Bluff-body drag reduction by extremum-seeking control

An actuator is used to control the drag exerted on a bluff-body at large Reynolds number (typically 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. The size of the low-pressure region behind the body is decreased and the drag reduced: the faster the rotation of the cylinder the lower the drag. The goal of the control is for the system to find itself an optimal set point, defined as the lowest cost of global energy consumption of the system (drag reduction versus energy used by the actuator). For this purpose an extremum-seeking control method is applied in order to deal with the large background noise due to turbulence. It consists in a synchronous detection of the response measured in the drag measurements to a modulation of the actuator. This experiment attests from the real efficiency for local active control to reduce autonomously the global energy consumption of a turbulent separated flow, taking into account drag and actuator's energy cost.