CHARACTERIZING LASER INDUCED CAVITATION: EFFECTS OF AIR CONTENT, BEAM ANGLE ...

110 Pages·2012·7.1 MB·English

by  Minna Ranjeva

The Pennsylvania State University

The Graduate School

College of Engineering

CHARACTERIZING LASER INDUCED CAVITATION: EFFECTS OF AIR CONTENT,

BEAM ANGLE, AND LASER POWER

A Thesis in

Mechanical Engineering

by

Minna L. Ranjeva

© 2012 Minna L. Ranjeva

Submitted in Partial Fulfillment

of the Requirements

for the Degree of

Master of Science

August 2012
The thesis of Minna L. Ranjeva was reviewed and approved* by the following:

Brian R. Elbing
Associate Research Faculty
Thesis Adviser

Dan Haworth
Professor of Mechanical Engineering
PIC of MNE Graduate Programs

Gary Settles
Professor of Mechanical Engineering

Karen A. Thole
Professor of Mechanical Engineering
Head of the Department of Mechanical Engineering

*Signatures are on file in the Graduate School.

  ii
Abstract
Laser-induced cavitation allows for cavitation bubbles to be systematically reproduced using
a high power laser and focusing the laser beam to a point. This provides the opportunity to study
the physics of the cavitation process under different circumstances in an experimental setting.
Laser-induced cavitation has many applications. It has been used successfully to study cavitation
near boundaries in an effort to understand the mechanisms of cavitation erosion. It has also found
new applications in the world of medicine, as well as other areas. Previous work has largely
focused on cavitation near a surface causing damage, but as new applications emerge,
characterization of bubbles in a bulk fluid will be useful, permitting a high level of control over
the bubble size, shape, and lifetime. With this in mind, laser-induced cavitation bubbles in a bulk
fluid are characterized. The focusing angle of the lens, the air content of water, and the laser
power are all varied to provide a comprehensive understanding of how these variables affect the
bubbles’ development. Scaling for the bubble behavior is developed.

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Table
 of
 Contents

List of Tables. .............................................................................................................................. vii
List of Figures ............................................................................................................................. viii
1 Introduction ............................................................................................................................. 1
1.1 Background .................................................................................................................................... 1
1.2 Physics ............................................................................................................................................. 2
1.2.1 General ...................................................................................................................................... 2
1.2.2 Laser-induced optical breakdown ............................................................................................. 3
1.2.3 Plasma expansion and production of cavitation bubble ........................................................... 4
1.2.4 Cavitation bubble growth and collapse ..................................................................................... 4
1.3 Earlier work ................................................................................................................................... 5
1.4 Recent attempts to characterize bubbles ..................................................................................... 8
1.5 Newer applications of cavitation .................................................................................................. 9
1.5.1 Laser cleaning ......................................................................................................................... 10
1.5.2 Micro-pumps .......................................................................................................................... 10
1.5.3 Lithotripsy .............................................................................................................................. 11
1.5.4 Drug delivery .......................................................................................................................... 12
1.6 Introduction summary ................................................................................................................ 13
1.6.1 Research Objectives ............................................................................................................... 13
2 Experimental Methods ......................................................................................................... 15
2.1 Test apparatus .............................................................................................................................. 15
2.1.1 Water Tank ............................................................................................................................. 15
2.1.2 Laser Optics ............................................................................................................................ 18
2.2 Instrumentation ........................................................................................................................... 22
2.2.1 Imaging setup ......................................................................................................................... 22
2.2.2 Laser power ............................................................................................................................ 23
2.2.3 Water dissolved gas content ................................................................................................... 24
2.3 Test Matrix ................................................................................................................................... 26
2.4 Measurement Uncertainty .......................................................................................................... 26
2.4.1 Camera .................................................................................................................................... 26
2.4.2 Laser ....................................................................................................................................... 27
2.4.3 Lenses ..................................................................................................................................... 27
2.4.4 Pressure ................................................................................................................................... 28
2.4.5 Air Content ............................................................................................................................. 28
3 Experimental Results ............................................................................................................ 30
3.1 General trends and overview ...................................................................................................... 30
3.2 Repeatability ................................................................................................................................ 35
3.3 Variation of bubble topology ...................................................................................................... 38
3.4 Bubble size .................................................................................................................................... 42
3.4.1 Bubble size: sensitivity to beam angle .................................................................................... 42

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3.4.2 Bubble size: sensitivity to air content ..................................................................................... 49
3.5 Bubble half-life ............................................................................................................................. 54
3.5.1 Bubble half-life: sensitivity to beam angle ............................................................................. 54
3.5.2 Bubble half-life: sensitivity to air content .............................................................................. 57
3.6 Bubble diameter time history ..................................................................................................... 60
4 Scaling of Laser Induced Cavitation ................................................................................... 70
4.1 Scaling ........................................................................................................................................... 70
4.2 Comparison with non-spherical bubbles ................................................................................... 80
4.2.1 Vertical diameter .................................................................................................................... 80
4.2.2 Horizontal diameter ................................................................................................................ 82
4.2.3 Bubble half-life ....................................................................................................................... 84
4.2.4 Behavior over time ................................................................................................................. 86
4.3 Error Propagation ....................................................................................................................... 89
5 Limitations and future work ................................................................................................ 91
5.1 Limitations ................................................................................................................................... 91
5.1.1 Pulsed laser ............................................................................................................................. 91
5.1.2 Equipment setup ..................................................................................................................... 91
5.1.3 Assumptions in deriving scaling ............................................................................................. 92
5.2 Future work .................................................................................................................................. 92
5.2.1 Pressure ................................................................................................................................... 92
5.2.2 Viscosity ................................................................................................................................. 93
5.2.3 Particulate matter .................................................................................................................... 93
5.2.4 Air content .............................................................................................................................. 94
6 Conclusions ............................................................................................................................ 95
Appendix: Uncertainty ............................................................................................................... 97
Bibliography .............................................................................................................................. 100

  v
List
 of
 Tables

Table 1 – Distances between components in experimental set up in Figure 1. ............................ 17
Table 2 – Lens specifications used to create bubbles ................................................................... 18
Table 3 – Average water-air content concentration (C) for each air content condition ............... 25
Table 4 – standard deviation for air content concentration measurements ................................... 28
Table 5 – Measured quantities and their corresponding measurement uncertainty ...................... 29
Table 6 – Lowest laser pulse energy for which bubbles could be seen for each of the test
conditions, E .................................................................................................................... 43
0
Table 7 – Coefficients for second order polynomial best fit curves based on nondimensionalized
lifetime profiles ................................................................................................................. 67
Table 8 – Beam waist for lenses ................................................................................................... 71
Table 9 – Variables of interest used to derive scaling rules for spherical cavitation bubbles. ..... 72
Table 10 – Results of error propagation through calculations for nondimensional variables ...... 90

  vi
List
 of
 Figures

Figure 1 – Experimental setup used to generate laser-induced cavitation bubbles. The laser beam
is expanded and then focused through a lens. A camera was used to capture the cavitation
bubbles. The dashed box in the side view represents the camera’s nominal field-of-view.
........................................................................................................................................... 16
Figure 2 – This figure illustrates the relationship between beam divergence and angular spread.
In this work, the convergence or focusing angle refers to the angular spread. ................. 18
Figure 3 – Snell’s law applied to a ray path passing through air (n =1.00), glass (n =1.55) and
1 2
water (n =1.33). Increasing index of refraction bends the beam towards the axis
3
perpendicular to the interface between the two mediums. ................................................ 21
Figure 4 – Energy per pulse as a function of attenuator setting for the laser used in the current
study. The flash lamp was fixed at the maximum level. ................................................... 24
Figure 5 – Example of a bubble lifetime produced at relatively low laser power (7.3 mJ per
pulse) with the wide-angle lens configuration and intermediate air content level. Labelled
data points correspond to labeled images at the top of the figure. In image A the bright
white spot is produced from plasma generated by the focused laser beam, while the
remaining images show the shadow produced from the backlighted bubble. .................. 32
Figure 6 – Example of a bubble lifetime produced at relatively high laser power (42 mJ per
pulse) with the wide-angle lens configuration and intermediate air content level. Labelled
data points correspond to labeled images at the top of the figure. The growth, collapse
and rebound of the bubble is apparent from the plot. ....................................................... 33
Figure 7 – Comparison between low and high power bubble behavior. At lower pulse energies
single, spherical bubbles are produced. At higher powers the bubble shape depends on the
lens angle. The smaller angle lens produces one larger bubble formed from bubble
coalesence. The wider angle lenses produce smaller, more elongated bubbles. In the
figure SA, MA and WA refer to the small, medium and wide-angle lens configurations,
respectively. ...................................................................................................................... 35
Figure 8 – Comparing bubble diameter, there is reasonable agreement between data collected in
different setups from different points in time. This indicates that comparing results
between the two different setups is appropriate. ............................................................... 37
Figure 9 – Narrow angle lens bubble patterns at high power (42 mJ) in high air content water.
Smaller bubbles formed along the path of the laser beam coalesce to form a single, larger,
elliptical bubble. ................................................................................................................ 39
Figure 10 – Medium angle lens bubble patterns at high power (42 mJ) in intermediate air content
water. On the left is the bubble formation shortly after the laser pulse, on the right is the
bubble formation at the time of maximum bubble diameter. ............................................ 41
Figure 11 – Wide-angle bubble patterns at high power. On the left hand side of (A), (B), and (C)
is the bubble formation shortly after the laser pulse, and on the right hand side of (A), (B)
or (C) are the bubble formations at the points of maximum bubble diameter. These
images illustrate the variety of bubble patterns that can be formed. ................................ 42
Figure 12 – Vertical bubble diameter as a function of laser energy per pulse for (A) low, (B)
intermediate and (C) high air content conditions. The solid lines represent the best fit
curves for each condition. ................................................................................................. 47

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Figure 13 – Ratio of the horizontal to vertical bubble diameters plotted as a function of the laser
pulse energy with (A) low, (B) intermediate and (C) high air content levels. .................. 49
Figure 14 – Vertical bubble diameter plotted versus the laser pulse energy for (a) narrow, (b)
medium and (c) wide-angle lens configurations. Solid lines represent the best fit curves to
the data. ............................................................................................................................. 52
Figure 15 – Ratio of horizontal to vertical diameter plotted versus laser pulse energy for (a)
narrow (b) medium and (c) wide beam angles. ................................................................. 54
Figure 16 – Bubble half-life (time from laser pulse to achieve maximum bubble diameter) plotted
versus the laser pulse energy with (a) low, (b) intermediate and (c) high or saturated air
content levels. ................................................................................................................... 57
Figure 17 – Bubble half-life as a function of laser pulse energy with the (A) narrow, (B) medium
and (C) wide-angle lens configuration. ............................................................................. 60
Figure 18 – Small angle, low air content lifetime curve ............................................................... 61
Figure 19 – Small angle, intermediate air content lifetime curve ................................................. 62
Figure 20 – Small angle, saturated condition lifetime curve ........................................................ 62
Figure 21 – Medium angle lens, low air content lifetime curve ................................................... 63
Figure 22 – Medium angle, intermediate air content lifetime curve ............................................. 63
Figure 23 – Medium angle, saturated air content lifetime curve .................................................. 64
Figure 24 – Wide-angle, low air content condition lifetime curve ............................................... 64
Figure 25 – Wide-angle, intermediate air content lifetime curve ................................................. 65
Figure 26 – Wide-angle, saturated lifetime curve ......................................................................... 65
Figure 27 – Comparison of lifetime polynomial fit for each condition. SA/L stands for small
angle, low air content. MA stands for medium angle, WA for wide-angle, I for
intermediate air condition, H for high air content. ............................................................ 66
Figure 28 – Single lifetime curve for nondimensionalized bubble diameter vs. time. This curve
describes the bubble diameter’s growth over time for t ≤ 2. The different colors represent
h
different laser energy ranges, while the shapes indicate the air content condition (squares
are low air content, circles are intermediate, and triangles are the high air content
condition). ......................................................................................................................... 69
Figure 29 – An illustration of the beam profile of a Gaussian beam near the focal point. ........... 71
Figure 30 – The scaled horizontal diameter (D *) is plotted versus the scaled vertical diameter
h
(D *),holding Π constant. This plot shows a linear relationship between Π andΠ for the
v 6 1 2
majority of bubbles. The outliers in this plot represent bubbles produced at high energies
that do not maintain a spherical shape. These bubbles are indicated by open symbols and
are not included when looking at the scaling relationships. ............................................. 73
Figure 31 – Relationship between t */D * and ΔE* shows a logarithmic relationship that appears
h v
to be only minimally affected by air content for spherical bubbles. The average ratio for
each condition (lens, air content, laser power) is plotted. ................................................. 76
Figure 32 – Average th* versus Dv* for various ranges of ΔE*. The relationship appears to be
linear. ................................................................................................................................ 78
Figure 33 – The relationship between beam waist and E appears to be linear. ........................... 79
o
Figure 34 – Predicted D * versus actual D * when applying scaling to non-spherical bubbles. The
v v
solid line shows where the predicted and actual D * values are equal. The scaling
v
guidelines over predict the vertical diameter of the non-spherical bubbles. ..................... 81

  viii
Figure 35 – Predicted vertical diameter plotted against the actual vertical diameter when scaling
relationships are applied to non-spherical bubbles. The black line indicates where the
predicted and actual values are equal. ............................................................................... 82
Figure 36 – Predicted Dh* versus actual Dh* when applying scaling to non-spherical bubbles.
The solid line shows where the predicted and actual Dh* values are equal. The horizontal
bubble diameter appears to be predicted relatively well with the scaling relationships used
here. ................................................................................................................................... 83
Figure 37 - Predicted horizontal diameter plotted against the actual horizontal diameter when
scaling relationships are applied to non-spherical bubbles. The black line indicates where
the predicted and actual values are equal. ......................................................................... 84
Figure 38 – Predicted t * versus actual t * when applying scaling to non-spherical bubbles. The
h h
solid line shows where the predicted and actual t * values are equal. The scaling
h
relationships used over predict the scaled half-life t * for non-spherical bubbles. .......... 85
h
Figure 39 – Predicted half-life versus actual half-life for scaling relationships applied to non-
spherical bubbles. The black line indicates where the predicted and actual values are
equal. ................................................................................................................................. 86
Figure 40 – Non-spherical bubble behavior over time. Each run represents the development of a
single, non-spherical bubbles. The blue curve represents the curve in Figure 28, the
generic lifetime curve obtained in chapter 3. .................................................................... 88
Figure 41 – The ratio of horizontal to vertical diameter for non-spherical bubbles. This graph
represents the average ratio of vertical to horizontal diameter for six non-spherical
bubbles. This illustrates that non-spherical bubbles begin as more elliptical shapes and
then become more spherical over time. It also shows the oscillation in size that non-
spherical bubbles exhibit. .................................................................................................. 89

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1 Introduction
1.1 Background
Cavitation is a phenomenon that occurs in nature when fluids develop areas of high speed
or local pressure drops. These forces cause the rapid formation and then collapse of a cavity
within a liquid, and this process is referred to as cavitation. Cavitation has traditionally been an
area of interest in the scientific community due to its erosive effects on mechanical parts, such as
propellers, and the fact that loud noise resulting from cavitation can cause an issue when vehicles
desire to go undetected.
Since people began to study cavitation for its erosive consequences, new techniques to
both produce and record cavitation events have been developed. In the early stages of cavitation
study, scientists had difficulty controlling the occurrence of cavitation events due to the
statistical nature of this phenomenon in both space and time (Lauterborn & Bolle, 1975). Using a
laser allows cavitation bubbles to be produced at a known location. Early research focused on
cavitation events occurring near a boundary, since cavitation is well known for causing damage
to propellers. More recently laser-induced cavitation has been used in a variety of medical
applications such as lithroscopy (Kokaj et al., 2008). It is also being explored for use in other
areas, for example laser cleaning of surfaces or micropump technologies (Song et al., 2004;
Dijkink & Ohl, 2008).
As more and more applications for laser-induced cavitation bubbles emerge, a greater
understanding of how the environmental factors and controllable variables (such as laser power
or beam angle) affect the growth and development of cavitation bubbles is needed. As much
previous research has focused on the erosive consequences of cavitation bubble collapse, more
information on bubble formation away from a boundary in an infinite medium, or how changes

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