sets are obtained with direct numerical simulations for two types of fluid flow: an isotropic case initialized with a Taylor-Green vortex and a channel flow. Results are in agreement with Newcomb-Benford's law in nearly homogeneous cases and the discrepancies are related to intermittent events. Thus the scale invariance for the first significant digits, which supports Newcomb-Benford's law, seems to be related to an equilibrium turbulent state, namely with a significant inertial range. A matlab/octave program is provided in appendix in such that part of the presented results can easily ...
Steady streamwise transpiration control in turbulent pipe flow
Gómez, F; Rudman, M; Sharma, AS; McKeon, BJ
2016-01-01
A study of the the main features of low- and high amplitude steady streamwise wall transpiration applied to pipe flow is presented. The effect of the two transpiration parameters, amplitude and wavenumber, on the flow have been investigated by means of direct numerical simulation at a moderate turbulent Reynolds number. The behaviour of the three identified mechanisms that act in the flow: modification of Reynolds shear stress, steady streaming and generation of non-zero mean streamwise gradients, have been linked to the transpiration parameters. The observed trends have permitted the identification of wall transpiration configurations able to reduce or increase the overall flow rate in -36.1% and 19.3% respectively. A resolvent analysis has been carried out to obtain a description of the reorganization of the flow structures induced by the transpiration.
Statistics of polymer extensions in turbulent channel flow
Bagheri, Faranggis; Perlekar, Prasad; Brandt, Luca
2012-01-01
We present direct numerical simulations~(DNSs) of turbulent channel flow with passive Lagrangian polymers. To understand the polymer behavior we investigate the behavior of infinitesimal line elements and calculate, for the first time, the PDF of finite-time Lyapunov exponents and from them the corresponding Cramer's function for the channel flow. We study the statistics of polymer elongation for both the Oldroyd-B model (for Weissenberg number Wi 1 (FENE model) the polymer are significantly more stretched near the wall than at the centre of the flow. Furthermore near the wall the polymers show a strong tendency to orient along the stream-wise direction of the flow but near the centerline the statistics of orientation of the polymers is consistent with analogous results obtained recently in homogeneous and isotropic flows [2].
Flow Enhancement due to Elastic Turbulence in Channel Flows of Shear Thinning Fluids
Bodiguel, Hugues; Beaumont, Julien; Machado, Anaïs; Martinie, Laetitia; Kellay, Hamid; Colin, Annie
2015-01-01
We explore the flow of highly shear thinning polymer solutions in straight geometry. The strong variations of the normal forces close to the wall give rise to an elastic instability. We evidence a periodic motion close the onset of the instability, which then evolves towards a turbulentlike flow at higher flow rates. Strikingly, we point out that this instability induces genuine drag reduction due to the homogenization of the viscosity profile by the turbulent flow.
VELOCITY PROFILES OF TURBULENT OPEN CHANNEL FLOWS
WANG Dianchang; WANG Xingkui; YU Mingzhong; LI Danxun
2001-01-01
The log-law and the wake law of velocity profile for open channel flows are discussed and compared in this paper. Experimental data from eight sources are used to verify the velocity distribution models.The effect of bed level on the velocity profile is analyzed. A formula to calculate the maximum velocity is proposed. In the region of y ＜δm , the velocity profile approximately follows the log-law. For the region of y ＞δm , the effect of the aspect ratio is considered. A new velocity profile model on the basis of log-law that can unify all of the hydraulic bed roughness is presented.
Secondary flow, turbulent diffusion and mixing in axial-flow compressors
Wisler, D. C.; Bauer, R. C.; Okiishi, T. H.
1987-05-01
The relative importance of convection by secondary flows and diffusion by turbulence as mechanisms responsible for mixing in multistage, axial-flow compressors has been investigated by using the ethylene tracer-gas technique and hot wire anemometry. The tests were conducted at two loading levels in a large, low-speed, four-stage compressor. The experimental results show that considerable cross-passage and spanwise fluid motion can occur and that both secondary flow and turbulent diffusion can play important roles in the mixing process, depending upon location in the compressor and loading level.
An eddy viscosity calculation method for a turbulent duct flow
Antonia, R. A.; Bisset, D. K.; Kim, J.
1991-01-01
The mean velocity profile across a fully developed turbulent duct flow is obtained from an eddy viscosity relation combined with an empirical outer region wake function. Results are in good agreement with experiments and with direct numerical simulations in the same flow at two Reynolds numbers. In particular, the near-wall trend of the Reynolds shear stress and its variation with Reynolds number are similar to those of the simulations. The eddy viscosity method is more accurate than previous mixing length or implicit function methods.
RESEARCH ON TURBULENT FLOW WITHIN THE VORTEX PUMP
CHEN Hong-xun
2004-01-01
Based on the standard k-ε turbulence model and the RANS equations, the finite volume method and the SIMPLE algorithm were adopted to carry out the three-dimensional viscous numerical simulation of the internal flow within a vortex pump in double reference frames. According to the results of numerical simulation, the internal flow in the vortex pump was analyzed, and the calculated results of blade surface pressure of the impeller were compared with experimental results. The maximum relative error is 6.6% between calculated value and experimental value of the pump head under operation conditions.
Particle deposition in low-speed, high-turbulence flows
Reck, Mads; Larsen, Poul Scheel; Ullum, U.
2002-01-01
field measurements. Particle deposition is shown to be associated with near-wall coherent structures. Flow reversal, simulated by impulsive start, is shown to give higher deposition rates than steady mean flows. Key word index: Spoilage fungi; spores; food processing plant; deposition flux; large eddy......, active and passive samplers provide typical values of airborne concentrations and specific deposition fluxes. Velocity and turbulence data from field studies are used as input in LES simulations of the process, and estimates of deposition fluxes are of the same order of magnitude as those deduced from...... simulation....
On integrating large eddy simulation and laboratory turbulent flow experiments.
Grinstein, Fernando F
2009-07-28
Critical issues involved in large eddy simulation (LES) experiments relate to the treatment of unresolved subgrid scale flow features and required initial and boundary condition supergrid scale modelling. The inherently intrusive nature of both LES and laboratory experiments is noted in this context. Flow characterization issues becomes very challenging ones in validation and computational laboratory studies, where potential sources of discrepancies between predictions and measurements need to be clearly evaluated and controlled. A special focus of the discussion is devoted to turbulent initial condition issues.
Characterization of zonal flow generation in weak electrostatic turbulence
Negrea, M; Petrisor, I [Department of Physics, Association Euratom-MEdC, Romania, University of Craiova, 13 A.I. Cuza Street, 200585 Craiova (Romania); Weyssow, B [Physique Statistique-Plasmas, Association Euratom-Etat Belge, Universite Libre de Bruxelles, Campus Plaine, Bd. du Triomphe, 1050 Bruxelles (Belgium)], E-mail: mnegrea@yahoo.com
2008-05-15
The influence of the diamagnetic Kubo number, which is proportional to the diamagnetic drift velocity, on the zonal flow generation by an anisotropic stochastic electrostatic potential is considered from a semi-analytic point of view. The analysis is performed in the weak turbulence limit and as an analytical tool the decorrelation trajectory method is used. It is shown that the fragmentation of the drift wave structures (a signature of the zonal flow generation) is influenced not only by the anisotropy parameter and the electrostatic Kubo number as expected, but also by the diamagnetic Kubo number. Global Lagrangian averages of characteristic quantities are calculated and interpreted.
On soft stability loss in rotating turbulent MHD flows
Kapusta, Arkady [Center for MHD Studies, Ben-Gurion University of the Negev PO Box 653, Beer-Sheva 84105 (Israel); Mikhailovich, Boris, E-mail: borismic@bgu.ac.il [Department of Mechanical Engineering, Ben-Gurion University of the Negev PO Box 653, Beer-Sheva 84105 (Israel)
2014-08-01
The problem of the stability of turbulent flows of liquid metal in a cylindrical cavity against small velocity disturbances under the action of a rotating magnetic field (RMF) has been studied. The flow is considered in the induction-free approximation using the ‘external’ friction model. A system of dimensionless equations is examined in cylindrical coordinates. The results of computations performed on the basis of this mathematical model using the exchange of stabilities principle have shown a good consistency between the critical values of computed and experimental Reynolds numbers. (paper)
The Extraction of Anisotropic Contributions in Turbulent Flows
Arad, I; Kurien, S; Lvov, V S; Procaccia, I; Sreenivasan, K R; Arad, Itai; Dhruva, Brindesh; Kurien, Susan; L'vov, Victor S.; Procaccia, Itamar
1998-01-01
We analyze turbulent velocity signals measured by two probes in the atmosphere, both at the height of 35 meters but displaced by 40 cm nominally orthogonal to the mean wind. Choosing a suitable coordinate system with respect to that of the mean wind, we derive theoretical forms for second order structure functions, and fit them to experimental data. We show that the effect of flow anisotropy is small on the longitudinal component but significant on the transverse component. The data provide an estimate of a universal exponent from among a hierarchy that governs the decay of flow anisotropy with the scale-size.
3D critical layers in fully-developed turbulent flows
Saxton-Fox, Theresa; McKeon, Beverley
2016-11-01
Recent work has shown that 3D critical layers drive self-sustaining behavior of exact coherent solutions of the Navier-Stokes equations (Wang et al. 2007; Hall and Sherwin 2010; Park and Graham 2015). This study investigates the role of 3D critical layers in fully-developed turbulent flows. 3D critical layer effects are identified in instantaneous snapshots of turbulent boundary layers in both experimental and DNS data (Wu et al. 2014). Additionally, a 3D critical layer effect is demonstrated to appear using only a few resolvent response modes from the resolvent analysis of McKeon and Sharma 2010, with phase relationships appropriately chosen. Connections are sought to the thin shear layers observed in turbulent boundary layers (Klewicki and Hirschi 2004; Eisma et al. 2015) and to amplitude modulation observations (Mathis et al. 2009; Duvvuri and McKeon 2014). This research is made possible by the Department of Defense through the National Defense & Engineering Graduate Fellowship (NDSEG) Program and by the Air Force Office of Scientific Research Grant # FA9550-12-1-0060. The support of the Center for Turbulence Research (CTR) summer program at Stanford is gratefully acknowledged.
Aerosol deposition in bends with turbulent flow
McFarland, A.R.; Gong, H.; Wente, W.B. [Texas A& M Univ., College Station, TX (United States)] [and others
1997-08-01
The losses of aerosol particles in bends were determined numerically for a broad range of design and operational conditions. Experimental data were used to check the validity of the numerical model, where the latter employs a commercially available computational fluid dynamics code for characterizing the fluid flow field and Lagrangian particle tracking technique for characterizing aerosol losses. Physical experiments have been conducted to examine the effect of curvature ratio and distortion of the cross section of bends. If it curvature ratio ({delta} = R/a) is greater than about 4, it has little effect on deposition, which is in contrast with the recommendation given in ANSI N13.1-1969 for a minimum curvature ratio of 10. Also, experimental results show that if the tube cross section is flattened by 25% or less, the flattening also has little effect on deposition. Results of numerical tests have been used to develop a correlation of aerosol penetration through a bend as a function of Stokes number (Stk), curvature ratio ({delta}) and the bend angle ({theta}). 17 refs., 10 figs., 2 tabs.
History effect on the Reynolds stress in turbulent swirling flow
Hamba, Fujihiro
2017-02-01
The eddy-viscosity model for turbulence has some difficulty in predicting rotating and swirling flows. Turbulent swirling flow in a straight pipe is a typical example. A rapidly rotating core in the pipe decays too quickly in results obtained from the standard k-ɛ model. The eddy viscosity needs to be reduced to predict the velocity profiles well; the mechanism of the decrease in the eddy viscosity has not been clarified yet. In this work, the eddy-viscosity model was investigated using a temporally nonlocal expression for the Reynolds stress that represents the history effect. A simple transport equation for the Reynolds stress was integrated along a mean-flow pathline to obtain a temporally nonlocal model for the Reynolds stress. The nonlocal model was applied to simple swirling flows for which the time integral can be further calculated to investigate the history effect. It was shown that the history effect associated with the rotating motion gives rise to a small factor in the expression for the eddy viscosity. In order to confirm the history effect, the present model and the linear eddy-viscosity model were used to simulate a swirling pipe flow. The velocity profiles obtained from the present model agree well with experimental results; the reduced eddy viscosity can account for the slow decay of the swirling motion in the core region. The anisotropic nature of the eddy viscosity was also discussed in relation to the small factor caused by the history effect.
Randomness Representation of Turbulence in Canopy Flows Using Kolmogorov Complexity Measures
Dragutin Mihailović
2017-09-01
Full Text Available Turbulence is often expressed in terms of either irregular or random fluid flows, without quantification. In this paper, a methodology to evaluate the randomness of the turbulence using measures based on the Kolmogorov complexity (KC is proposed. This methodology is applied to experimental data from a turbulent flow developing in a laboratory channel with canopy of three different densities. The methodology is even compared with the traditional approach based on classical turbulence statistics.
Schröder, A.; Geisler, R.; Elsinga, G.E.; Scarano, F.; Dierksheide, U.
2007-01-01
In this feasibility study the tomographic PIV technique has been applied to time resolved PIV recordings for the study of the growth of a turbulent spot in a laminar flat plate boundary layer and to visualize the topology of coherent flow structures within a tripped turbulent flat plate boundary lay
The phase-locked mean impulse response of a turbulent channel flow
Luchini, P; Zuccher, S; Luchini, Paolo; Quadrio, Maurizio; Zuccher, Simone
2006-01-01
We describe the first DNS-based measurement of the complete mean response of a turbulent channel flow to small external disturbances. Space-time impulsive perturbations are applied at one channel wall, and the linear response describes their mean effect on the flow field as a function of spatial and temporal separations. The turbulent response is shown to differ from the response a laminar flow with the turbulent mean velocity profile as base flow.
Transition from laminar to turbulent flow in liquid filled microtubes
Sharp, K. V.; Adrian, R. J.
2004-05-01
The transition to turbulent flow is studied for liquids of different polarities in glass microtubes having diameters between 50 and 247 µm. The onset of transition occurs at Reynolds numbers of ~1,800 2,000, as indicated by greater-than-laminar pressure drop and micro-PIV measurements of mean velocity and rms velocity fluctuations at the centerline. Transition at anomalously low values of Reynolds number was never observed. Additionally, the results of more than 1,500 measurements of pressure drop versus flow rate confirm the macroscopic Poiseuille flow result for laminar flow resistance to within -1% systematic and ±2.5% rms random error for Reynolds numbers less than 1,800.
El Khoury, George K.; Schlatter, Philipp; Brethouwer, Geert; Johansson, Arne V.
2014-04-01
Direct numerical simulation data of fully developed turbulent pipe flow are extensively compared with those of turbulent channel flow and zero-pressure-gradient boundary layer flow for Reτ up to 1000. In the near-wall region, a high degree of similarity is observed in the three flow cases in terms of one-point statistics, probability density functions of the wall-shear stress and pressure, spectra, Reynolds stress budgets and advection velocity of the turbulent structures. This supports the notion that the near-wall region is universal for pipe and channel flow. Probability density functions of the wall shear stress, streamwise turbulence intensities, one-dimensional spanwise/azimuthal spectra of the streamwise velocity and Reynolds-stress budgets are very similar near the wall in the three flow cases, suggesting that the three canonical wall-bounded flows share many features. In the wake region, the mean streamwise velocity and Reynolds stress budgets show some expected differences.
Measurement of turbulent flow in a narrow open channel
Sarkar Sankar
2016-09-01
Full Text Available The paper presents the experimental results of turbulent flow over hydraulically smooth and rough beds. Experiments were conducted in a rectangular flume under the aspect ratio b/h = 2 (b = width of the channel 0.5 m, and h = flow depth 0.25 m for both the bed conditions. For the hydraulically rough bed, the roughness was created by using 3/8″ commercially available angular crushed stone chips; whereas sand of a median diameter d50 = 1.9 mm was used as the bed material for hydraulically smooth bed. The three-dimensional velocity components were captured by using a Vectrino (an acoustic Doppler velocimeter. The study focuses mainly on the turbulent characteristics within the dip that were observed towards the sidewall (corner of the channel where the maximum velocity occurs below the free-surface. It was also observed that the nondimensional Reynolds shear stress changes its sign from positive to negative within the dip. The quadrant plots for the turbulent bursting shows that the signs of all the bursting events change within the dip. Below the dip, the probability of the occurrence of sweeps and ejections are more than that of inward and outward interactions. On the other hand, within the dip, the probability of the occurrence of the outward and inward interactions is more than that of sweeps and ejections.
A Streamwise Constant Model of Turbulence in Plane Couette Flow
Gayme, D F; Papachristodoulou, A; Bamieh, B; Doyle, J C
2010-01-01
There is a consensus that turbulent flow is characterized by coherent structures. In particular, streamwise and quasi-streamwise elongated structures have been observed in both numerical simulations and experiments. Using this idea the mean behavior of fully turbulent plane Couette flow is modeled using a streamwise constant projection of the Navier Stokes equations. This assumption results in a two dimensional, three velocity component (2D/3C) model. We first use a steady state version of this 2D/3C model to demonstrate that the nonlinear coupling in the equations provides the mathematical mechanisms associated with the shape of the turbulent velocity profile. In simulating the full model we borrow some ideas from robust control and represent uncertainty as well as modeling errors using small amplitude noise forcing. Simulations of the 2D/3C model under small amplitude Gaussian forcing of the cross stream components is compared to DNS data. The results indicate that a streamwise constant projection of the Na...
Large Eddy Simulations of Turbulent Flow Over a Wavy Wall
Sundaram, Shivshankar; Avva, Ram
1997-11-01
Turbulent, separated flow over a wavy wall was simulated using CFD-ACE, a general purpose Navier-Stokes code. The code employs finite-volume formulation and body-fitted curvilinear (BFC) grids. The flow channel consists of a flat upper wall at a mean distance, H, from a sinusoidally varying lower wall (amplitude of 0.05H and a wavelength of 1H). The Reynolds number in terms of bulk velocity and H was 6760. Computations used both a coarse grid (40x40x20;4waves) and a fine grid (60x40x40;2 waves). The spanwise extent was 2H. Periodic boundary conditions were enforced in the streamwise and spanwise directions. Both Smagorinsky (with van Driest damping) and Dynamic models were employed. The Dynamic model yielded better overall results. Present separation and reattachment lengths of 0.13 and 0.64 are in excellent agreement with prior DNS and experiment. Pressure, friction velocity over the wavy wall and mean cross-channel profiles were indistinguishable from prior data. A turbulent mixing layer and a growing boundary layer downstream of reattachment were identified using peaks in turbulence intensities. The level and location of these peaks were in good agreement with DNS.
1982 AFOSR Research Meeting on Diagnostics of Reacting Flow, 25-26 February 1982.
1982-02-01
observations and computer deconvolution methods. * COHERENT ANTI-STOKES RAMAN SPECTROSCOPY (Byer) - Development of tech- niques and measurements of species...of axial velocities for both cold and combusting flows, comparisons of velocity data with FREP and TEACH code predictions, the successful evalua- tion...of laser induced fluorescence and photoacoustic spectroscopy to measure trace species concentrations in flames. The Ramjet Division is also sponsoring
AFOSR/ONR Contractors Meeting - Combustion, Rocket Propulsion, Diagnostics of Reacting Flow
1990-06-15
Section, The Combustion Institute, Oct. 1989. 5. McMillin, B. K., Lee, M. P., Palmer, J. L., Paul , P. H. and Hanson, R. K., "Planar Laser- Induced...K., Chang, A. Y., Seitzman, J. M., Lee, M. P., Paul , P. H. and Battles, B. E., "Laser-Induced Fluorescence Diagnostics for Supersonic Flows," AIAA-90...916)355-3087 289 I Dr Leanne Pitchford Dr Robert L Poeschel GTE Laboratories Plasma Physics Department 40 Sylvan Road Hughes Research Laboratories
Tiwari, S. N.; Szema, K. Y.
1979-01-01
The influence of change in the precursor region flow properties on the entire shock layer flow phenomena around a Jovian entry body was investigated. The flow in the shock layer was assumed to be steady, axisymmetric, and viscous. Both the chemical equilibrium and the nonequilibrium composition of the shock layer gas were considered. The effects of transitional range behavior were included in the analysis of high altitude entry conditions. Realistic thermophysical and radiation models were used, and results were obtained by employing the implicit finite difference technique in the shock layer and an iterative procedure for the entire shock layer precursor zone. Results obtained for a 45 degree angle hyperboloid blunt body entering Jupiter's atmosphere at zero angle of attack indicates that preheating the gas significantly increases the static pressure and temperature ahead of the shock for entry velocities exceeding 36 km/sec. The nonequilibrium radiative heating rate to the body is found to be significantly higher than the corresponding equilibrium heating. The precursor heating generally increases the radiative and convective heating of a body. That increase is slightly higher for the nonequilibrium conditions.
EFFECT OF NON-SPHERICAL PARTICLES ON THE FLUID TURBULENCE IN A PARTICULATE PIPE FLOW
SUN Lei; LIN Jian-zhong; WU Fa-li; CHEN Yi-min
2004-01-01
In the non-spherical particulate turbulent flows, a set of new fluid fluctuating velocity equations with the non-spherical particle source term were derived, then a new method, which treats the slowly varying functions and rapidly varying functions separately, was proposed to solve the equations, and finally the turbulent intensity and the Reynolds stress of the fluid were obtained by calculating the fluctuating velocity statistically. The equations and method were used to a particulate turbulent pipe flow. The results show that the turbulent intensity and the Reynolds stress are decreased almost inverse proportionally to the fluctuating velocity ratio of particle to fluid. Non-spherical particles have a greater suppressing effect on the turbulence than the spherical particles. The particles with short relaxation time reduce the turbulence intensity of fluid, while the particles with long relaxation time increase the turbulence intensity of fluid. For fixed particle and fluid, the small particles suppress the turbulence and the large particles increase the turbulence.
The Physics of Flow Instability and Turbulent Transition in Shear Flows
Dou, H S
2006-01-01
In this paper, the physics of flow instability and turbulent transition in shear flows is studied by analyzing the energy variation of fluid particles under the interaction of base flow with a disturbance. It is shown that it is the transverse energy gradient that leads to the disturbance amplification while the disturbance is damped by the energy loss due to viscosity along the streamline. For the first time, a theory derived strictly from physics, is used to show that the flow instability under finite amplitude disturbance leads to turbulent transition. It is also shown that flow instability in shear flows is a nonlinear phenomenon and it has a threshold related to the disturbance amplitude. The mechanism for velocity inflection and hairpin vortex formation are explained with reference to analytical results. The inverse Reynolds number dependence of the disturbance threshold, observed in recent experiments, is well explained. Following from this analysis, it can be demonstrated that the critical value of th...
Coherent structures and extreme events in rotating multiphase turbulent flows
Biferale, Luca; Mazzitelli, Irene M; van Hinsberg, Michel A T; Lanotte, Alessandra S; Musacchio, Stefano; Perlekar, Prasad; Toschi, Federico
2016-01-01
By using direct numerical simulations (DNS) at unprecedented resolution we study turbulence under rotation in the presence of simultaneous direct and inverse cascades. The accumulation of energy at large scale leads to the formation of vertical coherent regions with high vorticity oriented along the rotation axis. By seeding the flow with millions of inertial particles, we quantify -for the first time- the effects of those coherent vertical structures on the preferential concentration of light and heavy particles. Furthermore, we quantitatively show that extreme fluctuations, leading to deviations from a normal-distributed statistics, result from the entangled interaction of the vertical structures with the turbulent background. Finally, we present the first-ever measurement of the relative importance between Stokes drag, Coriolis force and centripetal forces along the trajectories of inertial particles. We discover that vortical coherent structures lead to unexpected diffusion properties for heavy and light ...
Coherent Structures and Extreme Events in Rotating Multiphase Turbulent Flows
L. Biferale
2016-11-01
Full Text Available By using direct numerical simulations (DNS at unprecedented resolution, we study turbulence under rotation in the presence of simultaneous direct and inverse cascades. The accumulation of energy at large scale leads to the formation of vertical coherent regions with high vorticity oriented along the rotation axis. By seeding the flow with millions of inertial particles, we quantify—for the first time—the effects of those coherent vertical structures on the preferential concentration of light and heavy particles. Furthermore, we quantitatively show that extreme fluctuations, leading to deviations from a normal-distributed statistics, result from the entangled interaction of the vertical structures with the turbulent background. Finally, we present the first-ever measurement of the relative importance between Stokes drag, Coriolis force, and centripetal force along the trajectories of inertial particles. We discover that vortical coherent structures lead to unexpected diffusion properties for heavy and light particles in the directions parallel and perpendicular to the rotation axis.
Large Eddy Simulations of turbulent flows at supercritical pressure
Kunik, C.; Otic, I.; Schulenberg, T., E-mail: claus.kunik@kit.edu, E-mail: ivan.otic@kit.edu, E-mail: thomas.schulenberg@kit.edu [Karlsruhe Inst. of Tech. (KIT), Karlsruhe (Germany)
2011-07-01
A Large Eddy Simulation (LES) method is used to investigate turbulent heat transfer to CO{sub 2} at supercritical pressure for upward flows. At those pressure conditions the fluid undergoes strong variations of fluid properties in a certain temperature range, which can lead to a deterioration of heat transfer (DHT). In this analysis, the LES method is applied on turbulent forced convection conditions to investigate the influence of several subgrid scale models (SGS-model). At first, only velocity profiles of the so-called inflow generator are considered, whereas in the second part temperature profiles of the heated section are investigated in detail. The results are statistically analyzed and compared with DNS data from the literature. (author)
Coherent Structures and Extreme Events in Rotating Multiphase Turbulent Flows
Biferale, L.; Bonaccorso, F.; Mazzitelli, I. M.; van Hinsberg, M. A. T.; Lanotte, A. S.; Musacchio, S.; Perlekar, P.; Toschi, F.
2016-10-01
By using direct numerical simulations (DNS) at unprecedented resolution, we study turbulence under rotation in the presence of simultaneous direct and inverse cascades. The accumulation of energy at large scale leads to the formation of vertical coherent regions with high vorticity oriented along the rotation axis. By seeding the flow with millions of inertial particles, we quantify—for the first time—the effects of those coherent vertical structures on the preferential concentration of light and heavy particles. Furthermore, we quantitatively show that extreme fluctuations, leading to deviations from a normal-distributed statistics, result from the entangled interaction of the vertical structures with the turbulent background. Finally, we present the first-ever measurement of the relative importance between Stokes drag, Coriolis force, and centripetal force along the trajectories of inertial particles. We discover that vortical coherent structures lead to unexpected diffusion properties for heavy and light particles in the directions parallel and perpendicular to the rotation axis.
Experimental evidence of conformal invariance in soap film turbulent flows
Thalabard, S; Artana, G; Mininni, P D; Pouquet, A
2010-01-01
We present experimental evidence of statistical conformal invariance in isocontours of fluid thickness in experiments of two-dimensional turbulence using soap films. A Schlieren technique is used to visualize regions of the flow with constant film thickness, and association of isocontours with Schramm-L\\"owner evolution (SLE) is used to identify conformal invariance. In experiments where an inverse energy cascade develops, statistical evidence is consistent with such an association. The diffusivity of the associated one-dimensional Brownian process is close to 8/3, a value previously identified in isocontours of vorticity in high-resolution numerical simulations of two-dimensional turbulence (D. Bernard et al., Nature Phys. 2, 124, 2006). In experiments where the inverse energy cascade is not sufficiently developed, no statistical evidence of conformal invariance is found.
Compressive sampling for energy spectrum estimation of turbulent flows
Adalsteinsson, Gudmundur F
2014-01-01
Recent results from compressive sampling (CS) have demonstrated that accurate reconstruction of sparse signals often requires far fewer samples than suggested by the classical Nyquist--Shannon sampling theorem. Typically, signal reconstruction errors are measured in the $\\ell^2$ norm and the signal is assumed to be sparse, compressible or having a prior distribution. Our spectrum estimation by sparse optimization (SpESO) method uses prior information about isotropic homogeneous turbulent flows with power law energy spectra and applies the methods of CS to 1-D and 2-D turbulence signals to estimate their energy spectra with small logarithmic errors. SpESO is distinct from existing energy spectrum estimation methods which are based on sparse support of the signal in Fourier space. SpESO approximates energy spectra with an order of magnitude fewer samples than needed with Shannon sampling. Our results demonstrate that SpESO performs much better than lumped orthogonal matching pursuit (LOMP), and as well or bette...
Simulation of inertial fibre orientation in turbulent flow
Njobuenwu, Derrick O.; Fairweather, Michael
2016-06-01
The spatial and orientational behaviour of fibres within a suspension influences the rheological and mechanical properties of that suspension. An Eulerian-Lagrangian framework to simulate the behaviour of fibres in turbulent flows is presented. The framework is intended for use in simulations of non-spherical particles with high Reynolds numbers, beyond the Stokesian regime, and is a computationally efficient alternative to existing Stokesian models for fibre suspensions in turbulent flow. It is based on modifying available empirical drag correlations for the translation of non-spherical particles to be orientation dependent, accounting for the departure in shape from a sphere. The orientational dynamics of a particle is based on the framework of quaternions, while its rotational dynamics is obtained from the solution of the Euler equation of rotation subject to external torques on the particle. The fluid velocity and turbulence quantities are obtained using a very high-resolution large eddy simulation with dynamic calibration of the sub-grid scale energy containing fluid motions. The simulation matrix consists of four different fibre Stokes numbers (St = 1, 5, 25, and 125) and five different fibre aspect ratios (λ = 1.001, 3, 10, 30, and 50), with results considered at four distances from a channel wall (in the viscous sub-layer, buffer, and fully turbulent regions), which are taken as a measure of the flow velocity gradient, all at a constant fibre to fluid density ratio (ρp/ρ = 760) and shear Reynolds number Reτ = 150. The simulated fibre orientation, concentration, and streakiness confirm previous experimentally observed characteristics of fibre behaviour in turbulence, and that of direct numerical simulations of fibres in Stokesian, or creeping flow, regimes. The fibres exhibit translational motion similar to spheres, where they tend to accumulate in the near-wall (viscous sub-layer and buffer) region and preferentially concentrate in regions of low
A flamelet model for turbulent diffusion combustion in supersonic flow
LEE; ChunHian
2010-01-01
In order to develop a turbulent diffusion combustion model for supersonic flow, the physical argument of the extension of the flamelet model to supersonic flow was presented, and the flow field of a hydrogen/air diffusion combustion generated by axisymmetric supersonic jets was numerically simulated by employing the flamelet model. Using the experimental data, value of the model coefficient of scalar dissipation in the flamelet model was revised specifically for supersonic flow. The computational results of the modified flamelet model were compared with the experimental results, and it was indicated that the precision of the modified flamelet model was satisfying. Based on the numerical results and flamelet theory, the influence mechanisms of turbulence fluctuation on the average state equation and chemical reaction rate were studied for the first time. It was found that the fluctuation correlation of species mass fractions and temperature has little effect on the averaged gas state equation; the temperature fluctuation decreases the product of H2O, but its effect is small; the fluctuation of species mass fractions increases the product of H2O in the region close to oxidizer while decreases the product of H2O in other regions; the fluctuation correlation of species mass fractions and temperature largely decreases the product of H2O.
Predicting Turbulent Convective Heat Transfer in Fully Developed Duct Flows
Rokni, Masoud; Gatski, Thomas B.
2001-01-01
The performance of an explicit algebraic stress model (EASM) is assessed in predicting the turbulent flow and forced heat transfer in both straight and wavy ducts, with rectangular, trapezoidal and triangular cross-sections, under fully developed conditions. A comparison of secondary flow patterns. including velocity vectors and velocity and temperature contours, are shown in order to study the effect of waviness on flow dynamics, and comparisons between the hydraulic parameters. Fanning friction factor and Nusselt number, are also presented. In all cases. isothermal conditions are imposed on the duct walls, and the turbulent heat fluxes are modeled using gradient-diffusion type models. The formulation is valid for Reynolds numbers up to 10(exp 5) and this minimizes the need for wall functions that have been used with mixed success in previous studies of complex duct flows. In addition, the present formulation imposes minimal demand on the number of grid points without any convergence or stability problems. Criteria in terms of heat transfer and friction factor needed to choose the optimal wavy duct cross-section for industrial applications among the ones considered are discussed.
Pipe Flow and Wall Turbulence Using a Modified Navier-Stokes Equation
L. Jirkovsky; A. Muriel
2012-01-01
We use a derived incompressible modified Navier-Stokes equation to model pipe flow and wall turbulence. We reproduce the observed flattened paraboloid velocity profiles of turbulence that cannot be obtained directly using standard incompressible Navier-Stokes equation. The solutions found are in harmony with multi-valued velocity fields as a definition of turbulence. Repeating the procedure for the flow of turbulent fluid between two parallel flat plates we find similar flattened velocity profiles. We extend the analysis to the turbulent flow along a single wall and compare the results with experimental data and the established controversial yon Karman logarithmic law of the wall.
Modification of Turbulent Boundary Layer Flows by Superhydrophobic Surfaces
Gose, James W.; Golovin, Kevin; Barros, Julio; Schultz, Michael P.; Tuteja, Anish; Perlin, Marc; Ceccio, Steven L.
2016-11-01
Measurements of near zero pressure gradient turbulent boundary layer (TBL) flow over several superhydrophobic surfaces (SHSs) are presented and compared to those for a hydraulically smooth baseline. The surfaces were developed at the University of Michigan as part of an ongoing research thrust to investigate the feasibility of SHSs for skin-friction drag reduction in turbulent flow. The SHSs were previously evaluated in fully-developed turbulent channel flow and have been shown to provide meaningful drag reduction. The TBL experiments were conducted at the USNA in a water tunnel with a test section 2.0 m (L) x 0.2 m (W) x 0.2 m (H). The free-stream speed was set to 1.26 m/s which corresponded to a friction Reynolds number of 1,500. The TBL was tripped at the test section inlet with a 0.8 mm diameter wire. The upper and side walls provided optical access, while the lower wall was either the smooth baseline or a spray coated SHS. The velocity measurements were obtained with a TSI FSA3500 two-component Laser-Doppler Velocimeter (LDV) and custom-designed beam displacer operated in coincidence mode. The LDV probe volume diameter was 45 μm (approx. one wall-unit). The measurements were recorded 1.5 m downstream of the trip. When the measured quantities were normalized using the inner variables, the results indicated a significant reduction in the near wall viscous and total stresses with little effect on the flow outside the inner layer.
Yoon, Min; Ahn, Junsun; Hwang, Jinyul; Sung, Hyung Jin
2016-08-01
The relationship between the frictional drag and the velocity-vorticity correlations in wall-bounded turbulent flows is derived from the mean vorticity equation. A formula for the skin friction coefficient is proposed and evaluated with regards to three canonical wall-bounded flows: turbulent boundary layer, turbulent channel flow, and turbulent pipe flow. The frictional drag encompasses four terms: advective vorticity transport, vortex stretching, viscous, and inhomogeneous terms. Drag-reduced channel flow with the slip condition is used to test the reliability of the formula. The advective vorticity transport and vortex stretching terms are found to dominate the contributions to the frictional drag.
GAO Zhen-yu; LIN Jian-zhong; LI Jun
2007-01-01
The rotational dispersion coefficient of the fiber in the turbulent shear flow of fiber suspension was studied theoretically. The function of correlation moment between the different fluctuating velocity gradients of the flow was built firstly. Then the expression, dependent on the characteristic length, time, velocity and a dimensionless parameter related to the effect of wall, of rotational dispersion coefficient is derived. The derived expression of rotational dispersion coefficient can be employed to the inhomogeneous and non-isotropic turbulent flows. Furthermore it can be expanded to three-dimensional turbulent flows and serves the theoretical basis for solving the turbulent flow of fiber suspension.
A Nonlinear k-ε Turbulence Model Applicable to High Pressure Gradient and Large Curvature Flow
Xiyao Gu
2014-01-01
Full Text Available Most of the RANS turbulence models solve the Reynolds stress by linear hypothesis with isotropic model. They can not capture all kinds of vortexes in the turbomachineries. In this paper, an improved nonlinear k-ε turbulence model is proposed, which is modified from the RNG k-ε turbulence model and Wilcox's k-ω turbulence model. The Reynolds stresses are solved by nonlinear methods. The nonlinear k-ε turbulence model can calculate the near wall region without the use of wall functions. The improved nonlinear k-ε turbulence model is used to simulate the flow field in a curved rectangular duct. The results based on the improved nonlinear k-ε turbulence model agree well with the experimental results. The calculation results prove that the nonlinear k-ε turbulence model is available for high pressure gradient flows and large curvature flows, and it can be used to capture complex vortexes in a turbomachinery.
Hybrid finite-volume/transported PDF method for the simulation of turbulent reactive flows
Raman, Venkatramanan
A novel computational scheme is formulated for simulating turbulent reactive flows in complex geometries with detailed chemical kinetics. A Probability Density Function (PDF) based method that handles the scalar transport equation is coupled with an existing Finite Volume (FV) Reynolds-Averaged Navier-Stokes (RANS) flow solver. The PDF formulation leads to closed chemical source terms and facilitates the use of detailed chemical mechanisms without approximations. The particle-based PDF scheme is modified to handle complex geometries and grid structures. Grid-independent particle evolution schemes that scale linearly with the problem size are implemented in the Monte-Carlo PDF solver. A novel algorithm, in situ adaptive tabulation (ISAT) is employed to ensure tractability of complex chemistry involving a multitude of species. Several non-reacting test cases are performed to ascertain the efficiency and accuracy of the method. Simulation results from a turbulent jet-diffusion flame case are compared against experimental data. The effect of micromixing model, turbulence model and reaction scheme on flame predictions are discussed extensively. Finally, the method is used to analyze the Dow Chlorination Reactor. Detailed kinetics involving 37 species and 158 reactions as well as a reduced form with 16 species and 21 reactions are used. The effect of inlet configuration on reactor behavior and product distribution is analyzed. Plant-scale reactors exhibit quenching phenomena that cannot be reproduced by conventional simulation methods. The FV-PDF method predicts quenching accurately and provides insight into the dynamics of the reactor near extinction. The accuracy of the fractional time-stepping technique in discussed in the context of apparent multiple-steady states observed in a non-premixed feed configuration of the chlorination reactor.
Large-eddy simulation of turbulent flow using the finite element method
McCallen, R.C.
1995-02-15
The equations of motion describing turbulent flows (in both the low and high Reynolds-number regimes) are well established. However, present day computers cannot meet the enormous computational requirement for numerically solving the governing equations for common engineering flows in the high Reynolds number turbulent regime. The characteristics that make turbulent, high Reynolds number flows difficult to simulate is the extreme range of time and space scales of motion. Most current engineering calculations are performed using semi-empirical equations, developed in terms of the flow mean (average) properties. These turbulence{open_quote} models{close_quote} (semi-empirical/analytical approximations) do not explicitly account for the eddy structures and thus, the temporal and spatial flow fluctuations are not resolved. In these averaging approaches, it is necessary to approximate all the turbulent structures using semi-empirical relations, and as a result, the turbulence models must be tailored for specific flow conditions and geometries with parameters obtained (usually) from physical experiments. The motivation for this research is the development of a finite element turbulence modeling approach which will ultimately be used to predict the wind flow around buildings. Accurate turbulence models of building flow are needed to predict the dispersion of airborne pollutants. The building flow turbulence models used today are not capable of predicting the three-dimensional separating and reattaching flows without the manipulation of many empirical parameters. These empirical parameters must be set by experimental data and they may vary unpredictably with building geometry, building orientation, and upstream flow conditions.
Sid, Samir; Terrapon, Vincent; Dubief, Yves
2015-11-01
Results of direct numerical simulation of turbulent channel flows under unstable stratification are reported. Two Reynolds number are considered: Reτ = 180 , 395 and the Rayleigh number ranges between Ra = [106 -109 ] . The Prandtl number is set to 1. The channel is periodic in both streamwise and spanwise directions and non-slip/isothermal boundary conditions are imposed at the walls. The temperature difference between the walls is set so that the stratification is unstable and the coupling between temperature and momentum is achieved using the Boussinesq approximation. The dependency of the typical large scale convective structures on both Reynolds and Rayleigh numbers are investigated through cross flow sectional statistics and instantaneous flow field visualizations. Moreover, the effects of the natural convection on the coherent structures associated to the cycle of wall-bounded turbulence (Jimenez, et al. JFM 1999), namely velocity streaks and streamwise vortices, are examined. Finally, macroscopic quantities such as friction coefficient and Nusselt number are reported as a function of the Rayleigh number and are compared for both Reynolds numbers. The Belgian Team acknowledges computational resources from CÉCI (F.R.S.-FNRS grant No.2.5020.11) and the PRACE infrastructure. YD acknowledges the support of NSF and DOE under grant NSF/DOE 1258697.
Turbulent flow and scalar transport in a large wind farm
Porte-Agel, F.; Markfort, C. D.; Zhang, W.
2012-12-01
Wind energy is one of the fastest growing sources of renewable energy world-wide, and it is expected that many more large-scale wind farms will be built and cover a significant portion of land and ocean surfaces. By extracting kinetic energy from the atmospheric boundary layer and converting it to electricity, wind farms may affect the transport of momentum, heat, moisture and trace gases (e.g. CO_2) between the atmosphere and the land surface locally and globally. Understanding wind farm-atmosphere interaction is complicated by the effects of turbine array configuration, wind farm size, land-surface characteristics, and atmospheric thermal stability. A wind farm of finite length may be modeled as an added roughness or as a canopy in large-scale weather and climate models. However, it is not clear which analogy is physically more appropriate. Also, surface scalar flux is affected by wind farms and needs to be properly parameterized in meso-scale and/or high-resolution numerical models. Experiments involving model wind farms, with perfectly aligned and staggered configurations, having the same turbine distribution density, were conducted in a thermally-controlled boundary-layer wind tunnel. A neutrally stratified turbulent boundary layer was developed with a surface heat source. Measurements of the turbulent flow and fluxes over and through the wind farm were made using a custom x-wire/cold-wire anemometer; and surface scalar flux was measured with an array of surface-mounted heat flux sensors far within the quasi-developed region of the wind-farm. The turbulence statistics exhibit similar properties to those of canopy-type flows, but retain some characteristics of surface-layer flows in a limited region above the wind farms as well. The flow equilibrates faster and the overall momentum absorption is higher for the staggered compared to the aligned farm, which is consistent with canopy scaling and leads to a larger effective roughness. Although the overall surface
Chemically reacting dusty viscoelastic fluid flow in an irregular channel with convective boundary
R. Sivaraj
2013-03-01
Full Text Available In this paper, we have studied the combined effects of free convective heat and mass transfer on an unsteady MHD dusty viscoelastic (Walters liquid model-B fluid flow between a vertical long wavy wall and a parallel flat wall saturated with porous medium subject to the convective boundary condition. The coupled partial differential equations are solved analytically using perturbation technique. The velocity, temperature and concentration fields have been studied for various combinations of physical parameters such as magnetic field, heat absorption, thermal radiation, radiation absorption, Biot number and chemical reaction parameters. The skin friction, Nusselt number and Sherwood number are also presented and displayed graphically. Further, it is observed that the velocity profiles of dusty fluid are higher than the dust particles.
Adjoint-based sensitivity of flames to ignition parameters in non-premixed shear-flow turbulence
Capecelatro, Jesse; Bodony, Daniel; Freund, Jonathan
2016-11-01
The adjoint of the linearized and perturbed compressible flow equations for a mixture of chemically reacting ideal gases is used to assess the sensitivity of ignition in non-premixed shear-flow turbulence. Direct numerical simulations are used to provide an initial prediction, and the corresponding space-time discrete-exact adjoint is used to provide a sensitivity gradient for a specific quantity of interest (QoI). Owing to the ultimately binary outcome of ignition (i.e., it succeeds or fails after some period), a QoI is defined that both quantifies ignition success and varies smoothly near its threshold based on the heat release parameter in a short-time horizon during the ignition process. We use the resulting gradient to quantify the flow properties and model parameters that most affect the initiation of a sustained flame. A line-search algorithm is used to identify regions of high ignition probability and map the boundary between successful and failed ignition. The approach is demonstrated on a non-premixed turbulent shear layer and on a reacting jet-in-crossflow.
On the origin of streaks in turbulent shear flows
Waleffe, Fabian; Kim, John
1991-01-01
The paper substantiates the notion that selective amplification and direct resonance, based on linear theory, does not provide a selection mechanism for the well-defined streak spacing of about 100 wall units observed in wall-bounded turbulent shear flows. For the direct resonance theory, it is shown that the streaks are created by the nonlinear self-interaction of the vertical velocity rather than that of the directly forced vertical vorticity. It is proposed that the selection mechanism must be inherently nonlinear and correspond to a self-sustaining process. For the case of plane Poiseuille flow the 100-wall-unit criterion corresponds to a critical Reynolds number of 1250, based on the centerline velocity and the channel half-width, which is close to the usually quoted value of about 1000. In plane Couette flow, it corresponds to a critical Reynolds number of 625, based on the half-velocity difference and the half-width.
Renormalization group analysis of anisotropic diffusion in turbulent shear flows
Rubinstein, Robert; Barton, J. Michael
1991-01-01
The renormalization group is applied to compute anisotropic corrections to the scalar eddy diffusivity representation of turbulent diffusion of a passive scalar. The corrections are linear in the mean velocity gradients. All model constants are computed theoretically. A form of the theory valid at arbitrary Reynolds number is derived. The theory applies only when convection of the velocity-scalar correlation can be neglected. A ratio of diffusivity components, found experimentally to have a nearly constant value in a variety of shear flows, is computed theoretically for flows in a certain state of equilibrium. The theoretical value is well within the fairly narrow range of experimentally observed values. Theoretical predictions of this diffusivity ratio are also compared with data from experiments and direct numerical simulations of homogeneous shear flows with constant velocity and scalar gradients.
V. Ramanujachari
1992-10-01
Full Text Available The dispersion of particles in the presence of Turbulent gas flow is studied theoretically using a stochastic separated flow model and the results compared with the available experimental data. As the particle loading in the jet is of the order of 0.1-0.4 per cent, the particles are assumed to have negligible effect on the mean and the turbulent gas phase properties (one-way coupling. The particle-turbulent eddy interactions are calculated by paying attention to the energy containing eddies, characterised by the integral length scale. The fluctuating velocities are sampled randomly from Gaussian distribution, and the particle trajectories are obtained using a procedure similar to random-walk computation. A large number of particle trajectories are averaged to obtain the statistical nature of the turbulent gas-particle jet. It is seen that the particles with less inertia, which are characterised by the Stokes number, tend to diffuse more. The turbulent diffusivities of the particles are in agreement with the available experimental data, when the time-averaged velocities of gas and particles are the same, obtained by the stochastic separated flow model.
Tripping Effects on the Friction Factor in Turbulent Pipe Flows
Al-Salaymeh, A.; Bayoumi, O. A.; Durst, F.; Gad-El-Hak, M.
2004-11-01
Tripping devices are usually installed at the entrance of laboratory-scale pipe test sections to obtain a fully developed turbulent flow sooner. The tripping of laminar flow to induce turbulence can be carried out in different ways, such as using cylindrical wires, sand papers, well-organized tape letters, fences, etc. Claims of tripping effects have been made periodically since the classical experiments of Nikuradse (1932), which covered a significant range of Reynolds numbers. NikuradseÂ's data have become the metric by which theories are established, and have also been the subject of intense scrutiny. Several subsequent experiments reported friction factors as much as 5% lower than those measured by Nikuradse, and the authors of those reports attributed the difference to tripping effects. In the present study, measurements with and without ring tripping devices of different blocking areas of 10%, 20%, 30% and 40% have been carried out to determine the effect of entrance condition on the developing flow field in pipes. Along with pressure drop measurements to compute the skin friction, both Pitot tube and hot-wire anemometry measurements have been used to accurately determine the mean velocity profile over the working test section at different Reynolds numbers in the range of 1× 10^5--4.5 × 10^5. The results we obtained suggest that the tripping technique has an insignificant effect on the wall friction factor, in agreement with Nikuradse's original data.
Proposed method for measurement of flow rate in turbulent periodic pipe flow
Werzner, E.; Ray, S.; Trimis, D.
2011-12-01
The present investigation deals with a previously proposed flow metering technique for laminar, fully-developed, time-periodic pipe flow. Employing knowledge of the pulsation frequency-dependent relationship between the mass flow rate and the pressure gradient, the method allows reconstruction of the instantaneous mass flow rate on the basis of a recorded pressure gradient time series. In order to explore if the procedure can be extended for turbulent flows, numerical simulations for turbulent, fully-developed, sinusoidally pulsating pipe flow with low pulse amplitude have been carried out using a ν2-f turbulence model. The study covers pulsation frequencies, ranging from the quasi-steady up to the inertia-dominated frequency regime, and three cycle-averaged Reynolds numbers of 4360, 9750 and 15400. After providing the theoretical background of the flow rate reconstruction principle, the numerical model and an experimental facility for the verification of simulations are explained. The obtained results, presented in time and frequency domain, show good agreement with each other and indicate a frequency dependence, similar to that used for the signal reconstruction for laminar flows. A modified dimensionless frequency definition has been introduced, which allows a generalised representation of the results considering the influence of Reynolds number.
Graeger, Helmut
Drag reduction in turbulent pipe flow is obtained by addition of polymeric flow accelerator. Turbulent flow pattern is described on the basis of existing theories and reduction of loss of pressure heads is discussed. A turbulence rheometer is developed permitting the measurement of friction reduction for Reynolds numbers 1100 to 90,000. Effectiveness of water soluble polymer systems like polyacrylamide and coacrylate is studied in dependence of concentration, chemical composition, product aging and polymer chain deformation.
Magnetohydrodynamic flows and turbulence: a report on the Fourth Beer-Sheva seminar
Branover, H.; Mond, M. (Ben-Gurion Univ. of the Negev, Beersheba (Israel). Dept. of Mechanical Engineering); Pierson, E.S. (Purdue Univ. Calumet, Hammond, IN (USA)); Walker, J.S. (Illinois Univ., Urbana (USA))
1984-11-01
This paper is a summary of the Fourth Beer-Sheva Seminar on Magnetohydrodynamic (MHD) Flows and Turbulence held in Israel during 27 February-2 March 1984 with 67 participants from 13 countries. Reviews and contributed papers were presented on laminar and turbulent single-phase and two-phase MHD flows, turbulent and two-phase flows without magnetic fields, and applications of MHD in power generation, in nuclear fission and fusion and in metallurgy.
Water circulation in non-isothermal droplet-laden turbulent channel flow
Russo, E.; Kuerten, J.G.M.; Geld, van der C.W.M.; Geurts, B.J.; Simos, T.; Psihoyios, G.; Tsitouras, Ch.
2013-01-01
We propose a point-particle model for two-way coupling of water droplets dispersed in turbulent flow of a carrier gas consisting of air and water vapor. An incompressible flow formulation is applied for direct numerical simulation (DNS) of turbulent channel flow with a warm and a cold wall. Compared
Befrui, Bizhan A.
1995-01-01
This viewgraph presentation discusses the following: STAR-CD computational features; STAR-CD turbulence models; common features of industrial complex flows; industry-specific CFD development requirements; applications and experiences of industrial complex flows, including flow in rotating disc cavities, diffusion hole film cooling, internal blade cooling, and external car aerodynamics; and conclusions on turbulence modeling needs.
Water circulation in non-isothermal droplet-laden turbulent channel flow
Russo, E; Kuerten, Johannes G.M.; van der Geld, C.W.M.; Geurts, Bernardus J.; Simos, T.; Psihoyios, G.; Tsitouras, Ch.
2013-01-01
We propose a point-particle model for two-way coupling of water droplets dispersed in turbulent flow of a carrier gas consisting of air and water vapor. An incompressible flow formulation is applied for direct numerical simulation (DNS) of turbulent channel flow with a warm and a cold wall. Compared
Oliver, Todd; Ulerich, Rhys; Topalian, Victor; Malaya, Nick; Moser, Robert
2013-11-01
A discretization of the Navier-Stokes equations appropriate for efficient DNS of compressible, reacting, wall-bounded flows is developed and applied. The spatial discretization uses a Fourier-Galerkin/B-spline collocation approach. Because of the algebraic complexity of the constitutive models involved, a flux-based approach is used where the viscous terms are evaluated using repeated application of the first derivative operator. In such an approach, a filter is required to achieve appropriate dissipation at high wavenumbers. We formulate a new filter source operator based on the viscous operator. Temporal discretization is achieved using the SMR91 hybrid implicit/explicit scheme. The linear implicit operator is chosen to eliminate wall-normal acoustics from the CFL constraint while also decoupling the species equations from the remaining flow equations, which minimizes the cost of the required linear algebra. Results will be shown for a mildly supersonic, multispecies boundary layer case inspired by the flow over the ablating surface of a space capsule entering Earth's atmosphere. This work is supported by the Department of Energy [National Nuclear Security Administration] under Award Number [DE-FC52-08NA28615].
Inverse cascades in turbulence and the case of rotating flows
Pouquet, A; Rosenberg, D; Mininni, P D; Baerenzung, J
2012-01-01
We first summarize briefly several properties concerning the dynamics of two-dimensional (2D) turbulence, with an emphasis on the inverse cascade of energy to the largest accessible scale of the system. In order to study a similar phenomenon in three-dimensional (3D) turbulence undergoing strong solid-body rotation, we test a previously developed Large Eddy Simulation (LES) model against a high-resolution direct numerical simulation of rotating turbulence on a grid of $3072^3$ points. We then describe new numerical results on the inverse energy cascade in rotating flows using this LES model and contrast the case of 2D versus 3D forcing, as well as non-helical forcing (i.e., with weak overall alignment between velocity and vorticity) versus the fully helical Beltrami case, both for deterministic and random forcing. The different scaling of the inverse energy cascade can be attributed to the dimensionality of the forcing, with, in general, either a $k_{\\perp}^{-3}$ or a $k_{\\perp}^{-5/3}$ energy spectrum of slo...
Guertler, Niels
2011-01-01
Turbulence driven zonal flows play an important role in fusion devices since they improve plasma confinement by limiting the level of anomalous transport. Current theories mostly focus on flow excitation but do not self-consistently describe the nearly stationary zonal flow turbulence equilibrium state. First-principles two-fluid turbulence studies are used to construct a Reynolds stress response functional from observations in turbulent states. This permits, for the first time, a reliable charting of zonal flow turbulence equilibria.
A skin friction law for compressible turbulent flow
Barnwell, Richard W.; Wahls, Richard A.
1989-01-01
An algebraic skin friction law is derived for adiabatic, compressible, equilibrium, turbulent boundary layer flow. An outer solution in terms of the Clauser defect stream function is matched to an inner empirical expression composed of compressible laws of the wall and wake. The modified Crocco temperature-velocity relationship and the Clauser eddy viscousity model are used in the outer solution. The skin friction law pertains for all pressure gradients in the incompressible through supersonic range and for small pressure gradients in the hypersonic range. Excellent comparisons with experiment are obtained in the appropriate parameter ranges. The application to numerical computation is discussed.
Occurrence of turbulent flow conditions in supercritical fluid chromatography.
De Pauw, Ruben; Choikhet, Konstantin; Desmet, Gert; Broeckhoven, Ken
2014-09-26
Having similar densities as liquids but with viscosities up to 20 times lower (higher diffusion coefficients), supercritical CO2 is the ideal (co-)solvent for fast and/or highly efficient separations without mass-transfer limitations or excessive column pressure drops. Whereas in liquid chromatography the flow remains laminar in both the packed bed and tubing, except in extreme cases (e.g. in a 75 μm tubing, pure acetonitrile at 5 ml/min), a supercritical fluid can experience a transition from laminar to turbulent flow in more typical operation modes. Due to the significant lower viscosity, this transition for example already occurs at 1.3 ml/min for neat CO2 when using connection tubing with an ID of 127 μm. By calculating the Darcy friction factor, which can be plotted versus the Reynolds number in a so-called Moody chart, typically used in fluid dynamics, higher values are found for stainless steel than PEEK tubing, in agreement with their expected higher surface roughness. As a result turbulent effects are more pronounced when using stainless steel tubing. The higher than expected extra-column pressure drop limits the kinetic performance of supercritical fluid chromatography and complicates the optimization of tubing ID, which is based on a trade-off between extra-column band broadening and pressure drop. One of the most important practical consequences is the non-linear increase in extra-column pressure drop over the tubing downstream of the column which leads to an unexpected increase in average column pressure and mobile phase density, and thus decrease in retention. For close eluting components with a significantly different dependence of retention on density, the selectivity can significantly be affected by this increase in average pressure. In addition, the occurrence of turbulent flow is also observed in the detector cell and connection tubing. This results in a noise-increase by a factor of four when going from laminar to turbulent flow (e.g. going
Geotropic tracers in turbulent flows: a proxy for fluid acceleration
Boffetta, Guido; De Lillo, Filippo; Santamaria, Francesco
2013-01-01
We investigate the statistics of orientation of small, neutrally buoyant, spherical tracers whose center of mass is displaced from the geometrical center. If appropriate-sized particles are considered, a linear relation can be derived between the horizontal components of the orientation vector and the same components of acceleration. Direct numerical simulations are carried out, showing that such relation can be used to reconstruct the statistics of acceleration fluctuations up to the order of the gravitational acceleration. Based on such results, we suggest a novel method for the local experimental measurement of accelerations in turbulent flows.
Numerical Analysis of Turbulent Flows in Channels of Complex Geometry
Farbos De Luzan, Charles
The current study proposes to follow a systematic validated approach to applied fluid mechanics problems in order to evaluate the ability of different computational fluid dynamics (CFD) to be a relevant design tool. This systematic approach involves different operations such as grid sensitivity analyses, turbulence models comparison and appropriate wall treatments, in order to define case-specific optimal parameters for industrial applications. A validation effort is performed on each study, with particle image velocimetry (PIV) experimental results as the validating metric. The first part of the dissertation lays down the principles of validation, and presents the details of a grid sensitivity analysis, as well as a turbulence models benchmark. The models are available in commercial solvers, and in most cases the default values of the equations constants are retained. The validation experimental data is taken with a hot wire, and has served as a reference to validate multiple turbulence models for turbulent flows in channels. In a second part, the study of a coaxial piping system will compare a set of different steady Reynolds-Averaged Navier Stokes (RANS) turbulence models, namely the one equation model Spalart-Almaras, and two-equation-models standard k-epsilon, k-epsilon realizable, k-epsilon RNG, standard k-omega, k-omega SST, and transition SST. The geometry of interest involves a transition from an annulus into a larger one, where highly turbulent phenomena occur, such as recirculation and jet impingement. Based on a set of constraints that are defined in the analysis, a chosen model will be tested on new designs in order to evaluate their performance. The third part of this dissertation will address the steady-state flow patterns in a Viscosity-Sensitive Fluidic Diode (VSFD). This device is used in a fluidics application, and its originality lies in the fact that it does not require a control fluid in order to operate. This section will discuss the
The Pressure Distribution in Thermally Bistable Turbulent Flows
Gazol, Adriana; Vázquez-Semadeni, Enrique; Kim, Jongsoo
2005-09-01
We present a systematic numerical study of the effect of turbulent velocity fluctuations on the thermal pressure distribution in thermally bistable flows. The turbulent fluctuations are characterized by their rms Mach number M (with respect to the warm medium) and the energy injection (forcing) wavenumber kfor=1/l, where l is the injection size scale in units of the box size L=100 pc. The numerical simulations employ random turbulent driving generated in Fourier space rather than starlike heating, in order to allow for precise control of the parameters. Our range of parameters is 0.5kfor7.1 cm-3) with P>104 cm-3 K increases from roughly 0.1% at kfor=2 and M=0.5 to roughly 70% for kfor=16 and M=1.25. A preliminary comparison with the recent pressure measurements of Jenkins in C I favors our case with M=0.5 and kfor=2. In all cases, the dynamic range of the pressure in any given density interval is larger than one order of magnitude, and the total dynamic range, summed over the entire density range, typically spans 3-4 orders of magnitude. The total pressure histogram widens as the Mach number is increased, and moreover develops near-power-law tails at high (low) pressures when γe~1), which occurs at kfor=2 (kfor=16) in our simulations. The opposite side of the pressure histogram decays rapidly, in an approximately lognormal form. This behavior resembles that of the corresponding density histograms, in spite of the large scatter of the pressure in any given density interval. Our results show that turbulent advection alone can generate large pressure scatters, with power-law high-P tails for large-scale driving, and provide validation for approaches attempting to derive the shape of the pressure histogram through a change of variable from the known form of the density histogram, such as that performed by Mac Low et al.
Sensing turbulent flow and heat transport in a cave conduit
Kurtzman, D.; Lucia, F. J.; Jennings, J. W.; Wilson, J. L.; Tyler, S. W.; Jorgensen, A. M.; Dwivedi, R.; Boston, P.; Burger, P.
2008-12-01
Cave systems provide an extreme example of complex subsurface porous media, dominated by flow through an interconnected network of conduits. Whether water or air-filled, these flow systems have been largely observed subjectively, with only a few simple quantitative measurements of flow and pressure. In the spring of 2008 a joint campaign of New Mexico Tech and the University of Nevada Reno entered the 210m deep, ~8m "diameter," keyhole shaped, subhorizontal, Left Hand Tunnel, a large air-filled conduit in Carlsbad Caverns, New Mexico, with the intent to observe fluid flow with modern thermally-based instruments. The conduit experiences countercurrent, thermally stratified flow, with mean velocities in each layer less than 0.1m/s. It is part of a geothermally forced, large-scale convection cell. Two instruments were deployed. A distributed temperature sensing (DTS) fiber optic cable was stretched over 1km of the tunnel, and partially suspended by balloons to the roof, to sense spatial and low-frequency (0.01Hz) temporal variations of temperature with a resolution approaching 0.05 degree C. The mean temperature difference between layers was on the order of 0.5 degree and the caveward (subhorizontal) temperature gradient was 1 degree/400m. Influences of connecting subvertical shafts, wet areas of the cave, human activity, and diurnal fluctuations were observed. The second instrument, a 7m tall tower with an array of eight 300Hz thermocouple temperature sensors, with a sensitivity approaching 0.005 degree, was deployed 200m into the tunnel and used to detect high- frequency temperature fluctuations associated with turbulence and the stratified flow. Turbulence structure of each layer was similar. Temperature fluctuation (and turbulence intensity?) was significantly greater near the boundary between layers and its steep vertical gradient of mean temperature. Results from this 3-day campaign, as limited as they are, suggest that there is a wealth of information and
Experimental investigation of turbulent flow over a permeable rough wall
Kim, T.; Blois, G.; Best, J.; Christensen, K. T.
2015-12-01
Permeable walls are encountered in a variety of geophysical flows, including alluvial river beds, canopies and urban environments. Permeable walls possess very different boundary conditions as compared to classic impermeable walls (i.e. the slip condition and penetration of flow into the bed). Permeability allows flow interactions across the wall interface, resulting in notable mass, momentum and energy exchange. Such exchange takes place in the so-called transition layer and often occurs through turbulent flow mechanisms. It is increasingly recognized that turbulence plays a key role in a number of important natural functions, including biogeochemical as well as geomorphological processes. However, the flow physics of the transition layer are still poorly understood due to a lack of quantitative investigation of these permeable systems within which physical and optical access are severely compromised. This is particularly true for state-of-the-art flow measurement techniques such as particle image velocimetry (PIV) that require unaberrated optical access to the measurement locations. To overcome optical limitations, a refractive index matching (RIM) technique was employed herein to gain full optical access to the transition layer. Sodium Iodide aqueous solution (63% by weight and RI ~ 1.496 at 20°C) served as a working fluid, and an acrylic resin (RI ~ 1.499) was chosen for fabricating wall models. Measurements were performed using high-resolution planar PIV in different configurations to characterize the turbulent boundary layer and the transition layer. The wall models comprised uniform spheres packed in a cubic arrangement, and two cases were modeled - impermeable and permeable walls that were both rough. To eliminate the effect of roughness, and thus isolate the effect of permeability, the surface roughness of the two wall models was kept identical. This allowed us to obtain a more meaningful comparison and highlight the impact of wall permeability in natural
Koutsona, Maria
This work is a numerical study of the design and operation of two reacting flow systems, each with great potential in their fields. The design of reacting flow systems by computer simulations are successfully used in science and engineering to evaluate design geometries and operation, without resorting to experimental trial and error that is expensive, time consuming and, in some cases, dangerous. The models of the two systems described in this work are based on fundamental conservation equations for momentum and mass transfer coupled with chemical reaction kinetics and particle dynamics. The first part of this work is a study aiming to elucidate the transport phenomena and chemical reactions that control the size of ZnSe nanoparticles formed by a new vapor-phase synthesis route. The nanoparticles are synthesized by reacting vapors of (CH3)2Zn:N(C2H 5)3 adduct with H2Se gas (diluted in hydrogen) fed continuously from opposite sides into a counterflow jet reactor. The nuclei of the nanocrystals are formed by a direct condensation reaction near the stagnation point. The nuclei grow into nanoparticles by coalescence/coagulation and by surface growth reactions. A 2D model of an axially symmetric reactor was developed that includes descriptions of flow, mass transfer by convection and diffusion, chemical kinetics, particle nucleation, coagulation and surface growth. The coupled nonlinear partial differential equations of the model were solved using the Galerkin Finite Element Method. The model was used to study the relative importance of the underlying physical and chemical phenomena in controlling particle size and particle size distribution. Model predictions compared well with the limited experimental data available for this system. The model was also used for model-assisted design of the experimental counterflow jet reactor, where vapor-phase synthesis of ZnSe nanoparticles was demonstrated for the first time. The second part of this work involves the development of
Low Reynolds number turbulence modeling of blood flow in arterial stenoses.
Ghalichi, F; Deng, X; De Champlain, A; Douville, Y; King, M; Guidoin, R
1998-01-01
Moderate and severe arterial stenoses can produce highly disturbed flow regions with transitional and or turbulent flow characteristics. Neither laminar flow modeling nor standard two-equation models such as the kappa-epsilon turbulence ones are suitable for this kind of blood flow. In order to analyze the transitional or turbulent flow distal to an arterial stenosis, authors of this study have used the Wilcox low-Re turbulence model. Flow simulations were carried out on stenoses with 50, 75 and 86% reductions in cross-sectional area over a range of physiologically relevant Reynolds numbers. The results obtained with this low-Re turbulence model were compared with experimental measurements and with the results obtained by the standard kappa-epsilon model in terms of velocity profile, vortex length, wall shear stress, wall static pressure, and turbulence intensity. The comparisons show that results predicted by the low-Re model are in good agreement with the experimental measurements. This model accurately predicts the critical Reynolds number at which blood flow becomes transitional or turbulent distal an arterial stenosis. Most interestingly, over the Re range of laminar flow, the vortex length calculated with the low-Re model also closely matches the vortex length predicted by laminar flow modeling. In conclusion, the study strongly suggests that the proposed model is suitable for blood flow studies in certain areas of the arterial tree where both laminar and transitional/turbulent flows coexist.
A Gas-Kinetic Scheme For The Simulation Of Compressible Turbulent Flows
Righi, Marcello
2013-01-01
A gas-kinetic scheme for the continuum regime is applied to the simulation of turbu- lent compressible flow, by replacing the molecular relaxation time with a turbulent relaxation time in the BGK model. The turbulence dynamics is modelled on the basis of a standard, linear two-equation turbulence model. The hydrodynamic limit of the resulting turbulence model is linear in smooth flow and non-linear in the presence of stronger flow gradients. The non-linear correction terms in the numerical flux are weighed as a function of "rarefaction" - referred to turbulence dynamics and not to molecular dynamics, i.e. measured by the ratio of turbulence to mean flow scales of motion. Even though no assumptions on the nature of the turbulence have been made and a linear two-equation turbulence model is used, the turbulence gas-kinetic scheme seems able to correct the turbulent stress tensor in an effective way; on the basis of a number of turbulence modelling benchmark flow cases, characterized by strong shock - boundary l...
Xingtuan Yang
2015-01-01
Full Text Available This study investigates the anisotropic characteristics of turbulent energy dissipation rate in a rotating jet flow via direct numerical simulation. The turbulent energy dissipation tensor, including its eigenvalues in the swirling flows with different rotating velocities, is analyzed to investigate the anisotropic characteristics of turbulence and dissipation. In addition, the probability density function of the eigenvalues of turbulence dissipation tensor is presented. The isotropic subrange of PDF always exists in swirling flows relevant to small-scale vortex structure. Thus, with remarkable large-scale vortex breakdown, the isotropic subrange of PDF is reduced in strongly swirling flows, and anisotropic energy dissipation is proven to exist in the core region of the vortex breakdown. More specifically, strong anisotropic turbulence dissipation occurs concentratively in the vortex breakdown region, whereas nearly isotropic turbulence dissipation occurs dispersively in the peripheral region of the strong swirling flows.
Investigation of particle-laden turbulent flow in free shear turbulent combustion
Buckingham, A.C.; Siekhaus, W.J.; Ellzey, J.; Daily, J.W.
1983-01-01
Explicit numerical mixed phase simulations are described which couple random gasdynamic motions to inertiallly interactive gas borne particles. Theses simulations are numerical experiments intended to provide data for investigating the interaction between a developing turbulent free shear layer and gas borne solid particles it entrains. The simulations predict most probable distributions of dispersed phase trajectories, standard deviations, and gas phase mixing dynamics which include the concomitant back-influences of the particle phase on the carrier gas flow. Data for refinement of the computational scheme and physical verification are provided by experiment. The experimental evidence is developed in a splitter plate divided, two-channel free shear mixing combustion tube. A variety of particle concentrations and particle size distributions are admitted into non-combusting or combusting flows with selected heat release levels. The computations, in turn, provide guidance on design and selection of new experiments.
Measurements of turbulent flow overlying impermeable and permeable walls
Kim, Taehoon; Blois, Gianluca; Best, James; Christensen, Kenneth
2016-11-01
There exist an array of natural and industrial flow systems wherein the flow is bounded by a surface that is both permeable and rough (e.g. river beds, bed reactors). In such scenarios, the wall boundary condition is complex as it involves both slip and penetration which together significantly modify the statistical and structural modifications the overlying flow owing to momentum exchange across the wall. The current investigation explores the individual roles of topography and permeability in such flows by systematically decoupling one from the other with a number of wall models having the same porous structure (i.e. cubically arranged spheres; two and five layers, respectively, to highlight the effect of turbulence penetration depth) but with different surface topography (smooth versus cubically arranged hemispheres). High resolution particle-image velocimetry measurements were conducted in the streamwise-wall-normal (x - y) plane and refractive-index matching was employed to optically access the flow within the permeable wall. First- and second-order velocity statistics are used to assess the flow modifications associated with the different wall models and thus ascertain the individual impacts of permeability and topography. NSF.
Experimental study of particle-driven secondary flow in turbulent pipe flows
Belt, R.J.; Daalmans, A.C.L.M.; Portela, L.M.
2012-01-01
In fully developed single-phase turbulent flow in straight pipes, it is known that mean motions can occur in the plane of the pipe cross-section, when the cross-section is non-circular, or when the wall roughness is non-uniform around the circumference of a circular pipe. This phenomenon is known as
Experimental study of particle-driven secondary flow in turbulent pipe flows
Belt, R.J.; Daalmans, A.C.L.M.; Portela, L.M.
2012-01-01
In fully developed single-phase turbulent flow in straight pipes, it is known that mean motions can occur in the plane of the pipe cross-section, when the cross-section is non-circular, or when the wall roughness is non-uniform around the circumference of a circular pipe. This phenomenon is known as
Turbulent behaviour of non-cohesive sediment gravity flows at unexpectedly high flow density
Baker, Megan; Baas, Jaco H.; Malarkey, Jonathan; Kane, Ian
2016-04-01
Experimental lock exchange-type turbidity currents laden with non-cohesive silica-flour were found to be highly dynamic at remarkably high suspended sediment concentrations. These experiments were conducted to produce sediment gravity flows of volumetric concentrations ranging from 1% to 52%, to study how changes in suspended sediment concentration affects the head velocities and run-out distances of these flows, in natural seawater. Increasing the volumetric concentration of suspended silica-flour, C, up to C = 46%, within the flows led to a progressive increase in the maximum head velocity. This relationship suggests that suspended sediment concentration intensifies the density difference between the turbulent suspension and the ambient water, which drives the flow, even if almost half of the available space is occupied by sediment particles. However, from C = 46% to C = 52% a rapid reduction in the maximum head velocity was measured. It is inferred that at C = 46%, friction from grain-to-grain interactions begins to attenuate turbulence within the flows. At C > 46%, the frictional stresses become progressively more dominant over the turbulent forces and excess density, thus producing lower maximum head velocities. This grain interaction process started to rapidly reduce the run-out distance of the silica-flour flows at equally high concentrations of C ≥ 47%. All flows with C 9%. Yet, the critical flow concentration at which turbulence modulation commenced for these silica-flour laden flows appeared to be much higher. We suggest that Bagnold's 9% criterion cannot be applied to flows that carry fine-grained sediment, because turbulent forces are more important than dispersive forces, and frictional forces start to affect the flows only at concentrations just below the cubic packing density of spheres of C = 52%. These experimental results also imply that natural flows may be able to transport vast volumes of non-cohesive sediment with relative ease, especially
Turbulent flow as a cause for underestimating coronary flow reserve measured by Doppler guide wire
Richartz Barbara M
2006-03-01
Full Text Available Abstract Background Doppler-tipped coronary guide-wires (FW are well-established tools in interventional cardiology to quantitatively analyze coronary blood flow. Doppler wires are used to measure the coronary flow velocity reserve (CFVR. The CFVR remains reduced in some patients despite anatomically successful coronary angioplasty. It was the aim of our study to test the influence of changes in flow profile on the validity of intra-coronary Doppler flow velocity measurements in vitro. It is still unclear whether turbulent flow in coronary arteries is of importance for physiologic studies in vivo. Methods We perfused glass pipes of defined inner diameters (1.5 – 5.5 mm with heparinized blood in a pulsatile flow model. Laminar and turbulent flow profiles were achieved by varying the flow velocity. The average peak velocity (APV was recorded using 0.014 inch FW. Flow velocity measurements were also performed in 75 patients during coronary angiography. Coronary hyperemia was induced by intra-coronary injection of adenosine. The APV maximum was taken for further analysis. The mean luminal diameter of the coronary artery at the region of flow velocity measurement was calculated by quantitative angiography in two orthogonal planes. Results In vitro, the measured APV multiplied with the luminal area revealed a significant correlation to the given perfusion volumes in all diameters under laminar flow conditions (r2 > 0.85. Above a critical Reynolds number of 500 – indicating turbulent flow – the volume calculation derived by FW velocity measurement underestimated the actual rate of perfusion by up to 22.5 % (13 ± 4.6 %. In vivo, the hyperemic APV was measured irrespectively of the inherent deviation towards lower velocities. In 15 of 75 patients (20% the maximum APV exceeded the velocity of the critical Reynolds number determined by the in vitro experiments. Conclusion Doppler guide wires are a valid tool for exact measurement of coronary flow
Velocity kinematic relations in a turbulent flow past a grid
Liberzon, Alex; Gurka, Roi; Kopp, Gregory; Sarathi, Partha; Tsinober, Arkady
2009-11-01
We present velocity kinematic relations, involving average and difference of the longitudinal velocity component of the two points at distance r: u+= u(x+r) + u(x) and u-= u(x+r)-u(x), obtained using PIV measurements in a turbulent flow of water past a grid. The present study follows recent numerical and experimental studies, that demonstrated analytical and empirical evidence of the relations, their validity and it emphasizes the physical meaning of the relations. The relations that contain both the large (u+) and small (u-) scale quantities emphasize the non-local aspects of turbulent flows. For example, the pure kinematic relation of Hosokawa in conjunction with the the Kolmogorov 4/5 law leading to the = r/30 shows that the that the large and small scale quantities are correlated contrary to what is suggested by the commonly used sweeping decorrelation hypothesis. Some relations are purely kinematic and some are dynamic, i.e. involving , like the Kolmogorov 4/5 law. The most important aspect is that pure kinematic relations that emphasize the non-local effects, become dynamically significant. Furthermore, we suggest that many of these relations could be used for validation of experimental results.
Energy spectra and passive tracer cascades in turbulent flows
Jolly, Michael
2016-01-01
We study the influence of the energy spectrum on the extent of the cascade range of a passive tracer in turbulent flows. The interesting cases are when there are two different spectra over the potential range of the tracer cascade (in 2D when the tracer forcing is in the inverse energy cascade range, and in 3D when the Schmidt number Sc is large). The extent of the tracer cascade range is then limited by the width of the range for the shallower of the two energy spectra. Nevertheless, we show that in dimension $d=2,3$ the tracer cascade range extends (up to a logarithm) to $\\kappa_{d\\text{D}}^{p}$, where $\\kappa_{d\\text{D}}$ is the wavenumber beyond which diffusion should dominate and $p$ is arbitrarily close to 1, provided Sc is larger than a certain power (depending on $p$) of the Grashof number. We also derive estimates which suggest that in 2D, for Sc${}\\sim1$ a wide tracer cascade can coexist with a significant inverse energy cascade at Grashof numbers large enough to produce a turbulent flow.
Dynamic evolution process of turbulent channel flow after opposition control
Ge, Mingwei; Tian, De; Yongqian, Liu
2017-02-01
Dynamic evolution of turbulent channel flow after application of opposition control (OC), together with the mechanism of drag reduction, is studied through direct numerical simulation (DNS). In the simulation, the pressure gradient is kept constant, and the flow rate increases due to drag reduction. In the transport of mean kinetic energy (MKE), one part of the energy from the external pressure is dissipated by the mean shear, and the other part is transported to the turbulent kinetic energy (TKE) through a TKE production term (TKP). It is found that the increase of MKE is mainly induced by the reduction of TKP that is directly affected by OC. Further analysis shows that the suppression of the redistribution term of TKE in the wall normal direction plays a key role in drag reduction, which represses the wall normal velocity fluctuation and then reduces TKP through the attenuation of its main production term. When OC is suddenly applied, an acute imbalance of energy in space is induced by the wall blowing and suction. Both the skin-friction and TKP terms exhibit a transient growth in the initial phase of OC, which can be attributed to the local effect of and in the viscous sublayer. Project supported by the National Natural Science Foundation of China (Grant No. 11402088 and Grant No. 51376062) , State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources (Grant No. LAPS15005), and ‘the Fundamental Research Funds for the Central Universities’ (Grant No.2014MS33).
Image analysis techniques for the study of turbulent flows
Ferrari Simone
2017-01-01
Full Text Available In this paper, a brief review of Digital Image Analysis techniques employed in Fluid Mechanics for the study of turbulent flows is given. Particularly the focus is on the techniques developed by the research teams the Author worked in, that can be considered relatively “low cost” techniques. Digital Image Analysis techniques have the advantage, when compared to the traditional techniques employing physical point probes, to be non-intrusive and quasi-continuous in space, as every pixel on the camera sensor works as a single probe: consequently, they allow to obtain two-dimensional or three-dimensional fields of the measured quantity in less time. Traditionally, the disadvantages are related to the frequency of acquisition, but modern high-speed cameras are typically able to acquire at frequencies from the order of 1 KHz to the order of 1 MHz. Digital Image Analysis techniques can be employed to measure concentration, temperature, position, displacement, velocity, acceleration and pressure fields with similar equipment and setups, and can be consequently considered as a flexible and powerful tool for measurements on turbulent flows.
Mathematics of large eddy simulation of turbulent flows
Berselli, L.C. [Pisa Univ. (Italy). Dept. of Applied Mathematics ' ' U. Dini' ' ; Iliescu, T. [Virginia Polytechnic Inst. and State Univ., Blacksburg, VA (United States). Dept. of Mathematics; Layton, W.J. [Pittsburgh Univ., PA (United States). Dept. of Mathematics
2006-07-01
Large eddy simulation (LES) is a method of scientific computation seeking to predict the dynamics of organized structures in turbulent flows by approximating local, spatial averages of the flow. Since its birth in 1970, LES has undergone an explosive development and has matured into a highly-developed computational technology. It uses the tools of turbulence theory and the experience gained from practical computation. This book focuses on the mathematical foundations of LES and its models and provides a connection between the powerful tools of applied mathematics, partial differential equations and LES. Thus, it is concerned with fundamental aspects not treated so deeply in the other books in the field, aspects such as well-posedness of the models, their energy balance and the connection to the Leray theory of weak solutions of the Navier-Stokes equations. The authors give a mathematically informed and detailed treatment of an interesting selection of models, focusing on issues connected with understanding and expanding the correctness and universality of LES. This volume offers a useful entry point into the field for PhD students in applied mathematics, computational mathematics and partial differential equations. Non-mathematicians will appreciate it as a reference that introduces them to current tools and advances in the mathematical theory of LES. (orig.)
Logarithmic boundary layers in highly turbulent Taylor-Couette flow
Huisman, Sander G; Cierpka, Christian; Kahler, Christian J; Lohse, Detlef; Sun, Chao
2013-01-01
We provide direct measurements of the boundary layer properties in highly turbulent Taylor-Couette flow up to $\\text{Ta}=6.2 \\times 10^{12}$ using high-resolution particle image velocimetry (PIV). We find that the mean azimuthal velocity profile at the inner and outer cylinder can be fitted by the von K\\'arm\\'an log law $u^+ = \\frac 1\\kappa \\ln y^+ +B$. The von K\\'arm\\'an constant $\\kappa$ is found to depend on the driving strength $\\text{Ta}$ and for large $\\text{Ta}$ asymptotically approaches $\\kappa \\approx 0.40$. The variance profiles of the local azimuthal velocity have a universal peak around $y^+ \\approx 12$ and collapse when rescaled with the driving velocity (and not with the friction velocity), displaying a log-dependence of $y^+$ as also found for channel and pipe flows [1,2].
LES of Turbulent Flows Through a Butterfly Valve
Gong, J. F.; Ming, P. J.; Zhang, W. P.
2011-09-01
Large-eddy simulation (LES) is used to analyze the wake flow characteristics around a butterfly valve. The Smagorinsky- Lilly (SML) model is used for the investigation. The turbulent fluctuations and power spectrum of the wake flow are analyzed for Re = 5.23 × 105 based on the mean inlet velocity and the diameter of the pipe. Four dominating frequencies corresponding to the Strouhal number equal to 0.084, 0.114, 0.152 and 0.175 are found both in the near-wall region and in the midst of the pipe. The evolution of vortices around the valve is presented to illustrate the formation of the single vortex propagating downstream.
Sustaining processes from recurrent flows in body-forced turbulence
Lucas, Dan
2016-01-01
By extracting unstable invariant solutions directly from body-forced three-dimensional turbulence, we study the dynamical processes at play when the forcing is large scale and either unidirectional in the momentum or the vorticity equations. In the former case, the dynamical processes familiar from recent work on linearly-stable shear flows - variously called the Self-Sustaining Process (Waleffe 1997) or Vortex-Wave Interaction (Hall & Smith 1991; Hall & Sherwin 2010) - are important even when the base flow is linearly unstable. In the latter case, where the forcing drives Taylor-Green vortices, a number of mechanisms are observed from the various types of periodic orbits isolated. In particular, two different transient growth mechanisms are discussed to explain the more complex states found.
DNS of turbulent channel flow subject to oscillatory heat flux
Bukhvostova Anastasia
2014-01-01
Full Text Available In this paper we study the heat transfer in a turbulent channel flow, which is periodically heated through its walls. We consider the flow of air and water vapor using direct numerical simulation. We consider the fluid as a compressible Newtonian gas. We focus on the heat transfer properties of the system, e.g., the temperature difference between the walls and the Nusselt number. We consider the dependence of these quantities on the frequency of the applied heat flux. We observe that the mean temperature difference is quite insensitive to the frequency and that the amplitude of its oscillations is such that its value multiplied by the square root of frequency is approximately constant. Next we add droplets to the channel, which can undergo phase transitions. The heat transfer properties of the channel in the case with droplets are found to increase by more than a factor of two, compared to the situation without droplets.
Vincent Casseau
2016-12-01
Full Text Available hy2Foam is a newly-coded open-source two-temperature computational fluid dynamics (CFD solver that has previously been validated for zero-dimensional test cases. It aims at (1 giving open-source access to a state-of-the-art hypersonic CFD solver to students and researchers; and (2 providing a foundation for a future hybrid CFD-DSMC (direct simulation Monte Carlo code within the OpenFOAM framework. This paper focuses on the multi-dimensional verification of hy2Foam and firstly describes the different models implemented. In conjunction with employing the coupled vibration-dissociation-vibration (CVDV chemistry–vibration model, novel use is made of the quantum-kinetic (QK rates in a CFD solver. hy2Foam has been shown to produce results in good agreement with previously published data for a Mach 11 nitrogen flow over a blunted cone and with the dsmcFoam code for a Mach 20 cylinder flow for a binary reacting mixture. This latter case scenario provides a useful basis for other codes to compare against.
Vincent Casseau
2016-10-01
Full Text Available A two-temperature CFD (computational fluid dynamics solver is a prerequisite to any spacecraft re-entry numerical study that aims at producing results with a satisfactory level of accuracy within realistic timescales. In this respect, a new two-temperature CFD solver, hy2Foam, has been developed within the framework of the open-source CFD platform OpenFOAM for the prediction of hypersonic reacting flows. This solver makes the distinct juncture between the trans-rotational and multiple vibrational-electronic temperatures. hy2Foam has the capability to model vibrational-translational and vibrational-vibrational energy exchanges in an eleven-species air mixture. It makes use of either the Park TTv model or the coupled vibration-dissociation-vibration (CVDV model to handle chemistry-vibration coupling and it can simulate flows with or without electronic energy. Verification of the code for various zero-dimensional adiabatic heat baths of progressive complexity has been carried out. hy2Foam has been shown to produce results in good agreement with those given by the CFD code LeMANS (The Michigan Aerothermodynamic Navier-Stokes solver and previously published data. A comparison is also performed with the open-source DSMC (direct simulation Monte Carlo code dsmcFoam. It has been demonstrated that the use of the CVDV model and rates derived from Quantum-Kinetic theory promote a satisfactory consistency between the CFD and DSMC chemistry modules.
M. M. Rashidi
2014-01-01
Full Text Available The optimal homotopy analysis method (OHAM is employed to investigate the steady laminar incompressible free convective flow of a nanofluid past a chemically reacting upward facing horizontal plate in a porous medium taking into account heat generation/absorption and the thermal slip boundary condition. Using similarity transformations developed by Lie group analysis, the continuity, momentum, energy, and nanoparticle volume fraction equations are transformed into a set of coupled similarity equations. The OHAM solutions are obtained and verified by numerical results using a Runge-Kutta-Fehlberg fourth-fifth order method. The effect of the emerging flow controlling parameters on the dimensionless velocity, temperature, and nanoparticle volume fraction have been presented graphically and discussed. Good agreement is found between analytical and numerical results of the present paper with published results. This close agreement supports our analysis and the accuracy of the numerical computations. This paper also includes a representative set of numerical results for reduced Nusselt and Sherwood numbers in a table for various values of the parameters. It is concluded that the reduced Nusselt number increases with the Lewis number and reaction parameter whist it decreases with the order of the chemical reaction, thermal slip, and generation parameters.
Recent progress in designing moving meshes for complex turbulent flows
Claudia Liersch
2014-09-01
Full Text Available This is concerned with an automated adaptive mesh design approach for Large Eddy Simulation (LES of turbulent flows. Based on a dynamic moving mesh partial differential equation (MMPDE, a fixed number of grid points is redistributed according to statistical quantities of interest (QoI selected to capture certain mean flow properties. Physically motivated LES-specific QoI, as the time-averaged gradient of streamwise velocity and the production rate, as well as more general QoI derived from the dual weighted residual method (DWRM for time-averaged statistics are investigated for a flow over periodic hills with Re=10595$Re=10\\,595$. Special emphasis is put on optimizing the grid adjustment phase. It is mainly determined by defining the length of the time interval for computing new time-averaged QoI, the number of time steps to solve the MMPDE, and the overall number of grid adjustment steps. The modifications proposed are nearly auto-adaptive with respect to the chosen QoI and lead to a reduction of the CPU time by more than one order of magnitude compared to the standard approach used so far. On the other hand, the DWRM not only provides suitable QoI to steer the grid movement, but also can be understood as a rigorous error analysis to assess the quality of numerical and subgrid modelling contributions of an LES. The numerical results compared to a highly resolved LES reference solution show the high potential of moving mesh methods to efficiently improve the resolution of turbulent flow features.
Turbulent Taylor-Couette flow with stationary inner cylinder
Ostilla-Monico, Rodolfo; Lohse, Detlef
2016-01-01
A series of direct numerical simulations of Taylor-Couette (TC) flow, the flow between two coaxial cylinders, with the outer cylinder rotating and the inner one fixed, were performed. Three cases, with outer cylinder Reynolds numbers $Re_o$ of $Re_o=5.5\\cdot10^4$, $Re_o=1.1\\cdot10^5$ and $Re_o=2.2\\cdot10^5$ were considered. The radius ratio $\\eta=r_i/r_o$ was fixed to $\\eta=0.909$ to mitigate the effects of curvature. The vertical aspect ratio $\\Gamma$ was fixed to $\\Gamma=2.09$. Being linearly stable, outer cylinder rotation TC flow is known to have very different behavior than pure inner cylinder rotation TC flow. Here, we find that the flow nonetheless becomes turbulent, but the torque required to drive the cylinders and level of velocity fluctuations was found to be smaller than those for pure inner cylinder rotation at comparable Reynolds numbers. The mean angular momentum profiles showed a large gradient in the bulk, instead of the constant angular momentum profiles of pure inner cylinder rotation. The ...
CFD Study of Deteriorated Turbulent Heat Transfer in Upward Flow
Nietiadi, Yohanes Setiawan; Lee, Jeong Ik [Korea Advanced Institute of Science and Technology, Daejeon (Korea, Republic of); Addad, Yacine [Khalifa University of Science and Technology and Research, Abu Dhabi (United Arab Emirates)
2014-10-15
DTHT regime can be induced by two effects: buoyancy and acceleration. Apart from these two deteriorating effects, another unique behavior of fluid in the DTHT regime is that the convective heat transfer rate will continue to deteriorate until it reaches certain point. The downstream of this point, is known as the recovery region, where the convective heat transfer rate returns back to the high values by recovering turbulence. We called this phenomena as re-turbulization.. The map of the DTHT regime can be seen from fig. 2, where the x-axis is the buoyancy parameter and y-axis is the acceleration parameter which is the agreed governing non-dimensional numbers among the researchers to illustrate the phenomena. The Buoyancy parameter is defind in Eq. (1) and the acceleration parameter is defined in Eq. (2), respectively. The threshold value for both effects to move from the forced turbulent heat transfer to the DTHT regime are found to be Bo* ≥ 2x10{sup -6}and Kv ≥ 2.5x10{sup -6} in the previous works. Bo{sup *}=Gr{sub q}/Re{sup 3}'.{sup 425} Pr{sup 0}'.{sup 8} (1). K{sub v}=4q{sup +}/Re (2). Many experiments and simulation have been done to investigate this phenomenon and the boundary of the regime. However, very limited number of experiment was conducted in the regime where buoyancy effect and acceleration effect are in the same order of magnitude and high enough to cause DTHT (mixed DTHT). Some important experimental researches that have been done in the gas DTHT regime is Lee et al. who investigated the heat transfer of gas flow in the range of buoyancy parameter from 3x10{sup -9} to 10{sup -5} and acceleration parameter span from 6x10{sup -8} to 5x10{sup -6} and presented the behavior of Nusselt number ratio from the experiment as fig. 3 and fig. 4. This paper will discuss a Computational Fluid Dynamics analysis on DTHT by assuming hypothetical boundary conditions especially on the mixed DTHT regime. It has been found that a gas cooled fast reactor
Measurements of laminar and turbulent flow in a curved duct with thin inlet boundary layers
Taylor, A. M. K. P.; Whitelaw, J. H.; Yianneskis, M.
1981-01-01
Laser Doppler velocimetry was used to measure the laminar and turbulent flow in a 90 deg square bend of strong curvature. The boundary layers at the inlet to the bend were approximately 25 percent and 15 percent of the hydraulic diameter for the laminar and turbulent flows, respectively. The development of the pressure driven secondary motion is more rapid for laminar flow: the maximum cross stream component measured was 60 percent of the bulk velocity in contrast to 40 percent for turbulent flow. The streamwise isotachs show that, for laminar flow, large velocities are found progressively nearer to the outer radius of the bend and along the sidewalls. For turbulent flow, the isotachs move towards the inner radius until about 60 deg around the bend where strong secondary motion results in a similar redistribution. Turbulence level and shear stress measurements are also presented.
Byron, M.; Meyer, C.; Bellani, G.; Variano, E. A.
2011-12-01
We present a method for simultaneously measuring both the fluid and particle phases in a dense particle-laden flow or slurry. There is no limit to the technique's ability to resolve very high particle number density. With this, we measure the motion of particles in high-Reynolds number turbulent water flow. The particles we consider include spheres, ellipsoids, and completely irregular geometries. The particles range from near neutrally buoyant to a specific gravity of 2. We measure the particles' translation and rotation, while simultaneously measuring the turbulent flow surrounding them. With this information, we can determine: the effect of particles on turbulent flow (e.g. enhanced dissipation); the effect of turbulent flow on particle (e.g. eddy diffusivity and modified settling velocity); and the dynamics at the particle interfaces which explain theses effects. In this presentation, we explain the method; present results on the rotational diffusion of particles; and present results on how particles affect the ambient turbulent flow.
Understanding the sub-critical transition to turbulence in wall flows
Manneville, Paul
2008-01-01
Contrasting with free shear flows presenting velocity profiles with inflection points which cascade to turbulence in a relatively mild way, wall bounded flows are deprived of (inertial) instability modes at low Reynolds numbers and become turbulent in a much wilder way, most often marked by the coexistence of laminar and turbulent domains at intermediate Reynolds numbers, well below the range where (viscous) instabilities can show up. There can even be no unstable mode at all, as for plane Couette flow (pCf) or for Poiseuille pipe flow (Ppf) that currently are the subject of intense research. Though the mechanisms involved in the transition to turbulence in wall flows are now better understood, statistical properties of the transition itself are yet unsatisfactorily assessed. A review of the situation is given. An alternative to the temporal theory of the transition to turbulence in terms of chaotic transients in such globally subcritical flows is proposed, which invokes spatio-temporal intermittence and the ...
CALCULATION OF THERMAL AND SALINE TURBULENT FLOW BY k-ε MODEL
Hu Zhen-hong; Shen Yong-ming; Zheng Yong-hong; Liu Cai-guang
2003-01-01
Based on the N-S equation, taking the character of thermal and saline stratified flow into account, and considering the effects of buoyancy on turbulence, the k-ε model of thermal and saline stratified flow is established.Density stratified flow with both the vertical temperature gradient and the vertical salinity gradient is simulated numerically, in which turbulent terms are calculated by the k-ε turbulent model.The distributions of velocity, temperature and salinity are given in this paper.The feature of stratification and turbulence is described correctly by the model.The computational results agree well with the experimental data.
Boundary-layer turbulence in experiments of quasi-Keplerian flows
Lopez, Jose M
2016-01-01
Most flows in nature and engineering are turbulent because of their large velocities and spatial scales. Laboratory experiments of rotating quasi-Keplerian flows, for which the angular velocity decreases radially but the angular momentum increases, are however laminar at Reynolds numbers exceeding one million. This is in apparent contradiction to direct numerical simulations showing that in these experiments turbulence transition is triggered by the axial boundaries. We here show numerically that as the Reynolds number increases turbulence becomes progressively confined to the boundary layers and the flow in the bulk fully relaminarizes. Our findings support that hydrodynamic turbulence cannot drive accretion in astrophysical disks.
He Zhi-guo; Mao Gen-hai; Yuan Xing-ming
2003-01-01
The 3-D turbulent flows in a valve pipe were described by the incompressible Reynolds-averaged Navier-Stokes equations with an RNG k-ε turbulence model. With the finite volume method and a body-fitted coordinate system, the discretised equations were solved by the SIMPLEST algorithm. The numerical result of a cut-off valve with curved inlet shows the flow characteristics and the main cause of energy loss when fluid flows through a valve. And then, the boundaries of valve were modified in order to reduce the energy loss. The computational results of modified valve show that the numerical value of turbulent kinetic energy is lower, and that the modified design of the 3-D valve boundaries is much better. The analysis of the result also shows that RNG k-ε turbulence model can successfully be used to predict the 3-D turbulent separated flows and the secondary flow inside valve pipes.
Shih, Tsan-Hsing; Liu, nan-Suey
2010-01-01
A brief introduction of the temporal filter based partially resolved numerical simulation/very large eddy simulation approach (PRNS/VLES) and its distinct features are presented. A nonlinear dynamic subscale model and its advantages over the linear subscale eddy viscosity model are described. In addition, a guideline for conducting a PRNS/VLES simulation is provided. Results are presented for three turbulent internal flows. The first one is the turbulent pipe flow at low and high Reynolds numbers to illustrate the basic features of PRNS/VLES; the second one is the swirling turbulent flow in a LM6000 single injector to further demonstrate the differences in the calculated flow fields resulting from the nonlinear model versus the pure eddy viscosity model; the third one is a more complex turbulent flow generated in a single-element lean direct injection (LDI) combustor, the calculated result has demonstrated that the current PRNS/VLES approach is capable of capturing the dynamically important, unsteady turbulent structures while using a relatively coarse grid.
Magnetohydrodynamic flow and turbulence: a report on the fifth Beer-Sheva seminar
Branover, H.; Moffatt, H.K.; Mond, M.; Pierson, E.S.; Sulem, P.S.; Yakhot, A.
1988-03-01
This paper is a summary of the Fifth Beer-Sheva Seminar on Magnetohydrodynamic (MHD) Flows and Turbulence, held in Jerusalem during 2-6 March 1987, with 99 participants from 12 countries. Reviews and research papers were presented on general problems of turbulence. MHD turbulence, fundamental MHD, two-phase flows with and without magnetic fields, and on different applications of liquid-metal MHD, especially in power generation nuclear fission and fusion, and in metallurgy.
The transition to turbulence in parallel flows: transition to turbulence or to regular structures
Pomeau, Yves
2015-01-01
We propose a scenario for the formation of localized turbulent spots in transition flows, which is known as resulting from the subcritical character of the transition. We show that it is not necessary to add 'by hand" a term of random noise in the equations, in order to describe the existence of long wavelength fluctuations as soon as the bifurcated state is beyond the Benjamin-Feir instability threshold. We derive the instability threshold for generalized complex Ginzburg-Landau equation which displays subcriticality. Beyond and close to the Benjamin-Feir threshold we show that the dynamics is mainly driven by the phase of the complex amplitude which obeys Kuramoto-Sivashinsky equation while the fluctuations of the modulus are smaller and slaved to the phase (as already proved for the supercritical case). On the opposite, below the Benjamin-Feir instability threshold, the bifurcated state does loose the randomness associated to turbulence so that the transition becomes of the mean-field type as in noiseless ...
Simulation of turbulent flow and temperature separation in a uni-flow vortex tube
Promvonge, P.
2007-03-01
Full Text Available The vortex tube is a mechanical device operating as a refrigerating machine without refrigerants, by separating a compressed gas stream into two streams; the cold air stream at the tube core while the hot airstream near the tube wall. Such a separation of the flow into regions of low and high total temperature is referred to as the temperature separation effect. In this paper, simulation of the turbulent compressible flowand temperature separation in a uni-flow vortex tube with the turbulence model and the algebraic Reynolds stress model (ASM is described. Steady, compressible and two-dimensional flows are assumed through outthe calculation. It has been found that the predicted results of velocity, pressure, and temperature fields are generally in good agreement with available experiment data. Moreover, it can be indicated that the highest temperature separation occurs near the inlet nozzle while the lowest temperature separation is found at the downstream near the control valve.
Surface Runoff in Watershed Modeling—Turbulent or Laminar Flows?
Mark E. Grismer
2016-05-01
Full Text Available Determination of overland sheet flow depths, velocities and celerities across the hillslope in watershed modeling is important towards estimation of surface storage, travel times to streams and soil detachment rates. It requires careful characterization of the flow processes. Similarly, determination of the temporal variation of hillslope-riparian-stream hydrologic connectivity requires estimation of the shallow subsurface soil hydraulic conductivity and soil-water retention (i.e., drainable porosities parameters. Field rainfall and runoff simulation studies provide considerable information and insight into these processes; in particular, that sheet flows are likely laminar and that shallow hydraulic conductivities and storage can be determined from the plot studies. Here, using a 1 m by 2 m long runoff simulation flume, we found that for overland flow rates per unit width of roughly 30–60 mm2/s and bedslopes of 10%–66% with varying sand roughness depths that all flow depths were predicted by laminar flow equations alone and that equivalent Manning’s n values were depth dependent and quite small relative to those used in watershed modeling studies. Even for overland flow rates greater than those typically measured or modeled and using Manning’s n values of 0.30–0.35, often assumed in physical watershed model applications for relatively smooth surface conditions, the laminar flow velocities were 4–5 times greater, while the laminar flow depths were 4–5 times smaller. This observation suggests that travel times, surface storage volumes and surface shear stresses associated with erosion across the landscape would be poorly predicted using turbulent flow assumptions. Filling the flume with fine sand and conducting runoff studies, we were unable to produce sheet flow, but found that subsurface flows were onflow rate, soil depth and slope dependent and drainable porosities were only soil depth and slope dependent. Moreover, both the sand
Coherent Turbulent Flow Structures in a Gravel-Bed River
Ashley, W.; Macmahan, J. H.; Reniers, A. J.; Thornton, E. B.; Brown, J.; Swick, W. A.
2010-12-01
The characteristics of coherent turbulent flow structures were examined during multi-day deployments with three different sensors in a gravel-bed river reach section of the Kootenai River, ID in August 2010. In-situ river velocities were measured using a custom 2 MHz Acoustic Doppler Current Profiler (ADCP) head, an Acoustic Doppler Velocimeter (ADV), and a lagged array of six electromagnetic current meters (ECM) mounted on a GPS-equipped portable aluminum frame. The frame was deployed in the river which varied from 0.6 to 1m water depths and 0.5 to 1.5 m/s velocities. It was also deployed in a small channel, near riffle pools, and on the lee of river obstructions. The ECM array was horizontally mounted on a 4m length pole attached to the frame, oriented in the stream-wise direction, with a sampling frequency of 16 Hz. The lagged spacing of the six ECMs was set to resolve coherent motions from up to 8m in length. The ADCP was fitted with a custom head to measure the along beam velocities in all three axes, with a sampling frequency of 1 Hz, 35cm bins, with a maximum range of 10m. The upstream beam is used to describe the coherent structures in the stream-wise velocity. An iterative maximum likelihood estimator is used to evaluate the streamwise wavenumber-frequency spectrum. The coherent structures measured by the ECM array and ADCP are compared to validate the results by the new ADCP head. Turbulent measurements from the 32 Hz sampled ADV are compared to the ADCP and ECM. Our unique approach provides spatial measurements in river reaches (depths) previously not examined. The flow structure as a function of river feature, bed roughness, and flow velocity are described in the stream-wise and lateral directions. This effort was supported by the Office of Naval Research Coastal Geosciences Program.
Flow in a Circular Expansion Pipe Flow: Effect of a Vortex Perturbation on Localized Turbulence
Selvam, Kamal; Willis, Ashley P
2016-01-01
We report the results of three-dimensional direct numerical simulations for incompressible viscous fluid in a circular pipe flow with a sudden expansion. At the inlet, a parabolic velocity profile is applied together with a finite amplitude perturbation in the form of a vortex with its axis parallel to the axis of the pipe. At sufficiently high Reynolds numbers the recirculation region breaks into a turbulent patch that changes position axially depending on the strength of the perturbation. This vortex perturbation is believed to produce a less abrupt transition than in previous studies with a tilt perturbation, as the localized turbulence is observed via the formation of a wavy structure at a low order azimuthal mode, which resembles an optimally amplified perturbation. For higher amplitude, the localized turbulence remains at a constant axial position. It is further investigated using proper orthogonal decomposition, which indicates that the centre region close to the expansion is highly energetic.
Flow in a circular expansion pipe flow: effect of a vortex perturbation on localised turbulence
Selvam, Kamal; Peixinho, Jorge; Willis, Ashley P.
2016-12-01
We report the results of three-dimensional direct numerical simulations for incompressible viscous fluid in a circular pipe flow with a sudden expansion. At the inlet, a parabolic velocity profile is applied together with a finite amplitude perturbation in the form of a vortex with its axis parallel to the axis of the pipe. At sufficiently high Reynolds numbers the recirculation region breaks into a turbulent patch that changes position axially, depending on the strength of the perturbation. This vortex perturbation is believed to produce a less abrupt transition than in previous studies, which applied a tilt perturbation, as the localised turbulence is observed via the formation of a wavy structure at a low order azimuthal mode, which resembles an optimally amplified perturbation. For large vortex amplitude, the localised turbulence remains at a constant axial position. It is further investigated using proper orthogonal decomposition, which indicates that the centre region close to the expansion is highly energetic.
Entrainment at a sediment concentration interface in turbulent channel flow
Salinas, Jorge; Shringarpure, Mrugesh; Cantero, Mariano; Balachandar, S.
2016-11-01
In this work we address the role of turbulence on entrainment at a sediment concentration interface. This process can be conceived as the entrainment of sediment-free fluid into the bottom sediment-laden flow, or alternatively, as the entrainment of sediment into the top sediment-free flow. We have performed direct numerical simulations for fixed Reynolds and Schmidt numbers while varying the values of Richardson number and particle settling velocity. The analysis performed shows that the ability of the flow to pick up a given sediment size decreases with the distance from the bottom, and thus only fine enough sediment particles are entrained across the sediment concentration interface. For these cases, the concentration profiles evolve to a final steady state in good agreement with the well-known Rouse profile. The approach towards the Rouse profile happens through a transient self-similar state. Detailed analysis of the three dimensional structure of the sediment concentration interface shows the mechanisms by which sediment particles are lifted up by tongues of sediment-laden fluid with positive correlation between vertical velocity and sediment concentration. Finally, the mixing ability of the flow is addressed by monitoring the center of mass of the sediment-laden layer. With the support of ExxonMobil, NSF, ANPCyT, CONICET.
Computation of a turbulent channel flow using PDF method
Minier, J.P. [Electricite de France (EDF), 78 - Chatou (France). Lab. National d`Hydraulique; Pozorski, J. [Polish Academy of Sciences, Gdansk (Poland). Inst. of Fluid-Flow Machinery
1997-05-01
The purpose of the present paper is to present an analysis of a PDF model (Probability Density Function) and an illustration of the possibilities offered by such a method for a high-Reynolds turbulent channel flow. The first part presents the principles of the PDF approach and the introduction of stochastic processes along with a Lagrangian point of view. The model retained is the one put forward by Pope (1991) and includes evolution equations for location, velocity and dissipation of a large number of particles. Wall boundary conditions are then developed for particles. These conditions allow statistical results of the logarithmic region to be correctly reproduced. Simulation of non-homogeneous flows require a pressure-gradient algorithm which is briefly described. Developments are validated by analysing numerical predictions with respect to Comte Bellot experimental data (1965) on a channel flow. This example illustrates the ability of the approach to simulate wall-bounded flows and to provide detailed information such as skewness and flatness factors. (author) 9 refs.
Small-scale motions in turbulent boundary-free shear flows
Fiscaletti, D.
2016-01-01
The present work is an experimental and numerical investigation of the small-scale motions in turbulent free-shear flows. In the far-field turbulence of a jet at high Reynolds number (Reλ = 350) hot-wire anemometry (HWA) is applied to measure time series of flow velocity. By filtering these time ser
Influence of turbulent grazing flow on the impedance of an opening
Golliard, J.
2003-01-01
In this paper, the impedance of a rectangular opening submitted to a turbulent grazing flow is investigated experimentally. The opening is located in a flat plate where a turbulent boundary layer flow develops. The impedance is measured with a two-microphone measuring pipe installed below the openin
Numerical Simulations of Competitive-Consecutive Reactions in Turbulent Channel Flow
Vrieling, A.J.
2003-01-01
This thesis deals with mixing of passive scalars in a turbulent flow. The passive scalars are released in a turbulent plane channel flow and interpreted as either non-reactive components or reactive components that are involved in a competitive-consecutive reaction system. The evolution of these pas
Influence of magnetic field on the turbulence of plane high temperature gas flow
Levitan, Yu.S.
1977-01-01
Expressions are obtained within the framework of the Prandtl hypothesis and proposition on local-isotropic turbulence for turbulent friction and thermal flow in a channel flow of an electrical-conducting medium with a current along the channel's axis in the internal magnetic field, and which account for temperature fluctuations of the medium. 6 references.