EXPERIMENTAL INVESTIGATION ON THE EVOLUTION MECHANISM OF UNIFORM MOMENTUM ZONES IN TURBULENT BOUNDARY LAYER

Uniform momentum zone is one type of local regions where the instantaneous momentum of fluid approaches, and its generation and distribution are closely related to the coherent structure. The study of uniform momentum zone contributes to further understanding of the coherent structure in turbulent boundary layers, but there is still a lack of experimental support and mechanism analysis for the evolution process of uniform momentum zone. The moving TRPIV system was designed and used to measure the velocity fields of the turbulent boundary layer on a smooth surface as it moved downstream. The data is denoised by filtering, and the fluctuating velocity signal is obtained by combining the interpolation results of the direct numerical simulation data. After the influence of non-turbulence removed by an improved method,this thesis detects the uniform momentum zone in turbulent boundary layer, and obtain the time series of its quantity. In combination with the change of the streamwise velocity probability density distribution, the stepwise variation in the number of uniform momentum regions from one steady state to another over large time scales is obtained. The fluctuating velocity of different scales was decomposed, and the conditional average of large-scale and small-scale fluctuation signals were carried out. The results show that the large-scale fluctuations play a major role in the quantity change of the uniform momentum zone, the action mode is that the probability density function distribution of streamwise velocity is changed by different burst events of fluid in different velocities. By analyzing the changes of different fluctuation regions in the streamwise large-scale fluctuation space, it is found that a uniform momentum zone often contains multiple large-scale fluctuation regions. The expansion, contraction, splitting and merging of different large-scale fluctuation regions affect the concentration degree of streamwise velocity, leading to significant changes of the number of uniform momentum zones.