Abstract: The fluid motion specialty has an important influence on the resistance at the near wall surface. The frictional resistance of the wall surface is linearly represented by the velocity gradient and the shear stress. The evolution of the shear eddy current and the distribution of eddy current in the self-excited oscillation chamber are analyzed. The numerical analysis method of large eddy simulation is used to find out instantaneous and average flow speeds of an axial section and normal plane in the outflow pipe, the shear stress of the near wall surface under different upstream and downstream pipe diameter ratios, and velocity gradient of normal plane under different aspect ratios in the outflow pipe. The results show that the self-excited oscillation pulse jet flow is strongly specialty of three-dimensional affected by the reverse boost vortex, forming a “wave-like” flow and circumferential deflection, and the flow velocity and flow pattern decreases with the change of position in a pulse-like manner; when the ratio of the diameter is larger than 1, the shear stress of the near wall surface in the outflow pipe is reduced, and as the ratios of upstream and downstream pipe diameter is gradually increased, the maximum reduction rate of wall shear stress reaches about 30% and the frictional resistance of the near wall surface is decreased; the maximum normal velocity gradient fluctuates when the length-diameter ratio is 0.55. Large fluctuations occur, but as the aspect ratio continues to increase, the jet velocity gradient gradually decreases and the frictional resistance will be reduced in the outflow pipe. The research results can provide theoretical basis and scientific basis for the design of self-excited oscillation pulse drag reduction and transmission device.

Key words: shear layer, self-oscillating, pulse jet, wall shear stress, normal speed gradient