Instability of Rotating-Cone Boundary Layer in Axial Inflow: Effect of Cone Angle

Boundary-layer instability on a rotating cone induces coherent spiral vortices that are linked to the onset of laminar-turbulent transition. This type of transition is relevant to several aerospace systems with rotating components, e.g., aeroengine nose cones. Because a variety of options exist for the nose-cone shapes, it is important to know how their shape affects the boundary-layer transition phenomena. This study investigates the effect of varying cone angle on the boundary-layer instability on rotating cones facing axial inflow. It is found that increasing cone angle has a stabilizing effect on the boundary layer over rotating cones in axial inflow. The parameter space of Reynolds number Re-l and local rotational speed ratio S is experimentally explored to find the spiral vortex growth on rotating cones of half angle ? = 22.5 degrees, 45 degrees, and 50 degrees. The previously addressed cases of ? = 15 degrees and 30 degrees are also revisited. Increasing half-cone angle is found to have a stabilizing effect on the boundary layer on the rotating cones with ? ? 45 degrees; i.e., the spiral vortex growth is delayed to higher Re-l and S. This effect diminishes when the half-cone angle increases from ? = 45 degrees to 50 degrees. The spiral vortex angle ? decreases with increasing rotational speed ratio S for all the investigated cones, irrespective of the half-cone angle. However, the instability on the broader cones is found to induce shorter azimuthal wavelengths.