Azeddine Kourta

Separated Flows Around the Rear Window of a Simplified Car Geometry

A 3D numerical simulation based on the lattice Boltzmann method is carried out on a simplified car geometry (initially proposed by Ahmed, Ramm, and Falting, 1984, SAE Technical Paper series No. 840300) to analyze and establish a method for controlling the near-wake flow topology of a generic blunt body model. The results indicate the existence of a complex flow topology consisting of transverse and longitudinal vortices emanating from flow separations that occur on the top and the lateral edges of the slanted rear window, respectively. The topology of each structure is detailed and the numerical results are compared with the experimental results in the literature. The results presented in this paper can then be used to develop and parametrize active control solutions conducive to improving the aerodynamic performances of automobile vehicles.

Drag reduction of a 3D bluff body using plasma actuators

The aim of this study is to reduce the drag of a simplified car geometry using surface dielectric barrier discharge actuators. Experiments were conducted in a wind tunnel for a low Reynolds number (6.7.105) with the Ahmed body reference (rear slant angle of 25°, zero yaw angle). The effect of steady and unsteady actuation on the flow topology was investigated carrying out 2C-PIV and 1D hot wire measurements. The efficiency of the actuators was characterised by stationary balance measurements. Drag reductions up to 8% were obtained by suppressing the separation bubble above the rear window. The results suggest that plasma actuators are simple to implement on a model and can provide useful information for automotive aerodynamics through parametric studies with parameters relevant for flow control (position, surface, frequency and duty cycle of the pulsed actuation).

Aerodynamic characterisation of a square back bluff body flow

This article deals with the flow characterisation around a square back Ahmed body. Results of time-averaged and instantaneous flow characteristics are compared to the literature. All the results presented in this article were obtained when the Reynolds number independency of the drag coefficient was reached. The first part of the study focuses on time-averaged results. The turbulent boundary layer on the roof of the model has been characterised and mean flow topology is investigated thanks to particle imaging velocimetry and pressure measurements. The second part of the paper is dedicated to the characterisation of the vortex shedding in the wake using hot wire anemometry. The phase difference of vortex shedding between two edges, the vortex pairing in the shear layers and the pumping effect are tackled.

Drag reduction by pulsed jets on strongly unstructured wake: towards the square back control

Experimental studies have been performed in a wind tunnel in order to control the flow separation on rear window of a generic vehicle shape (the Ahmed body with a scale of 0.7 and a slope angle of 35 degrees). The rear part of the geometry has been modified by replacing the sharp edge between roof and rear window by smooth curved surfaces. This model is equipped at the end of the roof with a strip of pulsed jets in order to control the flow with a velocity of 30 m.s−1. A 20% drag reduction has been obtained with a pulsed frequency at 500 Hz and a momentum coefficient Cµ = 2.75 10−3. This result confirms the interest in using pulsed jets in order to reduce aerodynamic drag and pollutant emission.

Closed-loop separation control over a sharp edge ramp using genetic programming

Abstract We experimentally perform open and closed-loop control of a separating turbulent boundary layer downstream from a sharp edge ramp. The turbulent boundary layer just above the separation point has a Reynolds number

Prediction of denosumab effects on bone remodeling: A combined pharmacokinetics and finite element modeling.

Re_{\theta }\approx 3500

Active flow control on an Ahmed body - An experimental study

Reθ≈3500 based on momentum thickness. The goal of the control is to mitigate separation and early re-attachment. The forcing employs a spanwise array of active vortex generators. The flow state is monitored with skin-friction sensors downstream of the actuators. The feedback control law is obtained using model-free genetic programming control (GPC) (Gautier et al. in J Fluid Mech 770:442–457, 2015). The resulting flow is assessed using the momentum coefficient, pressure distribution and skin friction over the ramp and stereo PIV. The PIV yields vector field statistics, e.g. shear layer growth, the back-flow area and vortex region. GPC is benchmarked against the best periodic forcing. While open-loop control achieves separation reduction by locking-on the shedding mode, GPC gives rise to similar benefits by accelerating the shear layer growth. Moreover, GPC uses less actuation energy.

Unsteady Experimental Characterization of the Natural Wake of a Squareback Ahmed Model

Local linear stability of swept and unswept incompressible boundary layers developing over compliant, fluid-saturated, porous plates is considered in the limit of small permeability. The analysis is meant to yield preliminary indications on the possible stabilization induced on the flows hydrodynamic and hydroelastic modes by poroelastic media, such as those occurring in many natural and technological settings. As far as hydrodynamic modes are concerned, the main stabilizing effect is that of compliance, which however couples weakly to low-frequency crossflow modes. Permeability plays a damping role on hydroelastic modes, which here take the form of travelling wave flutter instabilities. The passive control of instabilities through poroelastic coatings specifically designed to selectively exploit the effect of compliance and/or permeability is a subject worthy of future research efforts.