Flow control over a diamond-shaped cylinder using slits

Abstract

In this experimental study, flow control over a diamond-shaped cylinder using slits was investigated using Particle Image Velocimetry (PIV). The slits used to form self-generating jet flows were placed in the middle of the four sides of the diamond-shaped cylinder at a Reynolds number of 8.6 × 103. To investigate the effect of the slit width/diameter ratio (θ) on the flow over the diamond-shaped cylinder, different θs, such as 0.035, 0.07, 0.105, 0.14 and 0.175 were studied. Various physical parameters such as the time-averaged streamline topology, the streamwise velocity profile, the rms (root mean square) velocity components, the turbulent kinetic energy and the estimated drag coefficient have been presented. In addition to these parameters, Fast Fourier Transform (FFT) and Proper Orthogonal Decomposition (POD) analyses were performed to expand on the flow behaviors. Results showed that while values of θ up to 0.07 for slits did not significantly affect the flow, beyond θ = 0.105, significant changes were observed for the flow structure in the wake region. With increasing θ, the shear layers were smaller, thicker and the strength of the vortex shedding of the diamond-shaped cylinder was attenuated by the jet flows emanating from the slits. The slits caused a considerable decrease in the root mean square (rms) values of velocity and turbulence kinetic energy (TKE/U∞2) in the wake region, which resulted in a decrease of the estimated drag by 37%. FFT results demonstrated that as θ increased, the Strouhal number (St) of the first dominant peak increased, while the amplitudes of the first dominant peaks reduced because of vortex shrinkage in the wake. In the POD analysis, it was observed that θ was influential on the vortex shedding behavior, and higher rolling-ups formed when the control was applied. According to the results, using slits could be an effective passive control technique for diamond-shaped cylinders.