Hybrid Reynolds-averaged Navier-Stokes Research Articles

Slat noise suppression using upstream mass injection

A new approach, mass injection near the slat cusp, is proposed for the suppression of slat noise. The mechanism and efficiency of the slat noise suppression using upstream mass injection are numerically studied with hybrid Reynolds-averaged Navier-Stokes simulation/Large eddy simulation (RANS/LES) of a three-component high-lift model. Ffowcs-Williams and Hawking (FW-H) integration is employed to obtain far-field acoustic characteristics. Results show that with injection, aerodynamic capability of the high-lift devices is sustained. In addition, a considerable noise reduction is observed in the downward direction. With detailed analysis of the time-averaged and instantaneous flow fields, two mechanisms are identified as the reason accounting for the noise suppression: upstream mass injection significantly reduces the scale and strength of the vortices stemming from the cusp; the shear layer is lifted up, which relieves the shear layer impingement on the slat lower surface.

Evaluation of a Correlation-Based Transition Model and Comparison with the eN Method

The correlation-based γ-Reθ transition model has been implemented into a hybrid Reynolds-averaged Navier-Stokes solver and evaluated on various test cases. The original model formulation and recently published approaches of different authors for relevant empirical model functions as well as different transition criteria are compared. The prediction of transition with the correlation-based model was applied to a zero pressure gradient flat plate test case and some well known one -element airfoil test cases. Comparison with the standard approach for transition prediction in the flow solver used based on the e N method was accomplished. Simulation results in terms of transition locations and skin friction coefficient distributions , performance and arising difficulties of both models for the various test cases are presented and discussed.

DES of turbulent flow over wall-mounted obstacles using wall functions

An adaptive wall function is employed in Detached-Eddy Simulation (DES), a hybrid Reynolds-averaged Navier-Stokes (RANS) — Large Eddy Simulation (LES) model, of turbulent flow past two wall-mounted cubic obstacles in tandem at a Reynolds number Re = 22,000 based on the bulk velocity and the obstacle height. Numerical results are compared with previously published DES solutions obtained on wall integration grids and the experimental measurements of Martinuzzi and Havel (2000). The result shows that wallfunction approach in DES allows reasonable reproduction of coherent vortical structures massively separated from the wall-mounted obstacles to be achieved on wall function grids which require just the half of grid nodes of wall resolving grids. The numerical solutions computed by wall function computations reveal energetic unsteady flow fields with complex coherent vortical structures separated from obstacle edges, whose accuracy is better than those obtained by the unsteady RANS computations on the wall integration grid. Wall function solutions appear to be comparable to the wall-resolving DES solutions in most regions except at the junction of obstacle and the bottom wall where the flow is dominated by the horseshoe vortex with intense unsteadiness. The result confirms that DES with wall function approximation can reasonably resolves geometry-induced unsteady three-dimensional turbulent motions.

A dynamic hybrid Reynolds-averaged Navier Stokes–Large eddy simulation modeling framework

A dynamic framework for hybrid Reynolds-averaged Navier-Stokes (RANS)—large eddy simulation (LES) modeling is proposed, wherein the RANS-to-LES transition parameter is adjusted to maintain continuity in turbulence production. The model is applied for temporally developing plane channel flow at varying Reynolds numbers with different initial turbulence intensity and grid resolution. On sufficiently fine grids, dynamic hybrid RANS-LES model (DHRL) yields similar results to LES simulations. On coarse grids, DHRL activates RANS mode in the log layer, thus improving the mean flow predictions. The model framework addresses grid sensitivity issues observed in other hybrid RANS-LES approaches and may be used with any desired combination of LES and RANS basis models.

A hybrid RANS and kinematic simulation of wind load effects on full-scale tall buildings

Up till recent years, predicting wind loads on full-scale tall buildings using Large Eddy Simulation (LES) is still impractical due to a prohibitively large amount of meshes required, especially in the vicinity of the near-wall layers of the turbulent flow. A hybrid approach is proposed for solving pressure fluctuations of wind flows around tall buildings based on the Reynolds Averaged Navier–Stokes (RANS) simulation, which requires coarse meshes, and the mesh-free Kinematic Simulation (KS). While RANS is commonly used to provide mean flow characteristics of turbulent airflows, KS is able to generate an artificial fluctuating velocity field that satisfies both the flow continuity condition and the specific energy spectra of atmospheric turbulence. The kinetic energy is split along three orthogonal directions to account for anisotropic effects in atmospheric boundary layer. The periodic vortex shedding effects can partially be incorporated by the use of an energy density function peaked at a Strouhal wave number. The pressure fluctuations can then be obtained by solving the Poisson equation corresponding to the generated velocity fluctuation field by the KS. An example of the CAARC building demonstrates the efficiency of the synthesized approach and shows good agreements with the results of LES and wind tunnel measurements.

Detached-Eddy Simulation of a Coflow Jet Airfoil at High Angle of Attack

TO IMPROVE aircraft performance, innovative technology should be pursued to drastically reduce the weight and fuel consumption of the aircraft and to significantly increase aircraft mission payload and stall margin. Both military and commercial aircraft will benefit from the same technology. Flow control is the most promising route to bring significant performance improvement to aircraft [1–7]. Recently, Zha et al. [8–10] andWang et al. [11] have developed a promising airfoil flow control technique using a coflow jet, which significantly increases lift, stall margin, and drag reduction. The coflow jet airfoil is designedwith an injection slot near leading edge and a suction slot near trailing edge on the airfoil suction surface. The slots are opened by translating a large portion of the suction surface downward. A high-energy jet is injected tangentially near the leading edge and the same amount of mass flow is drawn in near the trailing edge. The turbulent shear layer between the main flow and the jet causes strong turbulence diffusion and mixing under the severe adverse pressure gradient, which enhances lateral transport of energy from the jet to the main flow and allows the main flow to overcome the severe adverse pressure gradient and remain attached at high angles of attack (AOA). The high-energy jet induces high circulation and hence generates high lift. The energized main flow fills the wake and therefore reduces drag. The coflow jet (CFJ) airfoil achieves net-zero mass-flux flow control and can significantly reduce the penalty to the propulsion system bydumping zero jetmass flow. In [11], the CFJ airfoils with three different slot sizes are computed using Spalart–Allmaras (S-A) model. It is observed that at lowAOA, the computed results agree well with the experiment. But at high AOA, the deviation of the lift and drag is large because a Reynoldsaveraged Navier–Stokes (RANS) model is not able to simulate the flow separation accurately. In particular, the predicted stall AOA by RANS is significantly smaller than the measured one. Recently, in [12], the DES predicts the stalled airfoil flow at high AOA significantly better than the RANS model. To overcome the disadvantages of RANSmodels and to avoid the intensive CPU requirement for LES, Spalart et al. developed the detached-eddy simulation (DES) strategy [13], which is a hybrid RANS and LES method. Near the solid surface within the wall boundary layer, the unsteady RANSmodel is realized in DES. Away from thewall surface, themodel automatically converts to large-eddy simulation (LES). By using the RANSmodel near thewall, the mesh size as well as the CPU time can be tremendously reduced. By using the LESmodel away from thewall, the regions of massive separation and other free shear flows are properly simulated. Its application for turbulence simulation has already achieved encouraging success, as shown in thework of Tarvin et al. [14], Spalart [15,16], Forsythe et al. [17], Viswanathan et al. [18], Squires et al. [19,20], Hansen and Forsythe [21], Subbareddy and Candler [22],Wang and Zha [23,24], and Im and Zha [12]. These flows using DES include those for airfoils, cylinders, forebodies, baseflows, etc. The results are qualitatively and quantitatively better than the solutions using RANS. The objective of this paper is to simulate the flowfield of a CFJ airfoil at high AOA using DES in order to better capture the jet mixing and flow separation. A low-diffusion E-CUSP scheme [25,26] with fifth-order weighted essentially nonoscillatory (WENO) scheme [27,28] is employed for the inviscid fluxes. The computed lifts atAOA 30, 37, and 39 agree better with the experiment than those using S-A one-equation RANS. The computed drag usingDES is also improved greatly, even though the deviation remains very large. The computed stall AOA using DES is 37 , which agrees very well with the experiment and is much better than the one using S-A one-equation RANS. The computed results using DES show significantly improvements at high AOA compared with those using S-A one-equation RANS.

Large-Eddy Simulation: Current Capabilities, Recommended Practices, and Future Research

Usage of large-eddy simulation (LES) methods for calculation of turbulent flows has increased substantially in recent years. This paper attempts to 1) provide an assessment of the current capabilities of LES, 2) outline some recommended practices for using LES, and 3) identify future research needs. The assessment considers flow problems for which LES can be successfully applied today and flow problems for which LES still has limitations. The availability of LES and hybrid Reynolds-averaged Navier-Stokes (RANS)/LES in general-purpose codes is discussed. Several important issues for which the LES community has not yet reached a consensus are discussed. These include grid sensitivity studies, application of unstructured grid methods, upwind-biased solvers, and turbulence (subgrid) modeling including continuous hybrid RANS/LES approaches. A section on recommended practices and key considerations tries to provide guidance on some of the important items that need to be addressed in using LES. The paper concludes with a discussion of future research directions, with a focus on work needed to advance the capabilities and reliability of LES for analysis of turbulent flows.

Implicit Large-Eddy Simulation of Swept-Wing Flow Using High-Resolution Methods

A numerical investigation of a fully three-dimensional swept-wing geometry featuring separation from a curved leading edge is presented. The implicit large-eddy-simulation strategy based on a third-order high-resolution method for discretizing the advective fluxes and a second-order Runge-Kutta time-stepping scheme with an extended stability region have been employed. No attempt to incorporate a wall model has been made. Instead, the boundary layer is fully resolved over the majority of the wing. Qualitative and quantitative comparisons with experimental oil-film visualizations and three-dimensional laser Doppler anemometry measurements show very good agreement between the experiment and the numerically predicted flow structures, as well as velocity and stress profiles near the wing. Furthermore, data from a hybrid Reynolds-averaged Navier-Stokes and large-eddy simulation have been included for comparison.

Evaluation of Various Turbulence Models in Predicting Airflow and Turbulence in Enclosed Environments by CFD: Part 2—Comparison with Experimental Data from Literature

Numerous turbulence models have been developed in the past two decades, and many of them can be used in predicting airflows and turbulence in enclosed environments. It is important to evaluate the generality and robustness of the turbulence models for various indoor airflow scenarios. This study evaluated the performance of eight turbulence models, potentially suitable for indoor airflow, in terms of accuracy and computing cost. These models cover a wide range of computational fluid dynamics (CFD) approaches, including Reynolds averaged Navier-Stokes (RANS) modeling, hybrid RANS and large-eddy simulation (or detached-eddy simulation [DES]), and large-eddy simulation (LES). The RANS turbulence models tested include the indoor zero-equation model, three two-equation models (the RNG k-∊, low Reynolds number k-∊, and SST k-ω models), a three-equation model ( model), and a Reynolds-stress model (RSM). The investigation tested these models for representative airflows in enclosed environments, such as forced convection and mixed convection in ventilated spaces, natural convection with medium temperature gradient in a tall cavity, and natural convection with large temperature gradient in a model fire room. The air velocity, air temperature, Reynolds stresses, and turbulent heat fluxes predicted by the models were compared against the experimental data from the literature. The study also compared the computing time used by each model for all cases. The results reveal that LES provides the most detailed flow features, while the computing time is much higher than for RANS models, and the accuracy may not always be the highest. Among the RANS models studied, the RNG k-ω and a modified model perform the best overall in four cases studied. Meanwhile, the other models have superior performance only in particular cases. While each turbulence model has good accuracy in certain flow categories, each flow type favors different turbulence models. Therefore, we summarize in the conclusions and recommendations both the performance of each particular model in different flows and the best suited turbulence models for each flow category.

Simulation of bluff body stabilized flows with hybrid RANS and PDF method

The motivation of this study is to investigate the turbulence–chemistry interactions by using probability density function (PDF) method. A consistent hybrid Reynolds Averaged Navier–Stokes (RANS)/PDF method is used to simulate the turbulent non-reacting and reacting flows. The joint fluctuating velocity–frequency–composition PDF equation coupled with the Reynolds averaged density, momentum and energy equations are solved on unstructured meshes by the Lagrangian Monte Carlo (MC) method combined with the finite volume (FV) method. The simulation of the axisymmetric bluff body stabilized non-reacting flow fields is presented in this paper. The calculated length of the recirculation zone is in good agreement with the experimental data. Moreover, the significant change of the flow pattern with the increase of the jet-to-coflow momentum flux ratio is well predicted. In addition, comparisons are made between the joint PDF model and two different Reynolds stress models.

Study of Delayed-Detached Eddy Simulation with Weakly Nonlinear Turbulence Model

This paper presents a parallel computation with hybrid Reynolds-averaged Navier-Stokes and large eddy simulation methods. Two types of hybrid approaches, referred to as detached eddy simulation and delayed-detached eddy simulation, are investigated through introducing length scales into the weakly nonlinear k-w turbulence model. Before the implementation of hybrid methods, the baseline weakly nonlinear model and k-w shear-stress-transport model are validated in the transonic RAE-2822 airfoil and ONERA-M6 wing cases. The numerical results show satisfactory agreement with the available experimental data. The Reynolds-averaged and hybrid methods based on the weakly nonlinear turbulence model are then applied to calculate the high Reynolds number transonic separation flows around NASA TN D-712 wing fulesage For the higher angle of attack case (a = 26.2 deg), both hybrid methods deliver good results when compared with the experiment; for the moderate angle of attack case (a = 12.5 deg), the results obtained with the delayed detached eddy simulation are satisfactory, whereas the starting point of the wing vortex breakdown predicted with the detached eddy simulation is too far upstream.

Zonal-Detached-Eddy Simulation of the Flow Around a High-Lift Configuration

A zonal hybrid Reynolds-averaged Navier-Stokes large-eddy simulation (RANS/LES) approach, called zonal-DES, used to handle a two-dimensional high-lift configuration with deployed slat and flap is presented. This method allows to reduce significantly the cost of an accurate numerical prediction of the unsteady flow around wings compared to a complete LES. Some issues concerning grid generation as well as the use of zonal-detached-eddy simulation for a multi-element airfoil are discussed. The basic planar grid has 250,000 points and the finest spanwise grid has 31 points with Az/c = 0.002. The effort is geared toward detailed comparison of the numerical results with the Europiv2 experimental particle image velocimetry data including both mean and fluctuating properties of the velocity field (Arnott and al)

Interfacing Statistical Turbulence Closures with Large-Eddy Simulation

Progress toward a general purpose hybrid Reynolds-averaged Navier-Stokes (RANS)/large-eddy simulation (LETS) framework is described, in which large-scale, statistically represented turbulence kinetic energy is converted automatically into resolved-scale velocity fluctuations wherever the local mesh resolution is sufficient to support them. Existing hybrid RANS/LES approaches alter the nature of the local partial differential equations according to the local mesh resolution, but they do not alter the nature of the data on which these equations operate. The implications of this are discussed. Subsequently, a simple mechanism is introduced to transfer statistical kinetic energy into resolved-scale fluctuations in a manner that preserves a given set of space/time correlations and set of second moments. This process, which can appropriately be termed Large-Eddy STimulation (LEST), generates the large-scale eddies needed to form the unsteady boundary conditions at RANS interfaces to LES regions, into which turbulence energy can be deposited either through mean convection or through turbulent transport

Detached-Eddy Simulations of Attached and Detached Boundary Layers

This article presents Detached-Eddy Simulations (DESs) of attached and detached turbulent boundary layers. This hybrid Reynolds Averaged Navier-Stokes (RANS) / Large Eddy Simulation (LES) model goes continuously from RANS to LES according to the mesh definition. We propose a parametric study of the model over two “academic” configurations, in order to get information on the influence of the mesh to correctly treat complex flow with attached and detached boundary layers. The first case is the simulation of a compressible temporally evolving turbulent boundary layer (TBL). We check here the DES behavior for an “over-resolved” attached TBL, in order to understand how the upstream-attached flow before separation would be modeled in a more complex configuration. Effects of change of the Cdes constant are scrutinized, and the use of an additional constant is assessed. The second case is the simulation of an established laminar compressible boundary layer suddenly exposed to an adverse pressure gradient. The mean flow deceleration is imposed by means of a suction at the upper side of the computational domain, leading to separation and transition to turbulence. The effect of increasing grid size (Dz) in the spanwise direction (leading to an extension of the region treated by the RANS model and s to an increase of eddy-viscosity) is scrutinized. Conclusions are then drawn from these two test cases for DES meshing strategy that should be used in more complex configurations.

Detached-Eddy Simulations and Reynolds-Averaged Navier-Stokes Simulations of Delta Wing Vortical Flowfields

An understanding of vortical structures and vortex breakdown is essential for the development of highly maneuverable vehicles and high angle of attack flight. This is primarily due to the physical limits these phenomena impose on aircraft and missiles at extreme flight conditions. Demands for more maneuverable air vehicles have pushed the limits of current CFD methods in the high Reynolds number regime. Simulation methods must be able to accurately describe the unsteady, vortical flowfields associated with fighter aircraft at Reynolds numbers more representative of full-scale vehicles. It is the goal of this paper to demonstrate the ability of detached-eddy Simulation (DES), a hybrid Reynolds-averaged Navier-Stokes (RANS)/large-eddy Simulation (LES) method, to accurately predict vortex breakdown at Reynolds numbers above 1×106. Detailed experiments performed at Onera are used to compare simulations utilizing both RANS and DES turbulence models.