Aeroacoustic Evaluation of an Axial Fan using CFD Methods

Aeroacoustic Evaluation of an Axial Fan using CFD Methods

Frederik Folke, Martin Hildenbrand (ITB Ingenieure GmbH)

ITB_SGC_AeroacousticEvaluationOfAnAxialFanUsingCFDMethods_160803.pptx ITB GmbH, Folke, Hildenbrand 1

Introduction

ITB_SGC_AeroacousticEvaluationOfAnAxialFanUsingCFDMethods_160803.pptx

ITB Ingenieure GmbH – engineering since 1998

ITB GmbH, Folke, Hildenbrand 2

Digital Development

CFD (1D/3D)

FEM

Process Automatization /

Programming

Thermo-Mechanical

Fatigue

Introduction


ITB Ingenieure GmbH – engineering since 1998


Engine Development

www.itb-ingenieure.de

Production Processes

Exhaust After-Treatment Turbomachinery

Introduction


Aeroacoustic evaluation of an axial blower • Blower is used in urban environments

• Statutory limitations of noise pollution

Straightforward numerical approach for

aeroacoustic evaluation

Motivation

Aeroacoustic

results

CFD-Simulation

(aerodynamics) model adjustments

+ simulation time

Test case geometry


• ITB Ingenieure GmbH

• Motivation

• Aeroacoustic Basics

• Computational Model

• Results

• Summary and Conclusion

Contents


TEST CASE

INTRODUCTION

BASICS

AEROACOUSTIC BASICS What is "Aeroacoustic Noise"?

Numerical Simulation of Aeroacoustic Noise

Numerical Challenge in Time

Estimation of Discretization


Aeroacoustic Basics


Flow induced (fluctuating) pressure waves in audible frequency range.

• Small magnitudes compared to flow field

• Wide frequency range ( depends on the receiver, e.g. human)

What is "Aeroacoustic Noise"?


Sound sources • monopole-sources

• interaction of flow and solid structures (dipoles)

• turbulence noise sources (quadrupoles)

Sound propagation • propagation from sources to far field

• damping

• interaction with solid structures (reflection, adsorption)

Sound propagation

• Resolution of the propagating pressure wave

• Elimination of boundary effects

• Propagation to locations outside of the

computational field

• Mesh

• sufficient mesh size in far-field

resolving propagating pressure waves

• Physics

• time-step

• boundary treatment

(non-reflecting boundaries)

Sound sources

• resolution of the aerodynamical flow field

• resolution / modelling of all relevant

mechanisms in space and time

(e.g. „blade passing“, turbulence)

• Mesh

• boundary layer resolution

• local mesh size

• moving mesh

• Physics

• turbulence models (resolution)

• time-step


Aeroacoustic Basics

Numerical Simulation of Aeroacoustic Noise


Aeroacoustic Basics


Numerical Challenge in Time


Frequency

Sound P

ressure

Low frequencies • long-time events in time-domain

• physical time of event has to be captured

𝜏𝑡𝑜𝑡𝑎𝑙 ≫ 1

High frequencies • short time events (turbulence)

• time-step has to capture event

Δ𝑡 ≪ 1

𝜏𝑡𝑜𝑡𝑎𝑙 ≥ 𝑛 ⋅ Δ𝑡 Large number of time-steps necessary

TEST CASE Computational Domain

Physics and Solver



Test Case

Computational Domain


Freestream

boundary conditions

FW-H Surface

Pressure outlet

Rotor

Sound propagation

model

Stator

FW-H = Ffowcs Williams - Hawkings

Test Case


Preconditions

rotational speed 16'000 min-1

frequency range 50 … 3000 Hz

Physics and Solver


Numeric Setup

solver CD-adapco® STAR-CCM+®

turbulence k-w SST (uRANS)

time step size 1e-5 s

boundary conditions freestream / pressure outlet

total number of cells ~ 10 million

RESULTS Flow field

Sound pressure level


TURBULENCE (microscopic) MEAN FLOW (macroscopic)


Results

Flow Field (Aerodynamic)


Results


Aeroacoustics


Main sound sources in region of rotor and stator

Surface sound

Volume sound

Results


Sound Spectrum


BPF

2 x BPF

Receiver 2

(far away)

Receiver 1

(close)

Sim

ula

ted p

hysic

al tim

e

[dB

] x x

R1 R2

Broadband noise sources

insufficiently captured by uRANS!

SUMMARY AND CONCLUSION Summary and Conclusion

Outlook



• Successful aerodynamic simulation of axial fan

(very good agreement with measurement)

• Successful aeroacoustical simulation of an axial fan (academic test case)

based on a well-resolved transient uRANS simulation

• Ability to identify noise sources (dipoles, quadrupoles)

• Ability to resolve tonal noise (blade passing frequency)

• Ability to evaluate the sound pressure level

By an adequate numerical effort!

Summary & Conclusion


Conclusion

Summary


Special thanks to CD-adapco for support!

• Hildenbrand (2014), Aeroacoustic simulation of rotating parts, ITB GmbH / Uni Stuttgart

• Mir (2016), Aeroacoustic simulation of an axial fan, ITB GmbH / Uni Stuttgart (in progress)

• Ongoing study

• Improvement of boundary treatment

(non-reflecting boundaries, minimization of disturbing noise)

• Higher resolution of boundary layer (shear flow)

Turbulence

• Validation by experimental data (not measured yet)

Outlook


Outlook

ITB Innovative Technische Berechnungen GmbH

Deckerstr. 37

D-70372 Stuttgart

+49 711 88 266 53 - 0 | [email protected] | www.itb-ingenieure.de


APPENDIX


Noise Sources

ITB_SGC_AeroacousticEvaluationOfAnAxialFanUsingCFDMethods_160803.pptx 22

Monopole Radial radiation

Noise in jet stream

Cavitation bubbles

𝑝 𝑥 , 𝑡 =𝑓 t −

𝑥 𝑐

𝑥 𝑝 𝑥 , 𝑡 =

𝜕

𝜕𝑥1

𝑓 t −𝑥 𝑐

𝑥

𝑝 𝑥 , 𝑡 =𝜕2

𝜕𝑥1𝜕𝑥2

𝑓 t −𝑥 𝑐

𝑥 𝑝 𝑥 , 𝑡 =

𝜕2

𝜕𝑥1𝜕𝑥1

𝑓 t −𝑥 𝑐

𝑥

Dipole Directed radiation

Two contrary

monopoles

Surface noise

(separations)

Turbulence-Structure

interaction

Quadrupole Longitudinal / Transverse /

arbitrary

Free turbulence / shear flow


Classification of Noise Sources

Acoustic Analogy Physical Phenomenon Numerical Treatment

Monopoles Jet stream, cavitation

bubbles

Not relevant in this case

Dipoles Interaction of flow with solid

structures

• Blade Passing

• Separation

• Stagnation Points

Sufficient resolution of the

boundary layer

Flow-Structure Interaction

Quadrupoles Turbulence Noise

• Shear flow

Resolution of turbulent

structures.

Scale resolving methods

Time Discretization

STAR-CCM+ Best Practice

Approx. 1° rotation per time-step or at least 15

time-steps per blade passage

Spatial Discretization

STAR-CCM+ Best Practice

20 cells per acoustic wavelength are

recommended


Estimation of Discretization


Example: 𝜔 = 16000 1/min

Time step size = 1e-5 s

Example: 𝑓 = 3000 𝐻𝑧

Max. mesh size < 5.7 mm

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Aeroacoustic Evaluation of an Axial Fan using CFD Methods