Triggering vortex shedding for flow past circular cylinder by acting on initial conditions: A numerical study

Abstract

Physically, when the Reynolds number exceeds some critical value, vortex shedding past a circular cylinder appears naturally and immediately. Numerically, if the domain geometry and the approaching flow conditions are symmetric, vortex triggering requires a long run-time, especially for low Reynolds numbers. The present study proposes to reduce this run-time by acting on the initial conditions instead of the classical approaches based on boundary conditions perturbation. In contrast to these, the proposed technique does not bring any energy to the flow. The vortex shedding is simply triggered by introducing a lateral gradient to the initial streamwise velocity. Simulations are performed using the ANSYS CFX 12® finite-element-based finite volume code. A two-dimensional laminar flow at Re=100 is first considered. Without perturbing the initial uniform flow, the obtained results are in good agreement with the experimental and numerical results available in the literature. With perturbed initial conditions, the main characteristics of the flow are properly found while the run-time required for triggering the ultimate regular periodic regime with vortex is considerably reduced. Simulation results show that the extent of the run-time reduction depends on the amplitude of the initial perturbation. The search of the optimal value corresponding to the largest run-time reduction led us to propose an analytical expression by assuming that the natural vortex shedding frequency is equal to the periodic lateral perturbation frequency. The validity of the expression was verified for Re=100 and confirmed for Re=60, 80 and 120. It was then used to perform many simulations in a reasonable time. All obtained results show that the proposed technique gives better results compared to the impulsive start technique and better mimics the physical reality, at least for low Reynolds numbers.