Numerical study on the effect of shape modification to the flow around circular cylinders

• LES is applied to study the 2- and 3-dimensional circular cylinders at Re=5000.
• The wavy cylinders could mitigate the aerodynamic forces.
• K–H instability frequency is different at the saddle and node for wavy cylinders.
• 2D cylinders have better correlated lift forces than the 3D ones.

Shape modification has been one of the effective ways of manipulating the flow past bluff bodies. Aiming at exploration of the flow control mechanisms of shape modified circular cylinders, a series of numerical simulations are conducted at the Reynolds number of 5000. The modifications adopted in the current paper could be divided into two categories, the 2-dimensional cross-sectional modifications (polygonal and ridged) and 3-dimensional span-wise modifications (linear wavy, sinusoidal wavy and O-ringed). By exploiting the numerical data obtained from the Large Eddy Simulations, several aspects, including the wake flow properties, the aerodynamic forces, the flow instabilities and span-wise correlations are compared in detail. The two wavy cylinders are found to modify the flow wake significantly, leading to considerable reduction in aerodynamic forces. The polygonal and ridged cylinders, however, are of no such effect in aerodynamic force mitigation. The O-rings only have slight effect on the flow wake and the forces. As for the flow instabilities, the modifications adopted in this paper barely affects the Kármán vortex shedding frequency. The shear layer frequency, however, differs from case to case. Particularly, in the cases of the two wavy cylinders, disparity exists in the shear layer frequency at the node and saddle. This is explained by the boundary layer properties at separation. While recognizing the insufficiency in the span-wise length of the cylinders, the span-wise correlations of the aerodynamic forces are inspected. It is found that the lift forces of the 3-dimensional cylinders are generally less correlated than that of the 2-dimensional ones.