Showing papers in "Journal of Turbulence in 2010"

The organization of near-wall turbulence: a comparison between boundary layer SPIV data and channel flow DNS data

Abstract: The vortical structures of near-wall turbulence at moderate Reynolds number are analyzed and compared in datasets obtained from stereoscopic particle image velocimetry (SPIV) in a turbulent boundary layer and from direct numerical simulations (DNS) in a turbulent channel flow. The SPIV data are acquired in the LTRAC water tunnel at Re=+=820 in a streamwise/wall-normal plane, and in the LML wind tunnel at Re=+=2590 in a streamwise/wall-normal plane and in a plane orthogonal to the mean flow. The DNS data is taken from DelAlamo et al. (Self-similar vortex clusters in the turbulent logarithmic region, J. Fluid Mech. 561 (2006), pp. 329-358), at Re=950. The SPIV database is validated through an analysis of its mean velocity profile and power spectra, which are compared with reference profiles. A common detection algorithm is then applied to both the SPIV and DNS datasets in order to retrieve the streamwise and spanwise vortices. The algorithm employed is based on the 2D swirling strength and on a fit of an Oseen vortex model, which allows to retrieve the vortex characteristics, including its radius, vorticity, and position of the center. At all Reynolds numbers, the near-wall region is found to be the most densely populated region, predominantly with streamwise vortices that are on average smaller and more intense than spanwise vortices. In contrast, the logarithmic region is equally constituted of streamwise and spanwise vortices having equivalent characteristics. Two different scalings were employed to analyze the vortex radius and vorticity: the wall unit scaling and the Kolmogorov scaling. In wall unit scaling, a good universality in Reynolds numbers of the vortices radius and vorticity is observed in the near-wall and logarithmic regions: the vorticity is found to be maximum at the wall, decreasing first rapidly and then increasing slowly with wall-normal distance; the radius is increasing slowly with wall-normal distance in both the regions, except for the streamwise vortices, for which a sharp increase in radius is observed in the near-wall region. However, wall units scaling is found to be deficient in the outer region, where Reynolds number effects are observed. The Kolmogorov scaling appears to be universal both in Reynolds number and wall-normal distance across the investigated three regions, with a mean radius of the order of 8 and a mean vorticity of the order of 1.5-1, but for the SPIV data only. In the DNS dataset, the radius in the Kolmogorov scaling slowly decreases with wall-normal distance. This difference between the SPIV and DNS datasets may be linked to a difference between the boundary layer flow and the channel flow, rather than to the techniques themselves. Finally, the distribution of the vorticity of the detected vortices seems to follow faithfully a log-normal distribution, in good agreement with Kolmogorov's theory (A refinement of previous hypothesis concerning the local structure of turbulence in a viscous incompressible fluid at high Reynolds number, J. Fluid Mech. 13 (1962), pp. 82-85).

Dynamics of sonic jet injection into supersonic crossflow

Abstract: The interaction between a sonic air jet and a supersonic air crossflow is simulated using large-eddy simulation (LES). A hybrid numerical methodology is used here to capture shock waves locally with minimal dissipation of the turbulent structures. The dynamic subgrid closure model employed for the LES permits a fully localized evaluation of the closure coefficients, such that there are no ad hoc adjustable parameters. Simulation of the experimental study of Santiago and Dutton (J. Propul. Power, vol. 13, 1997, pp. 264–273), where detailed measurements of the mean velocity and turbulent fluctuations have been acquired, is reported. The LES results show fairly good agreement with the experimental data for the mean and statistical fluctuations of the velocity field. The numerical study is then extended to two other jets in crossflow conditions to study the impact of the free-stream Mach number and of the jet to free-stream momentum ratio on the structure of the jet and on the dynamics of the interaction. The...

Turbulence structures over irregular rough surfaces

Abstract: Turbulent flow in a channel with irregular two-dimensional rough surfaces is analysed through wall-resolving large eddy simulation (LES). Both walls of the channel are roughened through the superimposition of sinusoidal functions having random amplitude and four different wavelengths. The downward shift of the velocity profile in the log region due to the roughness, known as roughness function, is well captured in the simulations. The spanwise and wall-normal turbulence intensities are found to increase with the roughness height, while the streamwise component decreases. The analysis of the Reynolds stress anisotropy tensor highlights a tendency towards isotropisation, confirmed by the vorticity rms. The analysis of the statistics shows that the effects of the roughness on the turbulent flow are greatly related to the increase of the height of the maximum peaks of the corrugations. Although the inner layer is dramatically affected by the wall irregularities, the outer layer appears not affected by the spe...

Analysis of the role of turbulence in curved open-channel flow at different water depths by means of experiments, LES and RANS

Abstract: In order to unravel the main flow and secondary flow characteristics and the role of turbulence in a curved single-bend open-channel flow, large-eddy simulations (LES) and Reynolds-averaged numerical simulations (RANS) were carried out and compared with experiments of the flow through a strongly bent laboratory flume. Turbulence was found to play an important role with respect to processes that are important in natural rivers. The strength of the curvature-induced secondary flow in the core of the flow domain, which is the most typical feature of curved open-channel flow, depends on the turbulence. Turbulence is especially important in the flow regions near the banks. Only the LES model is able to resolve accurately the boundary layer detachment and the formation of an internal shear layer at the inner bank as well as the outer-bank cell of secondary flow, whereas the RANS model is unable to reproduce these processes. Turbulence also conditions the magnitude of the bed shear stress, as indicated by the co...

Wind-tunnel study of surface boundary conditions for large-eddy simulation of turbulent flow past a rough-to-smooth surface transition

Abstract: A wind-tunnel experiment was performed to test surface boundary condition formulations for large-eddy simulation downwind of a rough-to-smooth surface transition in a turbulent boundary layer for (Reτ ≈ 1.5 × 104). Single and x-wire anemometers were used to obtain simultaneous high-resolution measurements of surface shear stress and wind velocity at different heights and positions downwind of the transition. One-dimensional filtering, using Taylor's hypothesis, was used to obtain filtered signals of both velocity and surface shear stress. Experimental results show substantial differences between measured and modelled shear stress using standard boundary conditions based on the direct application of the similarity theory (the log law under neutral conditions) with local fluctuating filtered velocities. Those errors affect both the average value as well as higher order statistics of the predicted surface shear stress. The best performance is obtained with a model that calculates the average surface shear st...

Velocity gradient invariants and local flow-field topology in compressible turbulence

Abstract: The effect of compressibility on velocity gradient structure and the related flow-field patterns/topology is investigated using direct numerical simulation data. To clearly isolate compressibility effects, the behavior is investigated as a function of local level of dilatation. Importantly, dilatation-conditioned behavior is found to be independent of Mach and Reynolds numbers. Not surprisingly, at low dilatation level, velocity gradient structure and local flow topology are similar to incompressible turbulence. At high dilatation levels, however, the behavior is quite different. A recently developed velocity gradient evolution equation – Homogenized Euler Equation (HEE) – qualitatively captures many of the observed features of compressible turbulence.

Large eddy simulation of free-stream turbulence effects on heat transfer to a high-pressure turbine cascade

Abstract: Large eddy simulation of an uncooled, transonic, linear high-pressure turbine vane cascade has been pursued using a high-order numerical method with about 83 million grid points. Heat transfer and boundary layer development on the surface of the blade are investigated in the presence of inflow turbulence. Quantitative comparisons with the experiments of Arts et al. (Tech. Note 174, von Karman Institute for Fluid Dynamics, Belgium, 1990) show excellent agreement. The distortion of vortices from the inflow turbulence in the cascade passage leads to the formation of long streamwise streaky structures, leading to transition of suction-side boundary layer. Significant heat transfer augmentation is seen on both the pressure and suction sides because of the presence of these structures.

Nature of sweep and ejection events in transitional and turbulent boundary layers

Abstract: This paper represents an experimental investigation of the physical nature of the turbulence production mechanisms in boundary layers associated with three fundamental phenomena known from previous studies: (i) sweep events, (ii) ejection events, and (iii) coherent vortical structures. The main goal of this study is to clarify connections between all these phenomena in transitional and turbulent flows. The experimental approach is based on a modified version of the hydrogen-bubble (HB) visualization technique called the synchronous visualization method, which is combined with quantitative processing of video images in a PIV-like manner. The results of investigation of sweep/ejection events and their connection with vortical structures are obtained at late and super-late stages of boundary-layer transition, as well as in fully turbulent boundary layers. The developed approach gives us the possibility to obtain detailed synchronous information about instantaneous flow characteristics in a great range of spa...

Wavelet analysis of the turbulent wake generated by an inclined circular cylinder

Abstract: Three-dimensional velocity and vorticity characteristics in the near wake of a stationary circular cylinder at an inclination angle α in the range of 0°–45° are analysed using the wavelet multiresolution technique, where α = 0° represents the case of the cylinder in a cross-flow. This study aims to examine the dependence of the velocity and vorticity characteristics at different wavelet levels on α as compared with that obtained at α = 0°. The validity of the independence principle (IP) for vortex shedding was also examined. It was found that the IP is only applicable for α < 45°. The energy spectra for the intermediate and large-scale structures decrease in terms of their maximum energy and disperse extensively over an enlarged frequency band with the increase of α. At α = 45°, the large-scale vortex dislocations may occur because of the increase of the three-dimensionality in the wake region. The consequences of vortex dislocations in the wake region can also be seen from the results of the velocity and...

A methodology to quantify the nonlinearity of the Reynolds stress tensor

Abstract: Turbulent models provide closure equations that relate the Reynolds stress with kinematic tensors. In this study, we present a methodology to quantify the dependence of the Reynolds stress tensor on mean kinematic tensor basis. The methodology is based upon tensor decomposition theorems which allow to extract from the anisotropic Reynolds stress tensor the part that is linear or nonlinear in the strain rate tensor D, and the parts that are in-phase (sharing the same eigenvectors) and out-of-phase with the strain rate. The study was conducted using direct numerical simulation (DNS) data for turbulent plane channel (from Reτ=180 to Reτ=1000) and square duct flows (Reτ=160). The results have shown that the tensorial form of the linear Boussinesq hypothesis is not a good assumption even in the region where production and dissipation are in equilibrium. It is then shown that the set of tensor basis composed by D, D2 and the persistence-of-straining tensor D·(W-ΩD)-(W-ΩD)·D, where W is the vorticity tensor and ΩD is the rate of rotation of the eigenvectors of D, is able to totally reproduce the anisotropic Reynolds stress. With the proposed methodology, the scalar coefficients of nonlinear algebraic turbulent models can be determined, and the adequacy of the tensorial dependence of the Reynolds stress can be quantified with the aid of scaled correlation coefficients.

Drag reduction by spanwise transversal surface waves

Abstract: A numerical simulation of a spatially evolving turbulent boundary layer impacted by a spanwise traveling transversal sinusoidal wall oscillation is performed Compared to boundary layer flow over a non-oscillating surface, ie a standard flat plate boundary layer, a reduction of friction drag of more than 6% is found A detailed discussion of the near-wall flow reveals similar effects as observed by other excitation methods, such as additionally implemented volume forces or longitudinal sinusoidal wall oscillation Secondary flow structures induced by the moving wall and a controlled formation of streamwise vorticity close to the wall are identified The wall-normal vorticity fluctuations are found to be damped such that streak instability and formation of new streaks are reduced yielding an evidently decreased friction drag

Statistics of the locally averaged thermal dissipation rate in turbulent Rayleigh–Bénard convection

Abstract: From the measured thermal dissipation rate in turbulent Rayleigh–Benard convection in a cylindrical cell, we construct a locally averaged thermal dissipation rate χ fτ by averaging over a time interval τ. We study how the statistical moments ⟨(χ fτ) p ⟩ depend on τ at various locations along the vertical axis of the convection cell. We find that ⟨(χ fτ) p ⟩ exhibits good scaling in τ, of about a decade long, with scaling exponents μ(p) for p = 1–6. For Rayleigh number (Ra) around 8×109, the scaling range is 1.4–21 s at the cell center and 4–21 s at the bottom plate. The dissipative and turnover times are about 0.8 s and 35 s respectively, while the timescale corresponding to the local Bolgiano scale is estimated to be about 31 s at the cell center and 3.5 s at the bottom plate. On the basis of several assumptions, we derive theoretical predictions for μ(p) at the different locations. The measured values of μ(p) are presented and shown to be in good agreement with our theoretical predictions.

Turbulence behavior of artificially generated vorticity

Abstract: Longitudinal vortices and hairpin-like structures are generated in an open loop flow by a row of vortex generators inserted on the inner wall of a circular pipe; the vortex generator row is made up of four diametrically opposed trapezoidal tabs tilted from the wall. Steady counter-rotating vortex pairs and periodic hairpin-like structures develop downstream from each tab. The flow pattern of these vortical structures has been studied extensively [D. Dong and H. Meng, Flow past a trapezoidal tab, J. Fluid Mech. 510 (2004), pp. 219–242]; nevertheless, the specific contributions of these structures to the mixing process have not yet been elucidated, especially with regard to global improvement of the transfer coefficients compared to a straight pipe. This study aims at exploring the turbulent mixing mechanisms caused by artificially generated vorticity, especially at the different mixing scales (macro-, meso- and micro-mixing), using both numerical simulations and laboratory experiments. Instantaneous veloci...

Direct numerical simulation of wind-driven turbulence and scalar transfer at sheared gas–liquid interfaces

Abstract: A three-dimensional direct numerical simulation is applied to wind-driven turbulence with sheared gas–liquid interface, and turbulence structure in interfacial boundary layers on both gas and liquid sides and scalar transfer mechanism across the gas–liquid interface are investigated. In order to capture the deforming gas–liquid interface, an arbitrary Lagrangian–Eulerian formulation method is employed. The results show that fluid motions are strongly affected by wind waves with ripples on the liquid side. The wind waves and ripples enhance the turbulence on the liquid side. For the present wind speed of several meters per second, the scalar transfer across the sheared wavy gas–liquid interface is mainly controlled by the streamwise vortices related to the downward bursting motions on the liquid side.

Multi-scale energy injection: a new tool to generate intense homogeneous and isotropic turbulence for premixed combustion

Abstract: Highly turbulent flow, generated by an original multi-scale device and nearly homogeneous and isotropic, is experimentally investigated. This multi-scale device is made of three perforated plates shifted in space such that the diameter of their holes and their blockage ratio increase with the downstream distance. The multi-scale turbulence injection (MuSTI) is compared with a mono-scale turbulence injection (MoSTI), the latter being only constituted of the last plate of the MuSTI. This comparison is done for both cold and reactive flows. For the cold flow the following are shown, in comparison with the classical mono-scale device, for the MuSTI device in the near-field: (i) The turbulent kinetic energy is larger, and the kinetic energy supply is distributed over the whole range of scales. This is emphasised by second- and third-order structure functions. (ii) The shear-stresses are enhanced. (iii) The homogeneity and isotropy are reached earlier (≈50%). (iv) The jet-merging distance is the relevant scalin...

Forcing large-scale coherent streaks in a zero pressure gradient turbulent boundary layer

Abstract: Large-scale coherent streaks are artificially forced in a well-developed turbulent boundary layer at using an array of cylindrical roughness elements. Measures of the velocity field with particle image velocimetry reveal the presence of well-reproducible, streamwise-oriented, steady coherent streaks. We show that the amplitude of these coherent streaks transiently grows in space. The position of the maximum amplitude is proportional to the spanwise wavelength of the streaks and the most amplified spanwise wavelength is of very large scale λ z ≈6δ0. These results are in good agreement with the recent predictions based on the optimal transient growth analysis of turbulent mean flows.

Simulations of heat transfer in a boundary layer subject to free-stream turbulence

Abstract: The present study investigates the effects of ambient free-stream turbulence (FST) on the momentum and heat transfer in a spatially developing, turbulent flat-plate boundary layer via large-eddy si ...

Embedded LES-to-RANS boundary in zonal simulations

Abstract: The RANS inflow boundary of a zonal LES/RANS simulation requires distributions of the averaged flow values and the turbulent viscosity. The mean flow data can be directly obtained by averaging the unsteady LES data, whereas the turbulent viscosity has to be reconstructed. Three methods to determine the turbulent viscosity are presented and tested using turbulent channel flow. The best method, i.e. the approach providing the smoothest transition from the LES to the RANS regime, being based on the turbulent kinetic energy k and the turbulent frequency ω, whereas the determination of ω is based only on derivatives of mean velocities, is used to compute a fully coupled LES/RANS channel flow to evidence the overall performance of the formulation. Finally, a zonal LES/RANS computation of a flat plate flow is performed, and the results are compared with a pure RANS simulation. The findings of the internal and the external flow problems show the formulation of the embedded boundary to be stable and to yield convi...

Transport and mixing of density in a continuously stratified shear layer

Abstract: Scalar transport and mixing by active turbulence in a high Reynolds number inhomogeneous stratified shear layer are investigated using three-dimensional Direct Numerical Simulation. Two density profiles are considered: (i) two layers of homogenous fluid with different density, namely the two-layer case, and (ii) a continuously stratified background ambient, namely the Jd case. The evolution of the mixing layer includes shear instability, formation of Kelvin–Helmholtz rollers, transition to turbulence, fully developed active turbulence, and, finally, decay toward a laminar state. In the Jd case, internal gravity waves carrying momentum and energy are observed to propagate away from the shear layer. Although different during the initial evolution, the eddy diffusivity and mixing efficiency when plotted as a function of buoyancy, Reynolds number takes similar values between the two cases later in time during the stage when turbulence decays. During this stage, the mixing efficiency computed based on the buoy...

Subfilter-scale transport model for hybrid RANS/LES simulations applied to a complex bounded flow

Abstract: The partially integrated transport modeling (PITM) method viewed as a continuous approach of hybrid Reynolds-averaged Navier–Stokes equations/large eddy simulations (RANS/LES) with seamless coupling is first recalled In the present work, the subfilter stress model derived from the PITM method is considered and developed in a general formulation valid for free as well as bounded flows Numerical simulations of the well-known, fully turbulent channel flows are first performed on coarse and medium grids for assessing the subfilter model and for studying the sharing out of the energy when the filter width is changed The practical flow over two-dimensional periodic hills is then simulated on coarse and medium grids for illustrating the performances of the subfilter stress model As a result, it is found that the subfilter stress model reproduces fairly well this complex flow governed by interacting turbulence mechanisms associated with separation, recirculation, reattachment, acceleration and wall effects O

Coherent structures in turbulent boundary layers with adverse pressure gradients

Abstract: The coherent structures in turbulent boundary layers (TBLs) subjected to adverse pressure gradients (APGs) were investigated by analyzing a database of direct numerical simulations. The equilibrium adverse pressure gradient flows were established by using a power law free-stream distribution. The population trends of the spanwise vortices show that the outer regions of the APG TBLs are densely populated with hairpin-like vortices. These vortical structures induce low-momentum regions in the middle of the boundary layers, which result in an outer peak in the Reynolds shear stress. The 3-D features of the hairpin packets were deduced from their spatial characteristics in the spanwise-wall-normal plane. The conditionally averaged velocity fields show that there are counter-rotating v−w swirling motions that represent cross-sectional evidences of the packets. Moreover, two-point correlations and linear stochastic estimations were used to provide statistical information about the hairpin packet motions in the ...

Large eddy simulations using truncated Navier–Stokes equations with the automatic filtering criterion

Abstract: We propose a large eddy simulation (LES) technique based on the previously developed Trancated Navier–Stokes (TNS) method. In TNS, the Navier–Stokes equations are solved through a sequence of direct numerical simulation runs and a periodic processing of small scales to provide the necessary dissipation. In the simplest case, the processing is accomplished by filtering the turbulent fields with a properly chosen filter. In the previous work, the period for processing was selected in advance for each case using heuristic arguments validated by trial and error. In this work, we develop a criterion that automates the selection of a time instant in simulations when the processing occurs. The criterion is based on the relationship between the energy of the flow field and the energy of the same field filtered with the chosen filter. The procedure is tested in LES of the turbulent channel flow performed at various Reynolds numbers and in domains of different sizes for which Direct Numerical Simulations (DNS) and ...

Analytical assessment of models for large eddy simulation of particle laden flow

Abstract: For large eddy simulation of particle-laden flow, the effect of the unresolved scales on the particles needs to be modelled. The present work contains an analysis of three such models, namely approximate deconvolution method (ADM) as proposed by Kuerten (Subgrid modeling in particle-laden channel flow, Phys. Fluids. 18 (2006), p.025108) and two stochastic models proposed by Shotorban and Mashayek (A stochastic model for particle motion in large-eddy simulations, J. Turbul. 7 (2006), p.N18) and Simonin, Deutsch and Minier (Eulerian prediction of the fluid/particle correlated Motion in turbulent two-phase flow, Appl. Sci. Res. 51, (1993), pp. 275–283). The purpose of the analysis is twofold. On the one hand, the results serve for model selection in dependence of the application and on the other hand, the analysis shows possibilities for model improvement. The present work contains for each model an analytical computation of averages (first moments) and root-mean square values (second moments) of particle ve...

Direct numerical simulation of unsheared turbulence diffusing towards a free-slip or no-slip surface

Abstract: The physics involved in the interaction between statistically steady, shearless turbulence and a blocking surface is investigated with the aid of direct numerical simulation. The original configuration introduced by Campagne et al. (G. Campagne, J.B. Cazalbou, L. Joly, and P. Chassaing, Direct numerical simulation of the interaction between unsheared turbulence and a free-slip surface, in ECCOMAS CFD 2006, P. Wesseling, E. Onate, and J. Periaux eds., TU Delft, The Netherlands, 2006) serves as the basis for comparing cases in which the blocking surface can be either a free-slip surface or a no-slip wall. It is shown that in both the cases, the evolutions of the anisotropy state are the same throughout the surface-influenced layer (down to the surface), despite the essentially different natures of the inner layers. The extent of the blocking effect can thereby be measured through a local (surface) quantity identically defined in the two cases. Examination of the evolution and content of the pressure–strain ...

LES approach coupled with stochastic forcing of subgrid acceleration in a high-Reynolds-number channel flow

Abstract: In this paper, the high-Reynolds-number channel flow is simulated by numerical approach at coarse resolution, in which the instantaneous acceleration is decomposed into filtered and subgrid parts, and then both components are modeled. The filtered acceleration is modeled in the framework of the large-eddy simulation approach. The model for the subgrid acceleration is based on two stochastic processes. The first is for its norm and is based on statistical universalities in fragmentation under scaling symmetry, providing correlation of subgrid forcing across the channel. The second is for its orientation and is based on the Brownian motion on a unit sphere in order to represent a stochastic relaxation toward full isotropy away from the wall. Two main parameters of the stochastic process include the Reynolds number based on the friction velocity and the channel half-width. In order to assess the capability of the model proposed, the paper illustrates its application versus recent high-Reynolds-number direct ...

Exploring the beta distribution in variable-density turbulent mixing

Abstract: In assumed probability density function (PDF) methods of turbulent combustion, the shape of the scalar PDF is assumed a priori and the PDF is parametrized by its moments for which model equations are solved. In non-premixed flows the beta distribution has been a convenient choice to represent the mixture fraction in binary mixtures or a progress variable in combustion. Here the beta-PDF approach is extended to variable-density mixing: mixing between materials that have very large density differences and thus the scalar fields are active. As a consequence, new mixing phenomena arise due to (1) cubic nonlinearities in the Navier–Stokes equation, (2) additional nonlinearities in the molecular diffusion terms and (3) the appearance of the specific volume as a dynamical variable. The assumed beta-PDF approach is extended to transported PDF methods by giving the associated stochastic differential equation (SDE). This enables the direct computation of the scalar PDF in a Monte–Carlo fashion. Using the moment equ...

Analysis of flow structures and energy spectra in chemical process equipment

Abstract: Studies have been carried out to extract relevant information on flow structures, their break-up distribution and the associated local turbulence phenomena using the 1D and 2D energy spectra. The 1D analysis uses the hot film anemometry (HFA) and large eddy simulation (LES) data sets, while the 2D analysis uses the PIV results. The chemical process equipment considered for the study are jet loop reactor (JLR), channel flow (CHA) and bubble column reactor (BCR). The work involves relative comparison of spectral methods, namely, fast Fourier transform (FFT), discrete wavelet transform (DWT), continuous wavelet transform (CWT), eddy isolation methodology (EIM) and proper orthogonal decomposition (POD) in evaluating the 1D and 2D spectra for the three equipment. FFT studies are used to identify the dominant frequencies, but they do not reveal the time dependent changes in the flow structure. This limitation is overcome by using the EIM, DWT and CWT and methodologies for identification of flow structures are d...

Direct numerical simulations of particle transport in a model estuary

Abstract: We investigate numerically the mixing of freshwater with ambient saltwater in a model estuary along with associated particle settling processes. We first discuss and specify two numerical setups that consider several relevant features to study the particle settling. The first configuration is a large rectangular basin with a small inlet for the (particle-laden) freshwater; the second is geometrically identical to the first except that the flow is laterally confined to the narrow inlet width. The two flows are computed until a statistically stationary solution is reached. We perform highly resolved direct numerical simulations using a high-order finite difference approach to yield reliable and accurate results. Accordingly, all relevant turbulent scales are resolved and turbulence modeling is not needed. The main target of this study is to describe and illustrate the fluid dynamics and the particle settling processes under the influence of turbulence arising in the freshwater/saltwater-stratified mixing la...

Eddy stress and shear in 2-D flow

Abstract: Upscale transfers of energy and momentum in quasi-2-D flows are seen in atmosphere, oceans and numerical experiments. Mechanisms that drive upscale transfer have been described in terms of sheared vortices becoming tilted and thinned, losing energy and developing eddy stresses which force, rather than resist, mean flow. Some of these concepts are mistaken. If initially isotropic eddy vorticity is passively advected in uniform shear, no eddy stress develops and there is no loss of eddy energy. When active vorticity self-advection is included, stresses arise yielding downgradient momentum transfer – just the opposite of what was hitherto expected.

Large-eddy simulations of control of a separated flow over a 2D bump by means of pulsed jets

Abstract: A numerical study of active flow control of a separated converging-diverging channel flow is presented. Several geometrical and operating parameters of the continuous and pulsed jets are investigated using large-eddy simulations (LES), which are highly resolved in the actuation region. The LES without control is validated with a direct numerical simulation of the same flow. Configurations with both counter-rotating and co-rotating jets are tested. The main statistical results are in general agreement with the literature. Thanks to the unsteady highly resolved simulation, both temporal and spatial organizations of the generated vortices are investigated. An optimal frequency associated with the convective time as well as an optimal duty cycle are obtained, leading to a control efficiency comparable to the cases with continuous jets and the same operating velocity and, consequently, to a significant saving in mass flow rate.