26 research outputs found
How micropatterns and air pressure affect splashing on surfaces
We experimentally investigate the splashing mechanism of a millimeter-sized
ethanol drop impinging on a structured solid surface, comprised of
micro-pillars, through side-view and top-view high speed imaging. By increasing
the impact velocity we can tune the impact outcome from a gentle deposition to
a violent splash, at which tiny droplets are emitted as the liquid sheet
spreads laterally. We measure the splashing threshold for different
micropatterns and find that the arrangement of the pillars significantly
affects the splashing outcome. In particular, directional splashing in
direction in which air flow through pattern is possible. Our top-view
observations of impact dynamics reveal that an trapped air is responsible for
the splashing. Indeed by lowering the pressure of the surrounding air we show
that we can suppress the splashing in the explored parameter regime.Comment: 7 pages, 9 figure
Bubble drag reduction requires large bubbles
In the maritime industry, the injection of air bubbles into the turbulent
boundary layer under the ship hull is seen as one of the most promising
techniques to reduce the overall fuel consumption. However, the exact mechanism
behind bubble drag reduction is unknown. Here we show that bubble drag
reduction in turbulent flow dramatically depends on the bubble size. By adding
minute concentrations (6 ppm) of the surfactant Triton X-100 into otherwise
completely unchanged strongly turbulent Taylor-Couette flow containing bubbles,
we dramatically reduce the drag reduction from more than 40% to about 4%,
corresponding to the trivial effect of the bubbles on the density and viscosity
of the liquid. The reason for this striking behavior is that the addition of
surfactants prevents bubble coalescence, leading to much smaller bubbles. Our
result demonstrates that bubble deformability is crucial for bubble drag
reduction in turbulent flow and opens the door for an optimization of the
process.Comment: 4 pages, 2 figure
Self-similar decay of high Reynolds number Taylor-Couette turbulence
We study the decay of high-Reynolds number Taylor-Couette turbulence, i.e.
the turbulent flow between two coaxial rotating cylinders. To do so, the
rotation of the inner cylinder (Re, the outer cylinder is at
rest) is stopped within 12 s, thus fully removing the energy input to the
system. Using a combination of laser Doppler anemometry and particle image
velocimetry measurements, six decay decades of the kinetic energy could be
captured. First, in the absence of cylinder rotation, the flow-velocity during
the decay does not develop any height dependence in contrast to the well-known
Taylor vortex state. Second, the radial profile of the azimuthal velocity is
found to be self-similar. Nonetheless, the decay of this wall-bounded
inhomogeneous turbulent flow does not follow a strict power law as for decaying
turbulent homogeneous isotropic flows, but it is faster, due to the strong
viscous drag applied by the bounding walls. We theoretically describe the decay
in a quantitative way by taking the effects of additional friction at the walls
into account.Comment: 7 pages, 6 figure
Azimuthal velocity profiles in Rayleigh-stable Taylor-Couette flow and implied axial angular momentum transport
We present azimuthal velocity profiles measured in a Taylor-Couette
apparatus, which has been used as a model of stellar and planetary accretion
disks. The apparatus has a cylinder radius ratio of , an
aspect-ratio of , and the plates closing the cylinders in the
axial direction are attached to the outer cylinder. We investigate angular
momentum transport and Ekman pumping in the Rayleigh-stable regime. The regime
is linearly stable and is characterized by radially increasing specific angular
momentum. We present several Rayleigh-stable profiles for shear Reynolds
numbers , both for
(quasi-Keplerian regime) and (sub-rotating regime)
where is the inner/outer cylinder rotation rate. None of the
velocity profiles matches the non-vortical laminar Taylor-Couette profile. The
deviation from that profile increased as solid-body rotation is approached at
fixed . Flow super-rotation, an angular velocity greater than that of
both cylinders, is observed in the sub-rotating regime. The velocity profiles
give lower bounds for the torques required to rotate the inner cylinder that
were larger than the torques for the case of laminar Taylor-Couette flow. The
quasi-Keplerian profiles are composed of a well mixed inner region, having
approximately constant angular momentum, connected to an outer region in
solid-body rotation with the outer cylinder and attached axial boundaries.
These regions suggest that the angular momentum is transported axially to the
axial boundaries. Therefore, Taylor-Couette flow with closing plates attached
to the outer cylinder is an imperfect model for accretion disk flows,
especially with regard to their stability.Comment: 22 pages, 10 figures, 2 tables, under consideration for publication
in Journal of Fluid Mechanics (JFM
Exploring the phase space of multiple states in highly turbulent Taylor-Couette flow
We investigate the existence of multiple turbulent states in highly turbulent
Taylor-Couette flow in the range of to ,
by measuring the global torques and the local velocities while probing the
phase space spanned by the rotation rates of the inner and outer cylinder. The
multiple states are found to be very robust and are expected to persist beyond
. The rotation ratio is the parameter that most strongly
controls the transitions between the flow states; the transitional values only
weakly depend on the Taylor number. However, complex paths in the phase space
are necessary to unlock the full region of multiple states. Lastly, by mapping
the flow structures for various rotation ratios in a Taylor-Couette setup with
an equal radius ratio but a larger aspect ratio than before, multiple states
were again observed. Here, they are characterized by even richer roll structure
phenomena, including, for the first time observed in highly turbulent TC flow,
an antisymmetrical roll state.Comment: 9 pages, 7 figure
Taylor-Couette turbulence at radius ratio : scaling, flow structures and plumes
Using high-resolution particle image velocimetry we measure velocity
profiles, the wind Reynolds number and characteristics of turbulent plumes in
Taylor-Couette flow for a radius ratio of 0.5 and Taylor number of up to
. The extracted angular velocity profiles follow a log-law more
closely than the azimuthal velocity profiles due to the strong curvature of
this setup. The scaling of the wind Reynolds number with the Taylor
number agrees with the theoretically predicted 3/7-scaling for the classical
turbulent regime, which is much more pronounced than for the well-explored
case, for which the ultimate regime sets in at much lower Ta. By
measuring at varying axial positions, roll structures are found for
counter-rotation while no clear coherent structures are seen for pure inner
cylinder rotation. In addition, turbulent plumes coming from the inner and
outer cylinder are investigated. For pure inner cylinder rotation, the plumes
in the radial velocity move away from the inner cylinder, while the plumes in
the azimuthal velocity mainly move away from the outer cylinder. For
counter-rotation, the mean radial flow in the roll structures strongly affects
the direction and intensity of the turbulent plumes. Furthermore, it is
experimentally confirmed that in regions where plumes are emitted, boundary
layer profiles with a logarithmic signature are created
Direct measurements of air layer profiles under impacting droplets using high-speed color interferometry
A drop impacting on a solid surface deforms before the liquid makes contact
with the surface. We directly measure the time evolution of the air layer
profile under the droplet using high-speed color interferometry, obtaining the
air layer thickness before and during the wetting process. Based on the time
evolution of the extracted profiles obtained at multiple times, we measure the
velocity of air exiting from the gap between the liquid and the solid, and
account for the wetting mechanism and bubble entrapment. The present work
offers a tool to accurately measure the air layer profile and quantitatively
study the impact dynamics at a short time scale before impact