Experimental Research On Controlling Coherent Structures In Wall Turbulence By Periodic Blowing And Suction From Streamwise Slit

The fluctuating velocity has been measured using hot wire anemometer with and without the local periodic blowing and suction disturbance coming from longitudinal thin slits periodically distributed in transverse direction of wall turbulence. The coherent structures in wall turbulence are controlled by the local sinusoidally oscillating jet flow issuing from the longitudinal thin slits. The fluctuating velocity signals have been analyzed by the comprehensive methods of probability statistics, Fourier transform, conditional sampling, wavelet and wavelet packet transform, etc in order to investigate the effects of disturbance to the coherent structures in wall turbulence. The variations of turbulence energy spectrum, intermittence and the mean flow parameters have been studied before and after blowing and suction disturbance introduced.The methods of local maximum of wavelet coefficients across related multi-scales and wavelet auto-correlation function have been put forward to detect and identify the coherent structures in wall turbulence. It is found that coherent structures in buffer layer fluctuates more actively than that in outer region of turbulent boundary layer and behaves temporally and spacially in multi-scales. Wavelet packet transform has been put forward to extract the disturbance component from the turbulent fluctuating velocity, which shows more accurate than the Fourier filter method; the wavelet function of Daubechies20 with high vanishing moments is employed to analyze turbulent spectrum so as to decrease the energy leakage of the signal effectively and improve the precision of turbulence spectrum analysis.The coherent structure detection results of wavelet transform are compared with that of VITA conditional sampling method. It is shown that blowing and suction disturbance can not only decrease the intensity of turbulent fluctuation at the most energetic scale in buffer layer and restrains the burst process in wall turbulence, but also prolong the averaged burst alternation and lessens the quantity and intensity of burst events. And then the amplitudes of conditionally averaged velocity as well as the wavelet restructured velocity are apparently decreased with the periodic blowing and suction disturbance. It can come to the conclusion that the periodic blowing and suction oscillating disturbance coming from longitudinal slit jet flow is an effective method of control wall turbulence. Through the investigation of turbulent energy spectrum, it is found that for wall turbulence, the variation of inertial wave number range is related to the vertical measurement position in turbulent boundary layer. When the measurement position is near to the wall, the inertial wave number range in wall turbulence shifts to the higher wavenumber and the spectrum gradually deviates from -5/3 exponent power, which stands for the homogeneous and isotropic turbulence. According to the velocity fluctuating waveform of coherent structures extracted from origin velocity signal, the fluctuation of coherent structure in buffer layer becomes more active than that in outer region.According to the effects of blowing and suction on turbulent energy spectrum by wavelet spectrum and maximum entropy spectrum method, it is discovered that the disturbance changes the turbulent kinetic energy distribution across scales in the near wall region, which implies that the coherent structures lose more kinetic energy and the small scale structures obtain more kinetic energy. The effect of disturbance disappears gradually when the measurement position is apart from the near wall region and the turbulent spectrum recovers to the situation without disturbance.Through investigating the intermittence of small-scale structures in wall turbulence with or without disturbance, the scaling exponent in inertial wavenumber range is lower than that in dissipative range. The scaling exponent in buffer layer extraordinarily deviates from the linear scaling law which shows intensive anomalous, whereas the scaling exponent in logarithmic region and outer region gradually approaches to the situation of homogeneous and isotropic turbulence. The blowing and suction further reduces the scaling exponent and enhances the intermittence of small scale structures in buffer layer.