The far-field noise and forces of a subscale two-blade rotor were measured in an anechoic wind tunnel for both hover and edgewise flight conditions. The mean thrust and torque coefficients increased with the edgewise advance ratio in relation to the hover coefficients. The tonal and broadband noise contributions were separated and analyzed in both the time and frequency domains. The sound pressure level (SPL) at the blade pass frequency (BPF) had the greatest contribution to the overall SPL, and the dominant broadband noise source changed from high-frequency trailing edge noise to midfrequency blade wake interaction noise with an increasing edgewise advance ratio. The tip Mach number scaling of the noise sources was examined and found to be highly dependent on the oncoming freestream velocity. The BPF SPL directivity showed a dependency on the advance ratio, with a peak magnitude below and upstream of the rotor’s retreating side. The broadband noise had a dipole directivity with a minimum in the rotor plane. The midfrequency broadband noise had signatures indicative of blade wake interaction noise, and the high-frequency noise had signatures indicative of amplitude modulated trailing edge noise. The results demonstrate the importance of unsteady loading noise and broadband noise for subscale urban air mobility rotors.

A common design of many of the proposed generation of electric Vertical Takeoff and Landing (eVTOL) vehicles involves fixed pitch rotors in edgewise flight. These applications typically have significantly lower tip Mach numbers than conventional rotor-craft and involve distributed smaller rotors making them operate at lower Reynolds numbers and hence likely having a greater sensitivity to gusts and environmental turbulence. This work will present results of studies with fixed pitch rotors conducted in the UF Anechoic Wind Tunnel Facility. The studies involve radiated noise, thrust and stereoscopic Particle Image Velocimetry to measure the inflow velocity to the rotor plane. The measurements were taken for un-altered free stream conditions as well as ones subjected to three different turbulence generation grids. The results provided show that there can be effects at moderate levels of free stream to turbulence to the rotor efficiency and radiated noise.

An experimental investigation of the noise and forces generated by tandem eVTOL rotors in simulated forward flight was performed in the University of Florida Anechoic Wind Tunnel. The campaign consisted of 48 test conditions sweeping through the tip Mach numbers and freestream velocity to cover a range of realistic eVTOL flight conditions for two rotors. The mean rotor forces and noise at 90 microphone locations below the rotor plane were measured for each test condition. The results showed that the noise and forces of the rear rotor were effected by the wake of the front rotor. Additionally, the inclusion of a wing and boom reduced overall noise levels and increased the rear rotor thrust coefficient. The results emphasize the importance of rotor-rotor and rotor-airframe interactions on eVTOL noise.

The induced velocity field of three different sub-scale eVTOL rotors were measured using a newly implemented stereoscopic particle image velocimetry (sPIV) system in the UF Anechoic Wind Tunnel Facility. The measurements were performed in the test section of the open jet wind tunnel for hover and edgewise flight conditions with advance ratios ranging from 0.065 to 0.268. The ensemble averaged sPIV measurements showed that an increase in advance ratio leads to a skewing of the maximum induced velocity towards the advancing side of the rotor disk and an increased area of upwash velocity at the front of the rotor disk. The results were further compared to the induced velocity field modeled with Peters-He Generalized Dynamic wake model. The modeled induced velocity was shown to be in poor agreement to the measured velocity field pointing to the limitations of the cylindrical wake assumption for a fixed pitch rotor in edgewise flight. The measured velocity field was utilized in both blade element theory and Sears unsteady airfoil theory to estimate the unsteady rotor blade forces. These forces were then utilized in an analytical noise model to predict unsteady loading noise and the results showed good agreement to measured sound pressure levels.

Electric vertical take-off and landing (eVTOL) aircraft are characterized by their unconventional wing and electric rotor configurations, which involve both side-by-side and tandem rotor configurations. These configurations create unique aerodynamic and acoustic flow-fields. We numerically investigate the interaction effects between rotor pairs as well as their individual and combined acoustic radiation. We examine horizontal and vertical spacing, rotor tilt angles, and forward flight effects. Performance is characterized by thrust coefficient, blade passage frequency (BPF) sound pressure level (SPL), and overall sound pressure level (OASPL). This study is performed with a mid-fidelity aerodynamic solver, Dust, which is used to predict the aerodynamic flow-field. The tonal acoustic pressure at observer positions is predicted via the Farassat F-1A solution of the Ffowcs Williams and Hawkings equation utilizing the aerodynamic flow-field. The configurations studied show strong aerodynamic interaction effects in thrust, as well as out-of-plane acoustic radiation from the aft rotor. Base predictions of thrust and noise are validated via experimental measurement. As rotor separation decreases, we observe that aft rotor thrust decreases and BPF SPL increases. The most forward rotor, however, is marginally impacted by the interactions. [This research is supported by Archer Aviation Inc.]

The acoustics and forces of a sub-scale 2-blade rotor were measured in an anechoic wind tunnel for both hover and edgewise flight conditions. The edgewise flight conditions included four tip Mach numbers, three freestream velocities, and two disk incidence angles representing realistic flight conditions for an Urban Air Mobility (UAM) vehicle. Forces were measured with a 6-axis load cell and far-field noise was measured with 1/4 in free-field microphones at 175 locations around the rotor. The tonal and broadband noise contributions were separated and analyzed in both the frequency and time domains. The mean thrust coefficient and torque coefficient were found to be proportional to the advance ratio squared. The sound pressure level (SPL) at the blade pass frequency (BPF) and the overall SPL (OASPL) increased for both increasing tip Mach number and increasing freestream velocity. The directivity of the noise displayed a dependency on advance ratio with a peak SPL below the retreating side. This mimics the unsteady loading noise directivity for a low tip speed rotor in edgewise flight. The broadband noise had a dipole directivity with a minimum in the rotor plane. The low/mid frequency broadband noise below30 times the BPF had time signatures indicative of blade wake interaction noise, while the high frequency noise above 30 times the BPF had time signatures and directivities indicative of trailing edge noise. These results demonstrate the importance of unsteady noise sources for an isolated UAM rotor in edgewise flight.

The noise from large-scale coherent turbulent structures within jets remains the dominant source. For the purpose of developing future control systems for the large-scale noise source, we investigate the statistics between upstream and downstream radiating waves. We investigate two off-design supersonic jet flows with instability theory and associated noise radiation, large-eddy simulation (LES), and experiments. We compare the auto-correlation, cross-correlation, coherence, and other statistics predicted by aeroacoustic instability theory. As instability waves are closely connected with the formation of large-scale turbulent structures, they yield insight into large-scale noise statistics. We investigate two nozzles at two supersonic off-design conditions. The first is a biconic nozzle operating at an unheated condition, and the second is a NASA nozzle operating at a heated condition. We find that for these jets, the noise from instability waves is coherent between 0.40 to 0.70 at large-scale radiation frequencies between the downstream and upstream radiation directions.