Detached-eddy simulation (DES) was first proposed in 1997 and first used in 1999, so its full history can be surveyed. A DES community has formed, with adepts and critics, as well as new branches. The initial motivation of high–Reynolds number, massively separated flows remains, for which DES is convincingly more capable presently than either unsteady Reynolds-averaged Navier-Stokes (RANS) or large-eddy simulation (LES). This review discusses compelling examples, noting the visual and quantitative success of DES. Its principal weakness is its response to ambiguous grids, in which the wall-parallel grid spacing is of the order of the boundary-layer thickness. In some situations, DES on a given grid is then less accurate than RANS on the same grid or DES on a coarser grid. Partial remedies have been found, yet dealing with thickening boundary layers and shallow separation bubbles is a central challenge. The nonmonotonic response of DES to grid refinement is disturbing to most observers, as is the absence of...

Detached-Eddy Simulation

An efficient generalized Zonal Detached Eddy Simulation method (ZDES) is presented, which aims at performing hybrid Reynolds Averaged Numerical Simulation (RANS)/Large Eddy Simulation (LES) calculations for both internal and external aerodynamics problems. It is based on a zonal formulation of the hybrid length scale that allows to combine the zonal approach with the best features of Delayed Detached Eddy Simulation (DDES) (Spalart et al. Theor Comput Fluid Dyn 20:181–195, 2006). In other words, the presumed weak point of a zonal approach, namely that the location of separation has to be known in advance, is now overcome. What is more, the problem of slow LES content development in mixing layers when they are treated neither in RANS nor in LES mode is investigated. It is argued that the subgrid length scale Δmax = max(Δx, Δy, Δz) entering DDES is physically justified to shield the boundary layer but is definitely not a good subgrid length scale in LES mode. Remedies are proposed based on new zonal subgrid length scales that depend not only on the grid spacing but also on the flow solution and especially on the local vorticity vector. The method is validated on a spatially developing mixing layer as well as in a backward facing step flow and then applied to a three-element airfoil. It is argued in this latter case that a precise control of the RANS mode thanks to a zonal approach is essential. More generally, in all simulated cases in this study, ZDES has proven to be very efficient as regards the behavior in LES mode while retaining the strongest asset of DDES, namely the treatment of the attached boundary layer in RANS mode. The issue of zonal or non-zonal treatment of turbulent flows is also briefly discussed.

Recent improvements in the Zonal Detached Eddy Simulation (ZDES) formulation

This review article addresses the dynamics and control of low-frequency unsteadiness, as observed in some aerodynamic applications. It presents a coherent and rigorous linearized approach, which enables both to describe the dynamics of commonly encountered open-flows and to design open-loop and closed-loop control strategies, in view of suppressing or delaying instabilities. The approach is global in the sense that both cross-stream and streamwise directions are discretized in the evolution operator. New light will therefore be shed on the streamwise properties of open-flows. In the case of oscillator flows, the unsteadiness is due to the existence of unstable global modes, i.e., unstable eigenfunctions of the linearized Navier-Stokes operator. The influence of nonlinearities on the dynamics is studied by deriving nonlinear amplitude equations, which accurately describe the dynamics of the flow in the vicinity of the bifurcation threshold. These equations also enable us to analyze the mean flow induced by the nonlinearities as well as the stability properties of this flow. The open-loop control of unsteadiness is then studied by a sensitivity analysis of the eigenvalues with respect to base-flow modifications. With this approach, we manage to a priori identify regions of the flow where a small control cylinder suppresses unsteadiness. Then, a closed-loop control approach was implemented for the case of an unstable open-cavity flow. We have combined model reduction techniques and optimal control theory to stabilize the unstable eigenvalues. Various reduced-order-models based on global modes, proper orthogonal decomposition modes, and balanced modes were tested and evaluated according to their ability to reproduce the input-output behavior between the actuator and the sensor. Finally, we consider the case of noise-amplifiers, such as boundary-layer flows and jets, which are stable when viewed in a global framework. The importance of the singular value decomposition of the global resolvent will be highlighted in order to understand the frequency selection process in such flows. © 2010 by ASME.

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Dynamics and Control of Global Instabilities in Open-Flows: A Linearized Approach

Predicting the onset of flow unsteadiness based on global instability

The paper addresses the issue of the significant delay of transition from RANS to LES in shear layers, which is known to affect the original version of Detached-Eddy Simulation (DES) on typical anisotropic grids. A common remedy has been to disable the subgrid scale model, leading to Implicit LES (ILES). Here, enhanced versions of DES are proposed based on new definitions of the subgrid length-scale. Unlike the original definition attached to DES (i.e., simply the maximum local grid spacing) the new ones include solution-dependent kinematic measures which serve as indicators of the nearly 2D grid-aligned flow regions which are typical of the initial region of free and separated shear layers. This brings about a significant reduction of the subgrid viscosity in such regions. This, in turn, unlocks the Kelvin-Helmholtz instability and drastically speeds-up transition to 2D and then 3D flow structures in shear layers. At the same time, the proposed length-scale is not influenced by the smallest grid dimension, unlike the cube root of the cell volume and other recently proposed definitions, and we view this as a physically plausible, safe and therefore preferable length-scale definition. The advantages of these enhanced versions are demonstrated on a set of numerical examples which include isotropic turbulence, a mixing layer, a jet, a boundary layer, and a backward-facing step. The new definitions are as successful as ILES in liberating the early instabilities, while being non-zonal and compatible with later interactions of the turbulent region with solid bodies. The turbulence statistics of the flows and the radiated noise of the jet are also considerably improved, especially with relatively coarse lateral grid spacings. The new definitions will also improve LES, particularly with the Smagorinsky model.

An Enhanced Version of DES with Rapid Transition from RANS to LES in Separated Flows

Shock wave/turbulent boundary-layer interaction and flow separation may induce self-sustained large-scale oscillations on a profile at transonic Mach number. This phenomenon, known as transonic buffet, is at the origin of intense pressure fluctuations which can have detrimental effects, both in external and internal aerodynamics. The present paper describes a new experiment executed in the ONERA S3Ch transonic wind tunnel on shock oscillations over the OAT15A supercritical profile. These experiments have allowed the precise definition of the conditions for buffet onset and the characterization of the properties of the periodic motion from unsteady surface pressure measurements. The flowfield behavior has been described in great detail thanks to high-speed schlieren cinematography and surveys with a two-component laser Doppler velocimetry along with a conditional sampling technique. The first aim of this study was to provide the computational fluid dynamics community with well-documented test cases to validate advanced computing methods. Concerning the physics of the phenomenon, it is suggested that it is mediated by acoustic waves which are produced at the trailing edge and which travel on the two sides of the airfoil. Also, the experimental results strongly suggest that the phenomenon is essentially two-dimensional, even if three-dimensional effects are also detected.

Experimental Study of Shock Oscillation over a Transonic Supercritical Profile

This paper presents zonal detached-eddy simulation of a round underexpanded sonic jet exhausting from a realistic aircraft afterbody and controlled by four radial injections. The comparison with a former experiment proves the capability of zonal detached-eddy simulation to reproduce mean flow induced by the complex interaction between the compressible waves and the turbulent jet shear layer. However, the very near-field jet structure indicates that the shear-layer transition to a fully turbulent state is delayed. Therefore, a modification of the characteristic length scale used in the modeling is proposed which is shown to improve the prediction of the shear-layer growth. The control injectors are aimed to enhance jet mixing. Experiment indicated the efficiency of this additional device but also expressed the need for more data to get a deeper and more complete insight into the whole flowfield. Both spatial and temporal information are provided by zonal detached-eddy simulation which is then a good candidate. This method is shown to correctly simulate streamwise vortices generated by secondary injections, their action in jet distortion, as well as their subsequent decay. Finally, computational results allow us to evaluate the action of the hypermixer on turbulent activity and its consequences on jet dilution.

Zonal Detached Eddy Simulation of a Controlled Propulsive Jet

Experiments and computations are used to investigate the mixing enhancement of an underexpanded sonic jet subjected to radial secondary injections in a realistic aircraft-afterbody configuration. In accordance with more academic studies, the present study confirms that secondary jets lead to a strong distortion of the jet interface under the action of the longitudinal vortices. Reynolds-averaged Navier-Stokes computations are validated on experimental data and then used to perform a parametric study. We investigate the influence of parameters such as the number of active injectors, their aspect ratio, and the total pressure and temperature of the injected air. These computations also allow us to investigate the drastic changes in the compressible structure of the flow due to the action of the secondary injection.

Laurent Jacquin

Papers

Experimental Study of Shock Oscillation over a Transonic Supercritical Profile

Zonal Detached Eddy Simulation of a Controlled Propulsive Jet

Numerical Study of Mixing Enhancement in a Supersonic Round Jet