An Experimental Investigation of the Near Wake of a Circular Cylinder in the Subcritical Regime.

Abstract

Abstract : The near wake of a circular cylinder in crossflow is experimentally investigated for Reynolds number 7600. The purpose is to develop techniques for characterizing the formation region of the Karman vortex street in presence of transition to turbulence and to use these techniques to examine the quasi-periodic vortex shedding phenomenon. A thermal tracer is used to tag vortical fluid in one of the cylinder boundary layers and this scalar is subsequently detected in the wake. The instantaneous phase of the amplitude and frequency modulated poly-chromatic vortex shedding phenomenon is computed using a digital Hilbert transform technique. This method is a generalization of the monochromatic complex rotating vector to processes which are modulated. The computed phase is used to conditionally sample the wake temperature using a six-sensor resistance thermo-meter array, and the resulting data are processed using statistical methods. Results demonstrate that the near wake region contains diffuse vortical fluid and this fluid has a role in the shar layer instability. The statistics also show the formation and shedding of a mean Karman vortex, and lead to a possible explanation of the modulation process. (Author)