Turbulent Ramp Flow Control Using Spanwise Unsteady Slot Jet

Backward-facing ramp,as a typical phenomenon of flow separation,has been extensively studied in both simulation and experiment in recent years.The control of a two-dimensional turbulent flow separation from a 25^° backward-facing ramp is experimentally studied using an unsteady upstream blowing through a spanwise slit with a view to reducing the recirculation bubble length.The Reynolds number,based on the ramp height is 9.4×10⁴.Three control parameters are examined,i.e.the blowing amplitude,frequency and duty-cycle.The flow is measured extensively with and without control using techniques such as hot-wire anemometry,particle image velocimetry and pressure scanner.And analyze the control mechanism of the actuator according to the measurement results.Results show that,optimal control parameters are f_e = 130 Hz,duty-cycle = 40%,Q_std = 55L/min.Compared with the non-controlled case,the length of recirculation bubble X_R/h can be reduced by 40% under the optimal control parameters even with a small momentum coefficient C_? = 1.3×10^-4,low velocity ratio ???_{jet max}/U?= 0.08.The main mechanism of the control is that the unsteady blowing of the actuator increases the spanwise structure,enhances vortex shedding periodicity,so that the separated shedding vortex can hit the downstream plate faster.Therefore the length of the recirculation bubble reduced.According to Particle Image Velocimetry data,the unsteady blowing of the actuator adds disturbance into the turbulent boundary layer at the upstream edge of the ramp,which increases the turbulent kinetic energy and increases the turbulence mixing intensity above the ramp.The Reynolds stress on the ramp increases correspondingly,promotes the momentum transfer in the shear layer,then the unsteady blowing causes the shear layer toward the ramp surface,increasing the negative pressure area of the ramp.So the pressure coefficient of the ramp will be significantly reduced compared to the case without control.