Influence of Time-Varying Freestream Velocity on the Flow Characteristics of Elongated Rectangular Cylinders

We perform Large-Eddy Simulations (LES) on the accelerating flow around streamwise-elongated rectangular cylinders with chord-to-depth ratios of 3:1 and 5:1 using Gaussian-type inflow accelerations of different intensities. The Reynolds numbers, defined with the freestream velocity and the crossflow dimension of the cylinder, range from Re = 17200 to Re = 65360. For both 3:1 and 5:1 rectangular cylinders the vortex shedding is characterized by constant-frequency time cells as observed in the literature for a square cylinder. For the 3:1 case, the Strouhal number variation range and the crossflow-force fluctuations within each time cell are the same for all cells. The results obtained under stationary inflow conditions for the rectangular 3:1 cylinder match well the statistical values computed in each time cell. On the other hand, for the 5:1 case, the cell-averaged recirculation region along the lateral side reduces in size during acceleration, leading to a narrower wake, decreased lift fluctuations, and higher Strouhal numbers. The shortening of the mean recirculation region with increasing Reynolds number for the 5:1 rectangular cylinder occurs at higher Reynolds numbers for accelerating inflows compared to stationary-inflow conditions. Finally, in agreement with what was observed for the square cylinder, for both considered aspect ratios the Strouhal number behaviors for accelerations of different severity collapse when plotted as a function of the Reynolds number.