Three-dimensional numerical simulation of oscillatory flow around a circular cylinder at right and oblique attacks

Abstract

Sinusoidal oscillatory flow around a circular cylinder at right and oblique attacks is investigated by three-dimensional numerical simulation. Calculations are carried out for oblique angles of α=0°, 15°, 30°, 45° and 60°, Reynolds number of 2000 and KC numbers ranging from 6.75 to 30. The oblique angle of α=0° corresponds to the right attack case. The predicted vortex shedding regimes for α=0° agree well with those found in physical experiments. It is found from the numerical simulations that when KC number is in the middle range of a vortex shedding regime (i.e. KC=10, 13, 17.5 and 26.2), the flow is in single mode. In each single mode, the number of vortices shed in each flow period is constant and the sectional hydrodynamic force in the cross-flow direction fluctuates at a unique frequency. As the KC number is close to the boundary between two vortex shedding regimes ( KC=6.75, 15, 20 and 30), the number of vortices shed from the cylinder varies from period to period and the time series of the transverse force contains more than one predominant frequencies, implying the flow switching from one mode to another. This flow mode is referred to as multi-modes. The spanwise correlation factor obtained according to the sectional transverse force is close to 1 for single-mode flows. The correlation length of multi-mode flows depends on the KC number and is generally smaller than that of single-mode flows. Comparison between the numerical results of α=0° and those of α>0° shows that the independent principle is applicable for the calculated KC number range and the oblique angle ( α≤45°). For α=60°, the maximum lift coefficient C L , max and mode-averaged lift force have distinct difference from their counterparts for α=0°.