Showing papers in "Journal of Turbulence in 2008"

A public turbulence database cluster and applications to study Lagrangian evolution of velocity increments in turbulence

Abstract: A public database system archiving a direct numerical simulation (DNS) data set of isotropic, forced turbulence is described in this paper. The data set consists of the DNS output on 10243 spatial points and 1024 time samples spanning about one large-scale turnover time. This complete 10244 spacetime history of turbulence is accessible to users remotely through an interface that is based on the Web-services model. Users may write and execute analysis programs on their host computers, while the programs make subroutine-like calls that request desired parts of the data over the network. The users are thus able to perform numerical experiments by accessing the 27 terabytes (TB) of DNS data using regular platforms such as laptops. The architecture of the database is explained, as are some of the locally defined functions, such as differentiation and interpolation. Test calculations are performed to illustrate the usage of the system and to verify the accuracy of the methods. The database is then used to analy...

Direct numerical simulation of a separated channel flow with a smooth profile

Abstract: A direct numerical simulation (DNS) of a channel flow with one curved surface was performed at moderate Reynolds number (Re τ = 395 at the inlet). The adverse pressure gradient was obtained by a wall curvature through a mathematical mapping from physical coordinates to Cartesian ones. The code, using spectral spanwise and normal discretization, combines the advantage of a good accuracy with a fast integration procedure compared to standard numerical procedures for complex geometries. The turbulent flow slightly separates on the profile at the lower curved wall and is at the onset of separation at the opposite flat wall. The thin separation bubble is characterized with a reversal flow fraction. Intense vortices are generated not only near the separation line on the lower wall but also at the upper wall. Turbulent normal stresses and kinetic energy budget are investigated along the channel.

Sand Motion induced by oscillatory flows: sheet flow and vortex ripples

Abstract: Shoaling short gravity waves at sea approaching the shore become asymmetric and are able to generate a net resulting sand transport in cross-shore direction (on-shore-offshore transport). The wave-related sand transport is still very difficult to predict due to the complexity of its underlying processes, which mainly take place in a thin layer near the sea bed in the wave boundary layer (thickness of order centimeters). The development of models for cross-shore sand transport heavily relies on experimental lab research, especially as taking place in large oscillating water tunnels (see, e.g., Nielsen, 1992). In oscillating water tunnels the near-bed horizontal orbital velocity, as induced by short gravity waves, can be simulated above fixed or mobile sandy beds (for a detailed description, see, e.g., Ribberink and Al-Salem, 1994). It should be realized that the vertical orbital flow and relatively small wave-induced residual flows as streaming and drift are not reproduced in flow tunnels. Research aimed at their contribution to the net sediment motion under surface waves is still ongoing (see Ribberink et al., 2000).

Elliptic relaxation second moment closure for the turbulent heat fluxes

Abstract: This study describes the development of near-wall second moment turbulent heat flux model and its application to various turbulent flows with heat transfer to test the performance of the model. The second moment models for turbulent heat fluxes based on the elliptic concept are proposed and closely linked to the elliptic blending model, which is used for the prediction of Reynolds stresses. The new models satisfy the near-wall balance between viscous diffusion, viscous dissipation and pressure–temperature gradient correlation, and also have the characteristics of approaching their respective conventional high Reynolds number model far away from the wall. On the other hand, the traditional heat flux model using the wall-normal vector expression in the wall-reflection model is tested in the present study with the new unit wall-normal vector formulation that appears in the elliptic blending model. To develop and calibrate the new heat flux models, firstly, the distributions of the mean temperature and the sc...

Evolutionary optimization of an anisotropic compliant surface for turbulent friction drag reduction

Abstract: Direct numerical simulation (DNS) of the channel flow with an anisotropic compliant surface is performed in order to investigate its drag reduction effect in a fully developed turbulent flow. The computational domain is set to be 3δ×2δ×3δ, where δ is the channel half-width. The surface is passively driven by the pressure and wall-shear stress fluctuations, and the surface velocity provides a boundary condition for the fluid velocity field. An evolutionary optimization method (CMA-ES) is used to optimize the parameters of the anisotropic compliant surface. The optimization identifies several sets of parameters that result in a reduction of the friction drag with a maximum reduction rate of 8%. The primary mechanism for drag reduction is attributed to the decrease of the Reynolds shear stress (RSS) near the wall induced by the kinematics of the surface. The resultant wall motion is a uniform wave traveling downstream. The compliant wall, with the parameters found in the optimization study, is also tested in...

Influence of the external conditions on transitionally rough favorable pressure gradient turbulent boundary layers

Abstract: Laser Doppler anemometry measurements are carried out in order to investigate the influences of the external conditions on a transitionally rough favorable pressure gradient turbulent boundary layer. The acquired data is normalized using the scalings obtained by the means of equilibrium similarity of the outer flow. The point at hand is to not only understand the interaction between the rough surface and the outer flow but also to include the external pressure gradient as the flow evolves in the streamwise direction. It is found that the velocity profiles show the effects of the upstream conditions imposed on the flow when normalized with the free-stream velocity. However, the profiles do collapse when normalized with U ∞ δ*/δ, demonstrating that this scaling absorbs the roughness effects and upstream conditions. An augmentation in the Reynolds stresses occurs with an increase in the roughness parameter and a decrease due to the external favorable pressure gradient. However, close to the wall, there is an...

Effects of subgrid-scale modeling on Lagrangian statistics in large-eddy simulation

Abstract: The application of large-eddy simulation (LES) to turbulent transport processes requires accurate prediction of the Lagrangian statistics of flow fields. However, in most existing SGS models, no explicit consideration is given to Lagrangian statistics. In this paper, we focus on the effects of SGS modeling on Lagrangian statistics in LES ranging from statistics determining single-particle dispersion to those of pair dispersion and multiparticle dispersion. Lagrangian statistics in homogeneous isotropic turbulence are extracted from direct numerical simulation (DNS) and the LES with a spectral eddy-viscosity model. For the case of longtime single-particle dispersion, it is shown that, compared to DNS, LES overpredicts the time scale of the Lagrangian velocity correlation but underpredicts the Lagrangian velocity fluctuation. These two effects tend to cancel one another leading to an accurate prediction of the longtime turbulent dispersion coefficient. Unlike the single-particle dispersion, LES tends to und...

Experimental study on clustering of large particles in homogeneous turbulent flow

Abstract: Results of simultaneous measurements of fluid flow and motion of large solid particles in homogeneous turbulent flow (at the Taylor microscale Reynolds number Re λ = 250) are presented. Velocity, acceleration and spatial distribution of particles in a three-dimensional volume along with the surrounding flow velocity and velocity derivatives fields (incl. vorticity and the strain-rate tensor) are obtained by means of two synchronized and cross-calibrated three-dimensional particle tracking velocimetry (3D-PTV) systems. The main focus of the present investigation is on the two-way coupling between the turbulent flow and solid particles which are significantly larger (∼900 μm) than the Kolmogorov length scale (∼200 μm) and heavier than the surrounding liquid (ρ p = 1400 kg m−3>ρ f = 1000 kg m−3). Joint statistics of the local particle concentration and the local strain and vorticity fields indicate that (1) large particles tend to cluster in strain-dominated regions and that (2) preferential concentration (c...

A low-dimensional stochastic closure model for combustion large-eddy simulation

Abstract: A novel formulation for the large-eddy simulation (LES) of turbulent combustion flows is presented. The formulation is based on coupling LES equations for mass and momentum, with the corresponding 1D stochastic governing equations using the One-Dimensional Turbulence (ODT) model. ODT domains, or elements, on which fine-grained ODT simulations are implemented, are embedded in the flow to represent unresolved scalar and momentum statistics. The formulation is designed to address important coupling between turbulent transport and molecular processes (reaction and diffusion) over a wide range of length and time scales. This coupling poses difficult challenges for the LES modeling of turbulent mixing and combustion flows. The LES-ODT approach is implemented for the problem of autoignition in non-homogeneous mixtures. The LES-ODT model yields excellent agreement with direct numerical simulations (DNS) of reactive scalars' statistics at different turbulence and Lewis number conditions. Comparisons of the LES-ODT...

Effect of grid discontinuities on large-eddy simulation statistics and flow fields

Abstract: We have performed large-eddy simulations of spatially developing homogeneous isotropic turbulence advected past an interface where the grid is suddenly coarsened or refined by a factor of two in each direction. We have compared simulations in which the filter width is proportional to the grid size and is discontinuous at the coarse–fine grid interface, to others in which the filter width varies gradually between the values corresponding to the coarse and fine grids. The Smagorinsky and Lagrangian-dynamic eddy viscosity (LDEV) models were used to parameterize the unresolved subgrid scales. A sudden refinement of the grid does not result in significant flow perturbation: small scales are gradually generated and the flow is generally quite smooth across the interface. When the grid is suddenly coarsened, on the other hand, a considerable energy pileup at small scales is observed near the interface. At lower resolutions, the extent of this high-energy zone grows. Increasing the eddy viscosity upstream of the ...

A turbulence model for magnetohydrodynamic plasmas

Abstract: The statistical theory of inhomogeneous turbulence is applied to develop a system of model equations for magnetohydrodynamic (MHD) turbulence. The statistical descriptors of MHD turbulence are taken to be the turbulent MHD energy, its dissipation rate, the turbulent cross helicity (velocity-magnetic field correlation), turbulent MHD residual energy (difference between the kinetic and magnetic energies), and turbulent residual helicity (difference between the kinetic and current helicities). Evolution equations for these statistical quantities are coupled to the mean-field dynamics. The model is applied to two MHD-plasma phenomena: turbulence evolution with prescribed mean velocity and magnetic fields in the solar wind, and mean flow generation in the presence of a mean magnetic field and cross helicity in tokamak plasmas. These applications support the validity of the turbulence model. In the presence of a mean magnetic field, turbulence dynamics should be subject to combined effects of nonlinearity and A...

Quasi-two-dimensional decaying turbulence subject to the β effect

Abstract: Freely decaying quasi-2D turbulence under the influence of a meridional variation of the Coriolis parameter f (β effect) is experimentally and numerically modelled. The experimental flow is generated in a rotating electromagnetic cell where the variation of f is approximated by a nearly equivalent topographical effect. In the presence of a high β effect, the initial disordered vorticity field evolves to form a weak polar anticyclonic circulation surrounded by a cyclonic zonal jet demonstrating the preferential transfer of energy towards zonal motions. In agreement with theoretical predictions, the energy spectrum becomes peaked near the Rhines wave number with a steep fall-off beyond, indicating the presence of a soft barrier to the energy transfer towards larger scales. DNS substantially confirmed the experimental observations.

Decrement of spanwise vortices by a drag-reducing riblet surface

Abstract: In this paper, the spanwise vortices in a turbulent flow over drag-reducing V-shaped riblet grooves have been investigated experimentally. The height and lateral spacing of each V-shaped riblet groove are 176.8 μm and 300 μm, respectively. A high-resolution PIV system was employed to measure the instantaneous velocity fields of the turbulent boundary layer developed over the riblet surface under a drag-reducing condition of s + = 10.4. The contours of the spanwise vortices which show a locally swirling motion were derived by using a vortex-identification technique. After extracting the cores of the spanwise vortices from instantaneous PIV velocity fields, the number distributions of the vortex cores were analyzed statistically. In addition, the spatial distributions of mean velocity, turbulence intensity and Reynolds stress were also obtained. All experimental results were compared with those over a flat smooth surface. As a result, the number distributions of the small-scale spanwise vortices were signif...

The structure and dynamics of coherent vortex tubes in the zero-absolute-vorticity state

Abstract: The structure and dynamics of coherent vortex tubes in the zero-absolute-vorticity state (ZAVS), which is realized in a rotating uniformly sheared flow with zero mean absolute vorticity, are investigated by the use of the results of direct numerical simulations. A coherent vortex tube consists of two parts; two end parts and an intermediate part between them. The end parts keep the inclination angle against the tilting mechanism by the mean shear flow and rotate the vortex tubes from the direction of the mean-shear vorticity at their birth period to that of the system rotation as time proceeds. The generation of the Reynolds stress is activated in the regions near the end parts. The change in the direction of the vortex tubes toward that of the system rotation contributes to the stabilization of the whole turbulent flow field. The vortex tubes are deformed and flattened as they are turned toward the direction of the system rotation, which leads to the decay of the vortex tubes. The deformation of the vort...

Quantitative investigation of vortical structures in the near-exit region of an axisymmetric turbulent jet

Abstract: High-resolution particle image velocimetry (PIV) measurements were made at the exit of an axisymmetric free jet at a Reynolds number of 21 900 with a top-hat, low-turbulence exit velocity profile. The data were analysed using the proper orthogonal decomposition (POD) technique to identify the main energy-containing vortices. The vortices so identified have been further quantified by computing their size, position, circulation and direction of rotation. The technique employed resolved vortices of radius 0.055 jet exit diameters and larger. This quantitative information has also been related to observations from flow visualisation images and measurements of the downstream evolution of the jet. The data clearly show the formation of alternating direction toroidal vortices identified in the fluctuating fields which begin to be resolved at a streamwise location of one-half the jet exit diameter. The vortices grow in number approximately linearly in the downstream direction until about one jet diameter. After t...

Lagrangian dynamic SGS model for stochastic coherent adaptive large eddy simulation

Abstract: Stochastic coherent adaptive large eddy simulation (SCALES) is an extension of large eddy simulation that uses a wavelet filter-based dynamic grid adaptation strategy to solve for the most energetic coherent structures in a turbulent flow field, while modeling the effect of the less energetic ones. A localized dynamic subgrid scale model is needed to fully exploit the ability of the method to track coherent structures. In this paper, new local Lagrangian models based on a modified Germano dynamic procedure, redefined in terms of wavelet thresholding filters, are proposed. These models extend the original path-line formulation of Meneveau et al. [J. Fluid Mech. 319 (1996)] in two ways: as Lagrangian path-line diffusive and Lagrangian path-tube averaging procedures. The proposed models are tested for freely decaying homogeneous turbulence with initial Re λ = 72. It is shown that the SCALES results, obtained with less than 0.4% of the total non-adaptive nodes required for a DNS with the same wavelet solver, ...

Refined vorticity statistics of decaying rotating three-dimensional turbulence

Abstract: The influence of background rotation on all nontrivial triple correlations of vorticity (VTCs) has been studied for an unbounded incompressible homogeneous turbulent flow, using pseudo-spectral direct numerical simulation. It is found that the time evolutions of the VTCs are in agreement with exact theoretical predictions for rotating turbulence presented herein. Furthermore, the effects of viscosity, initial value of the velocity derivative skewness S∂i u i (t ini), and background rotation rate on the vertical vorticity skewness Sω 3 have been thoroughly investigated. The initial growth rate of Sω3 is found to be proportional to t 0.75±0.1 for all considered cases, in agreement with recent experimental results by Morize et al. [Phys. Fluids 17, 095105 (2005)]. Also, it is found that higher background rotation rates — implying more linearity — result in lower final values of Sω3, while lower viscosities and higher S∂i u i (t ini) both yield higher final values of Sω3.

Large-eddy simulation of H2–air auto-ignition using tabulated detailed chemistry

Abstract: Large-eddy simulation (LES) of an igniting turbulent hydrogen/air jet for which experimental data are available is reported. A sub-grid scale closure is proposed that is based on a tabulation of detailed chemistry combined with a presumed probability density function, to be implemented into a fully compressible LES solver. The simulations are performed in a frame moving with the mean flow. The obtained results are shown to reproduce the experimental findings in terms of ignition length. A dynamic closure is used for the unknown sub-grid scale (SGS) scalar dissipation rate of mixture fraction and the sensitivity of the chemical response to SGS modelling is studied.

Direct numerical simulation of the motion of particles in rotating pipe flow

Abstract: In this paper the motion of particles in rotating pipe flow is studied for various flow cases by means of direct numerical simulation. Compared to flow in a non-rotating pipe, the Navier–Stokes equation contains as only extra term the Coriolis force when the equation is considered in a rotating frame of reference. Particles in the flow also experience a centrifugal force, which drives them to one side of the wall of the pipe. The flow is characterized by two Reynolds numbers for the mean axial velocity and the rotation rate, respectively. Among the cases studied are one in which the flow without rotation would be laminar and rotation leads to turbulence and another one for which Poiseuille flow is unstable but instead of transition to a turbulent state, a time-dependent laminar flow results. In all cases studied a counter-rotating vortex is present. The simulation results are used to calculate the collection efficiency of the rotational phase separator (RPS) under turbulent flow conditions. The RPS is a d...

Effects of aerodynamic particle interaction in turbulent non-dilute particle-laden flow

Abstract: Aerodynamic four-way coupling models are necessary to handle two-phase flows with a dispersed phase in regimes in which the particles are neither dilute enough to neglect particle interaction nor dense enough to bring the mixture to equilibrium. We include an aerodynamic particle interaction model within the framework of large eddy simulation together with Lagrangian particle tracking. The particle drag coefficients are corrected depending on relative positions of the particles accounting for the strongest drag correction per particle but disregarding many-particle interactions. The approach is applied to simulate monodisperse, rigid, and spherical particles injected into crossflow as an idealization of a spray jet in crossflow. A domain decomposition technique reduces the computational cost of the aerodynamic particle interaction model. It is shown that the average drag on such particles decreases by more than 40% in the dense particle region in the near-field of the jet due to the introduction of aerodynamic four-way coupling. The jet of monodisperse particles therefore penetrates further into the crossflow in this case. The strength of the counterrotating vortex pair (CVP) and turbulence levels in the flow then decrease. The impact of the stochastic particle description on the four-way coupling model is shown to be relatively small. If particles are also allowed to break up according to a wave breakup model, the particles become polydisperse. An ad hoc model for handling polydisperse particles under such conditions is suggested. In this idealized atomizing mixture, the effect of aerodynamic four-way coupling reverses: The aerodynamic particle interaction results in a stronger CVP and enhances turbulence levels.

One-equation RG hybrid RANS/LES computation of a turbulent impinging jet

Abstract: A one-equation variant of a previously developed two-equation, renormalization group based, hybrid RANS/LES model is presented. The model consists of a transport equation for the mean dissipation rate and an algebraically prescribed length scale. The length scale is proportional to the wall-distance in RANS regions of the flow and to the filter width in LES regions. A very simple, but efficient, near-wall model is also presented, in the manner of Yakhot and Orszag's RNG modeling. The only free parameter entering the low-Reynolds formulation has been calibrated against channel flow, and the value corresponds well with experimental values for the same parameter obtained for homogeneous isotropic turbulence. The model is then validated for a turbulent impinging jet heat transfer problem. Results are satisfactory and compare very favorably against DES and dynamic LES for the same computation.

Effects of Reynolds number and adverse pressure gradient on a turbulent boundary layer developing on a rough surface

Abstract: A detailed experimental study of turbulent boundary layers developing over rough surfaces subjected to both zero and adverse pressure gradients was undertaken. Two rough surfaces constituted by distributed random elements have been chosen. The experiments were conducted in the range of the Reynolds number R θ based upon the momentum thickness varying from 2200 to 5800 and 3200 to 3800, respectively, with and without pressure gradient. The roughness Reynolds number h + characteristic of the rough surfaces grew from about 25 to 80 in both zero and adverse pressure gradient, which corresponds to Reynolds number based upon the equivalent sand grain ranging from 75 to 225. The boundary layer was probed with a single hot wire. Skin friction coefficient was obtained from the semi-logarithmic profiles of the mean velocity. The aim of this work is to assess the relevance of the “equivalent sand grain” concept, and more precisely if the equivalent sand grain height only depends upon the surface characteristics or a...

Dual vortex structure shedding from low aspect ratio, surface-mounted pyramids

Abstract: The flow around surface-mounted, square-based pyramids, of apex angles ranging from 5° to 90°, placed in a thin boundary layer with one face normal to the on-coming flow was investigated for Reynolds numbers of 10 000–150 000 using surface pressure measurements and particle image velocimetry as well as oil-film surface-flow patterns. Based on the periodicity of the surface pressure on the pyramid side faces, four distinct shedding regimes were identified: constant local Strouhal number, discrete cellular, low aspect ratio and suppressed. Closer inspection of the low aspect ratio regime indicates that two distinct vortical structures are shed at the same frequency. Vortices are shed alternately from opposing faces in the obstacle base region, while hairpin vortices are shed in-phase and are most easily observed near the tip region of the pyramid. It is further shown that the existence of these structures is consistent with the mean surface-flow topology observed in the wake and side faces of the obstacles.

Anisotropy of a turbulent boundary layer

Abstract: A two-dimensional turbulent boundary layer with a Reynolds number 2000 based on the displacement thickness was calculated by using direct numerical simulation. The goal of this study was to establish turbulence models for the closure of the Reynolds stress tensor and the anisotropy tensor, distinguish the global and local anisotropy and calculate the orientation of turbulent structures occurring in the turbulent boundary layers. This paper proposes an anisotropic model, which approximates the anisotropy tensor bij using a normalized turbulent dissipation tensor dij . The multiscale behavior of anisotropy occurring in turbulent boundary layers is investigated and analyzed. The results reveal that the anisotropy does not decay as the scale decreases but persists in the local scales. Additionally, this paper analyzes the orientation of the turbulent energy transportation and the distribution of the positive and negative vorticities by using the skewness of the velocity fluctuations and turbulent vorticities.

Assessment of a film-cooling flow structure by large-eddy simulation

Abstract: Large-eddy simulation results using the approximate-deconvolution subgrid-scale model (ADM) are reported for a jet-in-crossflow configuration related to the film-cooling of turbine blades. Here, relatively cool air is ejected from a large isobaric plenum through an oblique round nozzle into a hotter turbulent boundary layer. The parameters are chosen in accordance with typical film-cooling conditions of turbines. For computational economy, a steady mean-turbulent inflow profile is used for the crossflow. The adequacy of this simplification is evaluated. The properties of the mean flow field are investigated and the mean film-cooling efficiency is compared to results from the literature. Special attention is paid to the flow structure inside the jet nozzle, which is considered to have a key influence on the mixing process. The physical understanding of film cooling is deepened with an illustration of the instantaneous flow field and its connection to the findings for the mean flow.

Improved turbulence generation techniques for hybrid RANS/LES calculations

Abstract: Hybrid turbulence simulations that use a combination of Reynolds-averaged Navier–Stokes (RANS) equations and large-eddy simulation (LES) are becoming increasingly popular to increase predictive accuracy in complex flow situations without the cost of full large-eddy simulations. In a typical application, the RANS equations are solved in the equilibrium or near-equilibrium regions of the flow, while LES equations are used in regions where the flow is away from equilibrium. A difficulty in performing hybrid simulations stems from the mismatch between resolved turbulent scales between the RANS and LES regions: in the RANS region all turbulent scales are modeled, while in the LES region the energy containing scales are resolved. Interfacing these two regions requires the generation of turbulent eddies capable of supporting the Reynolds stresses in the LES field. Previous work by the authors proposed a combination of synthetic turbulence and a controlled forcing method that was capable of quickly generating the...

Investigation using statistical closure theory of the influence of the filter shape on scale separation in large-eddy simulation

Abstract: The influence of the filter shape on scale separation in large-eddy simulation (LES) is investigated using the eddy-damped quasi-normal Markovian (EDQNM) modelling approach, for discrete filters of order 2–12. The LES subgrid-scale (SGS) stress tensor is split into a represented SGS tensor based on the scales resolved by the mesh, and a non-represented SGS tensor involving the unresolved scales. The investigation of the kinetic energy spectra associated with these quantities shows that the features of the SGS tensor strongly depend on the filter shape. In particular, for the second-order filter, the SGS stress tensor dynamics is dominated by the interactions between the resolved scales. The effective LES cut-off wavenumber is finally evaluated and recast in term of efficiency rates in order to estimate the computational cost required to achieve a given spectral resolution. Sharp cut-off filters appear to be well appropriate to perform an efficient scale separation in LES.

Thorough analysis of vortical structures in the flow around a yawed bluff body

Abstract: In the present study numerical investigations are used as a complementary approach to analyse large coherent structures in the flow around a bluff-body under cross-wind The results from the numerical simulation are validated through wind-tunnel experiments and used to analyse unresolved observations from these experimental investigations This underlines the duality of these two approaches in the analysis of the aerodynamics of a bluff-body Vortical structures in a three-dimensional, asymmetric and turbulent wake are detected through the use of different vortex detection schemes (vorticity, Q, λ2 and Γ3), permitting to isolate flow structures The vortex properties of two main lateral vortices are described and discussed on the basis of envelopes detected by the λ2 and the Γ3 criterion Although the flow is highly three-dimensional and complex, the vortex properties are quite simple and show mechanisms like vortex stretching and bursting These two criteria lead qualitatively to the same results; quanti

The influence of wavy walls on the transport of a passive scalar in turbulent flows

Abstract: We carry out an experimental study to investigate the turbulent transport of a passive scalar in turbulent flows between a flat top wall and two different complex surfaces and describe the influence of the different surface geometries on the mixing properties.In order to achieve a homogeneous and inhomogeneous reference flow situation two different types of surface geometries are considered: a sinusoidal bottom wall profile and a profile consisting of two superimposed sinusoidal waves.The laboratory investigations were performed by the use of a combined digital particle image velocimetry (DPIV) and planar laser-induced fluorescence (PLIF) technique to examine the spatial variation of the streamwise, spanwise and wall-normal velocity components and to assess the concentration field of the scalar. Statistically sufficient image sequences (frame rate 4 Hz) are obtained from both DPIV and PLIF to calculate time-averaged statistics of the velocity field and the scalar field. We discuss the influence of the com...

Time-evolution K–L analysis of coherent structures based on DNS of turbulent Newtonian and viscoelastic flows

Abstract: The turbulent dynamics corresponding to direct numerical simulation (DNS) data of Newtonian and viscoelastic turbulent channel flows is analyzed here through a projection of a time sequence of velocity fields into a set of Karhunen–Loeve (K–L) modes, large enough to contain, on the average, more than 90% of the fluctuating turbulence kinetic energy. Previous calculations of the K–L eigenstructure have demonstrated a dramatic decrease in the K–L dimensionality accompanying the presence of viscoelasticity in turbulent channel flows. Projection of turbulent velocity information into the most energetic K–L modes allows for a significant data reduction, most suitable for the study of the time-evolution dynamics. We exploit this feature here in analyzing the time series of the projection coefficients of the velocity field into K–L modes to gain further quantitative insights into the behavior of the overall flow dynamics. The presence of viscoelasticity induces significant changes to the turbulence dynamics as c...