Flow characterization using a laser Doppler vibrometer

11/3/2005 1Managing Uncertainties in Noise Measurements and Prediction, Le Mans,

France, 27-29 June 2005

Faculty of EngineeringDepartment of Mechanical Engineering

ACOUSTICS & VIBRATION RESEARCH GROUPPleinlaan 2 • B1050 Brussel • Belgium

[email protected] • http://avrg.vub.ac.be

Flow Characterization using a Laser Doppler Vibrometer

J. Vanherzeele, S. Vanlanduit & P. GuillaumeD. Ragni & M. Martarelli

Università Politecnica delle Marchec/o Dipartimento di Meccanica

Via Brecce Bianche - 60131 Ancona -Italia

Flow in a cylinder wake

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Acoustics & Vibration Research Group

Vrije Universiteit Brussel

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Outline

• Introduction• Theoretical principal LDV measurements• Experiment and Simulation Outline

• LDV• PIV• CFD (short)

• Measurement results• Frequency content• Visual content

• Conclusions

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Introduction

Visualizing acoustic waves• High intensity: Schlieren, holography, laser-speckle….• Low intensity, high spatial resolution or small measuring volumes ?

Visualizing flow• Intrusive: tracers (PIV, LDA…), pitot, hot wire …

Visualizing stress• Schlieren, shadow moiré, photoelasticity…• Image correlation

All applications measurable with one simple technique: LDV

Visualizing flow

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Theoretical principal LDV measurements

• Classical use LDV:Laser light (freq ν) is scattered by moving object (vel v), undergoes frequency shift ∆ν only dependant on optical path laser z

• Full view equationMeasured displacement s depends also on refraction index medium n

λθν cos2v=∆

dtdztyxn

dttyxdnz

dtztyxnd

dttyxdstyxv ),,(),,(]),,([),,(),,( +===

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• Measuring density fluctuationsKeep target steady; measure only turbulent effects (1st term full view eq)

Measurement volume containing flow

Retroreflective steady object

Laser Doppler Vibrometer

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• Validation?

• LDV measurement = interferometric technique

⇒signal is line integral across optical path

⇒tomography necessary in case of 3D flows

• Provides only visual information and frequency content

• Visual comparison with average flow calculated from PIV & CFD

• Frequency content comparison with PIV, CFD & St-number

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Experiment and Simulation Outline

Experimental set-up LDV•2 cylinders: ∅ 0.014 m & ∅ 0.062 m

•Glass wind tunnel (cross section 30 x 30 cm)

•3 velocities (11.2 m/s, 21.2 m/s and 31 m/s)

•Bandwidth 2kHz; freq resolution 5 Hz)

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Particle Image Velocimetry (Basics)

Light sheet opticsmirror

Light sheet

Illuminatedparticles

Flow direction

Flow with Tracer particles1st light pulse

2nd light pulse

Image plane

camera

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Particle Image Velocimetry (Basics)

X1

X2

X3 X1’

X2’

X3’

(a) (b)

X1

X2

X3X1’

X2’

X1’=X1

X2’=X2(c) (d)

framen-1

Camera

Light source

framen

framen+1

framen+2

∆t∆tShutterclosed

Cross Correlation

Timing diagram

∆t

δ

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Experimental set-up Particle Image Velocimetry (PIV)•Nd: YLF pulse laser; pulse energy 2 x 10 mJ at 1kHz

•1024 x 1024 pixel camera; 1500 Hz frame rate

•∆t laser: 20 µs

•Particle size: 4 µm - 5 µm (smoke)

•Velocity: 11.2 m/s

•Cylinder ∅ 0.014 m

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Processing Images Particle Image Velocimetry (PIV)

• Initial interrogation window (2000 image pairs): 32 x 32 pixels; 50% overlap (resolving vorticity); triple pass

• Frequency content extraction:

• Store velocity data vt in 3D- matrix vtime

• Fourier transform this matrix along the t-axis: vFour

• Rescale the frequency vector

),(),,( YXvtYXv ttime =

),(),,( tvFFTfYXv timeFour =

20001500

vecvec ff =

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Experimental set-up Particle Image Velocimetry (PIV)

PIV image

Velocity & Vorticitydistribution

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Computational Fluid Dynamics (CFD) settings• Numeca software; Euranus flow solver• Grid: O-type consisting of 1 block; 128 cells in circumferential & radial

direction; stretched mesh towards wall (viscous sub-layer + wake)

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Computational Fluid Dynamics (CFD) settings– Model: unsteady turbulent RANS calculation; turbulence modeled by

Baldwin-Lomax (∅ 0.062 m ∀ v & ∅ 0.014 m at 31 m/s); turbulence modeled by Spalart-Allmaras (∅ 0.014 m at 11.2 m/s & 21.2 m/s)

– 2nd order dual time stepping with 4-stage Runge-Kutta; multigrid on inner stages; physical time step: 0.0001 s

– Frequency content extraction: 2 possibilities– Store velocity data in 3D matrix and perform Fourier transform

– Fourier transforming the lift coefficient (at least 10 vortex sheddings)

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Measurement Results

Frequency content

v (m/s) freq LDV (Hz) freq St (Hz) freq CFD (Hz) freq PIV (Hz)11.2 190 175 188 19421,2 360 325 354 /31 510 476 540 /

cylinder: 14 mm

v (m/s) freq LDV (Hz) freq St (Hz) freq CFD (Hz)11.2 42 39 4321,2 80 74 8031 115 108 112

cylinder: 62 mm

error cylinder: 14 mm (%) error cylinder: 62 mm (%)LDV-PIV 2,1 /LDV-CFD 3 1,6LDV-St 7,3 6,8

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Measurement Results

Visual content: ∅ 0.014 m at 11.2 m/s LDV

PIV CFD

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Conclusions

• A Laser Doppler Vibrometer was used to characterize a cylinder wake (different cylinders+different velocities)

• The technique was validated by means of PIV-experiments and CFD-simulations

• LDV measurements proved able to extract frequency content quite well and provide a visual representation of the flow

• Advantages:• No tracer particles necessary• Full field view of flow, completely automated, almost no set-up• Ability to measure large bandwidth (1 Hz-several MHz)• Measurement time ≈ PIV, but far faster than CFD

Flow characterization using a laser Doppler vibrometer

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