吹吸扰动对壁湍流相干结构的影响

采用热线风速仪测量受吹吸扰动的壁湍流边界层的流向速度，用傅里叶变换和子波变换研究吹吸扰动对壁湍流能谱的影响，结果显示施加的低频扰动使边界层内层大尺度结构的能量减少，小尺度结构的能量有所增强，远离壁面时扰动强度逐步衰减直到在外层中消失；通过VITA法和子波变换法检测猝发事件，表明该扰动降低了猝发强度，使猝发周期延长，条件平均速度波形的幅值降低、持续时间变短，说明扰动明显抑制了相干结构的猝发过程. 利用子波变换可以实现湍谱分析，能有效检测猝发中的湍流结构，是一种客观的分析工具.

EFFECT OF BLOWING AND SUCTION ON WALL TURBULENT COHERENT STRUCTURES

The streamwise velocity component in the wall turbulent boundary layer disturbed by blowing and suction was measured with hot wire anemometer.FFT transform and wavelet transform were adopted to study the effect on the turbulent energy spectrum.The results show that the disturbance in low frequency makes the turbulent kinetic energy be transferred newly among different turbulent structures.One of the impact is that the energy of large scale turbulent structures in inner boundary layer obviously decreases,especially in wavelet scale j=-10,which corresponds to coherent structures.The other effect shows that the energy of small scale turbulent structures increases synchronously near the wall.When the measured position is leaving from the boundary layer,not only the energy of small structures keeps growing up but also the trend of increased energy gradually extends to larger scale structures until the change adds up to the maximum in the borderline between inner and outer (yv=0.1) boundary layers.According to the above-mentioned facts,one may assume that the large scale turbulent structures are broken up into many small scale structures arising from the effect of low frequent blowing and suction.Note that above the boundary layer at yv=0.15(the outer boundary layer), the turbulent spectrum graphs with and without disturbance are identical superposition,which reveals that the influence of blowing and suction far from the wall boundary layer decays gradually until it disappears in the outer boundary layer. Furthermore,by means of VITA method and wavelet transform to identify burst events,it is clear that the disturbance not only reduces the burst intensity and delays the averaged burst period but also diminishes the scope of conditionally averaged velocity signals and shortens the duration time,which indicates that the disturbance apparently restrains the burst process of coherent structures.Based upon the above described analysis,the disturbance of blowing and suction is perfectly suited to controU the turbulent flow. In summary,the two methods,wavelet transform and FFT transform,can both be regarded as perfect tools to analyze the turbulent energy spectrum.Compared with FFT,wavelet transform makes the energy spectrum graph much smooth due to its filter function.Another advantage is that it decomposes the turbulent flow into different frequencies (or scales) which represent multi scales of turbulent structures including coherent structures and dissipative structures in order to recognize their respective characteristics.With free parameters,wavelet transform is not only an objective and effective utility to identify coherent structures and burst events but also an important tool to validate and supplement the analysis of the wall turbulent flow with VITA method.