Recent Developments in Understanding the Sound Radia?on from Rotors Opera?ng Near Rigid Surfaces

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1 Recent Developments in Understanding the Sound Radia?on from Rotors Opera?ng Near Rigid Surfaces Stewart Glegg Florida Atlan,c University, Boca Raton FL William Devenport and W. Nathan Alexander, Virginia Tech, Blacksburg, VA Jus,n Grant (FAU, now NSWC), Armand Buono(FAU, now NSWC), Henry Murray(VT, now Lockheed Mar,n) Presented at FLINOVIA Conference, State College, PA April 2017 Work supported by the Office of Naval Research, under grant N and N

2 Sevik's Rotor Experiment In 1974 Sevik published the results of an experiment that measured and predicted the sound from a rotor operanng in grid turbulence. This experiment has been repeated many Nmes (e.g. Paterson and Amiet, Morris) for homogeneous or radially symmetric flows. Uniform Flow

3 Sevik's Rotor Noise Model Inflow Turbulence Measurement Turbulence Model (Von Karman) Blade Loading in Frequency Domain Assumes a Uniform TKE over the rotor Far Field Noise Spectrum

4 Effect of Loading on Sound RadiaNon haystacking Sevik PredicNon V Mean Flow Distor/on of the inflow turbulence occurs when the rotor is loaded causing haystacking (Hanson, Amiet, Mar/nez, Majumdar and Peake,Morris...)

5 Rotor OperaNng Near a Wall How is the noise affected by a rotor operanng in an inhomogeneous flow near a wall? Experimental/AnalyNcal Study

6 Virginia Tech AeroacousNc Wind Tunnel m Large test secnon 80m/s maximum flow speed AcousNc Test-secNon Flow Direction m Lexan Wall Inflow Microphones Kevlar Acoustic Window m Phased Array

7 Turbulent Boundary Layer IngesNon Acous/c Window Fairing and BL trip 7.3-m long test wall Rotor loca/on Inflow Boundary Layer 101 mm at rotor 11,500 at 20 m/s 16,600 and 30 m/s Complete 4D space-nme correlanon funcnon of undisturbed boundary layer known from measurement 2.25 scale Sevik (1971) rotor 10 blades, no lean, skew, sweep 457 mm diameter, 127 mm hub Blade pitch 55.6 (root), 21.2 (Np) 20mm Np gap

8 Rotor Noise Modeling for Inhomogeneous Flows Inflow Turbulence Measurement Non-Uniform TKE Blade Loading in the Time Domain Far Field Noise Spectrum

9 Time Domain FormulaNon Surface loading correla/on, (includes blade to blade) S pp (x,ω ) = 1 B 4πT n=1 B m=1 R max R min R max R min T _T T _T R ( n, m ) FF ( R, R, τ, τ ) n (n) i (R,τ )e iωr( n ) (τ )/c o n (m) j (R', τ )e iωr( m) (τ ')/c o e iω (τ τ ') drd R dτ dτ ' x i 4πr (n) (τ ) x i 4πr (m) (τ ') B = blade count Propaga/on expressed as a func/on of /me and integrated numerically, can include a hard wall reflec/on 9

10 The Blade Loading Cross CorrelaNon in the Time Domain R FF Measured or calculated turbulence correla/on func/on Velocity correla/on on path of each blade R (n,m) FF (R, R,τ, τ ) = τ τ s(r,τ τ )s( R, τ τ )R (n,m) (R, R,τ o o ww o, τ o )dτ o d τ o Time domain blade response func/on (Sears or Amiet)

11 Boundary Layer Detailed study was carried out by Morton 2012 The boundary layer thickness was found to be 101mm The study also analyzed the turbulent structures within the boundary layer This informanon was crincal part of later studies focusing on the noise radiated from the rotor A pair of hot wire probes were used to obtain complete 4D R ij (y,y',dz,dt)

12 Predicted and Measured Spectra 28º Zero Thrust J= rpm Max Thrust J= rpm BPF

13 Photo courtesy of Dr. Nathan Alexander Reverse Flow Observed at High Thrust

14 Reverse Flow at High Thrust 4/24/17 FLORIDA ATLANTIC UNIVERSITY 12

15 RESULTS CFD AERODYNAMIC RESULTS REVERSE FLOW VISUALIZATIONS J= mm 4/24/17 FAU 15

16 The Arch and Splaler VorNces Arch vortex Strong Blade Vortex InteracNon caused by splaler vortex (MarNo et al 2011) 16

17 PIV Measurements of Mean velocity field under the rotor J=1.44 J=1.05 J=0.58

18 Ω (rad/sec) Instantaneous Velocity and VorNcity J=0.58

19 Analyzing Instantaneous Structures J=0.58 Q value

20 Probability density of vortex posinon J=0.58

21 Modeling a Blade Vortex InteracNon for a Propeller Hull Vortex Fixed Vortex Γ Moving Blade Fixed Blade +Γ Moving Vortex

22 Howe's Theory of BVI contours of constant potennal F 2 Moving Vortex

23 Predicted and Measured Spectra at High Thrust BVI Predic?ons 70 28º db Hz -1 (re 20?Pa) Frequency (Hz) Max Thrust J= rpm Vortex Strength obtained from average of 5000 PIV measurements

24 Conclusions A rotor operanng near a wall has been shown to have a reverse flow at high thrust We have shown that this reverse flow can be characterized by an unstable vortex system Using PIV measurements to esnmate the strength of the vornces and their posinon the far field sound at the high thrust condinons has been well predicted using a simple BVI model