numerical and experimental investigation of the flow ... · • croaker et al. 2016 derived a...
TRANSCRIPT
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Numerical and Experimental Investigation of the Flow-Induced Noise
of a Wall Mounted Airfoil
Paul Croaker, Danielle Moreau, Manuj Awasthi, Mahmoud Karimi, Con Doolan, Nicole Kessissoglou
School of Mechanical and Manufacturing Engineering UNSW Australia
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• Motivation for current work
• Previous work on wall mounted airfoil flow-induced noise
– Experimental measurements
– Hybrid RANS-BEM technique
• Current experimental and numerical investigation
– Experimental and numerical flow data
– Numerical prediction of flow-induced noise
• Conclusions
Overview
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• Physically interesting case study
– Trailing edge noise
– Tip vortex noise
– Turbulence – leading edge interaction noise
• Moreau et al. 2016 demonstrated that the tip vortex noise scales with M 7.5
– Low to moderate Reynolds numbers and low Mach number flows
– Expect scaling to be closer to M 6 (surface dipoles) or M 5 (sharp edge)
– Noise generating mechanism clearly complex and interesting
Wall Mounted Airfoil Study
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• Croaker et al. 2016 derived a hybrid RANS-BEM technique to predict flow-induced noise produced by bodies in flow
– Successfully predicted the sound generated by the wall mounted airfoil
– Lack of experimental flow data made it difficult to fully validate the model
• Devised an experimental and numerical study into the flow around and noise generated by a finite wall mounted airfoil
– Gain a deeper understanding into the tip noise mechanism
– Generate validation data for the RANS-BEM technique
Wall Mounted Airfoil Study
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• Reynolds Averaged Navier Stokes
– Computational aeroacoustic methods based on LES are time demanding
– LES techniques have a significant data storage and processing burden
– RANS based approaches are computationally efficient
RANS-BEM Motivation
LES
RANS
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• Boundary Element Method
– Scattering from simple geometries using analytical Green’s function
– Extended by multiple scattering concept to include finite aerofoils
– BEM required to consider complex geometries or thick section aerofoil or hydrofoils
Hybrid RANS-BEM – Motivation
S831 Profile
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• Reynolds Averaged Navier Stokes
– Only provides a statistical representation of turbulence
– Lacks knowledge of phase relationship between flow noise sources
• Boundary Element Method
– Requires the incident field on the body due to the flow noise sources
– Predicts the acoustic field scattered by the body
– Phase relationship between flow noise sources vital in accurate calculation of scattered field
Hybrid RANS-BEM – Challenges
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• Reynolds Averaged Navier Stokes
– Only provides a statistical representation of turbulence
– Lacks knowledge of phase relationship between flow noise sources
• Boundary Element Method
– Requires the incident field on the body due to the flow noise sources
– Predicts the acoustic field scattered by the body
– Phase relationship between flow noise sources vital in accurate calculation of scattered field
Hybrid RANS-BEM – Challenges
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• RANS-based Statistical Noise Sources
– Accounts for phase relationship between flow noise sources through two point space-time correlations
– The space-time correlations are derived from the RANS-based turbulence statistics
Phase Relationship from RANS
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RANS-BEM Solution Procedure
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RANS-BEM Solution Procedure
Source, b
Far-field point,
,
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RANS-BEM Solution Procedure
Far-field point,
,
Source, c
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RANS-BEM Solution Procedure
, , ∗ ,
, Φ , , , ,
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RANS-BEM Solution Procedure
ls, ωs, us
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RANS-BEM Solution Procedure
• Velocity cross-spectra approximated from RANS turbulence statistics
– Requires extensive validation and empirical constant tuning
– Experimental and high-fidelity numerical flow and noise data required to achieve this
• Scattered sound field calculated for each flow noise source separately
• Total far-field acoustic power spectral density obtained by combining individual source contributions based on velocity cross-spectra
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Finite Wall Mounted Foil
Moreau, DJ, Doolan, CJ, Alexander, WN, Meyers, TW and Devenport, WJ 2016, ‘Wall-mounted finite airfoil-noise production and prediction’, AIAA Journal, vol. 54, 5, pp. 1637–1651.
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Flow Structures
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Flow Structures - Tip
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Characteristic Scales - Tip
z/s = 0.95
z/s = 0.975
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Characteristic Scales - Tip
z/s = 0.9875
z/s = 1.0
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Characteristic Scales - Tip
z/s = 1.0125
z/s = 1.025
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Far-Field Acoustic Power Spectral Density
Moreau, DJ, Doolan, CJ, Alexander, WN, Meyers, TW and Devenport, WJ 2016, ‘Wall-mounted finite airfoil-noise production and prediction’, AIAA Journal, vol. 54, 5, pp. 1637–1651.
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Source Contribution Regions
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Trailing and Leading Edge PSD
Trailing edge Leading edge
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Junction and Tip PSD
Junction Tip
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Conclusions of RANS-BEM work
• Hybrid RANS-BEM technique to predict flow-induced noise from wall mounted airfoil
• Velocity cross-spectra approximated from RANS turbulence statistics
• BEM used to calculate the scattered sound field
• Good comparison of results with experiment
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Conclusions of RANS-BEM work
• Lack of available hydrodynamic validation data
• Need an extensive experimental and high-fidelity numerical dataset to
– Validate RANS-BEM technique
– Inform RANS-BEM model parameters
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Current Numerical and Experimental Investigations
• Finite wall-mounted airfoil studied
– NACA0012 section airfoil
– Chord of 0.2m
– Span of 0.2m
– Trailing edge thickness of 3mm
• Flow velocity of 20 m/s
– Reynolds number of 274,000
– Mach number of 0.06
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Experimental Investigation at UNSW
• Perform flow and noise calculations on finite wall-mounted airfoils in the UNSW Anechoic Wind Tunnel
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Experimental Investigation at UNSW
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Experimental Investigation at UNSW
• Single hot wire measurements taken in the near wake of the airfoil at various spanwise locations
– Mean flow and turbulence statistics
– Spectral content of fluctuating velocities
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Experimental Investigation at UNSW
• Acoustic measurements are currently not possible due to excessive background noise levels
– New fan and motor are being installed
– Outlet ductwork modifications including muffler are being installed
– Facility will be ready for acoustic measurements from June 2017
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High-fidelity Numerical Simulations
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High-fidelity Numerical Simulations
• Structured mesh with approximately 11 million cells
– y+ ~ 1
– Spanwise and streamwise resolution less refined
• Numerical discretisation schemes
– Second order backward differencing in time
– Second order central differencing in space, with 10% blending of second order upwind differencing
• Pressure and velocity coupling achieved using the PISO algorithm
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Flow Structures
Tip Vortex
Transition to Turbulence
Trailing Edge Turbulence
Horseshoe Vortex Rollup
Laminar Flow over Leading Edge
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Mean flow statistics at 50% span
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Velocity Autospectral Density at 50% span
y = 0 mm y = 2 mm
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Mean flow statistics at 75% span
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Velocity Autospectral Density at 75% span
y = 0 mm y = 2 mm
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Mean flow statistics at 97.5% span
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Velocity Autospectral Density at 97.5% span
y = 0 mm y = 2 mm
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High-fidelity flow noise prediction
• Flow-induced noise sources extracted from LES data based on Lighthill’s acoustic analogy
• Pressure wave propagation from flow noise sources to the airfoil predicted
– Uses a near-field propagation algorithm which regularises the singular integrals
• Boundary element model used to predict the scattered and total acoustic fields
– Acoustic pressure predicted 1m perpendicular to airfoil surface at a height equal to the span
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Boundary element model
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Far-field pressure
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Far-field pressure
Blunt trailing edge vortex shedding
Low frequency noise caused by laminar to turbulence transition
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Conclusions
• Experimental and numerical investigation of flow around a wall mounted airfoil
– Experimental results exhibit a slight asymmetry which suggests airfoil is not aligned perfectly with 0 degrees
– Numerical results show reasonable results directly behind the airfoil, however a more rapid drop-off is observed transversely through the wake
• High-fidelity prediction of the flow induced noise
– Remains to be validated with experimental measurements
• Grid refinement study underway to improve grid resolution in streamwise and spanwise directions