Experimental investigation of flow field behind two tandem square cylinders with oscillating upstream cylinder

Abstract

In this paper, the fluid flow around two identical square cylinders arranged in tandem has been discussed. The upstream cylinder is oscillated in the transverse direction while the downstream cylinder is held stationary. The experiments are conducted at a Reynolds number of 295. Both the influences of spacing between two cylinders, as well as the effect of oscillation, of the upstream cylinder are investigated using Particle image velocimetry, hotwire anemometry and flow visualization techniques. The spacing ratios are varied from 1.5 to 5 while the upstream cylinder is oscillated in harmonics of the vortex shedding frequency of a stationary cylinder at the fixed amplitude. It is observed that there is a strong effect of spacing between the cylinders on vortex shedding mechanism and flow structures. When two cylinders are in tandem there is a critical distance below which vortex shedding of the upstream cylinder is suppressed. Three distinct flow regimes are captured and the detailed investigation of the intricate flow field is done both spatial and temporal domain. The vortex shedding frequency of a single stationary square cylinder is measured with hotwire anemometer and this frequency is being used to oscillate the upstream cylinder with its sub-harmonic, harmonic and super-harmonic frequency ratios. The effect of forcing frequency on flow interference, wake oscillation frequencies, aerodynamic forces and turbulence statistics have been studied for these spacing ratios. Flow fields are investigated in terms of the time-averaged drag coefficient, stream traces, vorticity contours, turbulence statistics and the time-dependent flow field is captured using flow visualization, vorticity contours, and Strouhal number.