Combined Particle Image Velocimetry and acoustic measurements on a rod-airfoil configuration

59 Pages·2008·11.28 MB·English

UNCLASSIFIED Nationaal Lucht- en Ruimtevaartlaboratorium
National Aerospace Laboratory NLR
Executive summary

Combined Particle Image Velocimetry and acoustic
measurements on a rod-airfoil configuration
Pilot test of the FLOVIST project

Left:Experimental setup
Right: PIV result at one time instant

Problem area leading edge. Combined Particle
Report no.
This report documents the results of Image Velocimetry (PIV) and
NLR-TR-2008-377
a pilot test, part of the FLOVIST acoustic measurements were

(Flow Visualization Inspired Aero- performed and the results are
Author(s)
acoustics with Time-resolved presented and discussed. V. Lorenzoni
Tomographic Particle Image M. Tuinstra
Velocimetry) project, conducted by Results and conclusions F. Scarano
National Aerospace Laboratories A data set, containing both acoustic
(NLR) in collaboration with Delft and PIV measurements, was Report classification
University of Technology (TU acquired for a rod-airfoil UNCLASSIFIED

Delft). The aim of the project is to configuration at three different free-
Date
predict the sound emission based on stream velocities: 10, 15 and 20
September 2008
flow measurements. m/s.

Knowledge area(s)
Description of work Applicability
Aeroacoustic & Experimental
The purpose of the experimental The PIV results will be used to Aerodynamics
campaign is to obtain the unsteady predict the noise emission based
flow field around an airfoil solely on velocity measurements. Descriptor(s)
downstream of a circular section As the sound emission was Rod-Airfoil
rod, generating a Karman vortex measured by microphone as well, Acoustic Analogy
PIV
wake impinging onto the airfoil the predictions can be validated.

UNCLASSIFIED
UNCLASSIFIED Combined Particle Image Velocimetry and acoustic measurements on a
rod-airfoil configuration
Pilot test of the FLOVIST project
Nationaal Lucht- en Ruimtevaartlaboratorium, National Aerospace Laboratory NLR

Anthony Fokkerweg 2, 1059 CM Amsterdam,

P.O. Box 90502, 1006 BM Amsterdam, The Netherlands
UNCLASSIFIED Telephone [phone], Fax [phone], Web site: www.nlr.nl
Nationaal Lucht- en Ruimtevaartlaboratorium
National Aerospace Laboratory NLR

NLR-TR-2008-377

Combined Particle Image Velocimetry and acoustic
measurements on a rod-airfoil configuration
Pilot test of the FLOVIST project
V. Lorenzoni1, M. Tuinstra and F. Scarano1

1 TU Delft

No part of this report may be reproduced and/or disclosed in any form or by any means without the prior
written permission of NLR (and contributing partners).
Customer National Aerospace Laboratory NLR
Contract number 6083205
Owner National Aerospace Laboratory NLR and TU Delft
Division NLR Aerospace Vehicles
Distribution Limited
Classification of title Unclassified
September 2008
Approved by:
Author Reviewer Managing department

NLR-TR-2008-377
Summary
This report documents the results of a pilot test, part of the FLOVIST (Flow Visualization
Inspired Aero-acoustics with Time-resolved Tomographic Particle Image Velocimetry) project,
conducted by National Aerospace Laboratories (NLR) in collaboration with Delft University of
Technology (TU Delft). The purpose of the experimental campaign is to obtain the unsteady
flow field around an airfoil downstream of a circular section rod, generating a Karman vortex
wake impinging onto the airfoil leading edge. Combined PIV and acoustic measurements were
performed and the results are presented and discussed. In a later stadium, the time resolved fluid
dynamics results will be used to predict the sound emission by means of aero acoustic
analogies. The predicted sound field will then be compared with the results of the acoustic
measurements.

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NLR-TR-2008-377
Contents
1 Introduction 5
2 Experimental apparatus 6
2.1 Experimental facility 6
2.2 Acoustic measuring system setup 7
2.3 Flow measuring system setup 8
3 Results and discussion 11
3.1 Acoustic measurements 11
3.1.1 Far-field check 11
3.1.2 Background noise 12
3.1.3 Airfoil alone noise 15
3.1.4 Rod alone noise 19
3.1.5 Rod-Airfoil configuration 20
3.2 PIV measurements 25
3.2.1 Large field of view: overall flow survey 25
3.2.2 View around the airfoil by two cameras 27
3.2.2.1 RUN 32: Velocity = 15 m/s, cylinder = 6 mm 28
3.2.2.2 RUN 34: Velocity=10 m/s, cylinder = 10 mm 34
3.2.3 Span wise visualization 35
3.3 Trailing Edge Zoom 38
3.4 Trailing Edge Span 41
4 Conclusions 44
References 45
Appendix A Test matrix acoustic measurements 47
Appendix B Test matrix PIV measurements 49
Appendix C Reference test plan 51
Appendix D PIV fields of view 53
Appendix E Microphone setup and location 55

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NLR-TR-2008-377
1 Introduction
Noise attenuation and suppression is becoming an increasingly important matter, as noise
regulation for public environments as well as for industrial applications is becoming more
demanding. Noise emission can be reduced once the sound sources are known and the noise
generating mechanisms are understood. Aero acoustics studies the noise generation mechanisms
by flows. The sound sources can be determined by means of an acoustic analogy. An analogy is
a reformulation of basic conservation equations of fluid dynamics, with the wave equation on
the left hand side of the equation and an acoustic source term, build up out of flow parameters,
on the right hand side of the equation. Commonly, the source terms are computed by means of
CFD techniques. However, in the present study it is attempted to construct the sound source
terms by experimentally obtained time-resolved velocity data for a rod-airfoil configuration.

For Reynolds numbers ranging from approximately 50 to 200000 the flow behind cylinders is
structured as a sequence of counter–rotating vortices known as the Von Karman street. The
phenomenology of vortex generation behind round cylinders has been widely described, e.g. by
Williamson in [1]. When the shedding frequency is normalized with respect to the free-stream
velocity and the cylinder diameter, it gives the Strouhal number. For the considered Reynolds
number range the Strouhal number is approximately constant at a value of 0.21. The periodic
impingement of vortices on the airfoil, located in the cylinder wake, generates rapid time
variations of the surface pressure on the airfoil, which is responsible for the emission of a tone.

This report documents the results of a pilot test, part of the FLOVIST (Flow Visualization
Inspired Aero-acoustics with Time-resolved Tomographic Particle Image Velocimetry) project,
conducted by National Aerospace Laboratories (NLR) in collaboration with Delft University of
Technology (TU Delft). The purpose of the experimental campaign is to obtain the unsteady
flow field around an airfoil downstream of a circular section rod, generating a Karman vortex
wake impinging onto the airfoil leading edge. Combined PIV and acoustic measurements were
performed. In a later stadium, the time resolved fluid dynamics results will be used to predict
the sound emission by means of aeroacoustic analogies. The predicted sound field will then be
compared with the results of the acoustic measurements.

The report is structured as follows. The first section describes the general rod-airfoil setup and
measurement apparatus. This is followed by the results and analysis of the acoustic
measurement data. The last section discusses the results of the flow measurements.

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NLR-TR-2008-377
2 Experimental apparatus
2.1 Experimental facility
The experiment was carried out at NLR-NOP in the small anechoic facility (KAT), configured
with a rectangular type nozzle. The nozzle has dimensions of 0.51 x 0.38 m2 The anechoic
.
chamber is 5.5 x 5.5 x 2.5 m3 large and covered with 0.5 m long foam wedges, yielding 99 %
acoustic waves absorption above 500 Hz.

A cylindrical rod was vertically mounted at an approximate distance of 22 cm (varying slightly
case by case) from the wind tunnel nozzle. A NACA0012 plexiglas airfoil, with a chord of
0.1 m, was vertically placed in the wake of the rod at zero angle of attack. Both rod and airfoil
were mounted on a rail system which allowed easy adjustment of the rod position and the rod-
airfoil distance. The top edges of rod and airfoil were held by a connection to the frame of the
wind tunnel exit nozzle. A generic sketch of the experimental arrangement is shown in the
following figure:
d
0.51 m
0.332 m
0.38 m
θ
0.102 m
0.10 m
0.25 m R = 1.25 m

Fig. 1 Schematic of the rod-airfoil configuration and dimensions
for V = 15 m/s and a rod diameter of 6mm
(run 32 FOV 2CAM of the PIV test matrix in App. B)

Two different cylinders of 6 mm and 10 mm diameter were used in order to assess the effect on
the sound generation, as a result of the variation of dimension and shedding frequency of the
vortices. Each configuration was examined for three different free stream velocities of 10 m/s,
15 m/s and 20 m/s respectively.

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NLR-TR-2008-377
2.2 Acoustic measuring system setup
The first phase of the experiment consisted of a preliminary investigation that was carried out
by means of acoustic measurements only. In this first phase a microphone array to conduct
beam forming and eight regular far-field microphones were used. The circular microphone-array
was placed 0.675m from the hart of the tunnel and was approximately 0.8 m in diameter. Two
GBM-Viper systems, yielding a total number of 96 available channels, were used for the
acoustic data acquisition. The 80 LinearX, M51 microphones used for the array were positioned
in an unstructured manner to avoid spurious source detection. Eight microphones were used to
determine the acoustic far-field. These were placed at half-span and perpendicular to the airfoil
at a distance of 0.35 , 0.6 m, 0.85 m, 1.1 m, 1.35 m, 1.6 m, 2.1 m and 2.6 m as shown in
figure 2.

Fig. 2 Laboratory configuration for acoustic measurements with far-field microphones and
planar microphone array

In the second phase, acoustic measurements were carried out simultaneously with PIV
measurements. Four free field microphones were placed at a fixed distance of 1.25 m from the
airfoil leading edge, at an angle, with respect to the airfoil chord, of 900, 1170, 1350, 1430, as
shown in figure 3.

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NLR-TR-2008-377
θ= 00 1800
900
m
25 1430 mic4
1.
=
R mic3
1350
1170
mic2
mic1

Fig. 3 Far field microphones set up (left), top view schematic of microphones disposition (right)

On all array microphones wind shields were placed to prevent the acoustic measurements to be
affected by air flow passing over the microphones. The far field microphones were covered with
wind shields as well, to ensure that if there were to be an influence of a wind shield on the
sound measurements, all microphones would be influenced similarly.

2.3 Flow measuring system setup
Two-component particle image velocimetry (2C-PIV) was used to obtain planar velocity field
measurements around the airfoil. By use of a Safex stage smoke generator the flow was seeded
with particles of approximately 1 μm diameter. The seeding was introduced at the inlet of the
wind tunnel system (see Fig. 4-left), which provided a homogeneously seeded flow at the test
section.

Fig. 4 Smoke generator placed at the wind tunnel inlet (left) and the laser and light sheet
optics (right)

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