A study of the application of air-jet vortex generators to intake ducts
Many modem combat aircraft have S-bend intake ducts supplying air to the engine compressor. At high Mach number and/or extreme manoeuvre conditions such ducts can produce excessive flow distortions at the engine face due to flow separating at the first bend of the duct. It has been proposed that vortex generators may be usefully employed in such intake ducts to enhance the homogeneity of the pressure distribution at the engine face, and hence, reduce the unsteady loading on the engine. Both vane and air jets have been tested experimentally as vortex generators and each has been found to reduce the flow distortion that would otherwise reach the engine face. The objective of this thesis was to construct a local numerical model reflecting the physical geometry and conditions of the fully turbulent flow field in the proximity of a vortex generator array. The location of the array is approximately at the first bend downstream in an S-bend intake duct. In this project, five different model geometries were tested. Two were used for model verification and the remaining three for investigation of the local flow structure in the vicinity of the vortex generators within the duct. Two of the local duct models neglected any curvature effects (referred to as flat plate models). The third duct model, referred to as a sector model took into account the circular nature of the duct's cross section. The flow is assumed to be incompressible and fully turbulent and was solved using the Finite Volume, Navier-Stokes Code CFX 4 (CFDS, AEA Technology, Harwell) on a non-orthogonal, body-fitted, grid using the k-c turbulence model and standard wall functions. The behaviour of the longitudinal vortices produced by the vanes and airjets is presented in terms of circulation and peak vorticity decay, peak vorticity paths in cross-stream and streamwise direction, cross-stream vorticity profiles, cross-stream shear stress distribution and streamwise and cross-stream velocity profiles. Negligible difference in results was observed for the flat plate models with and without the jet inlet tube; neither did we see significant differences between the flat plate model and the sector model, since the airjet momentum was not drastically altered. Comparing the predicted results provided by vanes and air jets reflected major differences in vortex circulation between the two but the enhancement in transverse skin friction was of similar magnitude. Experiments also showed that both types of vortex generator provided like enhancement of the flow field. The optimum pitch and skew angle configuration for the air jets in terms of maximum enhancement of the flow field was predicted with 30° pitch and 75° skew angle.