Jacques Borée

On the need for a nonlinear subscale turbulence term in POD models as exemplified for a high-Reynolds-number flow over an Ahmed body

We investigate a hierarchy of eddy-viscosity terms in proper orthogonal decomposition (POD) Galerkin models to account for a large fraction of unresolved fluctuation energy. These Galerkin methods are applied to large eddy simulation (LES) data for a flow around a vehicle-like bluff body called an Ahmed body. This flow has three challenges for any reduced-order model: a high Reynolds number, coherent structures with broadband frequency dynamics, and meta-stable asymmetric base flow states. The Galerkin models are found to be most accurate with modal eddy viscosities as proposed by Rempfer & Fasel (J. Fluid Mech., vol. 260, 1994a, pp. 351–375; J. Fluid Mech. vol. 275, 1994b, pp. 257–283). Robustness of the model solution with respect to initial conditions, eddy-viscosity values and model order is achieved only for state-dependent eddy viscosities as proposed by Noack, Morzynski & Tadmor (Reduced-Order Modelling for Flow Control, CISM Courses and Lectures, vol. 528, 2011). Only the POD system with state-dependent modal eddy viscosities can address all challenges of the flow characteristics. All parameters are analytically derived from the Navier–Stokes-based balance equations with the available data. We arrive at simple general guidelines for robust and accurate POD models which can be expected to hold for a large class of turbulent flows.

A seamless hybrid RANS-LES model based on transport equations for the subgrid stresses and elliptic blending

The aim of the present work is to develop a seamless hybrid Reynolds-averaged Navier–Stokes (RANS) large-eddy simulation (LES) model based on transport equations for the subgrid stresses, using the elliptic-blending method to account for the nonlocal kinematic blocking effect of the wall. It is shown that the elliptic relaxation strategy of Durbin is valid in a RANS (steady) as well as a LES context (unsteady). In order to reproduce the complex production and redistribution mechanisms when the cutoff wavenumber is located in the productive zone of the turbulent energy spectrum, the model is based on transport equations for the subgrid-stress tensor. The partially integrated transport model (PITM) methodology offers a consistent theoretical framework for such a model, enabling to control the cutoff wavenumber κc, and thus the transition from RANS to LES, by making the Ce2 coefficient in the dissipation equation of a RANS model a function of κc. The equivalence between the PITM and the Smagorinsky model is ...

Bluff body drag manipulation using pulsed jets and Coanda effect

The impact of fluidic actuation on the wake and drag of a three-dimensional blunt body is investigated experimentally. Jets blowing tangentially to the main flow allow to force the wake with variable frequency and amplitude. Depending on the forcing conditions, two flow regimes can be distinguished. First, in case of broadband actuation with frequencies comprising the natural wake time scale, the convection of the jet structures enhances wake entrainment, shortens the length of the recirculating flow and increases drag. Secondly, at higher actuation frequencies, shear-layer deviation leads to fluidic boat-tailing of the wake. It additionally lowers its turbulent kinetic energy thus reducing the entrainment of momentum towards the recirculating flow. The combination of both mechanisms produces a raise of the base pressure and reduces the drag of the model. Both actuation regimes are characterized by complementary velocity, pressure and drag measurements at several upstream conditions and control parameters. By adding curved surfaces to deviate the jets by the Coanda effect, periodic actuation is reinforced and drag reductions of about 20% are achieved. The unsteady Coanda blowing not only intensifies the flow deviation and the base pressure recovery but also preserves the unsteady high-frequency forcing effect on the turbulent field. The present results encourage further development of fluidic control to improve the aerodynamics of road vehicles and provide a complementary insight into the relation between wake dynamics and drag.

Analysis of the wall pressure trace downstream of a separated region using extended proper orthogonal decomposition

Multipoint wall pressure measurements and Laser Doppler Velocimetry (LDV) measurements of the velocity field generated over a 30° forward facing ramp are discussed in this paper. Mean and fluctuating signals are analyzed both in the mean separated region and in the recovering boundary layer flow. We present an analysis based on proper orthogonal decomposition (POD). The choice of a “POD region” as a subset of the sensor array is discussed. In a “POD region” restricted to the mean separated region, we show that the spatio-temporal coherence of the signal is efficiently captured by the POD decomposition. Extended POD is then introduced to extract in an objective way the part of the surface pressure signal, downstream of the mean reattachment point, which is correlated with the pressure signal under the separated region. The properties of this decomposition are demonstrated. We show that the power spectral density of the correlated part always displays a peak in the frequency domain associated with vortex sh...

Control of a three-dimensional blunt body wake using low and high frequency pulsed jets

We present experimental evidence of pressure changes in the wake of a three-dimensional blunt body by the use of periodic pulsed jets. The jets are pulsed tangentially to the shear layers separated from the trailing-edges of a classical square-back Ahmed body. The Reynolds number based on the models height is ReH = 3.105. Significant decrease (respectively increase) of the rear pressure are achieved considering low and high frequency pulsing. Low frequency actuation (St = Hf/Uo = 0.4, where f is the frequency and Uo the upstream velocity) is shown to enhance the global wake mode and to increase drag. On the contrary, dynamical effects associated to the particular flow control strategy provides a significant drag decrease for the higher frequency forcing (St = 11.5). Time-averaged pressure on the back surface of the model and velocity measurements on the wake illustrates the main effects of such actuation and introduce new strategies for drag control of three-dimensional geometries.

Resonances in the forced turbulent wake past a 3D blunt body

We study the resonances of a forced turbulent wake past a flat-based bluff body using symmetric and antisymmetric actuation modes. The natural, unforced wake flow exhibits broadband dynamics superimposed on oscillatory motions linked to the reminiscent laminar Benard-von Karman instability in the turbulent flow. Harmonic and subharmonic resonances can be controlled by the phase relationship of periodic forcing and are linked to the symmetry properties of vortex shedding. Symmetric forcing leads to a strong subharmonic amplification of vortex shedding in the wake, but no harmonic excitation. The robustness of the subharmonic resonance is confirmed at different Reynolds numbers. Antisymmetric actuation, however, promotes a harmonic resonance with very similar wake and drag features.

LES, Zonal and Seamless Hybrid LES/RANS: Rationale and Application to Free and Wall-Bounded Flows Involving Separation and Swirl

An overview is given of the activities in the framework of the German-French Research Group on ”LES of Complex Flows” (DFG-CNRS FOR 507) with respect to the development of zonal and seamless hybrid LES/RANS computational methods based on a near-wall Eddy-Viscosity Model (EVM) and a near-wall Second-Moment Closure (SMC) respectively. The zonal scheme represents a two layer model with a two-equation EVM-RANS model covering the near-wall layer and the true LES employing the zero-equation subgrid-scale (SGS) model of Smagorinsky resolving the core flow. Due attention was payed to the exchange of the variables between the ensemble-averaged RANS field and the spatially-filtered LES field across the discrete interface separating the two sub-regions. A procedure for controlling the interface position in the flow domain was also in focus of the present investigations. After considering a few introductory test cases (fully-developed channel flow, flows separating from sharp-edged surfaces) the feasibility of the method was validated against the available experiments in a single tubo-annular, swirl combustor configuration (Exp.: Palm et al., [39]) and in the separated flows in a 3-D diffuser (Exp. Cherry et al., [10]) and over a 2-D hump including the case with the separation control by steady suction (Exp.Greenblatt et al., [23]). The seamless LES/RANS method employs the so-called Elliptic-Blending Reynolds-Stress Model (EB-RSM, Manceau, [33]; Manceau and Hanjalic, [34]) being active in the entire flow field. This RANS-based SGS model represents a near-wall Second-Moment Closure model relying on the elliptic relaxation method. The model coefficient multiplying the destruction term in the transport equation for the scale-supplying variable e (dissipation rate of the turbulence kinetic energy) was made filter-width (corresponding to the grid spacing) dependent, i.e. dependent on the location of the spectral cutoff, by applying a multiscale modelling procedure originating from spectral splitting of filtered turbulence in line with the Partially Integrated Transport Model (PITM) proposed by Dejoan and Schiestel, [48] and Chaouat and Schiestel, [8]. Herewith, the dissipation rate level was obtained, which suppresses the turbulence intensity towards the subgrid (i.e. subscale) level in the regions where large coherent structures dominate the flow. The resulting model was validated by computing some free flows (a temporal mixing layer) and wall-bounded flows (a fully-developed channel flow). Finally, the PITM method applied to the high-Reynolds number RSM model due to Speziale et al., [53] was used to compute the flow separated from a 2-D hill (with reference LES by Frohlich et al., [19] and Breuer, [6]). In addition, all relevant cases were computed by the conventional LES method aiming at mutual comparison of the predictive capabilities of the afore-mentioned methods with respect to the quality of results and space-time resolution issues.

Finite time Lagrangian analysis of an unsteady separation induced by a near wall wake

Following the Lagrangian theory of unsteady flow separation on slip boundaries proposed by Lekien and Haller [“Unsteady flow separation on slip boundaries,” Phys. Fluids 20, 097101 (2008)], we use finite time Lagrangian analysis in order to educe large scale, unsteady flow separation downstream a near wall obstacle at a significant Reynolds number. By large scale flow separation, we mean here the ejection of fluid and vorticity outside a neighborhood of the wall at the scale of the obstacle. Indeed, while the separation point at the wall is not spatially resolved by the high speed particle image velocimetry measurements, free-slip boundary conditions are applied before educing unstable manifolds in the near wall region using finite time Lyapunov exponents. For this turbulent flow, conditional statistics are presented in order to discuss the relative contributions of the unsteady aerodynamics and of the turbulence in the separation region. The dynamics of the corresponding separation point has a very clear link with the fluctuating wall pressure induced by this unsteady turbulent flow.Following the Lagrangian theory of unsteady flow separation on slip boundaries proposed by Lekien and Haller [“Unsteady flow separation on slip boundaries,” Phys. Fluids 20, 097101 (2008)], we use finite time Lagrangian analysis in order to educe large scale, unsteady flow separation downstream a near wall obstacle at a significant Reynolds number. By large scale flow separation, we mean here the ejection of fluid and vorticity outside a neighborhood of the wall at the scale of the obstacle. Indeed, while the separation point at the wall is not spatially resolved by the high speed particle image velocimetry measurements, free-slip boundary conditions are applied before educing unstable manifolds in the near wall region using finite time Lyapunov exponents. For this turbulent flow, conditional statistics are presented in order to discuss the relative contributions of the unsteady aerodynamics and of the turbulence in the separation region. The dynamics of the corresponding separation point has a very clear...

Drag reduction of a car model by linear genetic programming control

We investigate open- and closed-loop active control for aerodynamic drag reduction of a car model. Turbulent flow around a blunt-edged Ahmed body is examined at