Three-dimensional wake effects on flow-induced forces

Numerical simulations are carried out for a long slender rigid circular cylinder in a cross-flow to examine three-dimensional (3-D) wake effects on the flow-induced forces. The aim is to assess the validity and extent of the two-dimensional (2-D) assumption for both the mean drag and the flow-induced forces. In order to simulate the practical situation correctly, wall boundary conditions are specified at both ends of the cylinder. The long slender cylinder has different aspect ratios. A finite volume method (FVM) and a lattice Boltzmann method (LBM) are used to carry out the computations, and their results are compared with each other and with available experimental and simulation data. As a first attempt to assess the 2-D assumption, a Reynolds number Re=100 and an aspect ratio a=16 are chosen. At this Re and a, conventional experimental and numerical studies assume that the time-averaged flow is homogeneous and 2-D over a relatively large portion of the central span. However, present simulations indicate that vortex shedding from the nominally 2-D cylinder strongly depends on the span location and the flow-induced forces exhibit strong three dimensionality. The calculated mean drag and root-mean-square lift and drag vary greatly along the span. These results indicate that the 2-D assumption is not valid for the flow-induced forces, not even within a small region of the central span, for the aspect ratio examined. The validity and extent of the periodic boundary conditions assumption for a 3-D simulation of the flow and induced forces on a cylinder in a cross-flow is examined next. It is found that, within the range of a investigated, an appropriate period could not be found for the numerical simulation. The results further show that a has a significant effect on the calculated wake flow and the flow-induced forces.