73 research outputs found
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
Enhancing thermal mixing in turbulent bubbly flow by adding salt
The presence of bubbles in a turbulent flow changes the flow drastically and
enhances the mixing. Adding salt to the bubbly aqueous flow changes the bubble
coalescence properties as compared to pure water. Here we provide direct
experimental evidence that also the turbulent thermal energy spectra are
strongly changed. Experiments were performed in the Twente Mass and Heat
Transfer water tunnel,in which we can measure the thermal spectra in bubbly
turbulence in salty water. We find that the mean bubble diameter decreases with
increasing concentration of salt (NaCl), due to the inhibition of bubble
coalescence. With increasing salinity, the transition frequency from the
classical scaling of the thermal energy spectrum to the bubble induced
scaling shifts to higher frequencies, thus enhancing the overall thermal
energy. We relate this frequency shift to the smaller size of the bubbles for
the salty bubbly flow. Finally we measure the heat transport in the bubbly
flow, and show how it varies with changing void fraction and salinity:
Increases in both result into increases in the number of extreme events.Comment: 18 pages, 10 figures, submitted to International Journal of
Multiphase Flo
Gerechtshof Arnhem-Leeuwarden 19 november 2019, AB 2020/116 (Overheidsverkoop, mededinging, gelijke kansen)
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Enhancing thermal mixing in turbulent bubbly flow by inhibiting bubble coalescence
The presence of bubbles in a turbulent flow changes the flow drastically and enhances the mixing. Adding salt to the bubbly aqueous flow changes the bubble coalescence properties as compared to regular demineralized water. Here we provide direct experimental evidence that also the turbulent thermal energy spectra are strongly changed. Experiments were performed in the Twente Mass and Heat Transfer water tunnel, in which we can measure the thermal spectra in bubbly turbulence in salty water. We find that the mean bubble diameter decreases with increasing concentration of salt (NaCl), due to the inhibition of bubble coalescence. With increasing salinity, the transition frequency from the classical −5/3 scaling of the thermal energy spectrum to the bubble induced −3 scaling shifts to higher frequencies, thus enhancing the overall thermal energy. We relate this frequency shift to the smaller size of the bubbles for the salty bubbly flow. Finally we measure the heat transport in the bubbly flow, and show how it varies with changing void fraction and salinity: Increases in both result into increases in the number of extreme events.</p
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