Experimental Research On Active Control Of Separated Flow Based On Spanwise Jet Array

Separation and reattachment flow is a common phenomenon in nature and industrial production.As a separation and reattachment flow with fixed separation points,backward-facing step(BFS)flow has strong representative flow characteristics,so the study of BFS flow mechanism is particularly important.The mean reattachment length X r,Reynolds stress and the correlation coefficient of pulsating velocity are important parameters to characterize the BFS flow.It has been widely proved that periodic excitation at the separation point of the BFS can control the separation flow well,but many flows do not have fixed separation points or it is difficult to predict the separation points in advance.There are great challenges when the existing flow control methods are applied in practice.In this paper,the jet exciter is arranged on the wall of the recirculation region of BFS to explore the influence of different excitation methods on the separated flow.In this paper,32 jets arranged on the wall of recirculation region were used to actively control the BFS separated flow.The length of jet holes from the corner was twice the height of the step and was evenly distributed in the spanwise direction.This paper focused on the effects of in-phase jet and spanwise traveling wave jet on BFS separation flow.The experiment was carried out in water tank and wind tunnel with the Reynolds number Re H = 5750,10600,jet excitation frequency St = 0.03 ～ 1.25 and wavelength of spanwise traveling wave λz/H = 0.78 ～ 12.5.The velocity distribution on the bottom and cross section of the BFS were measured by Particle Image Velocimetry(PIV)and the average pressure at the corner of the BFS were measured by pressure sensor.The experimental results show that periodic jet excitation on the wall of the BFS recirculation region has a positive effect on changing the separation flow.For in-phase jet excitation,the minimum average reattachment length is found at a specific jet frequency.The above phenomena are related to the vortices excited by the pulsating jet.These vortices make the peak of Reynolds stress occurs upper in the flow direction and increase the mixing,thus affecting the whole process of separation and reattachment.For the spanwise traveling wave jet excitation,the jet with specific spanwise wavelength makes the average pressure at the corner of BFS reach the minimum compared with other jets when the jet frequency was consistent.At the same time,the spanwise traveling wave jet changed the random motion of large-scale related structures in the recirculation region.