Experimental Investigation of Rotor Vortex Wake

An experimental study is performed on a three-bladed rotor model in a water tow tank. The rotor’s rotational velocity, the rotor plane angle of attack, and the carriage speed are all varied in order to simulate a wide range of rotorcraft operating states. The focus is on descent speeds and angles where the rotor is operating in or near vortex ring state. Circulation Reynolds numbers are of order 105 and chord Reynolds numbers are of order 104. Flow visualization is done using air bubbles and fluorescent dye injected tangentially from the blade tips to mark the vortex core, showing the development of both short-wave and long-wave instabilities on the helical vortices in the wake. Strain gages are used to record transient loads, allowing a correlation between the rotor thrust performance and the development of the vortex wake. The data indicate that as the instability develops, the adjacent vortices merge and form thick vortex rings, especially during descent. Periodic shedding of vorticity from the wake associated with vortex ring state is observed, resulting in peak-to-peak thrust fluctuations of up to 95 % of the mean and occurring at regular intervals of 20 − 50 rotor revolutions, depending on flow parameters. Nomenclature c = blade chord A = rotor disk area, πR2 CT = thrust coefficient, T/ρAV 2tip R = rotor radius Rec = Reynolds number based on chord, Vtipc/ν T = rotor thrust V = towing speed Vtip = rotor tip speed, ΩR Vx = rotor forward flight speed, V cosα Vz = rotor descent speed, V sinα α = descent angle λ = short-wave instability wavelength µ = rotor advance ratio, Vx/Vtip ν = kinematic viscosity ξ = rotor descent ratio, Vz/Vtip ρ = water density σ = standard deviation from mean rotor thrus