Flow past a rotating hydrophobic/nonhydrophobic circular cylinder in a flowing soap film

Flow past a rotating cylinder of various surface characteristics is investigated experimentally in a flowing soap film of a soap film tunnel. Experiments are carried out at Reynolds numbers similar to 200 and 250 with cylinders of different levels of hydrophobicity and normalized rotation rates (ratio of surface speed of cylinder to free-stream velocity) varying from 0 to 4.36. The diagnostic is done with flow visualization using interference technique, and quantitative measurements are obtained with particle image velocimetry. Global wake structure of a rotating hydrophobic/nonhydrophobic cylinder at various rotation rates is presented in this work. Reynolds number and hydrophobicity are found to have a substantial effect on the onset of the transition regime. Interestingly, the transition of von Karman vortex shedding to single-signed vortex shedding occurred at a transition zone with no evidence of region of suppression in soap film tunnel experiments. The shed single-signed vortices at a rotation rate above the transition regime are of an opposite sense to the cylinder rotation direction. The experimental evidence of this new vortex shedding mode in a soap film tunnel is presented using flow visualization and particle image velocimetry. In addition, inequality in the shedding frequency of counterclockwise and clockwise vortices is observed whose value reaches maximum before the transition zone and then decreases. A higher rotation rate of 4.36 made the counterclockwise vorticity completely wrap around the cylinder at Reynolds number similar to 250. Consequently, the stagnation point got lifted off from the surface at similar to 135 degrees. The relative circulation of shed clockwise vortices to counterclockwise vortices is observed to be increasing with rotation rate and reaches maximum at the transition zone. The variation of the Strouhal number with rotation rate for shed clockwise vortices is observed to be increasing until the transition regime, and subsequently it decreases. The Strouhal number for hydrophobic cylinders was found to be less compared to nonhydrophobic cylinder and was also observed to decrease with increase in hydrophobicity. The study of the far-wake structure shows the evidence of distortion in the wake structure due to the elongation of vortex pairs for both hydrophobic and nonhydrophobic cylinders.