Shear layer vortices and longitudinal vortices in the near wake of a circular cylinder

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Abstract

Results are presented on (1) the shear layer vortices developing in the free shear layers located between the separation point and the first shed Strouhal vortex of a circular cylinder and (2) the longitudinal vortices developing in the braid region of the Strouhal vortices in the wake. Evidence based on flow visualization shows that the longitudinal vortices develop independently at low Reynolds numbers. An analysis is developed to show that the spanwise length scale of the longitudinal vortices in the separated shear layers and the longitudinal vortices in the Strouhal vortices are comparable near Re ≈ 1000. At higher Reynolds number, with increasing Re, the spanwise scale of the former reduces faster than that of the latter.

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    Citation Excerpt :

    Evolution of three dimensionality in the wake behind nominally two dimensional bluff bodies is a natural phenomenon and inherent characteristic of periodic flows dominated by the Karman vortices (also known as rollers having spanwise vorticity). Numerous early investigations of flow over circular cylinder, both experimental (Huang et al., 2006; Lin et al., 1995; Scarano and Poelma, 2009; Scarano et al., 2006; Williamson, 1992, 1996; Wu et al., 1994, 1995, 1996a,b) and numerical (Leweke and Williamson, 1998; Mittal and Balachandar, 1995, 1997; Thompson et al., 1996; Zhang et al., 1995) have confirmed emergence of longitudinal braids or ribs (streamwise vortices) due to spanwise instability of rollers in their formation phase even at the Reynolds numbers as low as a couple of hundred. The occurrence of these streamwise vortices along the span of the rollers is temporally and spatially random with their spanwise length scale (spacing) being strongly dependent on the Reynolds number.

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