减阻工况下壁面周期扰动对湍流边界层多尺度的影响

通过在平板壁面施加不同频率振幅的压电陶瓷振子周期性扰动,进行了湍流边界层主动控制减阻的实验研究.在压电陶瓷振子最大减阻工况下(80 V和160Hz),使用单丝边界层探针对压电振子自由端下游2mm处进行测量,得到不同法向位置流向速度信号的时间序列.通过对比施加控制前后的多尺度分析,发现压电振子产生的扰动只对近壁区产生影响,使得近壁区大尺度脉动降低,小尺度脉动强度增大,而对边界层的外区则基本没有影响.进一步对大尺度和小尺度的脉动信号进行条件平均,发现压电振子产生的扰动对小尺度脉动的影响在时间相位上并不均匀,小尺度脉动强度在大尺度脉动为正时比在大尺度脉动为负时具有更明显的增加.这表明壁面周期扰动主要通过使大尺度高速扫掠流体破碎为小尺度结构,来影响相应的高壁面摩擦事件,从而达到减阻效果. 

THE EFFECT OF PERIODIC PERTURBATION ON MULTI SCALES IN A TURBULENT BOUNDARY LAYER FLOW UNDER DRAG REDUCTION1)

This study reports the drag reduction of a turbulent boundary layer flow under the active control by a pair of wall-mounted piezoelectric oscillators with different working frequency and amplitude. Under the maximum local skin drag reduction case of 80V and 160 Hz, a miniature boundary layer hot-wire probe of single sensor was used to measure velocity signals at the streamwise location 2 mm downstream of the oscillators. The time series of streamwise velocity signals at different wall normal positions were obtained. By comparing the turbulent fluctuations at different scales in the perturbed cases and canonical turbulent boundary layer flow, it was found that the perturbation produced by piezoelectric oscillators have an effect on the near-wall region, which attenuates the large-scale intensity and enhances the small-scale turbulence. Meanwhile, the outer layer is insensitive to the oscillator perturbation. Furthermore, conditional average of small-scale fluctuations shows that the impact of piezoelectric oscillators on small-scale fluctuations is not uniform in the condition of the large scales. The small-scale amplitude is increased more obviously as the large-scale fluctuations are positive than that the large-scale fluctuations are negative. It indicates that the current periodic perturbation dominantly modifies the high skin friction events by breaking the large-scale high-speed sweeping fluids into small-scale structures, which results in the local skin drag reduction.