Performance of active and passive cylinder in controlling base flows

Abstract

Flow control for drag reduction results in substantial fuel savings, thus contributing to low-cost greener industrial processes. A major problem at transonic Mach numbers is the base drag. This study focuses on the control of base drag by controlling the base pressure. Till date, there are either passive or active control methods for all flow regimes resulting in ineffective control. The present work is an attempt to control the base pressure by passive as well as active means. The Mach numbers considered are for the subsonic, sonic, transonic, and supersonic regimes. Experiments were conducted for the nozzle pressure ratio (NPR) in the range from 2 to 10. The geometric parameters considered were the L/W ratio and area ratios. The base pressure (Pb) and the flow development along the duct wall was measured. Flow visualization was performed for all the cases of the present study. To assess the influence of the control mechanism on base pressure as well as the flow development in the enlarged duct a stationary or rotating cylinder of 2 mm diameter located at various positions from separation to the reattachment point inside the recirculation zone was employed. The investigation on base flows indicates that the base pressure is dependent on the length-to-width ratio, the level of expansion, Mach number, and the location as well as the orientation of the static and rotating cylinder as the control mechanism in the recirculation zone. For subsonic, sonic, transonic and low supersonic Mach numbers, the active, as well as the passive controls, increase the base pressure, thus decreasing the base drag but at higher supersonic flow say Mach 2, the control results in a decrease of base pressure for most of the cases of the present study. L/W = 4 seems to be the minimum length needed for the flow to remain attached with the enlarged duct. The results of different Mach number regimes have been validated with the published data from the National Aeronautics and Space Administration (NASA) and Sandia National Laboratories wind tunnels. While working in sudden expansion, it is mandatory to ensure that the control mechanism does not disturb the main flow field in the duct and it did remain same for with and without control cases. The results reported in this thesis are in an uncertainty band of ± 2.3%.