Effect of Forced Boundary Layer Transition on Hovering Rotor Aerodynamics and Aeroacoustics

This study experimentally examines the effect of forced boundary layer (BL) transition on the aerodynamic and aero-acoustic performance of a low Reynolds number rotor in hover. An APC 15×4E two-bladed rotor was tested in three configurations: clean, upper-surface trip (U.S.T.), and combined upper- and lower-surface trip (U.S.T./L.S.T.). Surface oil flow visualization was used to characterize the BL structure. A hover test rig was used to measure the static thrust and torque. Acoustic measurements were conducted in an anechoic chamber, with tonal and broadband noise components separated during post-processing. Results show that surface trips effectively force BL transition, increasing turbulent attachment over the blade. Tripped configurations reduced thrust and increased torque but mitigated Reynolds-number sensitivity. Forced transition reduced the tonal noise for all but one case. For the broadband noise, the forced transition increased the noise in the frequency range where turbulent boundary layer-trailing edge (TBLTE) mechanisms dominate, while decreasing the noise in the frequency range where laminar boundary layer vortex shedding (LBL-VS) occurs.