Experimental Investigation of an External Aerodynamic Diffuser

Improvement in the thrust/power ratio for shrouded propellers and fan-in-wing units, which are widely used for V/STOL aircraft propulsion, can be achieved by using an external aerodynamic diffuser during hover. Large diffuser divergence angles are required which necessitate a technique of controlling the flow separation from the internal diffuser wall. The purpose of this experimental investigation was to obtain a correlation between experimental and analytical prediction of performance improvement in order to generate a design technique for optimization of external diffusion, and investigate the potential of several techniques which offer a method of forcing the flow to diffuse externally. A static test facility for measuring the external flow field and the thrust of the model was constructed. All tests were performed with uniform, axial flow at the diffuser inlet. Two-dimensional tests were conducted employing either turning vanes, coanda effect, or rotating cylinder walls. Axisymmetric tests were performed using boundary layer control by suction or blowing. (Modified author abstract)