24 research outputs found
Multiphase wall-bounded turbulence
In many geophysical situations and in all industrial applications, turbulent flows are wall-bounded. Many of these flows are multi-phase, i.e. flows consisting of one or multiple inclusions. The current understanding of these flows is still limited and this makes it important to study them. In this thesis we study these wall-bounded multi-phase flows in two canonical systems: Taylor-Couette flow (TC) and Rayleigh-Bénard convection (RBC). In this work we used spherical and cylindrical particles to investigate if we have reduced skin friction similar to bubbly drag reduction. The global torque measurements showed that these particles barely alter the drag, even at very large particle volume fractions. Surprisingly, we found a preferential alignment for the cylindrical particles with respect to the inner cylinder wall. Using oil and water we are able to create deformable inclusions. Increasing the oil volume fraction over a critical point results in phase inversion with water droplets in oil. In this regime we found drag reduction due to the large water droplets in the flow. This is confirmed with in-situ microscopic imaging. In the last two chapters of this thesis we study the effect of non-homogeneous boundaries in both TC and RBC. Using bands of sandgrain roughness we were able to control the secondary flows in TC. This means that for example roughness like barnacles on the hull of a ship can induce secondary flows that push air bubbles away and thereby, reducing the drag reducing effect
Mixed insulating and conducting thermal boundary conditions in Rayleigh-B\'enard convection
A series of direct numerical simulations of Rayleigh-B\'enard convection, the
flow in a fluid layer heated from below and cooled from above, were conducted
to investigate the effect of mixed insulating and conducting boundary
conditions on convective flows. Rayleigh numbers between and
were considered, for Prandtl numbers and
. The bottom plate was divided into patterns of conducting and
insulating stripes. The size ratio between these stripes was fixed to unity and
the total number of stripes was varied. Global quantities such as the heat
transport and average bulk temperature and local quantities such as the
temperature just below the insulating boundary wall were investigated. For the
case with the top boundary divided into two halves, one conducting and one
insulating, the heat transfer was found to be approximately two thirds of the
fully conducting case. Increasing the pattern frequency increased the heat
transfer which asymptotically approached the fully conducting case, even if
only half of the surface is conducting. Fourier analysis of the temperature
field revealed that the imprinted pattern of the plates is diffused in the
thermal boundary layers, and cannot be detected in the bulk. With
conducting-insulating patterns on both plates, the trends previously described
were similar, however, the half-and-half division led to a heat transfer of
about a half of the fully conducting case instead of two-thirds. The effect of
the ratio of conducting and insulating areas was also analyzed, and it was
found that even for systems with a top plate with only conducting
surface, heat-transport of of the fully conducting case can be seen.
Changing the 1D stripe pattern to 2D checkerboard tessellations does not result
in a significantly different response of the system.Comment: Submitted to JF
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
Controlling secondary flow in Taylor-Couette turbulence through spanwise-varying roughness
Highly turbulent Taylor-Couette flow with spanwise-varying roughness is
investigated experimentally and numerically (direct numerical simulations (DNS)
with an immersed boundary method (IBM)) to determine the effects of the spacing
and axial width of the spanwise varying roughness on the total drag and
{on} the flow structures. We apply sandgrain roughness, in the form of
alternating {rough and smooth} bands to the inner cylinder. Numerically, the
Taylor number is and the roughness width is varied between
, where is the gap width. Experimentally,
we explore and .
For both approaches the radius ratio is fixed at , with
and the radius of the inner and outer cylinder respectively. We
present how the global transport properties and the local flow structures
depend on the boundary conditions set by the roughness spacing .
Both numerically and experimentally, we find a maximum in the angular momentum
transport as function of . This can be atributed to the
re-arrangement of the large-scale structures triggered by the presence of the
rough stripes, leading to correspondingly large-scale turbulent vortices.Comment: 20 pages, 7 figures, draft for JF
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
Catastrophic phase inversion in high-Reynolds number turbulent Taylor--Couette flow
Emulsions are omnipresent in the food industry, health care, and chemical
synthesis. In this Letter the dynamics of meta-stable oil-water emulsions in
highly turbulent () Taylor--Couette
flow, far from equilibrium, is investigated. By varying the oil-in-water void
fraction, catastrophic phase inversion between oil-in-water and water-in-oil
emulsions can be triggered, changing the morphology, including droplet sizes,
and rheological properties of the mixture, dramatically. The manifestation of
these different states is exemplified by combining global torque measurements
and local in-situ laser induced fluorescence (LIF) microscopy imaging. Despite
the turbulent state of the flow and the dynamic equilibrium of the oil-water
mixture, the global torque response of the system is found to be as if the
fluid were Newtonian, and the effective viscosity of the mixture was found to
be several times bigger or smaller than either of its constituents.Comment: 5 pages, 4 figure
Effect of axially varying sandpaper roughness on bubbly drag reduction in Taylor-Couette turbulence
We experimentally investigate the influence of alternating rough and smooth
walls on bubbly drag reduction (DR). We apply rough sandpaper bands of width
between and , and roughness height ,
around the smooth inner cylinder (IC) of the Twente Turbulent Taylor-Couette
facility. Between sandpaper bands, the IC is left uncovered over similar width
, resulting in alternating rough and smooth bands, a constant pattern in
axial direction. We measure the DR in water that originates from introducing
air bubbles to the fluid at (shear) Reynolds numbers ranging
from to . Results are compared to bubbly DR
measurements with a completely smooth IC and an IC that is completely covered
with sandpaper of the same roughness . The outer cylinder is left smooth for
all variations. Results are also compared to bubbly DR measurements where a
smooth outer cylinder is rotating in opposite direction to the smooth IC. This
counter rotation induces secondary flow structures that are very similar to
those observed when the IC is composed of alternating rough and smooth bands.
For the measurements with roughness, the bubbly DR is found to initially
increase more strongly with , before levelling off to reach a
value that no longer depends on . This is attributed to a more
even axial distribution of the air bubbles, resulting from the increased
turbulence intensity of the flow compared to flow over a completely smooth wall
at the same . The air bubbles are seen to accumulate at the
rough wall sections in the flow. Here, locally, the drag is largest and so the
drag reducing effect of the bubbles is felt strongest. Therefore, a larger
maximum value of bubbly DR is found for the alternating rough and smooth walls
compared to the completely rough wall