Turbulence Control With Particle Image Velocimetry in a Backward-Facing Step1

Abstract

A new concept of particle image velocimetry (PIV) for turbulence control is introduced. The PIV method is used to measure the flow and supply information on the turbulence quantities on-line for a control station connected to an actuator for flow manipulation. With this “closed-loop” system of PIV and PID controllers, it is possible to control increase or decrease turbulence quantities or length scales. Special techniques for the on-line measurement are developed. The mean velocity is computed with a moving time average operator and the Reynolds decomposition is applied for the calculation of the Reynolds stresses, velocity fluctuations, or other instantaneous turbulence quantities. The control concept is tested in a backward-facing step with a DC-motor based actuator for mixing of the near wall flow. A length-scale estimate similar to the integral scale and the Reynolds shear stress are calculated on-line. The results show it is possible to control the turbulence and for example to compensate the disturbance on the Reynolds shear stress caused by a manual change in flow velocity. The control frequency is quite slow (e.g., 0.1–100 Hz), limited primarily by the image-grabbing operations and the computation of the velocity vectors in the PIV station. For this reason the method is applicable for slow processes, e.g., to steer the mixing processes or more generally to manage the turbulence level or the length scales.