Spiral instability-modes on rotating cones in high-Reynolds number axial flow

Abstract

This work shows the behaviour of an unstable boundary-layer on rotating cones in high-speed flow conditions: high Reynolds number Rel > 106, low rotational speed ratio S < 1–1.5, and inflow Mach number M = 0.5. These conditions are most-commonly encountered on rotating aero-engine-nose-cones of transonic cruise aircraft. Although it has been addressed in several past studies, the boundary-layer instability on rotating cones remained to be explored in high-speed inflow regime. This work uses infrared-thermography with POD approach to detect instability-induced flow structures by measuring their thermal footprints on rotating cones in high-speed inflow. Observed surface temperature patterns show that the boundary-layer instability induces spiral modes on rotating cones, which closely resemble the thermal footprints of the spiral vortices observed in the past studies at low-speed flow conditions: Rel < 105, S > 1, and M ≈ 0. Three cones with half-cone angles y = 15◦, 30◦, and 40◦ are tested. For a given cone, the Reynolds number relating to the maximum amplification of the spiral vortices is found to follow an exponential relation with the rotational speed ratio S, extending from low- to high-speed regime. At a given rotational speed ratio S, the spiral vortex angle appears to be as expected from the low-speed studies, irrespective of the half-cone angle.