Controlling the Jet of Overexpanded Nozzle Using Coanda Effect

Abstract

The supersonic nozzle of rockets becomes overexpanded during takeoff i.e., sea level operation, because such nozzles are designed for specific higher altitudes conditions. The overexpansion of nozzle results into inevitable flow separation inside the diverging section via separation shock. For a range of Nozzle Pressure Ratio (NPR), the separated flow becomes asymmetric, thereby resulting into a free shock separation (FSS) and restricted shock separation (RSS) regions along the top and bottom walls of diverging section of planar nozzles. Such asymmetric separated flow results in a thrust vector which is offset at certain angle from its symmetric axis, as well as generate side loads onto the walls of nozzle which may cause structural damages. This research study is thus dedicated to control such asymmetric separated jet flows using the Coanda effect. a novel approach is presented by studying the influence of Coanda effect on asymmetric shock separation of overexpanded nozzles by inclining the sonic plane at the throat of supersonic nozzle. This is achieved by displacing the top or bottom wall of planar nozzle (depending upon the NPR and configuration of asymmetric separation) so as to create a geometric offset at the start of diverging section of nozzle. This research study is performed numerically in ANSYS Fluent ® 19.0 environment using transient SST k-ꙍ turbulence model. Due to the Coanda effect, the asymmetry switches its configuration, thereby ascertaining the idea of controlling the jet. The sonic plane inclination angle is also varied to study its influence and it is revealed that for each NPR there exists a limiting degree of inclination beyond which the control of asymmetric jet is lost accompanied by reduction in thrust.