Jean-François Beaudoin

Drag reduction of a bluff body using adaptive control methods

A classical actuator is used to control the drag exerted on a bluff body at large Reynolds number (Re=20000). The geometry is similar to a backward-facing step whose separation point is modified using a rotating cylinder at the edge. The slow fluctuations of the total drag are directly measured by means of strain gauges. As shown by visualizations, the actuator delays the separation point. The size of the low-pressure region behind the body is decreased and the drag reduced. It is found that the faster the rotation of the cylinder, the lower the drag. In a first study, the goal of the control is for the system to reach a drag consign predetermined by the experimentalist. The control loop is closed with a proportional integral correction. This adaptive method is shown to be efficient and robust in spite of the large fluctuations of the drag. In the second method, the system finds itself its optimal set point. It is defined as the lowest cost of global energy consumption of the system (drag reduction versus...

Thorough analysis of vortical structures in the flow around a yawed bluff body

In the present study numerical investigations are used as a complementary approach to analyse large coherent structures in the flow around a bluff-body under cross-wind. The results from the numerical simulation are validated through wind-tunnel experiments and used to analyse unresolved observations from these experimental investigations. This underlines the duality of these two approaches in the analysis of the aerodynamics of a bluff-body. Vortical structures in a three-dimensional, asymmetric and turbulent wake are detected through the use of different vortex detection schemes (vorticity, Q, λ2 and Γ3), permitting to isolate flow structures. The vortex properties of two main lateral vortices are described and discussed on the basis of envelopes detected by the λ2 and the Γ3 criterion. Although the flow is highly three-dimensional and complex, the vortex properties are quite simple and show mechanisms like vortex stretching and bursting. These two criteria lead qualitatively to the same results; quantitative differences are due to the envelope diameter and are therefore observed only on integral flow quantities.

Effect of unsteady separation on an automotive bluff-body in cross-wind

This paper is treating the dynamic effects of cross-wind on a 3D bluff-body. It is shown that contrary to the mean flow, it is not the upper vortex that predominates the forces, but the flow generated and piloted by the front lee-strut wake. It is made responsible for the appearance of a frequency peak in the bluff-body force spectra. Furthermore it is shown that the intensity as well as the frequency of this peak depend on the yawing angle. Experimental and numerical results are used to analyse the flow dynamics.

Feedback Control Using Extremum Seeking Method for Drag Reduction of a 3D Bluff Body

The flow around a modified Ahmed body with a curved rear section is studied at Re > 2.106 and a line of vortex generators (VG) is used as actuators. By varying the angle α of the VG we observe an optimal value of α defining a minimum of aerodynamic drag and correspondingly a maximum base pressure coefficient. As this optimum value is shown to be Reynolds dependent we use an extremum control strategy for the system to find this optimal condition autonomously. It consists of the synchronous detection of the response measured in either the base pressure signal or in the drag and a slow sinusoidal modulation of the angle of the VG. It is finally demonstrated that the closed-loop system is robust and reacts successfully to unpredictable changes in the external flow conditions.

Drag fluctuations of a disk in a turbulent jet: Effect of turbulent scales averaging

The drag fluctuations of a disk placed on the axis of a turbulent incompressible jet are studied at Re=70000. Statistics and spectra have been obtained for different disk sizes. A significant spatial averaging effect is observed in the symmetrization of probability distribution function and in the low-pass filtering of spectra. These effects are associated with a redistribution of the high-frequencies energy to the low frequencies. It is shown that this redistribution is done in such a way that the rms value of the drag fluctuations increases linearly with the disk surface. These results concerning the drag fluctuations are compared and found to be consistent with fluctuations of a global kinetic energy extracted from the turbulent field in front of the disk.