Large eddy simulation of flow past a forced oscillating square cylinder

Abstract

A finite volume incompressible flow solver has been developed to simulate a moving body in fluid and the immersed boundary method is implemented to construct the geometry. A square cylinder is forced to oscillate with a prescribed sinusoidal motion is studied by using large eddy simulation (LES). Although the geometry of the square cylinder is simple, it is very challenging to LES, for example, the estimation of transition and the vortex formation length in the wake region.

In the past decades, with the advancement of measurement and computer techniques, a lot of work has been done on the VIV of square cylinder. Bearman et. al. [1] conducted an experiment of a forced oscillating square cylinder at a range of reduced velocity and motion amplitude ratio (A/D, where A is amplitude and D is side length of cylinder). In their experiment, they focused on the measurements of surface pressure fluctuation. The fluctuating surface pressure is correlated across the span when the cylinder is at lock-in region. Daniels et. al. [2] used LES to analyze VIV of an elongated rectangular cylinder with side ratio B/D = 4 (breadth/thickness) in one degree of freedom, i.e. pitching or heaving. In their simulations, the upstream turbulence with 6% turbulence intensity (TI) was imposed. Results are compared between uniform incoming flow and flow with free upstream turbulence. It is found that the upstream turbulence can reduce the spanwise correlations and subsequently reduce the oscillation amplitude compared to the smooth incoming flow. Tamura et. al. [3] simulated VIV of cylinders with square section and rectangular section (B/D = 2) in turbulent flow by using LES. In the case of rectangular cylinder, it is found that the unstable oscillation is strongly affected and disappears in the flow with high turbulence intensity.

In our previous work, flow past a stationary square cylinder at Reynolds number 21,400 was validated. The global integral quantities such as lift and drag coefficients and Strouhal number are in good agreement with experimental measurement. The turbulent statistics, in particular in the shear layer and wake regions are compared rigorously as well. The developed solver is able to predict surface pressure fluctuation, which is very challenging in LES. Following the previous work, a square cylinder, constructed by the immersed boundary method, with a prescribed sinusoidal motion in uniform incoming flow is studied. The immersed boundary method is very efficient in simulating moving body in fluid as it needs to generate mesh only once. The Reynolds number based on free stream velocity and side length D of cylinder is from 5*10^3 to 3*10^4. The reduced velocity is chosen from 5 to 12. The oscillation amplitude is set to 0.10. In this paper, the vortex shedding frequency versus reduced velocity result and time averaged surface pressure fluctuation distribution are validated. Additionally, phase average of flow field and surface pressure fluctuation at different phase angles are calculated. The phase averaged flow field is used to identify and analyze the spanwise vortical structures. The phase averaged surface pressure fluctuation is used to analyze the turbulence characteristics due to motion of cylinder. Moreover, the result of the oscillating square cylinder with inflow turbulence will be reported in the conference.

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Identifiers

ORCID for Zheng-Tong Xie: orcid.org/0000-0002-8119-7532

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