Automotive Fan Noise Modelling using STAR-CCM+mdx2.plm.automation.siemens.com/sites/default/files/Presentation/... · Automotive Fan Noise Modelling using STAR-CCM+ Anders ... •

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




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 - 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




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.




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.




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




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




The Computational Model

Model facts:

Region #Cells

Fan Region 800 000

Outer Region 1 900 000

Radiator 100 000

TOTAL: 2 800 000




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)




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).




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




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.




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




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




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).




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.




Thank you for listening!

Noisy fans…

Documents

Automotive Fan Noise Modelling using STAR-CCM+mdx2.plm.automation.siemens.com/sites/default/files/Presentation/... · Automotive Fan Noise Modelling using STAR-CCM+ Anders ... •