Numerical study of flow characteristics around a 30° yawed circular cylinder at Re=104

Abstract

Unstable motions of bridge stay cables have been observed on site and in wind tunnel tests when a cable is yawed at certain orientations to wind. To uncover the underlying mechanisms, flow around a circular cylinder at a yaw angle of 30° has been numerically analyzed in the current study using delayed detached eddy simulation (DDES) at Re=104. A comparison with the reference normal flow case indicates the presence of a more coherent span-wise flow structure when the cylinder is yawed at 30°. The application of proper orthogonal decomposition further reveals that at this orientation, a synchronized flow structure exists, which is characterized by continuous anti-symmetric pressure blocks. In addition, a low-frequency flow fluctuation has been identified, the Strouhal number of which is roughly a quarter of that of the conventional Kármán vortex shedding. The pivotal role of axial flow in the intermittent amplification of cylinder sectional lift and the subsequent span-wise propagation of this enhanced local lift event has been revealed. The former is evident from the low-frequency sectional lift peaks occurred during vortex shedding, whereas the propagation speed associated with the latter is in good agreement with the span-wise component of the incoming flow speed. The temporal and spatial impact of axial flow on the surrounding flow structure of the cylinder may serve as a periodic excitation source, which could trigger an unstable response of a cylinder. This, in the context of bridge stay cables, could possibly contribute to the onset mechanism of dry cable galloping.