Visualization Study of Vorticity Amplification in Stagnation Flow.
PURDUE UNIV LAFAYETTE IND PROJECT SQUID HEADQUARTERS
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A visualization study of stagnation flow round a circular cylinder was carried out in order to gain a physical insight into its model put forward by the vorticity-amplification theory. The visualization was conducted at a Reynolds number of 8000 utilizing white smoke composed of titanium dioxide for an approaching flow containing turbulence at scales larger than the neutral wavelength. An in-depth dynamic qualitative apperception of the flow events was obtained by means of motion pictures taken from three different viewpoints. In addition, a frame by frame examination of selected movie strips led to the acquisition of a reasonably quantitative interpretation of the gross flow structure. The analysis of the flow events focused on tracing the temporal and spatial evolution of a cross-vortex tube outlined by the entrained smoke filaments. The visualization supplied striking evidence concerning 1 the selective stretching of cross-vortex tubes which is responsible for the amplification of vorticity and, hence, of turbulence 2 the streamwise tilting of stretched cross-vortex tubes 3 the existence of a coherent array of vortices near the stagnation zone 4 the interaction of the amplified vorticity with the body laminar boundary layer and, finally, 5 the growth of a turbulent boundary layer. These are basically the flow characteristics and events advanced by the vorticity-amplification theory. It is thus apparent that the amplification of turbulence in stagnation flow around a bluff body and its effects upon the body boundary layer are satisfactorily explained by the vorticity-amplification theory. Author
- Fluid Mechanics