Experimental study for stratified flow around an impulsively started rotating cylinder using combined PIV and LIF

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An experimental study has conducted for density-stratified circular flow induced by an impulsively started rotating cylinder. The objective of the present study is qualitative classification of visualized flow pattern and understanding of relationship between the variation of velocity field and that of density field. To achieve them, several experimental techniques, such as electrolytic precipitation and dye injection method, laser induced fluorescent (LIF), particle image velocimetry (PIV), and shadowgraph, are employed. The Reynolds number (Re=r^<2>ω)/v=100-300, where r is radius and ω angular velocity of rotating cylinder) and the Froude number (Fr=ω/N=4-12, N is buoyancy frequency) are varied independently. At the beginning of the rotation, instability of dyed flow pattern becomes significant, and then rolls up the mushroom-type vortex ring array. The evolutions of dyed vortex pattern lead to subsequent large-scale layered formation. During the layered formation and the rearrangement of the flow pattern, the dyed flow exhibits the behavior of oscillating motion. The vorticity contours show the corresponding occurrence of vortex ring array around the cylinder. In addition, occurrence of pairs of vorticity array nearby the cylinder is observed even in the situation where the large-scale layered formation is maintained. The oscillating dyed flow motion far from the cylinder, therefore, is due to the occurrence of vorticity nearby the cylinder. The Reynolds number and the Froude number affected the radial length scale of the layered formation, while the wavelength was almost independent of Reynolds number. The density field exhibits corresponding variations with those of dyed flow pattern and the vorticity within certain time period. However, variations of density gradient become negligible with time progress, while the remarkable layered motion of dyed pattern is still observed. It means that the significant time scale exists in the present mixing phenomenon.