基于单个压电振子的湍流边界层主动控制

利用安装在壁面上的单个压电振子周期振荡,采用开环主动控制方案,实现了对平板湍流边界层相干结构猝发的主动控制和壁湍流减阻.根据不同的输入电压幅值和频率,完成了10种工况的实验.在压电振子下游2mm处,用热线风速仪和迷你热线单丝探针,精细测量湍流边界层不同法向位置瞬时流向速度信号的时间序列,分析了在Re_?=2183压电振子振动对湍流边界层平均速度剖面、减阻率和相干结构猝发过程的影响.实验结果表明,施加控制的工况使平均速度剖面对数律层上移,产生减阻效果;压电振子振幅越大,减阻率越高,减阻效果越明显;通过对施加控制前后流向瞬时速度的多尺度湍涡结构脉动动能的尺度分析,当压电振子振动频率与壁湍流能量最大尺度的猝发频率相近时,减阻率达到最大,为25%,说明控制壁湍流能量最大尺度相干结构的猝发是实现壁湍流减阻的关键;通过对比相干结构猝发的流向速度分量条件相位平均波形,发现施加控制的工况中相干结构猝发流向速度分量的波形幅值明显降低,且流向速度在扫掠后期高速阶段迅速衰减,缩短了高速流体的下扫过程,说明压电振子的振动能抑制相干结构的高速流体下扫过程,减弱高速流体与壁面的强烈剪切过程,并使近壁区域相干结构的振幅显著减弱,迁移速度加快,从而减小壁面摩擦阻力. 

ACTIVE CONTROL OF TURBULENT BOUNDARY LAYER BASED ON A SINGLE PIEZOELECTRIC OSCILLATOR

Open-loop active control of a turbulent boundary layer has been achieved in skin-friction reduction and sup-pression of coherent structure bursting process by means of periodic oscillating of a piezoelectric oscillator embedded on the surface of a flat plate wall. Ten experimental cases were carried out under variable input voltage amplitudes and frequencies. At 2 mm downstream of the piezoelectric oscillator, the simultaneous time series of streamwise velocity component at different wall-normal positions in the turbulent boundary layer were finely measured by hot-wire anemome-ter and a mini single-sensor probe. The effects of piezoelectric oscillation on the mean velocity profile, drag-reduction rate and conditional phase-average waveform of coherent structure burst were investigated at Reθ=2 183. An upward shift in the log-law of mean velocity profile is observed, which indicates the reduction of skin-friction. With the larger amplitude of vibration, the higher drag reduction rate is achieved. Furthermore, a maximum rate of 25%can be reached when the vibration frequency is very close to the burst frequency of maximum-energy scale, which indicates that the manipulation of energetic-scale coherent structure burst is the key of wall-bounded turbulence drag reduction. In addition, by com-paring the conditional phase-average waveforms of manipulated and unmanipulated cases, the waveform for manipulated conditions has more decreased amplitude with its wave crest damping rapidly in the later stage of high-speed sweep event and the sweep process of high-speed fluids are shorten. The vibration of piezoelectric oscillator can suppress the coherent structure sweep process of high-speed fluids, weaken the shear process of the high-speed fluids with the surface of the wall, bate the amplitude of coherent structure burst in the near-wall region, and as a result, reduce the skin-friction drag.