Wind peak pressures on a square-section cylinder: Flow mechanism and standard/conditional POD analyses

This study emphasizes the flow mechanism of negative peak pressures on a square-section cylinder at Reynolds number of 22,000, particularly of those immediately upstream of the trailing edges (trailing-edge peak pressure). The minimum trailing-edge pressure coefficients are observed as low as -5 and have a close correlation in a streamwise length of approximately 0.1D (D is the width of the cylinder). The occurrence of small trailing-edge vortex, which has highly concentrated vorticity, is responsible for the trailing-edge peak pressures. The trailing-edge peak pressure occurs immediately before the shear layer moves far from the cylinder and recovers to the symmetric state. Standard proper orthogonal decomposition (POD) is applied to discover the inherent spatial modes with sharp peak pressures upstream of the trailing edges. Instead of the 1st mode (corresponding to vortex shedding), the higher-order modes contribute significantly to the trailing-edge peak pressures, whose mode coefficients have harmonics of vortex-shedding frequency. More than nine POD modes are necessary to reconstruct the trailing-edge peak pressures. As a variant of POD proposed recently, conditional space–time POD is valid in studying the average evolution of wind peak pressures. It further provides the relative phase and frequency ratio between the trailing-edge peak pressure and vortex shedding.