The dynamics of large-scale structures in the near wake downstream a square-back Ahmed body [1] are experimentally studied then controlled at Re=3.9 ×10e5, based on the body height. The barycenters of the rear pressure and the recirculation intensity are tracked based respectively on pressure and PIV measurements. Both quantities track large-scale structures and exhibit strong bimodal distributions characteristic of a stochastic behavior [4]. However their low frequencies dynamics seem to contain a chaotic aspect which is characterized through standard properties of such dynamics: structure function, correlation dimension, largest Lyapunov exponent and telegraph-like signal.

Using asymmetric jets tangentially to the upper rear surface of the bluff body a reactive control is applied, based on the recirculation intensity position as control variable. The jets can be set to continuous or pulsed mode (cµ = 0.03%). An optical flow algorithm computes in real-time the velocity fields [2, 3], which enables such experimental approach. The controlled flow results in more stabilized bi-modal dynamics. The wake switches between two highly preferred modes with a shorter transition time. The dynamics are fixed by the time scale of the control law.

[1] S.R. Ahmed, G. Ramm, and G. Faltin. Some Salient Features Of The Time-Averaged Ground Vehicle Wake. Technical report, SAE Technical Paper 840300, 1984.

[2] F. Champagnat, A. Plyer, G. Le Besnerais, B. Leclaire, S. Davoust, and Y. Le Saint. Fast and accurate piv computation using highly parallel iterative correlation maximization. Exp. Fluids, 50:1169–1182, 2011.

[3] N. Gautier and J.-L. Aider. Real-time planar flow velocity measurements using an optical flow algorithm implemented on GPU. J. Vis., 18:277–286, 2015.

[4] M. Grandemange, M. Gohlke, and O. Cadot. Turbulent wake past a three-dimensional blunt body. Part 1. Global modes and bi-stability. J. Fluid Mech., 722:51–84, 2013.