19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 16 – 19, 2018

Comparative study of optical experimental methods for micro-bubble sizing
R. Boucheron1,*, V. Aumelas2, M. Donnet1, D. Fréchou1, A. Poidatz3

1: DGA Techniques hydrodynamiques, Chaussée du Vexin, 27105 Val-de-Reuil, France
2: YLEC Consultants, 24 Bd de la chantourne, 38700 La Tronche, France
3 : DANTEC Dynamics, 8 rue Gutenberg, 91620 Nozay, France* Correspondent author: romuald.boucheron@intradef.gouv.fr
Keywords: Micro-bubbles sizing, Shadowgraphy, Defocus imaging, Phase Doppler Anemometry

ABSTRACT
A comparative study of three optical experimental methods for micro-bubble sizing is presented. In order to ensure
to measure the same bubbles with all the techniques, a dedicated injector has been designed and provided by the
YLEC Company. This injector is able to produce small bubbles with very low rate and is briefly presented. It has
been installed inside an optical water box. Three techniques (shadowgraphy, Phase Doppler Anemometry,
Interferometric Laser Imaging Technique) have been performed with the same working point of the injector. Image
processing techniques for both the shadowgraphy and ILIT have been designed specifically for this experiment. A
particular attention has been made to all thresholds, parameters and calibration needed to process the acquired
images. The calibration coefficient for the ILIT k , here processed from Lorenz-Mie theory has been found to bew

slightly diameter dependent for small micro-bubbles. This behavior is taken into account in the diameter estimate
process. Nevertheless, the comparison is quite tricky because one has to deal with the different sampling features of
all these techniques. The distribution obtained in our experiment is thinner with the ILIT and PDA techniques while
the shadowgraphy one is a little more spread out. Bias of about 15% between these techniques could be observed.

1. Introduction
Cavitation is an important phenomenon influencing the performance of ship propellers in termsof thrust, integrity and noise. Measuring the size and the concentration of micro-bubbles haslong been recognized as a key point since these nuclei influence directly the cavitation inception[5]. Different techniques have been developed in the past to measure both size distribution andconcentration of cavitation nuclei. The application of such techniques in a test section of acavitation tunnel presents many difficulties especially in large cavitation such as the GTH [5].
The first part of the communication presents the injector designed by YLEC Consultants, its mainfeatures and the experimental apparatus designed for the three techniques tested. The second
part of the presentation will briefly review these different techniques focusing on the limitations
we encounter for their applications in cavitation tunnel. In order to develop an appropriated19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 16 – 19, 2018technique and to assess its reliability, we have set up a simple experiment to performmeasurements using the micro-bubble injector. The last part of the paper is dedicated to theresults obtained and discussion.
2. Experimental apparatus
2.1 The injector from YLEC Company
Figure 1a presents the injector used in our experiment and provided by YLEC Company.
abFig. 1 a: T-junction injector by YLEC Consultants. b: The supply device for air/water pressures control.

This injector is supplied in air and demineralized water and was designed in order to produce
bubbles with a controlled size and number. This control is simply performed by adjusting thepressures of water and air separately. For a general point of view about injector behavior, highpressures produces high number of bubbles and high differences between air and waterpressures produces small bubbles (see [1] for more details on the LAMYLEC injector used). A
dedicated supply device was designed and manufactured specifically for this experiment. Figure
1b presents a schematic drawing of this device. Due to very low air and water flows, thestabilized working point of injector takes several couples of minutes, also due to high pressure
loss in supply ducts. Typically, just after adjusting the objective pressures, a decrease in pressure
up to 15% were observed during the first hour. Generally, since two hours of continuous
injection without any change in pressures objectives, variations less than 1% could be reported.
This behavior imposes us to be patient before acquiring data from each optical technique but
ensures us that the given working point of injector is reasonably the same even if each19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 16 – 19, 2018measurement technique is performed consecutively and not simultaneously. YLEC provides amodel for size and number rate of bubbles produces by their injectors. Figure 2 gives an example
of results provided by the YLEC theoretical model with different conditions that clearly shows
the influence of inlet pressures.

Fig. 2 Evolution of the diameter of micro-bubbles produced by YLEC injector.

2.2 Optical set up
An experimental apparatus has been set up in order to generate micro-bubbles of a given size. A
picture of this experimental set up is shown in Figure 3. The injector is mounted in a water box
filled with demineralized water and equipped on each face with glass windows allowing easy
optical accesses. The outlet of the device is oriented according the vertical axis and microbubbles are produced inside a no motion fluid. For the shadowgraphy technique, a powerful
LED device is used to floodlight the box through a tracing paper to homogenize the backgroundof an image. A high resolution camera (6000x4000 pixels²) is used to take shadow images of the
micro-bubble injection. A macro-lens of 200mm focal length is used with a f-number around 40 toget a high depth of field. The Phase Doppler Anemometer has been performed with a receiverloaned by DANTEC©. The Laser source used was a 660nm wavelength with a 500mm focal
length (in air). A forward configuration has been set up to take into account the first order ofrefraction mode near 70°.

19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 16 – 19, 2018

Fig. 3 Picture of the experimental set up showing shadowgraphy, Phase Doppler Anemometer (working) andInterferometric Laser Imaging Technique around the water box.

The third technique used is the ILIT (Interferometric Laser Imaging Technique). It combines theuse of a single Laser beam with a defocused camera. A Laser beam of a Laser Doppler
Velocimetry system (488nm and 514.5nm) without Bragg frequency shift has been used withanother high resolution camera equipped with a 85mm-lens. Figure 4 presents a top view of theexperimental apparatus with optical axis of each technique.

Fig. 4 Picture of experimental set up with optical axis of each technique (color coded).

19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 16 – 19, 2018

We carry out the three techniques consecutively. Even if each technique has been performed
alone, the working point has been adjusted and left constant during few hours in order to ensure
it to be stable. Each technique, included the image processing techniques, is presented in thefollowing parts.
3. Bubble sizing techniques
3.1 Shadowgraphy
The shadowgraphy technique is widely used due to its simplicity. This technique consists inadjusting an optical device in an `in-line' configuration (see Fig. 4). Each air bubble will beshown as a shadow pattern. Even if, with an appropriated calibration, it is easy to measure thesize of a bubble, this technique suffers mainly from threshold adjustment. Here, an image
processing technique is proposed. A typical image from shadowgraphy technique is shown by
figure 5.

Fig. 5 A typical acquisition image of micro-bubbles generated by the LAMYLEC injector.

The different impage processing steps are described in Figure 6.
a

b

c

d

e

f

g

h

Fig. 6 Illustrations of the different steps for shadowgraphy image processing used.

19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 16 – 19, 2018

A captured Image (Fig. 6a) shows a homogeneous background with small micro-bubbles in grey
levels. To detect these bubbles and measure their diameter, we first impose a classicalnormalization (Fig 6c) using a background image (Fig. 6b). Then, a classical threshold (Fig. 6d)and edge detection (Fig. 6e) is processed. A convolution with a synthetic image of a perfect circleof given radius is used to detect the center of each bubble of this given size (Fig. 6f). The
diameter estimate (described later is performed (Fig. 6g). All bubbles detected and measured aremasked and the process is done again until no bubble could be detected. One of most important
step is the convolution process. It yields an image which maxima are the center of the microbubbles of the given radius, as shown in figure 6f. Due to the shape of the convolution image a
sub-pixel algorithm using Gaussian shape could be applied to refine the position of the
maximum (that is the center of the bubble). At this moment, no diameter is evaluated even if theradius of the convoluted image is known to be close to the real micro-bubble radius. To estimate
the radius of each bubble, we plot the radial grey level profile with all the pixels around thecenter detected in cylindrical coordinates. Then a Gaussian-shape model is adjusted to fit thegrey level measured as shown by figure 7.

Fig. 7 Example of radial grey level pattern around a micro-bubble.
O : pixels around the center. Red : adjusted model.

It can be shown that for the considered bubble, the grey level model fit quite well the measured
one. With this model, we take 60% of grey level step between the dark and clear value toestimate the radius of each bubble. Such a technique is useful because all thresholds used before19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 16 – 19, 2018

do not influence the diameter estimate (only the bubble detection), except the 60% of the grey
level model threshold. This seems to be correct when we plot the diameter estimated by thealgorithm on the raw image (see Fig. 8).

Fig. 8 Center (red cross) and diameter (green) estimated on a raw image of micro-bubble.

The technique is also robust to overlapped micro-bubbles. Because the center detection is madewith a perfect circular shape by the convolution process, we can easily detect two bubbles which
are overlapped. All pixels around the bubble are used, making the grey level profile with large
scattering effect beyond the convolution radius. Then, the algorithm does not take into accountthese significant errors and the model can be adjusted without major problems. Moreover, to be
not influenced by the low number of pixels and with the different grey level encountered inside
the bubbles, the algorithm is adjusted considering an error proportional to the distance between
the center and the considered pixel. The most important errors considered by the algorithm are
the values around the bubble radius and beyond. Fig 9 presents an example of a high overlappedcase and with a bubble near the edge of camera sensor, well detected and sized.

Fig. 9 Bubbles detected with overlapping case and with half-bubble acquired at the edge of camera sensor.

19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 16 – 19, 2018

3.2 Interferometric Laser Imaging Technique (ILIT)
The second technique used to evaluate the size of micro-bubbles is the Interferometric Laser
Imaging Technique, developed by several authors in different domains (and also with different
names: IPI, ILIDS, ILIT, IPS …). This technique uses a Laser beam (or a Laser sheet) which islighting a bubble, viewed by a defocused imaging system. For micro-bubbles with diameter
higher than Laser wavelength, the scattered light could be described by the Lorenz-Mie theory[3]. An example of Lorenz-Mie pattern for light scattered by a 80µm is presented by figure 10.

Fig. 10 Example of scattered light pattern by an 80µm bubble against viewed angle.

It could be observed that the light pattern against viewed angle presents oscillations in lightintensity which could be observed by a camera if this latter is defocused, as presented by theschematic principle shown in figure 11.

Fig. 11 Optical set up for defocused image to measure fringe pattern (adapted from [6]).

19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 16 – 19, 2018

With such an experimental apparatus, for a given micro-bubble, the pattern of scattered lightagainst viewed angle presents fringes (dark and clear fringes) which are generally pretty much
regular with the angle. The fringes spacing is directly driven by the micro-bubble diameter. In
first approximation, the diameter could be expressed by𝑑 = 𝜅$

% '(

,

)

(1)

with d the bubble diameter, N the number of fringes, n the refraction index of water and a theweffective collecting angle. The calibration factor K has to be computed with Lorenz-Mie theoryw

for the optical configuration used (Laser wavelength, polarization and scattered angle). Besides,
as shown by figure 12, the effective collecting angle a of the equivalent lens (used in equation 1)
is different from the angle b in water. But, as noticed in [6], the number of fringes inside bothcollecting angles is the same, which allows a straightforward calibration procedure in air, easy
performed by a simple distance measurement.

Fig. 12 Relationship between collecting angles a b and number of fringes.

To calibrate an ILIT system, the coefficient K has to be computed for a given configuration. Withw

a Lorenz-Mie theory algorithm implemented in Scilab©, it is possible to evaluate this factor for agiven particle. The principle consists of calculate the Mie response for a given diameter and tocompute the K coefficient simply by dividing the number of fringes contained in the collectingw

angle near the scattered angle. Performing these computations for different micro-bubbles
diameters provides the calibration coefficient K . The Figure 13 presents such evolution for ourwconfiguration.

19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 16 – 19, 2018

Fig. 13 K against micro-bubble diameter for a 85° scattering angle with a 488nm wavelength.
w

It can be observed that the calibration coefficient, generally considered constant over the wholediameter range (see [1,6] for example) presents a small evolution which couldn't be neglected
especially for the small micro-bubbles. An adapted signal processing method is then proposed toestimate the diameter of micro-bubbles. From the data obtained with the Lorenz-Mie theory(black and blue curves in Figure 13), we fit a polynomial expression to smooth the curves (redand pink curves). Then, the diameter estimate consists simply of measuring the fringes number
N, computing the d/ K value with equation (2) and referring to the model abacus thanks to
w

% '(
)

+

./+01

= *, - 2⎯⎯⎯4 𝑑(.

Moreover, Figure 14 presents the sensitivity of this computation with the scattering angle.

Fig. 14 K against angle for a micro-bubble of 50µm-diameter for the three wavelength used.
w(2)

19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 16 – 19, 2018

Even if the calibration factor K is quasi constant with bubble diameter, its behavior againstw

scattering angle is non-monotone. The curves in Figure 14 allow predicting a sensitivity of K tow

scattered angle about 0.2µm.deg/deg (for all our configurations, the scattered angles arecomprised in the [70-100]° band, depicted by the black dashed curves in figure 14). This error
could be responsible of a bias in the measurement if the scattering angle is not perfectly
estimated. An image processing technique has been designed in order to ensure high accuracy
on diameter estimate. The principle is briefly described in the following and illustrated in Figure
15.
a

b

e

i

f

j

k

c

d

g

h

l

m

Fig. 15 Image processing steps for ILIT. a: acquired image. b : extension to 2 image. c: FFT image. d: low-passnfiltered image. e: edge of circle. f: center detection. g: center and diameter. h: bubble. i: FFT of bubble. j: zoom on FFTimage. k: angle selection. l: example of synchronous detection base. m: refinement near wanted frequency.

The first step consists of detection of circle due to defocus. This is made in Fourier domain (15c)by low-filtering the acquired image (15d). The obtained image presents circular shape (15e)which can be detected by classical Hough transform or edge/threshold processes (15f). A
convolution process could also be used because the size of circles only depends on defocusing
rate and will be the same for all bubbles detected. After detecting the circle for bubbles, we19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 16 – 19, 2018

perform a Fourier transform with the original image (15i). A first estimate of frequency is thendone with a simply peak-locking estimate. The researching area in Fourier domain is restricted toa given area avoiding the high frequencies and others artifacts which can occur (15k)(interferences, speckle pattern, etc.). The direction of fringes depends on optical set up and could
be an initial parameter of the image processing technique. The peak-locking estimate could be
not accurate enough to ensure high precision on the evaluation of fringe number N (in equation(2)). In order to refine the frequency estimate, a two dimensional synchronous detection has been
designed. It consists mainly of correlating experimental defocused image with a synthetic 2D
image (15l) containing pure sine/cosine grey level evolution. The refinement image ofsynchronous detection allows peak locking (15m). A sub-pixel refinement (as depicted forexample by Nobach [7]) could also be used instead of synchronous detection directly with the
FFT image of a bubble (15k).
3.3 Phase Doppler Anemometry
The Phase Doppler Anemometry is an interference technique using a LDV (Laser Doppler
Velocimetry) probe. Two Laser beams are crossing making interferences pattern. The scattered
light presents bright and dark fringes which can be linked to the particle velocity projected to theoptical axis (perpendicular to fringes). If two detectors are used with different viewed angles,
phase shift between the two light signals could be measured and linked to the bubble diameter.
Figure 16 presents the definitions of the different angles used in PDA applications.

P

P’
2q

y
j

LDV probey'j'aFig. 16 Angle definitions for PDA signal model with a vertical probe.

19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 16 – 19, 2018

In our case, we use a forward configuration with a scattering angle j near 70° that is a first orderrefraction mode (air bubble in water). For this optical configuration, the diameter of microbubble can be computed with the following equation𝑑=(67869 ) ;
<

=

=

7

9

*> − > -with 𝛽A = 2 CD1 + 𝑛H − 𝑛I2(1 + cos 𝜃 cos 𝜑 cos 𝜓A + sin 𝜃 sin 𝜓A ) −D1 + 𝑛H − 𝑛I2(1 + cos 𝜃 cos 𝜑 cos 𝜓A − sin 𝜃 sin 𝜓A )R ,(3)

with f the phase measured by receiver i, l the Laser wavelength, n the optical refraction indexiratio (𝑛 = 𝑛STSS10 ⁄𝑛.0+AT. ), q the semi-angle of Laser probe, j the scattering angle and y theielevation angle of receiver i. The optical set up is one of the key points for this technique. A slightshift in alignment could produce bias/error in diameter estimate. Different errors have been
investigated. Figure 17 presents an order of magnitude of the expected diameter error fordifferent misalignment.

a

b

c

d

Fig. 17 Illustrations of expected diameter errors for PDA for different optical set up errors.

19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 16 – 19, 2018

Figure 17a is the result for a shift of the scattering angle j, 17b for a global elevation angle shiftdy (for all the three receivers), 17c for an elevation angle error y (just for one receiver) and 17dfor a rotation of the receiver probe around its optical axis a (see Figure 16 for its definition fromP'). Indeed, if the receiver probe is slightly rotated by an angle a, the scattering angles andelevation angles are both modified according to
YZ[ \]YZ[ ) ^_Y \ `a[ b𝜑′ = atan *^_Y \8YZ[ ) YZ[ \ `a[ bc^_Y ) YZ[ \c.

(4)𝜓 = atan *tan 𝜓 YZ[ \]YZ[ ) ^_Y \ `a[ bThe expected diameter errors (in percent) for these given misalignments are small, excepted forthe elevation angle error which is fortunately fixed and measured by the manufacturer.
Nevertheless, a global bias of several percent could arise in our experiment. All these equations
and results are available only for spherical particles which is our case with micro-bubbles. The
aspect ratio of observed micro-bubbles is close to one. We then consider that this non-sphericalerror could be neglected [4].
4. Results
Comparison between all techniques is interesting but is still nowadays a challenge. The different
techniques presented in the previous sections have different advantages and drawbacks. The
PDA technique is now available with a "real-time" process allowing a direct estimate of diametercoupled with velocity of each bubbles. This technique is a point technique which requires
particular optical accesses. High sampling rate could be easily achieved. With a three receiver’s
device, it could be able to discriminate different micro-bubble size range. The shadowgraphy issimple and is a plane measurement. All bubbles can be measured if the sampling frequency ishigh. Unfortunately, the camera used gets a high resolution but a very low sampling frequency.
The ILIT technique requires a Laser point or a Laser sheet. In our application, we use a Laserbeam (for convenience) which imposes a point measurement coupled with a high resolution
camera which in return imposes a very low sampling frequency too. The following Table 1 gives
an overview of the characteristics of our experimental apparatus.

19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 16 – 19, 2018

Tab. 1 Technical features of our experimental three techniques.
Shadowgraphy

PDA

Sampling mode

Plane

Point

Sampling rate

~1 im./10s

100 HzNumber of bubbles~40000~20000Signal/Image

Timeprocessing

consuming

Characteristic

Blue
Point

1

Green

Red

Point

Point

1

1

~1 im./10s
100

100

'Real-time'

100

Quick

d ≈ 9.5µm

few %

1.2µm

1.1µm

0.8µm

Viewed angle

in-line

70°

85°

85°

95°

Advantages

Simplicity

Real-time

Threshold

Fixed opticaldependingconfiguration

accuracy

Drawbacks
1

ILIT

Technique

pix

+

*

*

*

Optical adaptation easy
Theoretical calibration: ILIT is also available with a Laser sheet even if it was not our optical design.

+: see § 3.3, Figure 17 and discussion.

: Theoretical accuracy estimated by the peak-locking in Fourier domain with SD.

*

Statistics presented in the following can suffer from these different space/time sampling
configurations. Figure 18 presents a picture taken with the shadowgraphy camera with a highshutter duration time (2s) during injection process with the two Laser used by other techniques
in order to see the location of probe volumes of each technique compare to the shadowgraphy
one.

Fig. 18 Location of probe volumes for PDA (red) and ILIT (blue) taken with a long shutter time.

19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 16 – 19, 2018

We can then estimate the location of PDA probe volumes (red location near the injector outlet) inthe shadowgraphy image and the location of ILIT probe volume (the green/blue location). Then
it is possible to filter the data obtained by the shadowgraphy in space to compare with the PDA(or ILIT) data. Figure 19 presents the size distribution evolution in the shadowgraphy image. The
size distribution obtained presents important evolution far from the injector outlet with bubbles
coalescence.

Fig. 19 Evolution of the micro-bubble size distribution along the micro-bubble train.

The test case number 5 is particularly interesting because the micro-bubbles present pretty muchthe same size along a large distance far from the injector outlet, which is important for thecomparison of the techniques. This is not the case for all the test cases performed. Figure 20
presents the results of such comparison. It can be observed that the shadowgraphy results are
spread out, probably due to the non-optimal optical set up (a global spread about 15 µm is
observed, that is around 1.5pixels). Both PDA and ILIT obtain thinner distributions. The
differences between PDA and shadowgraphy is about 17µm (~15%) and the difference between
ILIT and shadowgraphy is about 15µm (~15%).

19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 16 – 19, 2018

Fig. 20 Comparison of micro-bubble size distribution for a "single size" case.

The other test cases present multi-mode distributions from the injector outlet. The way adopted
to compare these distributions is to peak the average size of the "single bubble" and to comparewith the theory. Figure 21 presents an example of a 3 main sizes distribution. The mean values ofeach Gaussian shape peak correspond to a single-bubble and coalescence of several singles
bubbles. For the test case 4, the mean values are respectively 96.4, 138.7 and 161.3µm. These twolast values correspond to a coalescence of 3 and 5 single bubbles (96.4* √3=139.0, 96.4* √5=164.8).
3

3

Fig. 21 Distribution for a multi-mode test case. Mean values observed: 96.4µm, 138.7µm, 161.3µm.

19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 16 – 19, 2018

Finally, the comparison with the theoretical model could be done only for the single mode.
Figure 22 summarizes the global results for the comparison. All the test cases performed arereported as an index point (with different water/air pressures conditions). The average diameter,
expressed in µm is reported in y-coordinate. The results from preliminary tests using only
shadowgraphy technique are also reported [index 1 to 13]. A good agreement during thesequalification tests could be observed. It has to be noticed that a parameter has to be adjusted forthe theoretical model. This has been done only with the qualification test data (the pixel size ofthe shadowgraphy set up was better and magnification factor close to one).

Fig. 22 Mean diameter for all configurations performed.

The 4 other tests case (index point 15, 17-19) correspond to a comparison working points for the
injector. No clear trend could be observed between each technique. No determinist bias could beobserved for a given technique nor global fail to follow the global behavior of micro-bubble
diameter for the different configurations. The scattering observed between results of one
technique seems to be random and not determinist. A global scattering (around 15-20%) remains
between the three techniques. Moreover, it has to be noticed that angles estimate (for PDA andILIT) is very difficult in such configuration because of refraction due to glass and water. The
Laser does not enter in water with perpendicular incident angle which causes refraction. In
addition, the scattered light used by ILIT and PDA is also refracted. It is the obvious that ourestimate of all these angles should be corrupted by these refraction effects and appears as themain source of discrepancies for these techniques in our applications.

19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 16 – 19, 2018

5. Conclusion
An experiment has been set up in order to evaluate three bubble sizing techniques. Particularattention has been made on image processing effect on estimate. The susceptibility of thresholds
for shadowgraphy could be partially avoided using the proposed method. It consists ofobtaining the center of each detected bubble and fitting a Gaussian shape model to radial grey
level profiles. It remains nevertheless for this latter an improvement to evaluate the appropriatedthreshold for grey level gradient, which could be doubtless provided by a theoretical analysis.
The ILIT method has been used and gives good results. The optical set up didn't allow reachinghigh number of particles which inhibits good statistical moments but it could be easily improved
using high speed cameras for example. However, we pay attention to calibration process of the
ILIT technique. Lorenz-Mie theory has been used and an improved method has been designed
which were not required for our bubbles measured in this study but which could be
advantageously used in other cases with smaller micro-bubbles. The fringe number has been
estimated with accuracy using a two dimension synchronous detection. The comparison
between the three techniques shows that scattered results remain.
6. Acknowledgments
Authors are grateful to Martijn van Rijsbergen for loaning the water box. Authors want also
thank the DANTEC Company for loaning the PDA receiver and technical support. We are also
grateful to Loïc Méès for fruitful discussion about the ILIT techniques and the associated image
processing.
7. References[1] Aumelas V, Lecoffre Y, Maj G, Franc J-P (2016) Micro-bubbles seeding for flow
characterization.

28th

IAHR

symposiumon

Hydraulic

Machinery

andSystems

doi:10.1088/1755-1315/49/6/062005.
[2] Birvalski M, van Rijsbergen MX (2017) Exploratory measurements of cavitation nuclei in thewake of a ship model. 5th International Conference on Advanced Model Measurement
Technology for the Maritime Industry (AMT'17), Glasgow, UK.
[3] Bohren CF, Huffman DR (1963) Absorption and scattering of light by small particles. Wiley
New York.

19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 16 – 19, 2018[4] Damaschke N, Gouesbet G, Grehan G, Mignon H and Tropea C (1998) Response of Phase
Doppler Anemometer systems to nonspherical droplets. Applied Optics 37(10) 1752-1761.
[5] Fréchou D, Dugué C, Briançon-Marjollet . et al (2000) Marine propulsor noise investigationsin the hydroacoustic water tunnel GTH, 23th Symposium on Naval Hydrodynamics (ONR),
France.
[6] Méès L, Lebrun D, Allano D, Walle F, Lecoffre Y, Boucheron R, Fréchou D (2010) Developmentof interferometric techniques for nuclei size measurement in cavitation tunnel. 28th symposium
on Naval Hydrodynamics, Pasadena, US.
[7] Nobach H, Damaschke N and Tropea C (2005) High-precision sub-pixel interpolation inparticle image velocimetry image processing. Exp. in Fluids 30 299-304.