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Vehicle CFD Team at VTEC
Anders Tenstam, Volvo Technology AB
Vehicle CFDSTAR European Conference March 22-23 2010
Automotive Fan Noise Modelling using
STAR-CCM+
Anders Tenstam
Volvo Technology AB
Vehicle CFD Team at VTEC
Anders Tenstam, Volvo Technology AB
Vehicle CFDSTAR European Conference March 22-23 2010
Volvo Technology – a business unit within the Volvo Group
AB VolvoBusiness Areas
Business Units
Volvo 3P
Volvo Powertrain
Volvo Parts
Volvo Information Technology
Volvo Technology
Volvo Logistics
Renault Trucks Mack TrucksVolvo Trucks Nissan Diesel Buses Volvo Penta Volvo AeroFinancial
Services
Construction
Equipment
• Volvo Technology is the center for innovation, research and developmentin the Volvo Group
• The customer base is limited, focusing on the Volvo Group, Volvo Cars, selected suppliers and public bodies
• Secures hard & soft product & process innovation for superior end customer solutions
• Established 1969
• Locations: Gothenburg, Lyon, Greensboro, Chesapeake, Hagerstown, Los Angeles
• ~500 employees
Vehicle CFD Team at VTEC
Anders Tenstam, Volvo Technology AB
Vehicle CFDSTAR European Conference March 22-23 2010
Vehicle CFD Team - Main Focus :
Support Volvo Group with Methods
Development & Resources in:
- Aerodynamics
- UTM (Heat management & Fan operation)
- Aeroacoustics
- Climate / HVAC
- Internal flows
- Fuel Cell Technology
- In-cylinder model development
TrucksConstruction Equipment
Buses
Vehicle CFD Team at VTEC
Anders Tenstam, Volvo Technology AB
Vehicle CFDSTAR European Conference March 22-23 2010
Understanding Fan Noise – the Motive:
• Fan noise is one of the main contributors to noise pollution from
heavy-duty vehicles.
• The overall noise level from a heavy-duty vehicle is an entity
controlled by legislation.
• Given a certain discharge flow, the link between hydraulic efficiency
for a fan and emitted noise is quite strong. Hence a quiet fan has
also normally low losses.
• A large truck fan at high load can consume 30-35kW of power from
the crank shaft, a substantial portion of the mechanical power from
the engine.
Vehicle CFD Team at VTEC
Anders Tenstam, Volvo Technology AB
Vehicle CFDSTAR European Conference March 22-23 2010
The Project - Goal:
• To predict and understand noise sources using CFD simulations.
• To investigate the propagation of noise sources to far-field; directivity.
• Increase the in-house understanding of fan noise, i.e. what is important
in low noise emission fan design?
The Project - Facts:
• The project was sponsored by the Volvo Group Key Technology
Comittee and performed during 2008-2009
• The numerical part of the project was performed by staff from Volvo
Technology and Volvo Aero Corporation.
• Masurements were made by the NVH lab at Volvo 3P.
Vehicle CFD Team at VTEC
Anders Tenstam, Volvo Technology AB
Vehicle CFDSTAR European Conference March 22-23 2010
The Project - Scope:
• To predict the major noise sources in two fan installations
of an Articulated hauler: Low-range frequencies from LES
High-range broad-band signatures from URANS
• To predict propagated noise using acoustics analogies
• Propose ways to further reduce noise in the installations
A: In-house developed AH fan at VCE B. Conventional heavy-duty fan
Vehicle CFD Team at VTEC
Anders Tenstam, Volvo Technology AB
Vehicle CFDSTAR European Conference March 22-23 2010
The Project – Scope contd.
• The computational time frame for each geometry must not exceed 1
week. This way, the developed method may allow for fan noise
calculations to be fit into product development chain.
• Perform LES simulations of each fan, and extract noise sources and
frequency spectrum up to threshold given by maximum model size.
• Propagate sources to far-field by a FWH (Ffowcs-Williams Hawkins)
integration routine.
• Perform URANS simulations to allow for additional broad-band
character noise prediction
• Analyse and Compare with measured data
Vehicle CFD Team at VTEC
Anders Tenstam, Volvo Technology AB
Vehicle CFDSTAR European Conference March 22-23 2010
The Computational Model
Model facts:
Region #Cells
Fan Region 800 000
Outer Region 1 900 000
Radiator 100 000
TOTAL: 2 800 000
Vehicle CFD Team at VTEC
Anders Tenstam, Volvo Technology AB
Vehicle CFDSTAR European Conference March 22-23 2010
Results – URANS Simulation (Broadband Sources):
Volume Noise Sources (Proudman): Logarithmised values (Isosurface
corresponding to 100 dB):
[W/m3]
Where
AP = Acoustic power [W/m3]
ε = constant
ρ0 = density [kg/m3]
a0 = speed of sound [m/s]
U = Turbulent velocity scale
L = Turbulent length scale
This equation is implemented in STAR-CCM+ 4.06.011
Similar regions are depicted by adopting the Curle
Expression for surface sources
5
0
53
0a
U
L
UAP c
refP
APdBAP log10
Derived from k, ε
Suction side (upstream) Pressure side (downstream)
Vehicle CFD Team at VTEC
Anders Tenstam, Volvo Technology AB
Vehicle CFDSTAR European Conference March 22-23 2010
Results – Broadband Sources contd.:
• Broadband sources are strong in the tip region, especially for fan B. This
leads to blade-to-blade interaction, which is a strong noise contributor.
• For the less noisy fan (A), additional sources exist at mid-radius due to partial
separation (improvement potential).
• Tip leakage leads also to performance degradation (improvement potential).
Vehicle CFD Team at VTEC
Anders Tenstam, Volvo Technology AB
Vehicle CFDSTAR European Conference March 22-23 2010
Results – LES Simulation (Narrow Band Sources):
• Simulations run for 2-3 revolutions for quasi-stationary behaviour
• Pressure data is collected for 2 revolutions
• For each surface cell, a discrete Fourier Transform is performed, and
data is mapped back to the model surface directly or summed up by
frequency bands (3rd octave or octave) using ProAm+shell scripts
Maximum theoretically resolved frequency (Nyqvist Frequency):
fmax = 1/(2*t) where t = sampling time (s)
Maximum attainable frequency is however limited by the grid spacing (Mesh cut-
off Frequency). Derived from URANS simulations as a relation between turbulent
velocity scales and length scales
Minimum resolved frequency:
(and also the frequency resolution to be captured by the Discrete Fourier Transform) is
the inverse of the interval length T
fmin = f =1/(N*t) = 1 / T where N = number of samples
Near blade surface: max. 2 kHz
Blade wakes: max. 1 kHz
Vehicle CFD Team at VTEC
Anders Tenstam, Volvo Technology AB
Vehicle CFDSTAR European Conference March 22-23 2010
Results – LES Simulation (Narrow Band Sources):
RMS values of pressure series (logarithmised):
•Distinctly lower sources
are present for the less
noisy fan (A)
•Sources are mostly
concentrated to area
near tips, these are
linked to the tip vortex.
• Stator (shroud)
interaction levels are
stronger for fan B.
•Frequency
decomposition reveals
strongest contributions
around BPF.
Vehicle CFD Team at VTEC
Anders Tenstam, Volvo Technology AB
Vehicle CFDSTAR European Conference March 22-23 2010
Results – LES Simulation (Narrow Band Sources) – Moved Fan:
RMS values of pressure series (logarithmised):
When the A type fan
is moved axially, a
substantial increase
in surface pressure
fluctuations is
encountered!
Fan A in Nominal position Fan A moved partially out of shroud
Vehicle CFD Team at VTEC
Anders Tenstam, Volvo Technology AB
Vehicle CFDSTAR European Conference March 22-23 2010
Fan A
Fan A moved
Fan B
Fan A
Fan A moved
Fan B
Fan A
Fan A moved
Fan B
Directivity (power spectrum), BPF marked with arrow
Beside fan:
High levels
at
frequencies
above BPF
Fan A
Fan A moved
Fan B
On axis (+):
Fan A: BPF
dominates.
Fan B:
0.5*BPF
dominates
on pressure
side
(fan/shroud
interaction)!
Downstream Upstream
Results – LES Simulation (Propagation - FWH):Time series in a listener
location (on axis)
Fan B
Fan A
Fan A,
moved
Vehicle CFD Team at VTEC
Anders Tenstam, Volvo Technology AB
Vehicle CFDSTAR European Conference March 22-23 2010
Results – Summation over whole observer sphere, A-weighted, third
octave band filtered. Experimental results from echo chamber:
Fan A moved (PWL)
Fan B (PWL)
Fan A (PWL)
Fan A Measured (SPL)
Fan B Measured (SPL)
+20
+10
0
-10
-20
-30
-40
-50
Conclusions:
•BPF is well resolved for all fans
•Broadband character above BPF is less well represented, the contribution
over ~800-1000 Hz is exaggerated (probably caused by the tip vortex and
blade-to-blade interaction. Overall, the results are promising.
Remarks:
•Different results definitions required for simulations (PWL) and
experiments (SPL).
•Fan A position was not specified to 100% in experiments.
•Due to Sliding mesh interpolation routine, simulations were slow. No
significant speedup detected above 4 CPU:s on single quadcore machine.
A significant speedup should be expected if this is resolved or the model
can then be made finer (problem could be parallelized more).
Vehicle CFD Team at VTEC
Anders Tenstam, Volvo Technology AB
Vehicle CFDSTAR European Conference March 22-23 2010
Summary
•LES Calculated noise level (PWL) fan A compared to fan B: -5dB
•URANS Calculated noise level (PWL) fan A compared to fan B: -6 dB
•Measured noise level (PWL) fan A compared to fan B: ~ -10dB
Results are very direction sensitive. If the noise level is evaluated on the
rotation axis, the simulation difference between the two fans is 6-9dB.
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