Effect of acoustic perturbation on the transition of vortex street in a flowing soap film

An experimental study of the effect of the acoustic perturbation on the evolution of wakes of cylinders with different cross sections and the same projected length (D) is conducted in a soap-film tunnel. Parameters for acoustic perturbation include different perturbation amplitude, frequency, and location. The formation of a secondary street structure is seen in the immediate downstream (similar to 21D-25D) of the triangular and rhomboid cylinder, while it is absent for the circular, square, and inverted triangular cylinder within the domain of the test section (similar to 135D). This study observed that the acoustic perturbation influences the location of the transition point at which the primary street transforms into the secondary street. For the triangular cylinder, the maximum shift in the location of the transition point in the downstream direction is 16.4%, and the maximum shift in the upstream direction is 16% when compared with the unperturbed flow. Similarly, for the rhomboid cylinder at two different acoustic perturbation conditions, the maximum shift in the downstream direction is 12.5%, and in the upstream direction, it is 25.4%, compared with the unperturbed flow. Further, the Strouhal number variation of the primary and secondary wake with perturbation frequency ratio is calculated for different perturbation cases. In both triangular and rhomboid cross-section cylinders, the Strouhal number of the secondary wake is almost 50% of that of the primary wake.