arXiv subjects

Emily S. C. Ching

Publications and source records attributed to Emily S. C. Ching.

At least 19 recordsLinked to original sources

arXiv2026

Buoyancy-driven melt rate of a vertical ice surface in seawater

Reliable estimates of the melt rate of tidewater glaciers require accurate knowledge of salt and heat fluxes at the near-vertical ice-ocean interface and such knowledge is currently lacking. In this paper, we present a theory for salt and heat fluxes in the idealized system of turbulent double-diffusive convection in an infinite vertical channel and show how this theory can be extended to give theoretical estimate of the buoyancy-driven melt rate of a vertical ice surface in seawater. Our theoretical results are shown to be in good agreement with direct numerical simulation data and measurements from laboratory experiments.

physics.flu-dyn↗

arXiv2026

Eddy thermal diffusivity model and mean temperature profiles in turbulent vertical convection

In this paper, we propose a space-dependent eddy thermal diffusivity model for turbulent vertical natural convection in a fluid between two infinite vertical walls at different temperatures. Using this model, we derive analytical results for the mean temperature profile. Our results reveal that mean temperature profiles for different Rayleigh and Prandtl numbers are described by two universal scaling functions in the inner region next to the walls and the outer region near the centerline between the two walls and the characteristic temperature scales in the inner and outer regions are expressed in terms of the two parameters of the model which determine the characteristic velocities for heat transfer in the two regions. We show that these results are in good agreement with direct numerical simulation data.

physics.flu-dyn↗

arXiv2025

Higher-order Tuning of Interface Physics in Multiphase Lattice Boltzmann

Tuning the interface properties of multiphase models is of paramount importance to the final goal of achieving a one-to-one matching with nucleation and cavitation experiments. The surface tension, at the leading order, and the Tolman length, at higher order, play a crucial role in the estimation of the free-energy barrier determining the experimentally observed nucleation rates. The lattice Boltzmann method allows for a computationally efficient modelling approach of multiphase flows, however, tuning results are concerned with the surface tension and neglect the Tolman length. We present a novel perspective that leverages all the degrees of freedom hidden in the forcing stencil of the Shan-Chen multiphase model. By means of the lattice pressure tensor we determine and tune the coefficients of higher-order derivative terms related to surface tension and Tolman length at constant interface width and density ratio. We test the method by means of both hydrostatic and dynamic simulations and demonstrate the dependence of homogeneous nucleation rates on the value of the Tolman length. This work provides a new tool that can be integrated with previously existing strategies thus marking a step forwards to a high-fidelity modelling of phase-changing fluid dynamics.

cond-mat.stat-mech↗

arXiv2024

Emergence of a dynamical state of coherent bursting with power-law distributed avalanches from collective stochastic dynamics of adaptive neurons

Spontaneous brain activity in the absence of external stimuli is not random but contains complex dynamical structures such as neuronal avalanches with power-law duration and size distributions. These experimental observations have been interpreted as supporting evidence for the hypothesis that the brain is operating near a critical point of phase transition between two states, and attracted much attention. Here, we show that a dynamical state of coherent bursting, with power-law distributed avalanches and features as observed in experiments, emerges in networks of adaptive neurons with stochastic input when excitation is sufficiently strong and balanced by adaptation. We demonstrate that these power-law distributed avalanches are direct consequences of stochasticity and coherent bursting, which in turn is the result of a balance between excitation and adaptation. Our work thus shows that the observed dynamical features of neuronal avalanches can arise from collective stochastic dynamics of adaptive neurons under suitable conditions and need not be signatures of criticality.

physics.bio-ph↗

arXiv2023

Heat Flux and Wall Shear Stress in Large Aspect-Ratio Turbulent Vertical Convection

We present a theoretical analysis of large aspect-ratio turbulent vertical convection that yields two relationships between heat flux and wall shear stress, measured respectively by the Nusselt number ($\text{Nu}$) and shear Reynolds number ($\text{Re}_τ$), in terms of the Rayleigh ($\text{Ra}$) and Prandtl numbers ($\text{Pr}$): $\text{Re}_τ^2 \text{Nu} = f(\text{Pr}) \text{Pr}^{-1} \text{Ra}$ in the high-$\text{Ra}$ limit and $\text{Nu} \approx C \text{Pr}^{\varepsilon} \text{Re}_τ$ with $\varepsilon=1/3$ for $\text{Pr} \gg1$ and $\varepsilon=1$ for $\text{Pr} \ll 1$, where $f(\text{Pr})$ is not a power law of $\text{Pr}$ and $C$ is a constant. These relationships imply $\text{Nu} \approx [C^2f(\text{Pr})]^{1/3} \text{Pr}^{-(1-2\varepsilon)/3} \text{Ra}^{1/3}$ and $\text{Re}_τ\approx[f(\text{Pr})/C]^{1/3}\text{Pr}^{-(1+\varepsilon)/3}\text{Ra}^{1/3}$ for high $\text{Ra}$.

physics.flu-dyn↗

arXiv2022

Revealing directed effective connectivity of cortical neuronal networks from measurements

In the study of biological networks, one of the major challenges is to understand the relationships between network structure and dynamics. In this paper, we model in vitro cortical neuronal cultures as stochastic dynamical systems and apply a method that reconstructs directed networks from dynamics [Ching and Tam, Phys. Rev. E 95, 010301(R), 2017] to reveal directed effective connectivity, namely the directed links and synaptic weights, of the neuronal cultures from voltage measurements recorded by a multielectrode array. The effective connectivity so obtained reproduces several features of cortical regions in rats and monkeys and has similar network properties as the synaptic network of the nematode C. elegans, the only organism whose entire nervous system has been mapped out as of today. The distribution of the incoming degree is bimodal and the distributions of the average incoming and outgoing synaptic strength are non-Gaussian with long tails. The effective connectivity captures different information from the commonly studied functional connectivity, estimated using statistical correlation between spiking activities. The average synaptic strengths of excitatory incoming and outgoing links are found to increase with the spiking activity in the estimated effective connectivity but not in the functional connectivity estimated using the same sets of voltage measurements. These results thus demonstrate that the reconstructed effective connectivity can capture the general properties of synaptic connections and better reveal relationships between network structure and dynamics.

q-bio.NC↗

arXiv2019

Effects of hidden nodes on the reconstruction of bidirectional networks

Much research effort has been devoted to developing methods for reconstructing the links of a network from dynamics of its nodes. Many current methods require the measurements of the dynamics of all the nodes be known. In real-world problems, it is common that either some nodes of a network of interest are unknown or the measurements of some nodes are unavailable. These nodes, either unknown or whose measurements are unavailable, are called hidden nodes. In this paper, we derive analytical results that explain the effects of hidden nodes on the reconstruction of bidirectional networks. These theoretical results and their implications are verified by numerical studies.

physics.data-an↗

arXiv2018

Polymers in Fluid Flows

The interaction of flexible polymers with fluid flows leads to a number of intriguing phenomena observed in laboratory experiments, namely drag reduction, elastic turbulence and heat transport modification in natural convection, and is one of the most challenging subjects in soft matter physics. In this paper we review our present knowledge on the subject. Our present knowledge is mostly based on direct numerical simulations performed in the last twenty years, which have successfully explained, at least qualitatively, most of the experimental results. Our goal is to disentangle as much as possible the basic mechanisms acting in the system in order to capture the basic features underlying different theoretical approaches and explanations.

physics.flu-dyn↗

arXiv2017

Mean Temperature Profiles in Turbulent Thermal Convection

To predict the mean temperature profiles in turbulent thermal convection, the thermal boundary layer (BL) equation including the effects of fluctuations has to be solved. In Shishkina et al., Phys. Rev. Lett. 114 (2015), the thermal BL equation with the fluctuations taken into account as an eddy thermal diffusivity has been solved for large Prandtl-number fluids for which the eddy thermal diffusivity and the velocity field can be approximated respectively as a cubic and a linear function of the distance from the plate. In the present work we make use of the idea of Prandtl's mixing length model and relate the eddy thermal diffusivity to the stream function. With this proposed relation, we can solve the thermal BL equation and obtain a closed-form expression for the dimensionless mean temperature profile in terms of two independent parameters for fluids with a general Prandtl number. With a proper choice of the parameters, our predictions of the temperature profiles are in excellent agreement with the results of our direct numerical simulations for a wide range of Prandtl numbers from 0.01 to 2547.9 and Rayleigh numbers from 10^7 to 10^9.

physics.flu-dyn↗

arXiv2016

Reconstructing Links in Directed Networks from Noisy Dynamics

In this Letter, we address the longstanding challenge of how to reconstruct links in directed networks from measurements, and present a general method that makes use of a noise-induced relation between network structure and both the time-lagged covariance of measurements taken at two different times and the covariance of measurements taken at the same time. For coupling functions that have additional properties, we can further reconstruct the weights of the links.

nlin.AO↗

arXiv2015

Thermal Boundary Layer Equation for Turbulent Rayleigh-Benard Convection

We report a new thermal boundary layer equation for turbulent Rayleigh-Benard convection for Prandtl number Pr>1 that takes into account the effect of turbulent fluctuations. These fluctuations are neglected in existing equations, which are based on steady-state and laminar assumptions. Using this new equation, we derive analytically the mean temperature profiles in two limits: (a) Pr>1, Pr~1 and (b) Pr>>1. These two theoretical predictions are in excellent agreement with the results of our direct numerical simulations for Pr=4.38 (water) and Pr=2547.9 (glycerol) respectively.

physics.flu-dyn↗

arXiv2011

Polymer Effects on Heat Transport in Laminar Boundary Layer Flow

We consider a laminar Blasius boundary-layer flow above a slightly heated horizontal plate and study the effect of polymer additives on the heat transport. We show that the action of the polymers can be understood as a space-dependent effective viscosity that first increases from the zero-shear value then decreases exponentially back to the zero-shear value as one moves away from the boundary. We find that with such an effective viscosity, both the horizontal and vertical velocities near the plate are decreased thus leading to an increase in the friction drag and a decrease in the heat transport in the flow.

nlin.CD↗

arXiv2009

Effects of particle-size ratio on jamming of binary mixtures

We perform a systematic numerical study of the effects of the particle-size ratio $R \ge 1$ on the properties of jammed binary mixtures. We find that changing $R$ does not qualitatively affect the critical scaling of the pressure and coordination number with the compression near the jamming transition, but the critical volume fraction at the jamming transition varies with $R$. Moreover, the static structure factor (density correlation) $S(k)$ strongly depends on $R$ and shows distinct long wave-length behaviors between large and small particles. Thus the previously reported behavior of $S(k)\sim k$ in the long wave-length limit is only a special case in the $R\to 1$ limit, and cannot be simply generalized to jammed systems with $R>1$.

cond-mat.soft↗

arXiv2009

Relations between the Material Mechanical Parameters and the Inter-particle Potential in Amorphous Solids

The shear-modulus and yield-stress of amorphous solids are important material parameters, with the former determining the rate of increase of stress under external strain and the latter being the stress value at which the material flows in a plastic manner. It is therefore important to understand how these parameters can be related to the inter-particle potential. Here a scaling theory is presented such that given the inter-particle potential, the dependence of the yield stress and the shear modulus on the density of the solid can be predicted in the athermal limit. It is explained when such prediction is possible at all densities and when it is only applicable at high densities. These results open up exciting possibilities for designing in principle new materials with desirable mechanical properties.

cond-mat.mtrl-sci↗

arXiv2008

Refined similarity hypotheses in shell models of turbulence

A major challenge in turbulence research is to understand from first principles the origin of anomalous scaling of the velocity fluctuations in high-Reynolds-number turbulent flows. One important idea was proposed by Kolmogorov [J. Fluid Mech. {\bf 13}, 82 (1962)], which attributes the anomaly to the variations of the locally averaged energy dissipation rate. Kraichnan later pointed out [J. Fluid Mech. {\bf 62}, 305 (1973)] that the locally averaged energy dissipation rate is not an inertial-range quantity and a proper inertial-range quantity would be the local energy transfer rate. As a result, Kraichnan's idea attributes the anomaly to the variations of the local energy transfer rate. These ideas, generally known as refined similarity hypotheses, can also be extended to study the anomalous scaling of fluctuations of an active scalar, like the temperature in turbulent convection. In this paper, we examine the validity of these refined similarity hypotheses and their extensions to an active scalar in shell models of turbulence. We find that Kraichnan's refined similarity hypothesis and its extension are valid.

nlin.CD↗

arXiv2008

Understanding the different scaling behavior in various shell models proposed for turbulent thermal convection

Different scaling behavior has been reported in various shell models proposed for turbulent thermal convection. In this paper, we show that buoyancy is not always relevant to the statistical properties of these shell models even though there is an explicit coupling between velocity and temperature in the equations of motion. When buoyancy is relevant (irrelevant) to the statistical properties, the scaling behavior is Bolgiano-Obukhov (Kolmogorov) plus intermittency corrections. We show that the intermittency corrections of temperature could be solely attributed to fluctuations in the entropy transfer rate when buoyancy is relevant but due to fluctuations in both energy and entropy transfer rates when buoyancy is irrelevant. This difference can be used as a criterion to distinguish whether temperature is behaving as an active or a passive scalar.

nlin.CD↗

arXiv2008

Ultimate-state scaling in a shell model for homogeneous turbulent convection

An interesting question in turbulent convection is how the heat transport depends on the strength of thermal forcing in the limit of very large thermal forcing. Kraichnan predicted [Phys. Fluids {\bf 5}, 1374 (1962)] that the heat transport measured by the Nusselt number (Nu) would depend on the strength of thermal forcing measured by the Rayleigh number (Ra) as Nu $\sim$ Ra$^{1/2}$ with possible logarithmic corrections at very high Ra. This scaling behavior is taken as a signature of the so-called ultimate state of turbulent convection. The ultimate state was interpreted in the Grossmann-Lohse (GL) theory [J. Fluid Mech. {\bf 407}, 27 (2000)] as a bulk-dominated state in which both the kinetic and thermal dissipation are dominated by contributions from the bulk of the flow with the boundary layers either broken down or playing no role in the heat transport. In this paper, we study the dependence of Nu and the Reynolds number (Re) measuring the root-mean-squared velocity fluctuations on Ra and the Prandtl number (Pr) using a shell model for homogeneous turbulent convection where buoyancy is acting directly on most of the scales. We find that Nu$\sim$ Ra$^{1/2}$Pr$^{1/2}$ and Re$\sim$ Ra$^{1/2}$Pr$^{-1/2}$, which resemble the ultimate-state scaling behavior for fluids with moderate Pr, but the presence of a drag acting on the large scales is crucial in giving rise to such scaling. This suggests that if buoyancy acts on most of the scales in the bulk of turbulent convection at very high Ra, then the ultimate state cannot be a bulk-dominated state.

nlin.CD↗

arXiv2007

Anomalous Scaling and Refined Similarity of an Active Scalar in a Model of Homogeneous Turbulent Convection

Anomalous scaling in the statistics of an active scalar in homogeneous turbulent convection is studied using a dynamical shell model. We extend refined similarity ideas for homogeneous and isotropic turbulence to homogeneous turbulent convection and attribute the origin of the anomalous scaling to variations of the entropy transfer rate. We verify the consequences and thus the validity of our hypothesis by showing that the conditional statistics of the active scalar and the velocity at fixed values of entropy transfer rate are not anomalous but have simple scaling with exponents given by dimensional considerations, and that the intermittency corrections are given by the scaling exponents of the moments of the entropy transfer rate.

nlin.CD↗