Experimental Investigation On Multi-scale Coherent Structures In Wall Turbulence By Periodic Disturbance

Experimental investigations on controlling multi-scale coherent structures by locally periodic disturbances of different frequency were carried out. The time sequence signals of instantaneous longitudinal, normal and spanwise velocity components at different vertical locations in turbulent boundary layer over a flat plate have been finely measured by constant temperature anemometry and the effects of periodic disturbances on the multi-scale coherent structures of boundary layer were studied in a flat plate.The longitudinal and normal regions comprising disturbed signals were extracted by the method of FFT. In these regions, it is found that the friction coefficients are reduced, the sub-layer becomes thicker and the turbulent kinetic distribution coefficients are reduced. The spactical ranges, in which the Reynolds stresses trend to zero, become larger. The mechanism of producing, diffusion and transporting is changed and the turbulent fluctuations are restrained to a certain degree. After disturbed, two unsteady, similar strength and large-scale spanwise vortexes came into being. These vortexes play the role of fluid rolling bearing, which reduce the wall friction drag.By the method of the module-maximum of wavelet coefficients, the conditional phased-averaged waveforms of coherent structures were extracted in the burst. The phased differences between the phased averaged waveforms of the longitudinal velocity and the normal velocity always remain blunt angles and almost not change with the normal locations. Thus the Reynolds stress u′v′remains negative and the amplitude is large. While the spanwise velocity lags to the longitudinal velocity apparently. The phased differences change with the normal locations. In the buffer layer, it is blunt angles and it gradually reduces to zero with far from the wall.Based on the concept of turbulent multi-scale and locally averaged velocity structure fuction, the effct of turbulent energy and coherent structure intermittence have been studied. The disturbances weaken the importance of small scale turbulent vorticity structures, while strengthen the importance of large scale turbulent vorticity structures. The turbulent system is restrained by the large-scale vorticity structures. The positive and negative alternations of vortical structures are enganced. After disturbed, the upward movement of low-velocity striation structures and the downward movement of high-velocity structures are restrained. The spanwise fringe became more ordering and the spanwise losting stability is restrained.By periodic disturbance, the phase angles between the phase-averaged waveforms of the longitudinal, vertical velocity and the spanwise velocity are changed. When their phase angles are about odd times ofπ/2, the Reynolds stress is about zero. At this point, the turbulent coherent structures are controlled and the burst in turbulence is restrained.