189 research outputs found
Vertical Coherence of Turbulence in the Atmospheric Surface Layer: Connecting the Hypotheses of Townsend and Davenport
Statistical descriptions of coherent flow motions in the atmospheric boundary
layer have many applications in the wind engineering community. For instance,
the dynamical characteristics of large-scale motions in wall-turbulence play an
important role in predicting the dynamical loads on buildings, or the
fluctuations in the power distribution across wind farms. Davenport (Quarterly
Journal of the Royal Meteorological Society, 1961, Vol. 372, 194-211) performed
a seminal study on the subject and proposed a hypothesis that is still widely
used to date. Here, we demonstrate how the empirical formulation of Davenport
is consistent with the analysis of Baars et al. (Journal of Fluid Mechanics,
2017, Vol. 823, R2) in the spirit of Townsend's attached-eddy hypothesis in
wall turbulence. We further study stratification effects based on two-point
measurements of atmospheric boundary-layer flow over the Utah salt flats. No
self-similar scaling is observed in stable conditions, putting the application
of Davenport's framework, as well as the attached eddy hypothesis, in question
for this case. Data obtained under unstable conditions exhibit clear
self-similar scaling and our analysis reveals a strong sensitivity of the
statistical aspect ratio of coherent structures (defined as the ratio of
streamwise and wall-normal extent) to the magnitude of the stability parameter
Planar bubble plumes from an array of nozzles:Measurements and modelling
Bubble curtains are widely used for sound mitigation during offshore pile driving to protect marine life. However, the lack of well validated hydrodynamic models is a major factor in the inability to predict the sound attenuation of a bubble curtain a priori. We present a new dataset resulting from bubble curtain measurements carried out in a 10 m deep and 31 m wide freshwater tank. The data describe the evolution of the void fraction profile and the bubble size distribution along the height of the bubble curtain. On this basis, a new relationship is developed for the dependence of the entrainment parameter of the bubble curtain on the air flowrate. In addition, we have extended a recently developed integral model for round bubble plumes to seamlessly capture the transition from initially individual round plumes to a planar plume after their merger. With additional modifications to the entrainment relation, the effective slip velocity and the initial condition for the bubble size distribution, the new model is found to be in good agreement with the data. In particular, the bubble size distribution sufficiently distant from the source is found to be independent of the gas flowrate, both in the data and in the model.</p
Rising and Sinking in Resonance: Probing the critical role of rotational dynamics for buoyancy driven spheres
We present experimental results for spherical particles rising and settling
in a still fluid. Imposing a well-controlled center of mass offset enables us
to vary the rotational dynamics selectively by introducing an intrinsic
rotational timescale to the problem. Results are highly sensitive even to small
degrees of offset, rendering this a practically relevant parameter by itself.
We further find that for a certain ratio of the rotational to a vortex shedding
timescale (capturing a Froude-type similarity) a resonance phenomenon sets in.
Even though this is a rotational effect in origin, it also strongly affects
translational oscillation frequency and amplitude, and most importantly the
drag coefficient. This observation equally applies to both heavy and light
spheres, albeit with slightly different characteristics for which we offer an
explanation. Our findings highlight the need to consider rotational parameters
when trying to understand and classify path properties of rising and settling
spheres.Comment: 7 pages, 4 figure
The large-scale footprint in small-scale Rayleigh-B\'enard turbulence
Turbulent convection systems are known to give rise to prominent large scale
circulation. At the same time, the `background' (or `small-scale') turbulence
is also highly relevant and e.g. carries the majority of the heat transport in
the bulk of the flow. Here, we investigate how the small-scale turbulence is
interlinked with the large-scale flow organization of Rayleigh-B\'enard
convection. Our results are based on a numerical simulation at Rayleigh number
in a large aspect ratio () cell to ensure a distinct
scale separation. We extract local magnitudes and wavenumbers of small scale
turbulence and find significant correlation of large scale variations in these
quantities with the large-scale signal. Most notably, we find stronger
temperature fluctuations and increased small scale transport (on the order of
of the global Nusselt number ) in plume impacting regions and
opposite trends in the plume emitting counterparts. This concerns wall
distances up to (thermal boundary layer thickness). Local
wavenumbers are generally found to be higher on the plume emitting side
compared to the impacting one. A second independent approach by means of
conditional averages confirmed these findings and yields additional insight
into the large-scale variation of small-scale properties. Our results have
implications for modelling small-scale turbulence.Comment: 19 pages, 9 figures, accepted at the Journal of Fluid Mechanic
Mass transport at gas-evolving electrodes
Direct numerical simulations are utilised to investigate mass-transfer processes at gas-evolving electrodes that experience successive formation and detachment of bubbles. The gas–liquid interface is modelled employing an immersed boundary method. We simulate the growth phase of the bubbles followed by their departure from the electrode surface in order to study the mixing induced by these processes. We find that the growth of the bubbles switches from a diffusion-limited mode at low to moderate fractional bubble coverages of the electrode to a reaction-limited growth dynamics at high coverages. Furthermore, our results indicate that the net transport within the system is governed by the effective buoyancy driving induced by the rising bubbles and that mechanisms commonly subsumed under the term ‘microconvection’ do not significantly affect the mass transport. Consequently, the resulting gas transport for different bubble sizes, current densities and electrode coverages can be collapsed onto one single curve and only depends on an effective Grashof number. The same holds for the mixing of the electrolyte when additionally taking the effect of surface blockage by attached bubbles into account. For the gas transport to the bubble, we find that the relevant Sherwood numbers also collapse onto a single curve when accounting for the driving force of bubble growth, incorporated in an effective Jakob number. Finally, linking the hydrogen transfer rates at the electrode and the bubble interface, an approximate correlation for the gas-evolution efficiency has been established. Taken together, these findings enable us to deduce parametrisations for all response parameters of the systems.</p
Coherence of temperature and velocity superstructures in turbulent Rayleigh-B\'enard flow
We investigate the interplay between large-scale patterns, so-called
superstructures, in the fluctuation fields of temperature and vertical
velocity in turbulent Rayleigh-B\'{e}nard convection at large aspect
ratios. Earlier studies suggested that velocity superstructures were smaller
than their thermal counterparts in the center of the domain. However, a
scale-by-scale analysis of the correlation between the two fields employing the
linear coherence spectrum reveals that superstructures of the same size exist
in both fields, which are almost perfectly correlated. The issue is further
clarified by the observation that in contrast to the temperature, and unlike
assumed previously, superstructures in the vertical velocity field do not
result in a peak in the power spectrum of . The origin of this difference is
traced back to the production terms of the - and -variance. These
results are confirmed for a range of Rayleigh numbers --, the
superstructure size is seen to increase monotonically with . Furthermore,
the scale distribution of particularly the temperature fluctuations is
pronouncedly bimodal. In addition to the large-scale peak caused by the
superstructures, there exists a strong small-scale peak. This `inner peak' is
most intense at a distance of from the wall and associated with
structures of size , where is the
thermal boundary layer thickness. Finally, based on the vertical coherence
relative to a reference height of , a self-similar structure is
identified in the velocity field (vertical and horizontal components) but not
in the temperature.Comment: 17 pages, 10 figure
Bubble-particle collisions in turbulence: insights from point-particle simulations
Bubble-particle collisions in turbulence are central to a variety of
processes such as froth flotation. Despite their importance, details of the
collision process have not received much attention yet. This is compounded by
the sometimes counter-intuitive behaviour of bubbles and particles in
turbulence, as exemplified by the fact that they segregate in space. Although
bubble-particle relative behaviour is fundamentally different from that of
identical particles, the existing theoretical models are nearly all extensions
of theories for particle-particle collisions in turbulence. The adequacy of
these theories has yet to be assessed as appropriate data remain scarce to
date. In this investigation, we study the geometric collision rate by means of
direct numerical simulations of bubble-particle collisions in homogeneous
isotropic turbulence using the point-particle approach over a range of the
relevant parameters, including the Stokes and Reynolds numbers. We analyse the
spatial distribution of bubble and particles, and quantify to what extent their
segregation reduces the collision rate. This effect is countered by increased
approach velocities for bubble-particle compared to monodisperse pairs, which
we relate to the difference in how bubbles and particles respond to fluid
accelerations. We found that in the investigated parameter range, these
collision statistics are not altered significantly by the inclusion of a lift
force or different drag parametrisations, or when assuming infinite particle
density. Furthermore, we critically examine existing models and discuss
inconsistencies therein that contribute to the discrepancy.Comment: 29 pages, 18 figures to be published in Journal of Fluid Mechanic
Electrolyte design for the manipulation of gas bubble detachment during hydrogen evolution reaction
During electrochemical gas evolution reactions, the continuous and vigorous formation of gas bubbles hugely impacts the efficiency of the underlying electrochemical processes. In particular, enhancing the detachment of gas bubbles from the electrode surface has emerged as an effective strategy to improve reaction efficiency. In this study, we demonstrate that the detachment of H2 gas bubbles can be controlled by the electrolyte composition, which can be optimized. We employ a well-defined Pt microelectrode and utilize electrochemical oscillation analysis to elucidate the features of H2 gas bubble detachment. Our investigation explores how the behaviour of H2 gas bubbles responds to variations in electrolyte composition and concentration. The coalescence efficiency of electrochemically generated microbubbles, a critical factor determining the mode of H2 gas bubble detachment (random detachment vs. single H2 gas bubble detachment), is profoundly influenced by the electrolyte composition. Specifically, coalescence efficiency follows the Hofmeister series concerning anions and coalescence is consistently inhibited in the presence of alkali metal cations. Furthermore, we establish a comprehensive model that accounts for both thermal and solutal Marangoni effects, allowing us to rationalize the trend of detachment size and period of single H2 gas bubbles under various conditions.</p
3D-PTV measurements in a plane Couette flow
Genuine plane Couette flow is hard to realize experimentally, and no applications of modern spatially resolving measurement techniques have been reported for this flow so far. In order to resolve this shortcoming, we designed and built a new experimental facility and present our first results here. Our setup enables us to access the flow via 3D particle tracking velocimetry and therefore to obtain truly three-dimensional flow fields for the first time experimentally in plane Couette flow. Results are analyzed in terms of basic flow properties, and a clear distinction of flow regimes (laminar for Re400) could be made. Comparison with DNS data shows good agreement in the turbulent regime and builds trust in our data. Furthermore, vortical coherent structures are studied in detail with the additional help of kalliroscope imaging, and the typical vortex spacing is determined to be roughly one gap width. As a noteworthy result, we find that the onset of the turbulent regime coincides with the range of Reynolds numbers at which a distance of 100 wall units is comparable to the gap widt
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