Aero-Vibro Acoustics For Wind Noise Application David Roche and Ashok Khondge ANSYS, Inc.
Dec 04, 2015
Outline
1. Wind Noise
2. Problem Description
3. Simulation Methodology
4. Results
5. Summary
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Aerodynamics Noise Generation
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A-Pillar Vortex
Flow separation/Vortex shading
Cavity Resonance Flow separation/ vortex shading
Flow separation/ Vortex shading
Cavity Resonance
Wind Noise
• High Frequency (> 500 Hz) Noise generated by head wind and perceived inside automotive cabin @ highway driving speeds
• It is cost beneficial to address wind noise “UPFRONT” in Design Process 1. Highest degree of freedom to change exterior design causing wind
noise 2. Avoids expensive late countermeasures
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[1]
Problem Description
• Demonstrate Aero-vibro-acoustics coupling to predict noise at the interior of the Hyundai simplified model [2]
• HSM Model – Simplified model released by Hyundai Kia Motors for 2013 KSNVE Open Benchmark[2]
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Sound Source Transient Separated flow at
A-pillar
Transfer Path [Side glass, windshield]
Receiver Ear of a driver
Simulation Methodology [1]
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A-pillar Mirror Turbulence
Mic.
Glass Interior Walls
Outer Walls (Rigid)
Connection between Vibrating walls and Rigid walls
External CFD Model – Transient Flow
Vibrating Surfaces (Side Glass, Windshield)
Acoustics Model (Car Interior)
Vibroacoustics Modeling
Inflow
CFD modeling
Simulation Methodology [2]
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………
Solve Transient
CFD
Time Freq. domain
transform
Mapping Freq. Domain
Pressure Loading
Solve “Vibro-Acoustics”
Model
Workflow Studied
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ANSYS Fluent Harmonic Strongly
Coupled Vibro-acoustic
ANSYS Fluent Harmonic Structural
Harmonic Acoustic
Pressure Mapping after
FFT
Pressure Mapping after
FFT
Velocity Mapping
Strong Vibroacoustic Coupling
One Way Vibroacoustic Coupling
Test Measurement Points
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650mm
1000mm
200mm
90mm
Interior Microphone Locations
Accelerometers mounted on LH Side
Glass
Transient CFD Modeling
• CFD Domain consists of External HSM surfaces, side glasses and windshield & wind tunnel boundaries
• Configuration Studied : HSM – 0 deg. Yaw – Nozzle Inlet : Velocity Inlet : 130 kmph [Profile] – Tunnel Outlet : Pressure Outlet (Gauge Pressure = 0 Pa) – Tunnel Inlet : Pressure Inlet (Gauge Tot. Pressure = 0 Pa) – Tunnel Top, floor, side, wall BC (No-Slip)
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CFD Domain
Boundary Conditions Inlet Vel. Profile
Mesh Details
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• Total Cell Count = 45 Million • Prims Cell Count = 23 Million • No of Prism Layers = 12 • First Prism Layer Height = 0.05 mm • Surface Mesh Size : A-pillar 1.5 to 2.0 mm,
Side glasses, windshield = 2.0 to 3.0 mm
Solver Settings : Transient CFD Simulation
1. Solver : ANSYS Fluent, Pressure Based, Double Precision, Transient , Gradient – Green Gauss Node Based
2. Transient Formulation : 2nd Order Implicit, Time Step Size = 2e-5 s 3. Material - Air as Ideal Gas 4. Turbulence Model Steady State : SST K-Omega 5. Turbulence Model Transient : DDES – SST K-Omega 6. Pressure Velocity Coupling – SIMPLEC 7. Spatial Discretization
• Pressure : Second Order, Density : Second Order Upwind • Momentum : Bounded Central Differencing • TKE & Specific Dissipation Rate : Second Order Upwind • Energy : Second Order Upwind
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Solution Procedure
1. Run Steady State Simulation using SST K-Omega Turbulence Model
2. Steady State Simulation Solver Settings – Pressure Based Coupled Solver
3. Switch to Transient Simulation, Use Second Order Temporal Discretization
4. Switch to DDES SST K-Omega Turbulence Model
5. Run initial transient simulation to achieve dynamic steady state
6. Run Final transient simulation [time step size 2e-5, no. of iterations per time step = 8]
7. Export the ASD Data on Wind-shield, side-glass surfaces at every time step
8. Perform Surface Acoustics FFT to transform source data into Frequency Domain
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Transient Flow Field
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Velocity Contours at Z = 0.5 m Pressure Contours at Z = 0.5 m
Iso-surface of Q-Criterion colored by velocity magnitude
Source Data : Transformation Time Frequency Domain
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1
2 3
4
5
Acoustic Source FFT is a Beta Feature in R 16.0
1. Set Modes (Real /Img.)
2. Octave (SPL)
3. 1/3rd Octave (SPL)
4. Constant Band (SPL)
Surface dB Map
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1/3rd Octave 100 Hz 1/3rd Octave 1000 Hz
1/3rd Octave 500 Hz 1/3rd Octave 1600 Hz
Acoustics Pressure Loading in Freq. Domain
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Freq. 455 Hz Freq. 455 Hz
Freq. 1575 Hz Freq. 1575 Hz
Acoustics Source Mapping
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Mapping for 100 Hz ANSYS Fluent ANSYS Mechanical
Mapping for 1000 Hz
Vibroacoustics Modeling Structural Material Properties
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PROPERTIES GLASS AL 6061 HEAVY LAYER
Thickness (mm) 4 12 1
Density (kg/m3) 2500 2700 2000
Young’s Modulus (GPa) 70 69 0.04
Poisson's ratio 0.22 0.33 0.45
PROPERTIES Air Foam
Mass Density (kg/m3) 1.2 1.2
Sound Speed (m/s) 343 343
Fluid Resistivity (N s /m4) 6.83E+16
Porosity 0.879
Tortuosity 3.31
Viscous Length (m) 9.483e-10
Thermal Length (m) 1.2174e-10
Vibroacoustics Modeling
Strong Coupling: • Full Vibroacoustics harmonic analysis
from 50 to 1000 Hz (117 frequencies) Weak Coupling: • Full structural harmonic analysis from
50 to 1000 Hz (117 frequencies) • Full acoustic harmonic analysis from
50 to 1000 Hz (117 frequencies)
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Results : Acceleration vs Frequency
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Results : SPL(dB) vs 1/3rd Octave Freq. @ Interior Microphone
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Summary
1. Aero and Vibroacoustics coupling is demonstrated using transient CFD and Vibroacoustics modeling
2. ANSYS Fluent solver is used for transient aeroacoustics simulation 3. A Vibroacoustics simulation is done using ANSYS Mechanical using two
approaches – Strong Vibroacoustics coupling approach – Weak Vibroacoustics coupling approach
4. Simulation Results are fairly in good agreement with Test Data 5. Differences between strong and weak Vibroacoustics coupling are
observed – It doesn’t seem possible to neglected the effects of the acoustics cavity on the
deformation of the structure
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