Aeroelastic performance of a vertical lift rotor coated with sharkskin-inspired micropillars in laminar and transitional flow

A stationary vertical lift rotor was coated with a sharkskin-inspired micropillar coating on the suction side of the blades and tested under normal operating speeds, Ω between 400 and 1400 revolutions per minute (rpm), and collective pitch angles, θc, of 2°, 5°, and 8°. The novel micropillars contain a diverging tip with increasing surface area and contain hydrophobic characteristics. The micropillars are applied as a thin-film flexible coating where the pillar heights h were 70 and 140 μm tall. The two-bladed carbon fiber rotor contains a diameter of 1.58 m with a symmetric 10% thick airfoil and zero twist distribution. When operating at the test speeds, the rotor produces Reynolds numbers based on local freestream velocity and chord in the range of 4.02 × 104 ≤ Re∞ ≤ 4.22 × 105 between three radial points, r/R = 0.32, 0.64, and 0.96. The near-wall flow and blade bending displacement were measured using laser Doppler velocimetry and laser triangulation techniques. Rotor thrust and power were measured using an electronic load cell and a high-capacity power supply. The main contributions of the work are the findings to explain why micropillars have a distinct role when operating in laminar vs transitional flow. This “fine line” appears to be somewhere when the local Reynolds number on the blades is approximately 1.88 × 105. Below this Reynolds number, attained on the blade tip region at a speed of Ω ≈ 600 rpm, micropillars are not desired as they generally reduce the rotor thrust coefficient and figure of merit (FM), while slightly increasing the unsteady bending vibration of the blade as measured by the root mean square of blade acceleration, arms. Alternatively, at higher rotor speeds in a transitional flow regime, the thrust coefficient, CT and FM increase by up to 5.7% and 14.9%, respectively. The mean bending displacement, y/R of the blade increases due to higher lift production, while unsteady vibration decreases significantly at the blade tip. This is directly correlated with the micropillar's ability to mitigate unsteady boundary layer separation via modulated small-scale turbulence which produces streamwise momentum.