18 research outputs found
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
Air cavities at the inner cylinder of turbulent Taylor-Couette flow
Air cavities, i.e. air layers developed behind cavitators, are seen as a
promising drag reducing method in the maritime industry. Here we utilize the
Taylor-Couette (TC) geometry, i.e. the flow between two concentric,
independently rotating cylinders, to study the effect of air cavities in this
closed setup, which is well-accessible for drag measurements and optical flow
visualizations. We show that stable air cavities can be formed, and that the
cavity size increases with Reynolds number and void fraction. The streamwise
cavity length strongly depends on the axial position due to buoyancy forces
acting on the air. Strong secondary flows, which are introduced by a
counter-rotating outer cylinder, clearly decrease the stability of the
cavities, as air is captured in the Taylor rolls rather than in the cavity.
Surprisingly, we observed that local air injection is not necessary to sustain
the air cavities; as long as air is present in the system it is found to be
captured in the cavity. We show that the drag is decreased significantly as
compared to the case without air, but with the geometric modifications imposed
on the TC system by the cavitators. As the void fraction increases, the drag of
the system is decreased. However, the cavitators itself significantly increase
the drag due to their hydrodynamic resistance (pressure drag): In fact, a net
drag increase is found when compared to the standard smooth-wall TC case.
Therefore, one must first overcome the added drag created by the cavitators
before one obtains a net drag reduction.Comment: 14 pages, 13 figure
The influence of wall roughness on bubble drag reduction in Taylor-Couette turbulence
We experimentally study the influence of wall roughness on bubble drag
reduction in turbulent Taylor-Couette flow, i.e.\ the flow between two
concentric, independently rotating cylinders. We measure the drag in the system
for the cases with and without air, and add roughness by installing transverse
ribs on either one or both of the cylinders. For the smooth wall case (no ribs)
and the case of ribs on the inner cylinder only, we observe strong drag
reduction up to and , respectively, for a void fraction of
. However, with ribs mounted on both cylinders or on the outer
cylinder only, the drag reduction is weak, less than , and thus quite
close to the trivial effect of reduced effective density. Flow visualizations
show that stable turbulent Taylor vortices --- large scale vortical structures
--- are induced in these two cases, i.e. the cases with ribs on the outer
cylinder. These strong secondary flows move the bubbles away from the boundary
layer, making the bubbles less effective than what had previously been observed
for the smooth-wall case. Measurements with counter-rotating smooth cylinders,
a regime in which pronounced Taylor rolls are also induced, confirm that it is
really the Taylor vortices that weaken the bubble drag reduction mechanism. Our
findings show that, although bubble drag reduction can indeed be effective for
smooth walls, its effect can be spoiled by e.g.\ biofouling and omnipresent
wall roughness, as the roughness can induce strong secondary flows.Comment: 10 pages, 5 figure
Periodically driven Taylor-Couette turbulence
We study periodically driven Taylor-Couette turbulence, i.e. the flow
confined between two concentric, independently rotating cylinders. Here, the
inner cylinder is driven sinusoidally while the outer cylinder is kept at rest
(time-averaged Reynolds number is ). Using particle image
velocimetry (PIV), we measure the velocity over a wide range of modulation
periods, corresponding to a change in Womersley number in the range . To understand how the flow responds to a given modulation, we
calculate the phase delay and amplitude response of the azimuthal velocity.
In agreement with earlier theoretical and numerical work, we find that for
large modulation periods the system follows the given modulation of the
driving, i.e. the system behaves quasi-stationary. For smaller modulation
periods, the flow cannot follow the modulation, and the flow velocity responds
with a phase delay and a smaller amplitude response to the given modulation. If
we compare our results with numerical and theoretical results for the laminar
case, we find that the scalings of the phase delay and the amplitude response
are similar. However, the local response in the bulk of the flow is independent
of the distance to the modulated boundary. Apparently, the turbulent mixing is
strong enough to prevent the flow from having radius-dependent responses to the
given modulation.Comment: 12 pages, 6 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
Rough wall turbulent Taylor-Couette flow: the effect of the rib height
In this study, we combine experiments and direct numerical simulations to
investigate the effects of the height of transverse ribs at the walls on both
global and local flow properties in turbulent Taylor-Couette flow. We create
rib roughness by attaching up to 6 axial obstacles to the surfaces of the
cylinders over an extensive range of rib heights, up to blockages of 25% of the
gap width. In the asymptotic ultimate regime, where the transport is
independent of viscosity, we emperically find that the prefactor of the
scaling (corresponding to the drag coefficient
being constant) scales with the number of ribs and by the rib
height . The physical mechanism behind this is that the dominant
contribution to the torque originates from the pressure forces acting on the
rib which scale with rib height. The measured scaling relation of is slightly smaller than the expected scaling, presumably
because the ribs cannot be regarded as completely isolated but interact. In the
counter-rotating regime with smooth walls, the momentum transport is increased
by turbulent Taylor vortices. We find that also in the presence of transverse
ribs these vortices persist. In the counter-rotating regime, even for large
roughness heights, the momentum transport is enhanced by these vortices.Comment: 18 pages, 9 figure
Wall roughness induces asymptotic ultimate turbulence
Turbulence is omnipresent in Nature and technology, governing the transport
of heat, mass, and momentum on multiple scales. For real-world applications of
wall-bounded turbulence, the underlying surfaces are virtually always rough;
yet characterizing and understanding the effects of wall roughness for
turbulence remains a challenge, especially for rotating and thermally driven
turbulence. By combining extensive experiments and numerical simulations, here,
taking as example the paradigmatic Taylor-Couette system (the closed flow
between two independently rotating coaxial cylinders), we show how wall
roughness greatly enhances the overall transport properties and the
corresponding scaling exponents. If only one of the walls is rough, we reveal
that the bulk velocity is slaved to the rough side, due to the much stronger
coupling to that wall by the detaching flow structures. If both walls are
rough, the viscosity dependence is thoroughly eliminated in the boundary layers
and we thus achieve asymptotic ultimate turbulence, i.e. the upper limit of
transport, whose existence had been predicted by Robert Kraichnan in 1962
(Phys. Fluids {\bf 5}, 1374 (1962)) and in which the scalings laws can be
extrapolated to arbitrarily large Reynolds numbers