Numerical and experimental study of the flow around a 4:1 rectangular cylinder at moderate Reynolds number

This paper presents the results of investigations into the flow around a rectangular cylinder with a chord-to-depth ratio equal to 4. The studies are performed through wind tunnel dynamic pressure measurements along a cross section combined with two-dimensional Unsteady Reynolds-Averaged Navier-Stokes (URANS) and three-dimensional Delayed-Detached Eddy Simulation (DDES). These experimental and numerical studies are complementary and combining them allows a better understanding of the unsteady dynamics of the flow. These studies aim mainly at determining the effects of the rectangle incidence and freestream velocity on the variation of the flow topology and the aerodynamic loads, and at assessing the capability of the industrially affordable URANS and DDES approaches to provide a sufficiently accurate estimation of the flow for different incidences. The comparison of experimental and numerical data is performed using statistics and Dynamic Mode Decomposition. It is shown that the rectangular cylinder involves complex separation-reattachment phenomena that are highly sensitive to the Reynolds number. In particular, the time-averaged lift slope increases rapidly with the Reynolds number in the range 7.8e3 < Re < 1.9e4 due to the modification of the time-averaged vortex strength, thickness and distance from the surface. Additionally, it is shown that both URANS and DDES simulations fail to accurately predict the flow at all the different incidence angles considered. The URANS approach is able to qualitatively estimate the spatio-temporal variations of vortices for incidences below the stall angle alpha = 4°. Nonetheless, URANS does not predict stall, while DDES correctly identifies the stall angle observed experimentally.