Effect of the yaw angle and spanning length on flow characteristics around a near-wall cylindrical structure

Abstract

Three-dimensional direct numerical simulation is carried out to investigate the effects of the yaw angle and spanning length on the vortex structures and hydrodynamics of flow crossing a near-wall cylindrical structure. Two yaw angles (α = 0°, 45°), and three spanning length ratios (L* = 25, 50, 100) are investigated at Re = 500. Under the normal cylinder condition (α = 0°), a parallel vortex shedding pattern is observed in the wake. The vortex formation lengths are identical for all L*, whereas the spanwise correlation of the vortex shedding declines with the increasing L*, indicating the intensified three-dimensionality. A single dominant peak is found in the lift frequency spectra for each L* case. Under the yawed condition (α = 45°), the flow characteristics are fundamentally changed with negatively-yawed linear vortex shedding tubes being observed in the wake. Different from the normal cases, the vortex formation length for L* = 25 is larger than that of L* = 50 and 100, while the oblique angle of the vortex tube decreases with the increasing L*. It is shown that, at α = 45°, the three-dimensional flow features are fully developed when L* ≥ 50. The lift frequency spectra show multiple peaks while the magnitudes of the secondary peaks are significantly reduced by extending the spanning length. By comparing the results between α = 0° and α = 45°, it is found the Independence Principle (IP) is applicable for predicting the first-order statistics of the hydrodynamic forces and vortex shedding frequency, but fails in predicting the second-order force statistics. The effects of the yaw angle on the pressure and shear stress distributions of the bottom wall are also discussed.