The impact of inlet flow conditions on the aerodynamic performance of an NACA submerged intake for ground vehicle applications

Abstract

Results are presented following a series of experimental measurements on a submerged NACA-type intake oriented between −30° and +30° yaw to the free stream in an atmospheric boundary layer wind tunnel at a unit Reynolds number of nominally 1 × 106. The intake was subjected to a range of upstream wall boundary layer conditions, and the mass flow into the intake (as measured by an orifice plate) was monitored to assess the aerodynamic performance. The mass flow data are supported by qualitative flow visualisation within the duct, using a smoke filament illuminated in a laser light sheet in order to gain insight into the flow physics. The intake performance, expressed in terms of a non-dimensional flow momentum coefficient, is seen to degrade both with increasing intake orientation to the free stream (changes of nominally 40% are seen for the angle range tested) and with increasing upstream boundary layer displacement thickness (changes of nominally 30% are seen for the range tested). These data are presented as a graphical carpet plot; it is intended that this is used as a guide to performance prediction in non-aeronautical applications where there are often significant changes in both the local flow direction and the boundary layer thickness. Flow visualisation studies show that the degradation in the intake performance with increasing yaw angle can be attributed to a progressive change in the vortex-pair structure within the intake as the local flow angle is increased. Both an increase in the lateral separation and an increase in the size of the respective vortex cores are considered to act so as to reduce the magnitude of the induced inflow into the intake.