Effects of steady wake-jets on subcritical cylinder flow

We investigate the effects of active wake-jets (characterized by a dimensionless jet momentum coefficient C-mu) on the suppression of aerodynamic forces and the manipulation of wake flow topology behind a cylindrical model through wind tunnel tests. The active jets are positioned at the rear stagnation points of the cylindrical test model. The experimental campaign is conducted at a subcritical Reynolds number of Re = 3.33 x 10(4). The surface pressure distributions around the bare and controlled cylinders are obtained by using a pressure measurement system. Apart from pressure measurements, we also obtain the streamwise and spanwise flow structures around the circular cylinder with different C-mu (including C-mu = 0) by employing the particle image velocimetry (PIV) technique. Pressure measurement results demonstrate that the lift force acting on the cylindrical test model is greatly reduced and drag decreased with the implementation of active wake-jets. Besides, it is found that a higher C-mu contributes to a better control effectiveness in unsteady lift forces but not necessarily a better drag reduction. PIV measurement results indicate that the mechanism of the active jet control scheme is to impose steady and symmetric perturbations into the unsteady and asymmetric flows in the cylinder wake. Owing to the dynamic competition of the wake-jet flow and shear layer flows, the vortex shedding pattern behind the controlled cylinder is greatly modified, vortex formation length significantly elongated and the fluctuations of aerodynamic forces conceivably suppressed.