Detached Eddy Simulation Simulations Research Articles

Oscillatory flow around a vertical circular cylinder placed in an open channel: coherent structures, sediment entrainment potential and drag forces

Flow and turbulent structures generated by the interaction of forced incoming oscillatory flow with a circular, vertical cylinder placed in an open channel with a horizontal bed are investigated using eddy-resolving simulations. Validation simulations performed with a Keulegan–Carpenter (KC) number of 20 and multiple-mode forcing corresponding to the laboratory experiment of Sumeret al.(J. Fluid Mech., vol. 332, 1997, pp. 41–70) show that detached eddy simulation (DES) predicts more accurately the amplification of the bed shear stress beneath the horseshoe vortex system (upstream side of the cylinder) and the maximum magnitude of the bed shear stress at the downstream (wake) side of the cylinder compared with unsteady Reynolds-averaged Navier–Stokes simulations. High-Reynolds-number DES simulations are then conducted with 1.5 ≤ KC ≤ 30.8 and one-mode sinusoidal forcing of the streamwise velocity in the approaching flow to investigate the changes in the wake vortex-flow regimes, the coherence of the horseshoe vortices and the generation of other near-bed coherent structures in the wake during the oscillatory cycle. The flow is periodic, no horseshoe vortices form and no vortices are shed in the wake forKC = 1.5. By contrast, forKC ≥ 8 horseshoe vortices are present over part of the oscillatory cycle and up to three wake vortices are shed over each half-cycle asKCis increased to 30.8. For an intermediate range ofKCnumbers, one (KC = 15.4) or two (KC = 8) of the vortices forming at the back of the cylinder during each half-cycle are washed around it when the flow reverses. The main horseshoe vortex and other horizontal near-bed vortices have a large capacity to amplify the bed shear stresses when the incoming velocity magnitude is significantly less than its peak value. Assuming the depth-averaged velocity in the incoming (undisturbed) oscillatory flow is the same in simulations conducted with differentKCnumbers, the peak values of the sediment entrainment potential measured by the mean (cycle-averaged) volumetric flux of sediment entrained from the bed over one oscillatory cycle occur for 8 ≤ KC ≤ 15.4. For allKCnumbers, the in-line force variation over the oscillatory cycle is fairly well approximated by the Morison equation. ForKC = 1.5, the in-line force is only due to inertia effects. ForKC = 30.8, the maximum and minimum values of the phase-averaged in-line force are approximatively in phase with those of the incoming flow velocity. ForKC ≥ 15, the phase-averaged in-line force coefficients vary between 0.8 and 1.1 during most of the oscillatory cycle (e.g. when the incoming flow velocity is not close to zero). This is different from cases withKC ≤ 8 where the in-line force coefficient is equal to zero twice during the oscillatory cycle as the in-line force becomes equal to zero for non-zero values of the incoming velocity. The largest cycle-to-cycle variations of the in-line force coefficient and in-line force are observed aroundKC = 8. ForKC = 8 and 15.4, the cylinder is subject to relatively large phase-averaged spanwise drag forces that are comparable to the peak phase-averaged streamwise drag forces. AsKCis increased to 30.8, the phase-averaged spanwise drag force becomes zero over the whole oscillatory cycle but the cylinder is still subject to large instantaneous spanwise forces over part of the oscillatory cycle.

IDDES method based on differential Reynolds-stress model and its application in bluff body turbulent flows

A general construction approach for a class of detached-eddy simulation (DES) methods with the turbulent length-scale equation is presented in this study. It supports the rationality of the construction and provides a theoretical estimation of the model coefficients in DES simulation. By using the construction approach, a differential Reynolds-stress model (RSM), referred to as Speziale-Sarkar-Gatski (SSG)/Launder–Reece–Rodi (LRR)-ω RSM, is built into the improved delayed DES (IDDES) method. After calibration of the model parameters in the IDDES method and basic validation for decaying isotropic turbulence, the RSM-based IDDES approach is then applied to simulate the massively separated flows around the tandem cylinders and the transonic buffet flow over a hammerhead launch vehicle. The simulations are validated by the available experimental data, and the performance is evaluated by means of instantaneous, statistical, spectral analysis of the numerical data. It is found that the RSM-based IDDES method shows better performance comparing with the k-ω shear-stress transport (SST)-based IDDES method, especially for predicting the development of massively separated flows behind the bluff body.

Turbulence-Model Comparison for Aerodynamic-Performance Prediction of a Typical Vertical-Axis Wind-Turbine Airfoil

In this work, different turbulence models were applied to predict the performance of a DU-06-W-200 airfoil, a typical choice for vertical-axis wind turbines (VAWT). A compromise between simulation time and results was sought, focusing on the prediction of aerodynamic forces and the developed flow field. Reynolds-averaged Navier–Stokes equation (U-RANS) models and Scale-Resolving Simulations (SRS), such as Scale-Adaptive Simulation (SAS) and Detached Eddy Simulation (DES), were tested, with k − ω -based turbulence models providing the most accurate predictions of aerodynamic forces. A deeper study of three representative angles of attack (5 ° , 15 ° , and 25 ° ) showed that U-RANS models accurately predict aerodynamic forces with low computational costs. SRS modeling generates more realistic flow patterns: roll-up vortices, vortex packets, and stall cells have been identified, providing a richer unsteady flow-field description. The power spectrum density of velocity at 15 ° has confirmed a broadband spectrum in DES simulations, with a small peak at a Strouhal number of 0.486. Finally, indications regarding the selection of the turbulence model depending on the desired outcome (aerodynamic forces, airfoil flow field, or VAWT simulation) are provided, tending toward U-RANS models for the prediction of aerodynamic forces, and SRS models for flow-field study.

Delayed Detached Eddy Simulation of Subcritical Flow past Generic Side Mirror

In the present study, the subcritical flow past a generic side mirror on a base plane is investigated at the Reynolds number of 5.2×105 using delayed detached eddy simulation (DDES) turbulence model. Asides from the capability of capturing main features of the large recirculation vortex in the wake of the side mirror and the front horseshoe vortex, the accuracy of DDES estimation of recirculation length is significantly increased by over 20%, compared to the detached eddy simulation (DES) estimation using the same grid. And DDES prediction of pressure coefficient at the trailing edge of the mirror is in good agreement with the experiments, which is more accurate than both DES and large eddy simulation (LES) results. The results verify the capacity of DDES turbulence model to solve the turbulent flow around the side mirror. This is a key foundation for possible future study of full simulation of external flow field of vehicle.

Two-phase flow DES and URANS simulations of pump-intake bay vortices

ABSTRACTWell-resolved detached eddy simulation (DES) and unsteady Reynold-averaged Navier–Stokes (URANS) simulations are conducted for a water pump-intake bay. Three cases are analysed: a high-submergence, low discharge case (Case 1), a low-submergence, low-discharge case (Case 2) and a low-submergence, high-discharge case (Case 3). The numerical model uses the volume of fluid (VOF) to account for the free surface deformations. In Case 1, for which the free surface deformations in the experiment were small, DES predictions are in better agreement with previous experimental measurements, compared to URANS predictions of the mean flow position, circulation, mean azimuthal velocity and vorticity distribution inside the cores of the main free-surface and submerged vortices forming inside the pump-intake bay. Decreasing the submergence depth in Case 2 results in an intensification of the coherence and unsteadiness of the main free surface vortex and in notable deformations of the free surface compared to Case 1. In Case 3, where the discharge is five times larger compared to Case 2, DES predicts the formation of an even stronger free-surface vortex whose core, at times, penetrates inside the pump column and entrains air. The effects of decreasing submergence and increasing discharge on the mean position, strength and general structure of the main submerged vortices are discussed. DES results for Case 3 are also used to describe the dynamics of the main pump-bay vortices. Results also show that the differences between the DES and URANS predictions of the mean-flow vortical structure increase dramatically for Case 3. Despite large deformations of the free surface, no well-defined free-surface vortex is present in the URANS.

Hybrid RANS/LES Simulation of Corner Stall in a Linear Compressor Cascade

Current design-cycle Reynolds-averaged Navier–Stokes (RANS) based computational fluid dynamics (CFD) methods have the tendency to over-predict corner-stall events for axial-flow compressors operating at off-design conditions. This shortcoming has been demonstrated even in simple single-row cascade configurations. Here we report on the application of hybrid RANS/large eddy simulation (LES), or detached eddy simulation (DES), for simulating the corner-stall data from the linear compressor cascade work conducted at Ecole Centrale de Lyon. This benchmark data set provides detailed loss information while also revealing a bimodal behavior of the separation which, not surprisingly, is also not well modeled by RANS. The hybrid RANS/LES results presented here predict bimodal behavior similar to the data only when special treatment is adopted to resolve the leading-edge endwall region where the horseshoe vortex (HV) forms. The (HV) is shown to be unstable, which produces the bimodal instability. The DES simulation without special treatment or refinement in the HV region fails to predict the bimodal instability, and thus the bimodal behavior of the separation. This, in turn, causes a gross over-prediction in the scale of the corner-stall. The HV region is found to be unstable with rolling of the tertiary vortex (TV) over the secondary vortex and merging with the primary HV. With these flow dynamics realized in the DES simulations, the corner stall characteristics are found to be in better agreement with the experimental data, as compared to RANS and standard DES approaches.

Evaluation of computational and physical parameters influencing CFD simulations of pollutant dispersion in building arrays

Many CFD studies have investigated the influence of computational parameters on the predicted concentration distribution of pollutants around isolated buildings, but such studies for building arrays are still lacking. This study systematically evaluated the influence of four computational and two physical parameters on pollutant dispersion in building arrays, including turbulence models, grid resolution, discretization of time step size Δt, length of sampling period, aspect ratio of the arrays, and release rate of tracer gas. Throughout these evaluations, a set of published wind tunnel experimental data was used to validate the CFD models. For concentration simulations, the Large Eddy Simulation (LES) model gave the most accurate results but still had limitations in areas near the source, whereas the Detached Eddy Simulation (DES) and the Reynolds Averaged Navier-Strokes (RANS) RNG k−ε models underperformed in some areas. The results of the LES and DES simulations varied with changes in Δt∗ and sampling length until Δt∗ was less than 0.24 and the sampling length was higher than 2400 Δt∗ for LES and 1200 Δt∗ for DES. A larger aspect ratio did not necessarily result in a higher concentration field than a smaller ratio. An increase in the tracer gas release rate did not change the general dispersion characteristics, but it still affected the concentration distribution in the areas near the source and resulted in a larger polluted area. The findings of this study are intended to contribute to improvements in the quality of CFD simulations of pollutant dispersion in building arrays.

CFD simulation of the wind environment around an isolated high-rise building: An evaluation of SRANS, LES and DES models

The choice of proper turbulence models in computational fluid dynamics is still a challenging issue for accurately predicting outdoor microclimate and thermal comfort conditions in urban planning. This study compared the performances of the Steady Reynolds Averaged Navier–Stokes (SRANS) RNG k-ε, Large Eddy Simulation (LES) and Detached Eddy Simulation (DES) modeling approaches in simulating the wind flow around an isolated building (with a 1:1:2 shape). The effects of the computational parameters were analyzed, including the grid resolution for all cases, and the discretization time step (Δt) and non-dimensional sampling time (t*) for the LES and DES cases. The results of the LES and DES simulations were affected by the gradual decrease in Δt and increase in t* until the two parameters reached 0.005 s and 288, respectively. The mean velocity fields on the windward side of the building predicted by the three models were in good agreement with the wind tunnel results. However, the results of the LES and DES cases were in better agreement with the experimental results for the leeward and lateral regions in both vertical and horizontal planes. The DDES (Delayed Detached Eddy Simulation) and LES models predicted similar results in the wake region, but the DDES has a lower overall mesh requirement. It is encouraging that the DDES model provides not only the mean flow field, but also the instantaneous wind characteristics, which can be useful for more accurate analysis of outdoor wind and thermal comfort.

Unsteady RANS and detached eddy simulation of the multiphase flow in a co-current spray drying

A detached eddy simulation (DES) and a k-ε-based Reynolds-averaged Navier–Stokes (RANS) calculation on the co-current spray drying chamber is presented. The DES used here is based on the Spalart–Allmaras (SA) turbulence model, whereas the standard k-ε (SKE) was considered here for comparison purposes. Predictions of the mean axial velocity, temperature and humidity profile have been evaluated and compared with experimental measurements. The effects of the turbulence model on the predictions of the mean axial velocity, temperature and the humidity profile are most noticeable in the (highly anisotropic) spraying region. The findings suggest that DES provide a more accurate prediction (with error less than 5%) of the flow field in a spray drying chamber compared with RANS-based k-ε models. The DES simulation also confirmed the presence of anisotropic turbulent flow in the spray dryer from the analysis of the velocity component fluctuations and turbulent structure as illustrated by the Q-criterion.

High‐order detached eddy simulation, zonal LES and URANS of cavity and labyrinth seal flows

SUMMARYHybrid numerical large eddy simulation (NLES), detached eddy simulation (DES) and URANS methods are assessed on a cavity and a labyrinth seal geometry. A high sixth‐order discretization scheme is used and is validated using the test case of a two‐dimensional vortex. The hybrid approach adopts a new blending function. For the URANS simulations, the flow within the cavity remains steady, and the results show significant variation between models. Surprisingly, low levels of resolved turbulence are observed in the cavity for the DES simulation, and the cavity shear layer remains two dimensional. The hybrid RANS–NLES approach does not suffer from this trait.For the labyrinth seal, both the URANS and DES approaches give low levels of resolved turbulence. The zonal Hamilton–Jacobi approach on the other had given significantly more resolved content. Both DES and hybrid RANS–NLES give good agreement with the experimentally measured velocity profiles. Again, there is significant variation between the URANS models, and swirl velocities are overpredicted. Copyright © 2013 John Wiley & Sons, Ltd.

Detached-Eddy Simulation of Vortex Generator Jet Using Chimera Grids

This paper aims at numerically analysing the effect of an active flow control (AFC) by a vortex generator jet (VGJ) submerged in a boundary layer via Chimera Grids and Detached- Eddy Simulation (DES). The performance of DES results are judged against Reynolds-Averaged Navier-Stokes (RANS) and compared with the experiments that showed an unsteady vortex motion downstream of VGJ. Experimental results showed that the mechanism of embedding logitudinal vortex structure in the main stream flow is quite effective in increasing the near wall momentum of separated aircraft wing. In order to simulate such a flow configuration together with the VGJ, an efficient numerical approach is required. This requirement is fulfilled by performing the DES simulation over the flat plate using the DLR TAU Code. The DES predictions identify the vortex region via smooth hybrid length scale and predict the unsteady vortex motion observed in the experiments. The DES results also showed that the sufficient grid refinement in the vortex region resolves the turbulent scales downstream of the VGJ, the spatial vortex core postion and non- dimensional momentum coefficientRVx .

An investigation of the dynamics of coherent structures in a turbulent channel flow with a vertical sidewall obstruction

The physics of the flow around a vertical-wall obstruction attached to one of the sidewalls of a straight channel is numerically investigated using detached eddy simulation (DES) at a high channel Reynolds number, Re=5×105 (case HR). In particular, the study investigates the role played by the large-scale coherent structures in the sediment entrainment processes at the bed for conditions close to the initiation of scour (flat bed) in a loose-bed channel. Scale effects are investigated by comparing the results of the present DES simulation with results from a large eddy simulation performed at a much lower Reynolds number, Re=18 000 (case LR). Similar to laboratory flume studies of flow and scour around in-stream obstructions, the incoming flow in the simulations was fully turbulent and contained unsteady velocity fluctuations. The main necklace vortex of the horseshoe vortex (HV) system forming near the upstream base of the flow obstruction was subject to bimodal large-scale oscillations. The intensity of the bimodal oscillations peaked at vertical sections cutting through the tip of the obstruction. Present results show the size of the region of high turbulence amplification within the HV system decreases with the increase in the Reynolds number. Patches of vorticity were observed to detach from the leg of the main necklace vortex and to be convected at a small distance from the bed. Before dissipating, these patches could induce relatively large values of the bed shear stress beneath them. In case HR, the formation of these patches was primarily determined by the interaction of the main necklace vortex with the leg of the secondary necklace vortex rather than the interaction of the main necklace vortex with the tip of the obstruction, as was the case in the LR simulation. The degree of deformation of the cores of the vortex tubes shed in the upstream part of the separated shear layer (SSL) originating at the tip of the obstruction was much larger in the HR simulation. This provided an additional mechanism for the amplification of the horizontal vorticity in the near-bed region. Large-scale hairpinlike structures formed in the downstream part of the SSL. The legs of these vortices were oriented parallel to the interface between the high-speed outer flow and the recirculating flow past the obstruction. The lower leg of some of these hairpin vortices was situated, at times, at a small distance from the bed and was able to strongly amplify the local bed shear stress values. Consequently, the bed shear stress distribution in the instantaneous flow fields displayed a streaky structure over part of the SSL region. The changes in the relative position and size of the regions of high turbulence amplification inside the HV system and the SSL induced noticeable scale effects on the distributions of the shear stress and pressure root-mean-square (rms) fluctuations at the bed which control the sediment entrainment processes. In particular, due to the richer eddy content of the SSL, the intensity of the nondimensional pressure rms fluctuations beneath the SSL was found to be about two times larger in the HR simulation.

A Numerical Investigation of the Flow Past a Generic Side Mirror and its Impact on Sound Generation

The case investigated is the flow past a generic side mirror mounted on a flat plate at the Reynolds number of ReD=5.2×105 based on the mirror diameter. The present work studies both flow and acoustic sources by evaluating two second-order advection schemes, different levels of turbulence modeling, and three different grids. The advection schemes discussed in the present study are a second-order upwind scheme and a monotonic central scheme. The turbulence models investigated cover three levels of modeling. These are the original formulation of the detached eddy simulation (DES) model, the Smagorinsky–Lilly sub-grid scale (SGS) model with near-wall damping, and a dynamic Smagorinsky model. The different grids are as follows: a primary grid where all parameter studies are conducted and a second grid with significantly higher wake resolution and to some extent also increased plate resolution, while maintaining the resolution at the front side of the mirror. The final grid uses a significantly higher plate resolution and a wake resolution similar to that of grid two, but a comparably lower mirror front side resolution as compared with the two other grids. The general outcome of this work is that the estimation of the grid cutoff frequency through a relation of the velocity fluctuation and the grid size matches both the experimental results and trend lines perfectly. Findings from the flow field show that the horseshoe vortex in front of the mirror causes pressure fluctuations with a magnitude exceeding the maximum levels at the rear side of the mirror. Its location and unsteady properties are perfectly captured in the final simulation as compared with the experiments conducted by Daimler–Chrysler. A laminar separation at the front side of the mirror is more or less found for all wall resolved cases except the DES simulation. The third grid fails to predict this flow feature, but it is shown that this effect has no significant effect on either the static pressure sensors at the mirror surface or at the dynamic sensors located downstream of the mirror. The simulation also supports the fundamental frequency based on the eddy convection in the mirror shear layer, which is shown to be twice as high as the frequency peak found in the lateral force spectra.

Prediction of sound generated by a rod–airfoil configuration using EASM DES and the generalised Lighthill/FW-H analogy

Sound generated by an airfoil in the wake of a rod is predicted numerically by using a Detached-Eddy Simulation (DES) unsteady flow field and a Ffowcs Williams and Hawkings acoustic analogy formulation for the far field computation. Volume sources from the rod wake are found to play a non-negligible role at high frequencies and surface contributions might be flawed if the surfaces cross highly turbulent flow regions even if surrounding volume terms are accounted for. The DES approach is based on a novel cubic explicit algebraic stress turbulence model which is built on a two-equation k– ε model from Lien and Lechziner. This DES has been recently implemented at the Berlin University of Technology in the compressible Navier–Stokes flow solver ELAN. The aerodynamic results are compared to experimental data obtained at the ECL by Jacob et al., as well as to previous Large Eddy Simulations results from the Proust/Turbflow code by Boudet et al. and DES simulations from Greschner et al. based on standard turbulence models. The acoustic analogy is applied both with and without volume terms to rigid and permeable control surfaces surrounding the rod–airfoil system. Aeroacoustic results are compared to experimental data from the literature, showing that the inclusion of volume terms improves the aeroacoustic prediction in the broadband high frequency range.