William N. Alexander

Noise from a Rotor Ingesting a Planar Turbulent Boundary Layer

This study concerns rotor noise generated by the ingestion of anisotropic, inhomogenous turbulence produced by a planar turbulent boundary layer. Far field noise was recorded at multiple receiving angles and blade wake profiles were measured for advance ratios ranging J=0.48 to 1.44. A novel method to infer the turbulence characteristics from the measured far field noise is presented. The time-frequency distribution of the recorded noise is used to estimate the streamwise scale of the turbulence while the time-averaged spectra are used to estimate the lateral scale. Results show that the estimated streamwise scale doubles over the measured range of advance ratios and the lateral scale remains approximately constant. This suggests a complex distortion of the turbulence due to the sheared flow and presence of the wall, a topic addressed in a companion paper (Glegg et al., 2013).

Boundary Layer Ingestion Noise and Turbulence Scale Analysis at High and Low Advance Ratios

Far field sound has been measured at several receiving angles from a rotor partially embedded in a boundary layer. The haystacking pattern, or spectral humps that appear at multiples of the blade passage frequency, was studied for a wide range of advance ratios. At high advance ratios, evidence of vortex shedding from the blade trailing edges was observed. For low advance ratios, the haystacks narrowed, became more symmetric and increased in number. A method of determining the average acoustic signature of an eddy passage through a rotor was developed from time delay aligning multiple microphone signals and eddy passages detected using a short time Fourier transform. This detection technique was expanded upon through the use of the continuous wavelet transform. It was found that the eddy passage signatures were similar to a cosine wave with a Gaussian window. It was also found that normalized timescales obtained directly from the eddy passage signatures remained constant with advance ratio. A method of determining lateral timescales from the autocorrelation function of the eddy passage signatures was also developed. It was found that normalized lateral scales also remained constant with advance ratio. For advance ratios less than 0.6, the eddy passage signatures were dominated by a tonal component.

Experimental and Theoretical Analysis of Bio-Inspired Trailing Edge Noise Control Devices

© American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.This work focuses on new experimental investigations into the function of bio-inspired trailing edge noise reduction treatments, and complementary theoretical analysis. Aeroacoustic wind tunnel tests of an instrumented DU96-W180 airfoil model at high (~2.5 million) Reynolds number reveal various effects of these treatments, termed “finlets.” Data collected include far-field noise, measured using a phased array microphone system, unsteady surface pressure near the trailing edge, lift and mean surface pressure distribution over the entire airfoil, and drag measured in its wake. Downstream effects of the finlets are investigated by positioning them at various distances upstream of the trailing edge. Off-design conditions are investigated by skewing the finlets to simulate the presence of spanwise flow. The results demonstrate the robustness of the present design and reveal additional insight into the effects of finlets on the structure of the boundary layer turbulence approaching the trailing edge. A simple mathematical model is introduced which demonstrates the noise-reducing capability of finlets through the displacement of noise-producing vortices. The influence of shape and position is investigated through the model, and results are qualitatively similar to the experimental findings.

Experimental Study of Quadcopter Acoustics and Performance at Static Thrust Conditions

A quadcopter, DJI Phantom II, was tested in the Virginia Tech Anechoic Chamber to study its aeroacoustics performance. Noise and thrust measured by a single microphone and a load cell were acquired for 4 different rotor configurations, two plastic and two carbon fiber rotors. To study the effects of multi-rotor interaction, the quadcopter was also set to operate with 1, 2 and 4 rotors. Results of 4-rotor operation show that tones at the blade passing frequency, shaft rate and their harmonics dominate the quadcopter acoustic spectrum up to 6000 Hz without much deterioration. Also significant is a broadband hump present in the mid frequency range which increases over 10 dB above the broadband level at low frequencies. Motor noise is also noticeable in the mid frequency range. For a smallscaled rotor, thrust performance is greatly influenced by rotor configuration whereas its acoustic signature is only altered near mid and high frequencies resulting in 1-2 dB change in the OASPL for the same thrust setting. Having 1, 2 or 4 rotors operating does not affect the acoustic signature but a significant increase was found in broadband noise when switching from 2 non-adjacent rotors to 4 rotors.

Sound Radiation from a Rotor Partially Immersed in a Turbulent Boundary Layer

This paper we will consider the sound radiation from a rotor that is partially immersed in a wall boundary layer. Experimental measurements for this configuration have been previously presented by Alexander et al. (2013) and the theoretical framework for the broadband noise prediction, based on a time domain approach, is described by Glegg et al(2014). In this paper we will show results comparing the measurements with predictions. It is found that at low thrust the prediction method works well, but it fails to predict the sound levels at BPF that were measured for the high thrust conditions. RANS calculations of the flow around the propeller clearly identify that this additional sound is caused by a blade vortex interaction that can occur when rotors are located near flat surfaces.

Sound and Distortion Produced by a Braking Rotor Operating in a Planar Boundary Layer with Application to Wind Turbines

Acoustic, mean wall pressure, and PIV measurements are presented for a 10-bladed 457mm diameter rotor operating in a planar boundary layer at a variety of braking conditions and yaw angles. Turbulence ingestion and laminar/transitional boundary layer vortex trailing edge noise are important components in the total sound spectrum, the latter appearing where significant portions of the rotor disk are at low angle of attack. Mean wall pressure and PIV measurements show that the flow is accelerated through the gap between the rotor and wall and distorted by the rotor’s blockage. Acceleration through the tip gap is associated with reduction in the streamwise normal Reynolds stress. Phase-averaged velocity fields show the influence of tip leakage vortices shed from the blades.

High Reynolds Number Turbulent Boundary Layer Flow over Rough Walls: Wall Pressure Spectrum and Noise

Experiments have been performed on series of high Reynolds number zero pressure gradient turbulent boundary layers formed over rough and smooth walls. The boundary layers were fully rough yet the elements remained a very small fraction (<1.4%) of the boundary layer thickness ensuring conditions free of transitional effects. The wall pressure spectrum and its scaling were studied in detail. The radiated roughness noise was compared with predictions made using diffraction theory. One of the major findings is that the rough wall turbulent pressure spectrum at vehicle relevant conditions is comprised of three scaling regions. These include a newly discovered high frequency region where the pressure spectrum has a viscous scaling controlled by the friction velocity, adjusted to exclude the pressure drag on the roughness elements.

Broadband Noise from a Rotor at an Angle to the Mean Flow

The broadband noise from a rotor operating in a turbulent boundary layer and at an angle to the mean flow is discussed in this paper. A time domain method for prediction of the far field noise is compared to measurements of rotor noise at different yaw angles over a range of speeds. It is shown that rotor yaw can significantly impact the far field sound levels and that the broadband rotor noise prediction method developed by Glegg et al (2015) can be readily adapted to apply to a rotor at an angle to the mean flow. The measurements show that the effect of rotor yaw is most significant at blade passing frequencies and its harmonics, increases with higher thrust, and is strongly dependent on observer angle. The prediction methodology has been found to be very accurate at frequencies away from blade passing frequency, but does not follow the observed trends with changes of yaw for the blade passing harmonics. It is believed that this is the result of the rotor interacting with a secondary flow vortex system that is known to set up close to the wall of the wind tunnel, and can cause blade vortex interaction noise.

Noise from Boundary Layer Flow over Fabric Covered Perforate Panels

Aeroacoustic wind tunnels often employ fabric coverings to reduce instrumentation selfnoise or to improve the background noise of a facility by containing the flow. To investigate the impact of these fabric coverings, acoustic spectra were measured from open and fabric covered 3.2 mm diameter perforate panels. Noise from the perforate panels was reduced at lower frequencies by the fabric coverings, but additional high frequency noise was introduced. The acoustic spectra from the fabric covered surfaces had two maxima which scaled differently with velocity. The low frequency maximum was attributed to the perforate geometry and scaled with the sixth power of velocity. The high frequency maximum is speculated to be a result of the fabric covering and scales with the eleventh power of velocity. The addition of the coverings also affected the streamwise directivity of the noise. The open perforate geometry radiated noise symmetrically upstream and downstream about a minimum produced directly above the center of the perforate sample. The fabric covered surfaces produced a different streamwise spectral directivity pattern which was projected upstream and decreased monotonically with downstream receiving angle. The noise produced by a fabric surface with a solid backing was reduced by more than 20dB compared to spectra with an open perforate backing indicating that the noise produced by the fabric may be a complex function of porosity, elasticity, and/or a coupling with the underlying geometry.

Aeroacoustics of 2D and 3D Surface Discontinuities

Far field sound and pressure fluctuation measurements have been performed in a low Mach number wall-jet boundary layer flow over a swept forward facing step with sweep angles of 0 , 5 , 15 and 30 . For each sweep angle the effects of rounding of the external step corner were examined for corner radii to step height ratios of 0%, 12.5%, 25% and 50%. When scaled on the step-normal velocity, the sound produced by sharp and rounded forward steps appears almost sweep independent, at least for sweep angles up to 30 and rounding radii up to 25% of the step height. This behavior is obscured for observer locations fixed with respect to the approach flow, perhaps because of the unequal effects of the finite span and spanwise non-uniformity at different sweep angles. Surface pressure fluctuations immediately downstream of the step show approximate sweep independence limited to lower frequencies for rounding radii up to 12.5% of the step height.