关键词: 射流-横流相干, 声激励, 合成射流效应, 流动主动控制, 穿透和混合特性

Abstract: The present study performs schlieren-imaging tests and large-eddy simulations to illustrate the effects of forcing frequency on flow dynamics of acoustically-excited transverse jets in crossflow, at a fixed referenced jet-to-crossflow ratio of VRref=1.5 and a wide range of forcing frequencies (f=20Hz~100Hz). Of particular interest is the finding that a synthetic jet effect could be generated in acoustically-excited transverse jet, showing a significant enhancement mechanism on the jet penetration and mixing in crossflow. Distinct flow regimes are also identified for the acoustically-excited jets in crossflow, depending on forcing frequency. When the forcing frequency exceeds a specifically critical value, the outer surrounding fluid could be suctioned into the jet plenum by the acoustic excitation, resulting in an obvious increase of the peak instantaneous ejecting velocity, forming strong individual vortex rings in the starting flow regime, and bringing additional mixing process inside the jet plenum. With respect to the unforced steady jet, acoustic excitation effectively increases the jet penetration depth, spread domain and mixing uniformity. Particularly at a high forcing frequency, such as f=100Hz, the penetration depth could be increased up to 160% at x/D=2 and 130% at x/D=10 approximately. Evaluated in a specified zone sized by Dy∈(0, 14D) in normal direction and Dz∈(-5D, 5D) in spanwise direction, the mixing after 2D at f=100Hz could reach the SMD level of the steady jet experiencing up to 16D. By introducing a modified excitation Strouhal number based on the vibration amplitude of loudspeaker diaphragm, a criterion of Stm,cr=0.004 is suggested for manipulating the synthetic effect approximately in the cur-rent acoustically-excited jet configuration.

Key words: jet-in-crossflow, acoustic excitation, synthetic jet effect, active flow control, penetration and mixing characteristics