Effect of variable pitch on flow around a helically twisted elliptic cylinder

We propose a new geometric disturbance called a variable pitch helically twisted elliptical (VPHTE) cylinder that was obtained by combining helically twisted elliptic (HTE) and asymmetric geometries for passive flow control. We confirmed that the new geometric disturbance reduces the drag and lift fluctuations compared with the smooth and HTE cylinders at the Reynolds number (Re) of 3000. Large eddy simulation was used for the evaluation of the flow control performance of the VPHTE cylinder at Re = 3000, which corresponds to the subcritical regime. A parametric study was conducted on the variable pitch ratio (VP). The smallest value of the time-averaged drag coefficient of the VPHTE cylinder is lower by about 16.7% and 4.1% than those of the smooth and HTE cylinders, respectively. Furthermore, a VPHTE cylinder reduces the root-mean-square value of the lift coefficient by 96.2% compared with a smooth cylinder. The vortex formation length related to the stabilization of the shear layer of the VPHTE cylinders is longer than those of the smooth and HTE cylinders, and it is consistently elongated as the VP increases. In addition, time-averaged pressure coefficients for different cylinders were compared to analyze the reason for the drag reduction. We systematically investigated the additional flow control performance of the VPHTE cylinders and VP effects by analyzing physical values such as the swirl strength, turbulent kinetic energy, and vorticity components around the VPHTE cylinders.