HYDRODYNAMIC CHARACTERISTICS OF VORTEX MOTION INSIDE THE HEMISPHERICAL DIMPLE

Abstract

Experimental researches of the features of the generation and evolution mechanism of coherent vortex structures, which are formed inside the hemispherical cavity, and space-time characteristics of the fields of velocity and pressure fluctuations that they create, both within the cavity, and in its wake, were conducted. Experiments were carried out in hydrodynamic flume on the hydraulically smooth plate with hemispherical dimple. The symmetric and asymmetric large-scale vortical systems inside a dimple are found out depending on the flow regime, and location and periodicity of their ejections are shown. The evolution of tornado-like vortices subjected to a switch mechanism that is result in appearance of low-frequency modulating transversal oscillations of vortex motion inside the hemispherical dimple. The fields of velocity, dynamic and wall pressure fluctuations by a group of hot-films, miniature piezoelectric sensors of pressure fluctuations are studied. Space-time correlations of velocity and pressure fluctuations, which generate the coherent large-scale vortex structures, the circulation flow and the vortex structure of the shear layer, which are formed inside the hemispherical dimple and ejected out into the boundary layer, are obtained. It was determined that the largest values of the coefficient of the space-time correlations of the wall pressure fluctuations observed on the aft and side walls of the hemispherical dimple, and the lowest – in its forward bottom part. It was established that the cross-correlations of the longitudinal velocity fluctuations are higher than the cross-correlations of the velocity and wall pressure fluctuations, and the cross-correlations of the dynamic and wall pressure fluctuations. On the basis of short-time spectrum and correlation analyses, the non-stationary in tame and non-uniform in space characteristics of the coherent vortical systems inside and past the open dimple have been found. Temporal periodicity of cross-correlation characteristics of the wall pressure fluctuations between two sensors, located on the fixed distance corresponds to frequencies of the shear and wake modes of resonance oscillations in the dimple. Registration of the time lag, at which the cross-correlation or anticorrelation has maximum, enables to determine the group convective velocity of the coherent vortical structures during their formation and evolution, as well as the phase of development and direction of vortex motion inside the dimple.