Inlet ground vortex aerodynamics under headwind conditions

Abstract

Quantitative velocity flow field measurements of the vortex system for a small scale model intake have been taken using a three component PIV system along with fan face total pressure distortion measurements. The effect of velocity ratio, non-dimensional height and approaching boundary layer thickness have been assessed under quiescent and headwind conditions. A range of inlet vortex flow modes are identified which depend on the intake velocity ratio and are characterised by the primary vorticity source. As the headwind velocity increases, and the velocity ratio reduces, the vortex strength initially increases until a local maximum is reached. Further reductions in the velocity ratio leads to a strong reduction in the vortex strength until a ‘blow-away’ condition is achieved at which the vortex no longer forms. A clear link is established between the external vortical flow-field and the intake internal flow-field. Consequently, these characteristics are also observed for the total pressure distortion coefficient within the intake. As the ground clearance is increased the peak vortex strength reduces and the corresponding velocity ratio increases. At intermediate velocity ratios, the vortex strength is greater at higher ground clearances. At the selected external vortex measurement plane, the vortex strength is generally insensitive to the approaching boundary layer thickness. The intake total pressure distortion increases with boundary layer thickness due to increased total pressure loss for a given ingested mass-flow. The vortex strength and distortion distributions are shown to be self-similar when non-dimensionalised by the peak values and associated velocity ratio. This dataset establishes a quantitative map for headwind ground vortices which presents a new formation criterion as well as a measure of the vortex strength as a function of ground clearance and intake velocity ratio.