60 research outputs found
Double-diffusive transport in multicomponent vertical convection
Motivated by the ablation of vertical ice faces in salt water, we use
three-dimensional direct numerical simulations to investigate the heat and salt
fluxes in two-scalar vertical convection. For parameters relevant to ice-ocean
interfaces, we observe that the salinity field drives the convection and that
heat is essentially transported as a passive scalar. By varying the diffusivity
ratio of heat and salt (the Lewis number ), we identify how the different
molecular diffusivities affect the scalar fluxes through the system. Away from
the walls, we find that the heat transport is determined by a turbulent Prandtl
number of and that double-diffusive effects are practically
negligible. However, the difference in molecular diffusivities plays an
important role close to the boundaries. In the (unrealistic) case where salt
diffused faster than heat, the ratio of fluxes would scale as ,
consistent with classical nested scalar boundary layers. However, in the
realistic case of faster heat diffusion (relative to salt), we observe a
transition towards a scaling of the ratio of the fluxes. This
coincides with the thermal boundary layer width growing beyond the thickness of
the viscous boundary layer. We compare our results to similar studies of
sheared and double-diffusive flow under ice shelves, and discuss the
implications for fluxes in large-scale ice-ocean models.Comment: 24 pages, 6 figures, submitted to Phys. Rev. Fluid
Shear-induced breaking of internal gravity waves
Motivated by observations of turbulence in the strongly stratified ocean
thermocline, we use direct numerical simulations to investigate the interaction
of a sinusoidal shear flow and a large-amplitude internal gravity wave. Despite
strong nonlinearities in the flow and a lack of scale separation, we find that
linear ray tracing theory is qualitatively useful in describing the early
development of the flow as the wave is refracted by the shear. Consistent with
the linear theory, the energy of the wave accumulates in regions of negative
mean shear where we observe evidence of convective and shear instabilities.
Streamwise-aligned convective rolls emerge the fastest, but their contribution
to irreversible mixing is dwarfed by shear-driven billow structures that
develop later. Although the wave strongly distorts the buoyancy field on which
these billows develop, the mixing efficiency of the subsequent turbulence is
similar to that arising from Kelvin-Helmholtz instability in a stratified shear
layer. We run simulations at Reynolds numbers of 5000 and 8000, and vary the
initial amplitude of the internal gravity wave. For high values of initial wave
amplitude, the results are qualitatively independent of . Smaller initial
wave amplitudes delay the onset of the instabilities, and allow for significant
laminar diffusion of the internal wave, leading to reduced turbulent activity.
We discuss the complex interaction between the mean flow, internal gravity wave
and turbulence, and its implications for internal wave-driven mixing in the
ocean.Comment: 27 pages, 12 figures, accepted to J. Fluid. Mec
Quantifying mixing and available potential energy in vertically periodic simulations of stratified flows
Turbulent mixing exerts a significant influence on many physical processes in
the ocean. In a stably stratified Boussinesq fluid, this irreversible mixing
describes the conversion of available potential energy (APE) to background
potential energy (BPE). In some settings the APE framework is difficult to
apply and approximate measures are used to estimate irreversible mixing. For
example, numerical simulations of stratified turbulence often use triply
periodic domains to increase computational efficiency. In this setup however,
BPE is not uniquely defined and the method of Winters et al. (1995, J. Fluid
Mech., 289) cannot be directly applied to calculate the APE. We propose a new
technique to calculate APE in periodic domains with a mean stratification. By
defining a control volume bounded by surfaces of constant buoyancy, we can
construct an appropriate background buoyancy profile and
accurately quantify diapycnal mixing in such systems. This technique also
permits the accurate calculation of a finite amplitude local APE density in
periodic domains. The evolution of APE is analysed in various turbulent
stratified flow simulations. We show that the mean dissipation rate of buoyancy
variance provides a good approximation to the mean diapycnal mixing
rate, even in flows with significant variations in local stratification. When
quantifying measures of mixing efficiency in transient flows, we find
significant variation depending on whether laminar diffusion of a mean flow is
included in the kinetic energy dissipation rate. We discuss how best to
interpret these results in the context of quantifying diapycnal diffusivity in
real oceanographic flows.Comment: 28 pages, 10 figures, accepted to J. Fluid Mec
Towards the understanding of convective dissolution in confined porous media:thin bead pack experiments, two-dimensional direct numerical simulations and physical models
We consider the process of convective dissolution in a homogeneous and isotropic porous medium. The flow is unstable due to the presence of a solute that induces a density difference responsible for driving the flow. The mixing dynamics is thus driven by a Rayleigh-Taylor instability at the pore scale. We investigate the flow at the scale of the pores using Hele-Shaw type experiment with bead packs, two-dimensional direct numerical simulations and physical models. Experiments and simulations have been specifically designed to mimic the same flow conditions, namely matching porosities, high Schmidt numbers and linear dependency of fluid density with solute concentration. In addition, the solid obstacles of the medium are impermeable to fluid and solute. We characterise the evolution of the flow via the mixing length, which quantifies the extension of the mixing region and grows linearly in time. The flow structure, analysed via the centreline mean wavelength, is observed to grow in agreement with theoretical predictions. Finally, we analyse the dissolution dynamics of the system, quantified through the mean scalar dissipation, and three mixing regimes are observed. Initially, the evolution is controlled by diffusion, which produces solute mixing across the initial horizontal interface. Then, when the interfacial diffusive layer is sufficiently thick, it becomes unstable, forming finger-like structures and driving the system into a convection-dominated phase. Finally, when the fingers have grown sufficiently to touch the horizontal boundaries of the domain, the mixing reduces dramatically due to the absence of fresh unmixed fluid. With the aid of simple physical models, we explain the physics of the results obtained numerically and experimentally. The solute evolution presents a self-similar behaviour, and it is controlled by different length scales in each stage of the mixing process, namely the length scale of diffusion, the pore size and the domain height.</p
Convective dissolution in confined porous media
We consider the process of convective dissolution in a homogeneous and
isotropic porous medium. The flow is unstable due to the presence of a solute
that induces a density difference responsible for driving the flow. The mixing
dynamics is thus driven by a Rayleigh-Taylor instability at the pore scale. We
investigate the flow at the scale of the pores using experimental measurements,
numerical simulations and physical models. Experiments and simulations have
been specifically designed to mimic the same flow conditions, namely matching
porosities, high Schmidt numbers, and linear dependency of fluid density with
solute concentration. In addition, the solid obstacles of the medium are
impermeable to fluid and solute. We characterise the evolution of the flow via
the mixing length, which quantifies the extension of the mixing region and
grows linearly in time. The flow structure, analysed via the centre-line mean
wavelength, is observed to grow in agreement with theoretical predictions.
Finally, we analyse the dissolution dynamics of the system, quantified through
the mean scalar dissipation, and three mixing regimes are observed. Initially,
the evolution is controlled by diffusion, which produces solute mixing across
the initial horizontal interface. Then, when the interfacial diffusive layer is
sufficiently thick, it becomes unstable, forming finger-like structures and
driving the system into a convection-dominated phase. Finally, when the fingers
have grown sufficiently to touch the horizontal boundaries of the domain, the
mixing reduces dramatically due to the absence of fresh unmixed fluid. With the
aid of simple physical models, we explain the physics of the results obtained
numerically and experimentally
Shape effect on ice melting in flowing water
Iceberg melting is a critical factor for climate change, contributing to
rising sea levels and climate change. However, the shape of an iceberg is an
often neglected aspect of its melting process. Our study investigates the
influence of different ice shapes and ambient flow velocities on melt rates by
conducting direct numerical simulations. Our study focuses on the ellipsoidal
shape, with the aspect ratio as the control parameter. It plays a crucial role
in the melting process, resulting in significant variations in the melt rate
between different shapes. Without flow, the optimal shape for a minimal melt
rate is the disk (2D) or sphere (3D), due to the minimal surface area. However,
as the ambient flow velocity increases, the optimal shape changes with the
aspect ratio. We find that ice with an elliptical shape (when the long axis is
aligned with the flow direction) can melt up to 10\% slower than a circular
shape when exposed to flowing water. We provide a quantitative theoretical
explanation for this optimal shape, based on the competition between surface
area effects and convective heat transfer effects. Our findings provide insight
into the interplay between phase transitions and ambient flows, contributing to
our understanding of the iceberg melting process and highlighting the need to
consider the aspect ratio effect in estimates of iceberg melt rates
Shape effect on solid melting in flowing liquid
Iceberg melting is a critical factor for climate change. However, the shape of an iceberg is an often neglected aspect of its melting process. Our study investigates the influence of different ice shapes and ambient flow velocities on melt rates by conducting direct numerical simulations of a simplified system of bluff body flow. Our study focuses on the ellipsoidal shape, with the aspect ratio as the control parameter. We found the shape plays a crucial role in the melting process, resulting in significant variations in the melt rate between different shapes. Without flow, the optimal shape for a minimal melt rate is the disk (two-dimensional) or sphere (three-dimensional), due to the minimal surface area. However, as the ambient flow velocity increases, the optimal shape changes with the aspect ratio. We find that ice with an elliptical shape (when the long axis is aligned with the flow direction) can melt up to 10% slower than a circular shape when exposed to flowing water. Following the approach considered by Huang et al. (J. Fluid Mech., vol. 765, 2015, R3) for dissolving bodies, we provide a quantitative theoretical explanation for this optimal shape, based on the combined contributions from both surface-area effects and convective-heat-transfer effects. Our findings provide insight into the interplay between phase transitions and ambient flows, contributing to our understanding of the iceberg melting process and highlighting the need to consider the aspect-ratio effect in estimates of iceberg melt rates.</p
The Rat Medial Prefrontal Cortex Exhibits Flexible Neural Activity States during the Performance of an Odor Span Task
Medial prefrontal cortex (mPFC) activity is fundamental for working memory (WM), attention, and behavioral inhibition; however, a comprehensive understanding of the neural computations underlying these processes is still forthcoming. Toward this goal, neural recordings were obtained from the mPFC of awake, behaving rats performing an odor span task of WM capacity. Neural populations were observed to encode distinct task epochs and the transitions between epochs were accompanied by abrupt shifts in neural activity patterns. Putative pyramidal neuron activity increased earlier in the delay for sessions where rats achieved higher spans. Furthermore, increased putative interneuron activity was only observed at the termination of the delay thus indicating that local processing in inhibitory networks was a unique feature to initiate foraging. During foraging, changes in neural activity patterns associated with the approach to a novel odor, but not familiar odors, were robust. Collectively, these data suggest that distinct mPFC activity states underlie the delay, foraging, and reward epochs of the odor span task. Transitions between these states likely enables adaptive behavior in dynamic environments that place strong demands on the substrates of working memory
Nonlinear Photon Pair Generation in a Highly Dispersive Medium
Photon pair generation in silicon photonic integrated circuits relies on four wave mixing via the third order nonlinearity. Due to phase matching requirements and group velocity dispersion, this method has typically required TE polarized light. Here, we demonstrate TM polarized photon pair production in linearly uncoupled silicon resonators with more than an order of magnitude more dispersion than previous work. We achieve measured rates above 2.8 kHz and a heralded second order correlation of . This method enables phase matching in dispersive media and paves the way for novel entanglement generation in silicon photonic device
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