Turbulence and vorticity decay of propulsion jets produced by ducted fans coated with a sharkskin-inspired surface

Abstract

Turbulent jets are produced by many propulsion systems containing a ducted fan or rotor operating at high speed. In this experimental study, the blades of a 12 cm diameter ducted rotor system were coated with a sharkskin-inspired surface with diverging tip micropillars. Surfaces containing 40 μm and 70 μm tall micropillars were applied on the rotor blades in order to study their role on fan aerodynamics and downstream jet flow. The ducted rotor was operated at up to 30,000 revolutions per minute (rpm), creating a turbulent jet with a Reynolds number of 5.97x10⁵ and Mach number of 0.222 based on mean streamwise velocity. The inflow at the inlet of the rotor and the flow-field downstream was measured using high-speed laser Doppler velocimetry (LDV) techniques. The effect of the micropillar coatings on the rotor blades marginally increases the mean streamwise velocity and rotor figure of merit due to mitigating boundary layer separation at higher rotor speeds. Moreover, this occurs due to the micropillar's ability to increase wall-normal turbulence intensity in the boundary layer when the pillar height is scaled appropriately to the boundary layer thickness. The rotor hub and blade tip vortex structures become diffused and undergo breakup into smaller structures accompanied with an acceleration in the decay of absolute mean cross-stream vorticity. This quantity physically represents vortical structures with a lower magnitude of rotation. In a streamwise distance of 1.5 rotor diameters, the decay of mean cross-stream vorticity in the jet flow-field is 42.4% and 44.1% for the jets produced by the rotor blades coated with micropillars with h = 40 μm and 70 μm respectively. This is in comparison to the 38.1% cross-stream vorticity decay for the baseline jet generated by the ducted rotor coated with smooth kapton tape. However, it was found that the decay in turbulence intensity as well as turbulent kinetic energy is more localized near the inlet of the jet measurement domain, and becomes reorganized further downstream into circular-like contours along the jet centerline and path of the rotor hub vortex structures. The overall results indicate that shark-inspired surfaces are viable to enhance the operating efficiency of ducted fans for subsonic aircraft propulsion.