-
[show abstract]
[hide abstract]
ABSTRACT: In order to elucidate the mechanism of cavitation erosion, the
dynamics of a single
laser-generated cavitation bubble in water and the resulting surface damage
on a
flat metal specimen are investigated in detail. The characteristic effects
of bubble
dynamics, in particular the formation of a high-speed liquid jet and the
emission of
shock waves at the moment of collapse are recorded with high-speed photography
with framing rates of up to one million frames/s. Damage is observed
when the
bubble is generated at a distance less than twice its maximum radius from
a solid
boundary (γ=2, where γ=s/Rmax,
s is the distance between the boundary and
the bubble centre at the moment of formation and Rmax
is the maximum bubble
radius). The impact of the jet contributes to the damage only at small
initial distances
(γ[less-than-or-eq, slant]0.7). In this region, the impact velocity rises to 83 m s−1,
corresponding to a water hammer pressure of about 0.1 GPa, whereas at γ>1,
the impact velocity is smaller than 25 m s−1. The largest
erosive
force is caused by the collapse of a bubble
in direct contact with the boundary, where pressures of up to several GPa
act on
the material surface. Therefore, it is essential for the damaging effect
that bubbles are
accelerated towards the boundary during the collapse phases due to Bjerknes
forces.
The bubble touches the boundary at the moment of second collapse when γ<2
and
at the moment of first collapse when γ<1. Indentations on an aluminium
specimen
are found at the contact locations of the collapsing bubble. In the range
γ=1.7 to 2,
where the bubble collapses mainly down to a single point, one pit below
the bubble
centre is observed. At γ[less-than-or-eq, slant]1.7, the bubble shape has become toroidal,
induced by
the jet flow through the bubble centre. Corresponding to the decay of this
bubble
torus into multiple tiny bubbles each collapsing separately along the circumference
of the torus, the observed damage is circular as well. Bubbles in the ranges
γ[less-than-or-eq, slant]0.3
and γ=1.2 to 1.4 caused the greatest damage. The overall diameter
of the damaged
area is found to scale with the maximum bubble radius. Owing to the possibility
of
generating thousands of nearly identical bubbles, the cavitation resistance
of even
hard steel specimens can be tested.
Journal of Fluid Mechanics 04/1998; 361:75 - 116. · 2.46 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A series of coherent light pulses is generated by pumping a dye laser with the pulsed output of a copper-vapor laser at rates of as much as 20 kHz. Holograms are recorded at this pulse rate on a rotating holographic plate. This technique of high-speed holographic cinematography is demonstrated by viewing the bubble filaments that appear in water under the action of a sound field of high intensity.
Optics Letters 01/1993; 18(1):4-6. · 3.40 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: High-speed rotational angioplasty is being evaluated as an alternative interventional device for the endovascular treatment of chronic coronary occlusions. It has been postulated that this type of angioplasty device may produce particulate debris or cavitations that induce myocardial ischemia. To determine the clinical presence of myocardial ischemia during rotational angioplasty, echocardiographic monitoring for wall motion abnormalities was performed in 9 patients undergoing rotational atheroablation using the Auth Rotablator for 10-sec intervals at 150,000 and 170,000 rpm. No wall motion abnormalities were detected in 5 patients evaluated with transesophageal echocardiography or in 4 patients monitored transthoracically, although AV block developed in one patient. Video intensitometry of the myocardial contrast effect for rotation times ranging from 3 to 20 sec found transient contrast enhancement of the myocardium supplied by the treated vessel. Intensity varied over time with half-time decay between 5.6 and 40 sec, indicating the likelihood of microcavitation. An in vitro model was constructed to measure the cavitation potential of the Auth Rotablator. A burr of 1.25 mm diameter rotating at 160,000 rpm achieves a velocity in excess of the 14.7 m/sec critical cavitation velocity. Testing the device in fresh human blood and distilled water produced microcavitations responsible for the enhanced echo effect, with the intensity and longevity of cavitation more pronounced in blood and proportional to the rotation time and speed. The mean size of the microcavitation bubbles in water was 90 +/- 33 (52-145) microns measured from photographs taken with a copper vapour laser emitting light pulses of 50 nsec duration as light source. The mean velocity of bubbles was found to be 0.62 +/- 0.30 ranging from 0.23 to 1.04 m/sec. It was measured via the motion of the bubbles during 5 laser pulses within 800 nsec. Clearly, microcavitations are associated with enhanced myocardial echo contrast effect.
Catheterization and Cardiovascular Diagnosis 07/1992; 26(2):98-109.
-
[show abstract]
[hide abstract]
ABSTRACT: An arrangement for determining the size distribution of small particles (e.g., droplets or bubbles) is presented. It consists of taking high speed holograms with a frequency doubled Nd:YAG laser, reconstructing the real image with an argon-ion laser, and a digital image processing system with a random access image dissector camera and two cascaded computers for filtering, segmentation, and higher recognition tasks. Results are presented for cavitation bubbles as test objects. It is shown that the quality of the holograms is sufficient for detecting bubbles with a diameter of <20 mum by digital image processing with the present configuration.
Applied Optics 08/1990; 29(23):3365-8. · 1.41 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Cavitation bubble dynamics is investigated by the method of optic cavitation, i.e. the formation of single cavities in liquids by light. From the sound waves radiated upon collapse the pressure-time curve is obtained. Maximum bubble size and shock wave amplitudes are evaluated and an energy balance is considered. Numerical calculations with a modified Gilmore model taking into account the mass loss of the cavity can explain the rapid damping of the bubble oscillation observed in the experiments.
Applied Scientific Research 12/1981; 38(1):225-230.
-
[show abstract]
[hide abstract]
ABSTRACT: Four holograms are recorded superimposed on the same plate at maximum repetition rates of about 10 kHz. A multiply Q-switched ruby laser produces the series of coherent light pulses for hologram exposure. Image separation of different holograms is achieved by spatial frequency multiplexing. The reference beam direction is altered by a unique acoustooptic beam splitter and deflector unit. The operating principle is a sound pulse-light pulse interception technique. Only one ultrasonic transducer is necessary. The quality of the holograms is demonstrated by the reconstructed images taken of laser produced cavitation bubbles following optical breakdown in water.
Applied Optics 07/1978; 17(13):2071-6. · 1.41 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The phenomenon of ruby laser‐induced breakdown in liquids is investigated by high‐speed holography. The advantage of using holography instead of ordinary photography lies in the possibility to easily suppress the bright white light emitted during the breakdown process. This light constitutes an incoherent background on the holographic plate and does not reproduce upon reconstruction of the recorded scene.
Applied Physics Letters 11/1977; 31(10):663-664. · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: For a thorough study of bubble dynamics in cavitation bubble fields one has to resort to holography to circumvent the problems caused by bubbles moving out of focus during their three-dimensional motion. The development and application of high speed holography and holocinematography in cavitation physics is reviewed in this paper. The state of the art is that several thousand holograms with frame size a few square millimetres can be taken at rates up to 300 000 per second. Digital image processing, a valuable tool which is used to cope with the huge information content of series of holograms, is discussed. Results on bubble dynamics obtained via holography are presented, in particular on shock wave emission and bubble oscillation, splitting and interaction in a sound field. Global field dynamics and streamer stability are also discussed.
Ultrasonics.
-
[show abstract]
[hide abstract]
ABSTRACT: Single cavitation bubble luminescence induced by laser in contrast to single bubble sonoluminescence has no need in a sound field for a strong collapse and for light emission. The cavitation bubbles are produced by focused laser light and make the single strong collapse. As shown experimentally, the number of emitted photons from cavitation luminescence is much greater than it was observed in sonoluminescence due to the large bubble size during the final stage of collapse. To describe the process of laser-induced bubble collapse a mathematical model is used, which is based upon the spherically symmetric motion including compressibility, heat and mass transfer effects. The basic results of the numerical solution are presented for the bubbles with maximum radii of about 1 mm. According to the observed results the minimum bubble radius in collapse is about 15 μm, and the mass decreases up to 5% of the initial value. Calculations with a small amounts of noncondensable gas inside the bubble predict its strong influence on the dynamics. As shown numerically the theoretical model gives a good agreement with experimental measurements.
Experimental Thermal and Fluid Science.
-
[show abstract]
[hide abstract]
ABSTRACT: The bubble size distribution in a cavitation tunnel is obtained via holography by using a Q-switched ruby laser and diffuse illumination of the object volume in an off-axis arrangement. Analysis of the holograms taken is done by scanning their real image with an image dissector camera under computer control and by calculating size and location of the particles using specially designed software. With this digital image processing system nuclei as small as 20 μm in diameter can be detected in order to yield histograms of their size distribution.
Optics Communications.
-
[show abstract]
[hide abstract]
ABSTRACT: Up to eight holograms are successively recorded at maximum repetition rates of 20kHz. A multiply Q-switched ruby laser is used to produce the series of coherent light pulses for hologram exposure. All holograms are recorded on the same plate. Image separation of different holograms is achieved by spatial multiplexing using a rotating disk with apertures directly in front of the holographic plate. The performance of the technique is demonstrated by reconstructions of a hologram series taken of laser produced cavitation bubbles following optical breakdown in water.
Optics Communications.
