Experimental investigation of the effectiveness of Coanda reaction surface geometries for Super-Critical Circulation-Control

The maximum effectiveness of fluidic Super Critical Circulation Control (SCCC) actuators is limited by separation of the actuating jet at high pressure ratios. An important design goal for SCCC is to maximize actuator effectiveness relative to geometric implementation cost, for which trailing edge radius can be taken as a proxy. For experimental work, actuator effectiveness can be meaningfully assessed under quiescent conditions when the jet total pressure is significantly higher than the intended operational freestream total pressure. A jet with a nozzle pressure ratio of up to 15 was issued from a convergent-only rectangular nozzle with an aspect ratio of 22.5 between two walls to produce a two-dimensional flow into quiescent air. Circular and elliptical reaction surface geometries were tested, with ratios of the nozzle height h to the radius or semi-minor axis of the geometries respectively of 0.025, 0.05, or 0.1, and ellipse eccentricity 1.5 or 2. Geometries were offset to create a base step of height h_s between the nozzle lip and the surface of h_s/h = 0 to 2. Schlieren photography and analysis of static pressures measured on the reaction surfaces was used to investigate the effectiveness of the different geometries with different base step heights. For circular geometries, detachment nozzle pressure ratio agrees with the literature. A scaling of elliptic geometries based on the semi-major axis was shown to provide a collapse onto the commonly used h/R scaling for circular geometries. Jet separation location generally moves towards the slot with increasing NPR however is not monotonic due to shock interactions and changing shock cell length. This behaviour warrants further investigation in co-flow experiments.