85 research outputs found
Techniques for Generating Centimetric Drops in Microgravity and Application to Cavitation Studies
This paper describes the techniques and physical parameters used to produce
stable centimetric water drops in microgravity, and to study single cavitation
bubbles inside such drops (Parabolic Flight Campaigns, European Space Agency
ESA). While the main scientific results have been presented in a previous
paper, we shall herein provide the necessary technical background, with
potential applications to other experiments. First, we present an original
method to produce and capture large stable drops in microgravity. This
technique succeeded in generating quasi-spherical water drops with volumes up
to 8 ml, despite the residual g-jitter. We find that the equilibrium of the
drops is essentially dictated by the ratio between the drop volume and the
contact surface used to capture the drop, and formulate a simple stability
criterion. In a second part, we present a setup for creating and studying
single cavitation bubbles inside those drops. In addition, we analyze the
influence of the bubble size and position on the drop behaviour after collapse,
i.e. jets and surface perturbations
Scaling laws for jets of single cavitation bubbles
Fast liquid jets, called micro-jets, are produced within cavitation bubbles
experiencing an aspherical collapse. Here we review micro-jets of different
origins, scales and appearances, and propose a unified framework to describe
their dynamics by using an anisotropy parameter , representing a
dimensionless measure of the liquid momentum at the collapse point (Kelvin
impulse). This parameter is rigorously defined for various jet drivers,
including gravity and nearby boundaries. Combining theoretical considerations
with hundreds of high-speed visualisations of bubbles collapsing near a rigid
surface, near a free surface or in variable gravity, we classify the jets into
three distinct regimes: weak, intermediate and strong. Weak jets
() hardly pierce the bubble, but remain within it throughout the
collapse and rebound. Intermediate jets () pierce the
opposite bubble wall close to the last collapse phase and clearly emerge during
the rebound. Strong jets () pierce the bubble early during the
collapse. The dynamics of the jets is analysed through key observables, such as
the jet impact time, jet speed, bubble displacement, bubble volume at jet
impact and vapour-jet volume. We find that, upon normalising these observables
to dimensionless jet parameters, they all reduce to straightforward functions
of , which we can reproduce numerically using potential flow theory. An
interesting consequence of this result is that a measurement of a single
observable, such as the bubble displacement, suffices to estimate any other
parameter, such as the jet speed. Remarkably, the dimensionless parameters of
intermediate and weak jets only depend on , not on the jet driver. In
the same regime, the jet parameters are found to be well approximated by
power-laws of , which we explain through analytical arguments
Shock waves from non-spherical cavitation bubbles
We present detailed observations of the shock waves emitted at the collapse
of single cavitation bubbles using simultaneous time-resolved shadowgraphy and
hydrophone pressure measurements. The geometry of the bubbles is systematically
varied from spherical to very non-spherical by decreasing their distance to a
free or rigid surface or by modulating the gravity-induced pressure gradient
aboard parabolic flights. The non-spherical collapse produces multiple shocks
that are clearly associated with different processes, such as the jet impact
and the individual collapses of the distinct bubble segments. For bubbles
collapsing near a free surface, the energy and timing of each shock are
measured separately as a function of the anisotropy parameter , which
represents the dimensionless equivalent of the Kelvin impulse. For a given
source of bubble deformation (free surface, rigid surface or gravity), the
normalized shock energy depends only on , irrespective of the bubble
radius and driving pressure . Based on this finding, we
develop a predictive framework for the peak pressure and energy of shock waves
from non-spherical bubble collapses. Combining statistical analysis of the
experimental data with theoretical derivations, we find that the shock peak
pressures can be estimated as jet impact-induced hammer pressures, expressed as
at . The same approach is found to explain the shock energy quenching as a
function of .Comment: Accepted for publication in Physical Review Fluid
The Quest for the Most Spherical Bubble
We describe a recently realized experiment producing the most spherical
cavitation bubbles today. The bubbles grow inside a liquid from a point-plasma
generated by a nanosecond laser pulse. Unlike in previous studies, the laser is
focussed by a parabolic mirror, resulting in a plasma of unprecedented
symmetry. The ensuing bubbles are sufficiently spherical that the hydrostatic
pressure gradient caused by gravity becomes the dominant source of asymmetry in
the collapse and rebound of the cavitation bubbles. To avoid this natural
source of asymmetry, the whole experiment is therefore performed in
microgravity conditions (ESA, 53rd and 56th parabolic flight campaign).
Cavitation bubbles were observed in microgravity (~0g), where their collapse
and rebound remain spherical, and in normal gravity (1g) to hyper-gravity
(1.8g), where a gravity-driven jet appears. Here, we describe the experimental
setup and technical results, and overview the science data. A selection of
high-quality shadowgraphy movies and time-resolved pressure data is published
online.Comment: 18 pages, 14 figures, 1 tabl
Energy partition at the collapse of spherical cavitation bubbles
Spherically collapsing cavitation bubbles produce a shock wave followed by a
rebound bubble. Here we present a systematic investigation of the energy
partition between the rebound and the shock. Highly spherical cavitation
bubbles are produced in microgravity, which suppress the buoyant pressure
gradient that otherwise deteriorates the sphericity of the bubbles. We measure
the radius of the rebound bubble and estimate the shock energy as a function of
the initial bubble radius (2-5.6 mm) and the liquid pressure (10-80 kPa). Those
measurements uncover a systematic pressure dependence of the energy partition
between rebound and shock. We demonstrate that these observations agree with a
physical model relying on a first-order approximation of the liquid
compressibility and an adiabatic treatment of the non-condensable gas inside
the bubble. Using this model we find that the energy partition between rebound
and shock is dictated by a single non-dimensional parameter , where is the driving pressure, is the static pressure in
the liquid, is the vapor pressure, is the pressure of the
non-condensable gas at the maximal bubble radius, is the adiabatic
index of the non-condensable gas, is the liquid density, and is the
speed of sound in the liquid.Comment: 7 pages, 7 figure
Techniques for generating centimetric drops in microgravity and application to cavitation studies
This paper describes the techniques and physical parameters used to produce stable centimetric water drops in microgravity, and to study single cavitation bubbles inside such drops (Parabolic Flight Campaigns, European Space Agency ESA). While the main scientific results have been presented in a previous paper, we shall herein provide the necessary technical background, with potential applications to other experiments. First, we present an original method to produce and capture large stable drops in microgravity. This technique succeeded in generating quasi-spherical water drops with volumes up to 8ml, despite the residual g-jitter. We find that the equilibrium of the drops is essentially dictated by the ratio between the drop volume and the contact surface used to capture the drop, and formulate a simple stability criterion. In a second part, we present a setup for creating and studying single cavitation bubbles inside those drops. In addition, we analyze the influence of the bubble size and position on the drop behaviour after collapse, i.e., jets and surface perturbation
Confined Shocks inside Isolated Liquid Volumes -- A New Path of Erosion?
The unique confinement of shock waves inside isolated liquid volumes
amplifies the density of shock-liquid interactions. We investigate this
universal principle through an interdisciplinary study of shock-induced
cavitation inside liquid volumes, isolated in 2 and 3 dimensions. By combining
high-speed visualizations of ideal water drops realized in microgravity with
smoothed particle simulations we evidence strong shock-induced cavitation at
the focus of the confined shocks. We extend this analysis to
ground-observations of jets and drops using an analytic model, and argue that
cavitation caused by trapped shocks offers a distinct mechanism of erosion in
high-speed impacts (>100 m/s).Comment: 4 page letter, 4 figure
Detailed Jet Dynamics in a Collapsing Bubble
We present detailed visualizations of the micro-jet forming inside an aspherically collapsing cavitation bubble near a free surface. The high-quality visualizations of large and strongly deformed bubbles disclose so far unseen features of the dynamics inside the bubble, such as a mushroom-like flattened jet-tip, crown formation and micro-droplets. We also find that jetting near a free surface reduces the collapse time relative to the Rayleigh time
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