Active drag reduction for a D-shaped cylinder flow using Coanda pulsation jets

A D-shaped cylinder can be considered as one of the typical models for bluff bodies; flow separation from the trailing edges and near-wake flow structures are internally linked with aerodynamic forces acting on the D-shaped cylinder. Based on Coanda pulsation jets and Genetic Algorithms (GAs), this work is focused on the active control of the D-shaped cylinder flow for drag reduction. Wind tunnel experiments are conducted at a Reynolds number Re = 1.8 × 10⁴, which is based on the incoming freestream velocity and height of the D-shaped cylinder. Being placed on the upper and lower sides of the cylinder base, the Coanda pulsation jets are composed of 1/4-parts of a circular cylinder (radius is 0.2H) and horizontal slot jets. Control parameters include the driving pressure of the jet, pulsation frequency, duty cycle, and the phase shift of the lower and upper jets. The time-averaged base pressure of the D-shaped cylinder, which is connected with the drag force, is chosen to be the objective function of GAs. Results from this work indicate that GAs are robust to identify the optimum control parameters (i.e., driving pressure of the jet is 1.94 times atmospheric pressure, non-dimensional pulsation frequency is 0.27, duty cycle is 37% and phase shift is 136°), resulting in a recovery of the base pressure up to 61% (corresponding to a drag reduction up to 23%), associated with a high efficiency of 45%; meanwhile, it is observed that large-scale near-wake structures of the D-shaped cylinder are impaired, with altered shedding frequency and phase difference, by the Coanda pulsation jets.