12 research outputs found
Wall forces on a sphere in a rotating liquid-filled cylinder
We experimentally study the behavior of a particle slightly denser than the
surrounding liquid in solid body rotating flow. Earlier work revealed that a
heavy particle has an unstable equilibrium point in unbounded rotation flows.
In the confinement of the rotational flow by a cylindrical wall a heavy sphere
with density 1.05 g/cm describes an orbital motion in our experiments. This
is due to the effect of the wall near the sphere, i.e. a repulsive force
(). We model on the sphere as a function of the distance from the
wall (): as proposed by Takemura and Magnaudet (2003).
Remarkably, the path from the model including reproduce the
experimentally measured trajectory. In addition during an orbital motion the
particle does not spin around its axis, and we provide a possible explanation
for this phenomenon.Comment: 11 pages, 11 figure
Bubble size prediction in co-flowing streams
In this paper, the size of bubbles formed through the breakup of a gaseous
jet in a co-axial microfluidic device is derived. The gaseous jet surrounded by
a co-flowing liquid stream breaks up into monodisperse microbubbles and the
size of the bubbles is determined by the radius of the inner gas jet and the
bubble formation frequency. We obtain the radius of the gas jet by solving the
Navier-Stokes equations for low Reynolds number flows and by minimization of
the dissipation energy. The prediction of the bubble size is based on the
system's control parameters only, i.e. the inner gas flow rate , the outer
liquid flow rate , and the tube radius . For a very low gas-to-liquid
flow rate ratio () the bubble radius scales as , independently of the inner to outer viscosity
ratio and of the type of the velocity profile in the gas, which
can be either flat or parabolic, depending on whether high-molecular-weight
surfactants cover the gas-liquid interface or not. However, in the case in
which the gas velocity profiles are parabolic and the viscosity ratio is
sufficiently low, i.e. , the bubble diameter scales as
, with smaller than 1/2
Drag and lift forces on a counter-rotating cylinder in rotating flow
Results are reported of an experimental investigation into the motion of a
heavy cylinder free to move inside a water-filled drum rotating around a
horizontal axis. The cylinder is observed to either co- or, counter
intuitively, counter-rotate with respect to the rotating drum. The flow was
measured with particle image velocimetry (PIV), and it was found that the inner
cylinder significantly altered the bulk flow field from the solid-body rotation
found for a fluid filled drum. In the counter-rotation case, the generated lift
force allowed the cylinder to freely rotate without contact with the drum wall.
Drag and lift coefficients of the freely counter-rotating cylinder were
measured over a wide range of Reynolds numbers, 2,500 Re 25,000,
dimensionless rotation rates, 0.01.2, and gap to cylinder
diameter ratios 0.003 0.5. Drag coefficients were consistent with
previous measurements on a cylinder in a uniform flow. However, for the lift
coefficient considerable larger values were observed in the present
measurements. We found the enhancement of the lift force to be mainly caused by
the vicinity of the wall
Oscillations of a gas pocket on a liquid-covered solid surface
The dynamic response of a gas bubble entrapped in a cavity on the surface of
a submerged solid subject to an acoustic field is investigated in the linear
approximation. We derive semi-analytical expressions for the resonance
frequency, damping and interface shape of the bubble. For the liquid phase, we
consider two limit cases: potential flow and unsteady Stokes flow. The
oscillation frequency and interface shape are found to depend on two
dimensionless parameters: the ratio of the gas stiffness to the surface tension
stiffness, and the Ohnesorge number, representing the relative importance of
viscous forces. We perform a parametric study and show, among others, that an
increase in the gas pressure or a decrease in the surface tension leads to an
increase in the resonance frequency until an asymptotic value is reached
Energy spectra in turbulent bubbly flows
We conduct experiments in a turbulent bubbly flow to study the nature of the
transition between the classical 5/3 energy spectrum scaling for a
single-phase turbulent flow and the 3 scaling for a swarm of bubbles rising
in a quiescent liquid and of bubble-dominated turbulence. The bubblance
parameter, which measures the ratio of the bubble-induced kinetic energy to the
kinetic energy induced by the turbulent liquid fluctuations before bubble
injection, is often used to characterise the bubbly flow. We vary the bubblance
parameter from (pseudo-turbulence) to (single-phase flow)
over 2-3 orders of magnitude () to study its effect on the turbulent
energy spectrum and liquid velocity fluctuations. The probability density
functions (PDFs) of the liquid velocity fluctuations show deviations from the
Gaussian profile for , i.e. when bubbles are present in the system. The
PDFs are asymmetric with higher probability in the positive tails. The energy
spectra are found to follow the 3 scaling at length scales smaller than the
size of the bubbles for bubbly flows. This 3 spectrum scaling holds not only
in the well-established case of pseudo-turbulence, but surprisingly in all
cases where bubbles are present in the system (). Therefore, it is a
generic feature of turbulent bubbly flows, and the bubblance parameter is
probably not a suitable parameter to characterise the energy spectrum in bubbly
turbulent flows. The physical reason is that the energy input by the bubbles
passes over only to higher wave numbers, and the energy production due to the
bubbles can be directly balanced by the viscous dissipation in the bubble wakes
as suggested by Lance Bataille (1991). In addition, we provide an
alternative explanation by balancing the energy production of the bubbles with
viscous dissipation in the Fourier space.Comment: J. Fluid Mech. (in press
Melting of olive oil in immiscible surroundings: experiments and theory
We report on the melting dynamics of frozen olive oil in quiescent water for
Rayleigh numbers up to . The density difference results in an upward
buoyancy-driven flow of liquid oil forming a thin film around the frozen oil.
We experimentally investigate flat, cylindrical, and spherical shapes and we
derive theoretical expressions for the local film thickness, velocity, and the
local melt rate for these three canonical geometries. Our theoretical models
compare favourably with our experimental findings.Comment: 18 pages, 9 figures, to be submitte
Interplay between air and water
In the Prologue I recall, among others, the period of the Cold War in which, thanks to Polish colleagues, scientific contacts between East and West were maintained . After that, several aspects of the flow of mixtures of air and water will be discussed and illustrated by examples. Finally I will give some comments on the differences and similarities between fundamental and applied science and scientists