Diagnosing Interior Noise due to Exterior Flows in STAR-CCM+ Phil Shorter, CD-adapco
Overview • Problem of interest • Analysis process • Modeling direct field acoustic radiation from a panel • Direct fields for individual modes • Direct fields due to random vibration induced by flow • Modeling the random reverberant response in a cavity • Summary
Overview • Problem of interest • Analysis process • Modeling direct field acoustic radiation from a panel • Direct fields for individual modes • Direct fields due to random vibration induced by flow • Modeling the random reverberant response in a cavity • Summary
Typical sources for interior noise Airborne : 300 Hz to 10 kHz Structure-borne : 0 to 800 Hz Wind-noise : 50 to 10 kHz Antennas/Racks Rear turbulence Wipers Mirror/Greenhouse Front End HVAC Underbody Many sources of interior noise are caused by exterior flow
Simplified example Cross-Section AA Exterior flow 40 m/s Panel A A Interior acoustic cavity Experimental setup Sound package foam/fiber M. Smith et al “Validation tests for flow induced excitation and noise radiation from a car window ”, Proc. 33rd AIAA Aeroacoustics Conference.
Overview • Problem of interest • Analysis process • Modeling direct field acoustic radiation from a panel • Direct fields for individual modes • Direct fields due to random vibration induced by flow • Modeling the random reverberant response in a cavity • Summary
Analysis process 1. Model flow 2. Model direct field 3. Model reverberant field Sff Unsteady transient analysis Vibro-Acoustic model used performed to predict direct field in STAR-CCM+ radiation into acoustic space Vibro-Acoustic model used when random fluctuating to predict reverberant Pressure time history data pressure applied across response in cavity for a exported across surface of exterior surface of panel given input power in the panel direct field
Step # 1 : modeling the flow 1. Model flow 2. Model direct field 3. Model reverberant field Sff Unsteady transient analysis Vibro-Acoustic model used performed to predict direct field in STAR-CCM+ radiation into acoustic space Vibro-Acoustic model used when random fluctuating to predict reverberant For details see: pressure applied across response in cavity for a exterior surface of panel given input power in the M . Smith et al “Validation tests for flow induced excitation and noise radiation from a car window”, direct field Proc. 33rd AIAA Aeroacoustics Conference 2012. P. Bremner , “ Vibroacoustic Source Mechanisms under Aeroacoustic Loads” Proc. 33rd AIAA Aeroacoustics Conference 2012.
Overview • Problem of interest • Analysis process • Modeling direct field acoustic radiation from a panel • Direct fields for individual modes • Direct fields due to random vibration induced by flow • Modeling the random reverberant response in a cavity • Summary
Step # 2 : modeling panel direct field 1. Model flow 2. Model direct field 3. Model reverberant field Sff Unsteady transient analysis Vibro-Acoustic model used performed to predict direct field in STAR-CCM+ radiation into acoustic space Vibro-Acoustic model used when random fluctuating to predict reverberant Pressure time history data pressure applied across response in cavity for a exported across surface of exterior surface of panel given input power in the panel direct field
Modeling a glass panel • Choice of Vibro-Acoustic method (FE, BEM, SEA etc.) depends on wavelengths of interest and size of system • Small glass panel (0.4 x 0.2 x 5e-3 m) has approx. 45 modes below 10 kHz • This example therefore uses a deterministic (analytical) representation of the panel and its modes Mode 1 : ~380 Hz Mode 40: ~8.9 kHz Mode 27 : ~3.9 kHz Frequency (Hz)
“Modal” direct fields at 1 kHz Normalize mode shape to have maximum velocity of 0.1 mm/s, look at radiated sound at 1kHz when mode shape radiates in a baffle (in this example, acoustic radiation calculated using boundary integral) Abs(Re{P}) dB re:2e-5 Mode 1 : ~380 Hz Mode 40: ~8.9 kHz Mode 27 : ~3.9 kHz Different modes have very different “radiation efficiencies” (below coincidence)
“Modal” direct fields 1 kHz 5 kHz 500 Hz Mode 1 : ~380 Hz Mode 27 : ~3.9 kHz Mode 40: ~8.9 kHz Directivity of radiated field from a given mode shape changes with frequency
Overview • Problem of interest • Analysis process • Modeling direct field acoustic radiation from a panel • Direct fields for individual modes • Direct fields due to random vibration induced by flow • Modeling the random reverberant response in a cavity • Summary
Random modal forces due to flow (S ff ) Fluctuating surface pressure from STAR-CCM+ can be transformed to the frequency domain and averaged over overlapping segments to give modal cross-spectral force matrices : S ff (f) Re{S ff (1kHz)} FSP(x,y,t) F (t) (x,y) <S ff (f)>
Random modal responses In a random vibration analysis, the cross-spectral modal response (Sqq) is related to the cross-spectral modal forces (Sff) by Modal dynamic stiffness matrix
Direct field response (<S pp >) Total direct field pressure response within the cavity can be found from S qq and the modal direct fields calculated previously. S pp (dB re:4e-10 Pa 2 /Hz) (40dB dynamic range) (60dB dynamic range) (80dB dynamic range) 500 Hz 1 kHz 5 kHz Direct field radiation from flow induced random vibration
Look at spatial variation along a line A2 A1 (40dB dynamic range) (60dB dynamic range) (80dB dynamic range) 500 Hz 1 kHz 5 kHz
Direct field pressure at 500 Hz Typical drivers ear location A1 A2 Evanescent (‘sloshing’) in near -field, free-field propagation outside near field
Overview • Problem of interest • Analysis process • Modeling direct field acoustic radiation from a panel • Direct fields for individual modes • Direct fields due to random vibration induced by flow • Modeling the random reverberant response in a cavity • Summary
Step # 3 : modeling reverberant field 1. Model flow 2. Model direct field 3. Model reverberant field Sff Unsteady transient analysis Vibro-Acoustic model used performed to predict direct field in STAR-CCM+ radiation into acoustic space Vibro-Acoustic model used when random fluctuating to predict reverberant Pressure time history data pressure applied across response in cavity for a exported across surface of exterior surface of panel given input power in the panel direct field
SEA model Reverberant response within the cavity involves short wavelength system level response (function of sound package distribution within the cavity). FE/BEM typically frequency limited for many applications and so Statistical Energy Analysis (SEA) commonly used. P in P coupling E (reverberant energy) P dissipated SEA is based on a set of power balance equations for the reverberant field (expressions developed for power input, power dissipation and power transmitted to adjacent subsystems)
Direct and reverberant fields at 500 Hz Total field Direct field Reverberant field Decreasing absorption Increasing absorption (Ensemble average) response in the reverberant field is spatially uniform. Levels depend on sound package within vehicle. For typical sound package configurations the direct and reverberant field contributions can often both be important at the drivers ear location.
Overview • Problem of interest • Analysis process • Modeling direct field acoustic radiation from a panel • Direct fields for individual modes • Direct fields due to random vibration induced by flow • Modeling the random reverberant response in a cavity • Summary
Summary • Prediction of interior noise due to exterior flows of significant interest in many applications (“Aero -Vibro- Acoustics”) • Aero-Vibro-Acoustic analysis requires STAR-CCM+ model of exterior fluctuating surface pressures and vibro-acoustic models of interior noise • Simple numerical example presented in this paper (glass panel in wall of wind tunnel radiating into an acoustic cavity) • Vibro-Acoustic analysis performed: • Hybrid (deterministic+SEA) modeling approach used • Direct fields calculated for individual modes • Direct fields calculated for flow induced random vibration • Reverberant field (and total cavity response) calculated using SEA • Example highlights that both direct and reverberant field contributions may be important and may therefore need to be included in vibro- acoustic analysis of interior windnoise
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