Research On The Closed-loop Active Flow Control Technique

Reynolds number based on the airfoil chord length between 104 and 106 is considered to be the low Reynolds number,in which the airfoil shows some worse aerodynamic characteristics such as the non-linear increasing lift coefficients and small stall angle.The investigation of the NACA633-421 airfoil at low Reynolds number is carried out in NUAA low speed wind tunnel.The results show that the stall of the airfoil at the small angle is caused by the burst of the laminar separation bubble,which is able to maintain the pressure distribution,at the strong pressure gradient.Propose a new idea on researching the flow structure with flow control.The dynamic bursting procession of the laminar separation bubble is reconstructed and captured with flow control method at the angle of 10° and the Reynolds number of 120000.The research shows that the laminar separation bubble can keep stable naturally at the first short period before totally burst.During the whole procession,the laminar separation shear layer keeps moving away from the surface resulting in the growth of the laminar separation bubble and the turbulent separation area expands towards the leading edge.In the end,the connection of these two separation areas results in the burst of the laminar separation bubble.Propose a new flow control method named interval disturbance which determines the interval time between two neighbor disturbances referring to the stable time of the separation bubble,T_stable.Under the interval disturbance,the max lift coefficient is improved and the stall is delayed.Using the particle image velocimetry technique and the dynamic pressure measurement to record a whole control period,the different phases flow field and pressure distribution around the airfoil show that the interval disturbance breaks the naturally developing of the burst of the laminar separation bubble.At last,taking a physical perspective,design a reference method controller and a searching gradient method controller,both of which get use of the stability time of the laminar separation bubble,T_stable,and take verification experiments at fixed angle and continuously moving state.Furthermore,in order to improve the efficiency of the whole control system,propose an idea of modulating the interval signal which uses a signal driven by the shear layer unstable frequency.