Numerical investigation of Strouhal number discontinuity and flow characteristics around single rectangular cylinder at low aspect ratios and Reynolds numbers

Two-dimensional unsteady wake characteristics and Strouhal number discontinuity have been investigated numerically in flow past rectangular cylinder of different aspect ratio using lattice Boltzmann method. Effect of aspect ratio (AR) has been studied for Reynolds number (Re) in the range from 75 to 150. For investigation the aspect ratio range between 0.05 and 1 has been considered. Different flow regimes, such as two-rows flow regime, modulated flow regime, fully developed flow regime and irregular flow regime have been identified based on the values of Reynolds number for different aspect ratios of the rectangular cylinder. The effects of the aspect ratios and Reynolds numbers on fluid forces, flow separation and vortex shedding frequency have been examined in detail. The results reveal that there exists an aspect ratio where the Strouhal number shows discontinuity. Especially for Re = 145 and 150, a discontinuity in Strouhal number, is observed at around 0.5 ≤ AR ≤ 0.6 where a few extra small peaks are found in the power spectra result of the lift coefficient and irregular vortex pattern behind the rectangular cylinder with no fixed shedding frequency from vorticity contour and streamline visualization. The results further show that the flow characteristics change from two-rows flow regime to fully developed flow regime by changing the values of AR.