Viscous Transport Research Articles

Effects of heat release on turbulence characteristics in a three-dimensional spatially developing supersonic droplet-laden mixing layer

By direct numerical simulations, the effects of heat release on turbulence characteristics are systematically investigated in a three-dimensional spatially developing supersonic droplet-laden mixing layer with the convective Mach number of 1.2. A one-step Arrhenius global reaction mechanism is employed for the combustion of n-decane and the liquid fuel is supposed to be fully atomized point droplets. The reactive and inert simulations are both conducted for comparison. The thermal-runaway ignition occurs in the transition region and the flame surfaces distribute at the upper and lower edges of the mixing layer. The diffusion combustion happens on the hot side, but the flame tends to extend towards the cold fuel layer with more vigorous chemical reactions, where the premixed combustion mode dominates. The combustion heat release makes the droplets gradually spread downwards and the tendency that the droplets accumulate in the low-temperature and high-density regions is augmented. Under the effects of chemical reactions, the mixing layer asymmetry is enhanced with the shear layer center further skewing towards the hot side and turbulence anisotropy is augmented due to the reduction of the pressure-strain rate. The fluctuations of temperature, density and species mass fractions are also augmented and two peaks appear at the positions of flame surfaces on the hot and cold sides, where the lower peak is more significant. However, the Reynolds stresses are attenuated, but exhibit the inflection points with larger magnitude on the cold side, which is attributed to the viscous transport and the enhancement of thermodynamic fluctuations.

Lagrangian analysis of enstrophy dynamics in a highly turbulent premixed flame

A Lagrangian analysis approach is used to examine the effects of heat release on the dynamics of the enstrophy during highly turbulent premixed combustion. The analysis is performed using data from a direct numerical simulation of a statistically planar premixed methane–air flame at a Karlovitz number of 100. Through cumulative, conditional, and correlation analyses, we show, consistent with prior studies, that vortex stretching and baroclinic torque both increase enstrophy at these highly turbulent conditions, while viscous transport and dilatation both lead to enstrophy destruction. However, although vortex stretching and viscous transport are individually an order of magnitude greater than all other terms in the enstrophy budget, the cumulative and combined effect of these two terms along Lagrangian trajectories is roughly only twice as large as the combined cumulative effect of dilatation and baroclinic torque. Moreover, trajectories that exhibit an increase in enstrophy through the flame are found to frequently have cumulative contributions from budget terms outside a single standard deviation of the mean contribution, indicating that enstrophy production at such highly turbulent conditions is associated with relatively infrequent but large values of dynamical terms. Lagged correlations further reveal a small but measurable contribution of baroclinic torque in enstrophy production, but these increases are overwhelmed, on average, by concurrent decreases in enstrophy due to viscous transport and dilatation. Taken together, these results provide further understanding of enstrophy dynamics in highly turbulent premixed flames.

Superballistic electron flow through a point contact in a Ga[Al]As heterostructure

We measure electronic transport through point contacts in the high-mobility electron gas in a Ga[Al]As heterostructure at different temperatures and bulk electron densities. The conductance through all point contacts increases with increasing temperature in a temperature window around $T \sim 10 K$ for all investigated electron densities and point contact widths. For high electron densities this conductance exceeds the fundamental ballistic limit (Sharvin limit). These observations are in agreement with a viscous electron transport model and previous experiments in graphene.

Laser engraved microapillary pump paper-based microfluidic device for colorimetric and electrochemical detection of salivary thiocyanate.

A microcapillary grooved paper-based analytical device capable of dual-mode sensing (colorimetric and electrochemical detection) was demonstrated for analysis of viscous samples (e.g., human saliva). Herein, a hollow capillary channel wasconstructed via laser engraved micropatterning functions as a micropump to facilitate viscous fluidic transport, which would otherwise impede analysis on paper devices. Using salivary thiocyanate as a model analyte, the proposed device was found to exhibit a promising sensing ability on paper devices without the need for sample pretreatment or bulky instrumentation, as normally required in conventional methods used for saliva analysis. An extensive linear dynamic range covering detection of salivary thiocyanate for both high and trace level regimes (5 orders of magnitudeworking range) was collectively achieved using the dual-sensing modes. Under optimal conditions, the limit of detection was 6μmolL-1 with a RSD of less than 5%. An excellent stability for the μpumpPAD was also observed for over 30days. Real sample analysis using the proposed device was found to be in line with the standard chromatographic method. Benefitting from simple fabrication and operation, portability, disposability, low sample volume (20μL), and low cost (< 1 USD), the μpumpPAD is an exceptional alternative tool for the detection of various biomarkers in saliva specimens.

A two-step gravitational cascade for the fragmentation of self-gravitating discs

ABSTRACT Self-gravitating discs are believed to play an important role in astrophysics in particular regarding the star and planet formation process. In this context, discs subject to an idealized cooling process, characterized by a cooling time-scale β expressed in unit of orbital time-scale, have been extensively studied. We take advantage of the Riemann solver and the 3D Godunov scheme implemented in the code Ramses to perform high-resolution simulations, complementing previous studies that have used smoothed particle hydrodynamics (SPH) or 2D grid codes. We observe that the critical value of β for which the disc fragments is consistent with most previous results, and is not well converged with resolution. By studying the probability density function of the fluctuations of the column density (Σ-PDF), we argue that there is no strict separation between the fragmented and the unfragmented regimes but rather a smooth transition with the probability of apparition of fragments steadily diminishing as the cooling becomes less effective. We find that the high column density part of the Σ-PDF follows a simple power law whose slope turns out to be proportional to β and we propose an explanation based on the balance between cooling and heating through gravitational stress. Our explanation suggests that a more efficient cooling requires more heating implying a larger fraction of dense material which, in the absence of characteristic scales, results in a shallower scale-free power law. We propose that the gravitational cascade proceeds in two steps, first the formation of a dense filamentary spiral pattern through a sequence of quasi-static equilibrium triggered by the viscous transport of angular momentum, and second the collapse alongside these filaments that eventually results in the formation of bounded fragments.

Effect of solder joint size and composition on liquid-assisted healing

This work presents an experimental and numerical investigation of liquid-assisted healing with respect to solder joint composition and size. A damage-healing model is formulated based on entropy generation of damage and viscous material transport during healing. The model accounts for a composition-dependency of healing by introducing the liquid film thickness, the liquid viscosity and a microstructural mobility parameter. The size dependencies enter the healing model in form of the local capillary pressure of the solder joint. A viscous flow experiment illustrates the compositional dependency of local material transport and a cyclic tensile experiment shows the regain of mechanical properties, such as, stiffness and strength after healing. The flow experiment shows that material transport is retarded for solders of low liquid fraction and crack healing is limited due to partial filling of the crack. Simulation results of a solder array suggest the capillary pressure as the driving force for healing, which leads to a size-dependency of the healing evolution. The required time for complete healing increases with reduced solder dimensions due to higher capillary pressures. Microstructural mobility due to high liquid fractions also promotes healing.

Multi-objective optimization of thermophysical properties of f\u2013Al2O3 nano-dispersions in heat transfer oil

Nanofluids are proven to be the next-generation smart fluids with tunable thermal and viscous properties. Nanomaterial concentration plays a vital role in determining the heat transfer and viscous transport characteristics. An optimum concentration is generally required to regulate a feasible and economical heat transfer operation. This research involves the modeling and optimizing different temperature-dependent thermal and viscous parameters for varying concentrations of nanofluids. The nanofluids consist of functionalized alumina (f–Al2O3) nano-dispersions in thermal oil (highly refined mineral oil). The experimentally measured temperature-dependent nanofluids' properties are used to optimize thermophysical parameters using Response Surface Methodology. Two case studies/scenarios are considered in the present research, where the primary objective is to maximize thermal conductivity for heat transfer applications and minimize nanoparticle loadings for economical operation. The input parameters include temperature and nanoparticle loadings. The output parameters or response include thermal conductivity, viscosity, density, and specific heat of nanofluids. For case study 1, the optimal findings for the thermal conductivity, viscosity, density, and specific heat are 0.146061 W/m °C, 0.031889 Pa.s, 838.529 kg/m3 and 1533.9 j/kg °C, respectively. For case study 2, the optimal findings for thermal conductivity, viscosity, density, and specific heat are 0.13476 W/m °C, 0.0226062 Pa.s, 831.071 kg/m3 and 1791.14 j/kg °C, respectively. Although the optimal value for thermal conductivity decreased in case study 2, the nanoparticle weight % was reduced from 1 to 0.322473%.

On the Existence of Global Weak Solutions to 1D Sediment Transport Model

This paper is devoted to the study of the existence of weak solution in time with a periodic domain of sediment transport model. We consider a one-dimensional viscous sediment transport model which combines a viscous Shallow-Water system with a transport equation that describes the bottom evolution. The model studied does not take into account all the regularizing terms used by Roamba Brahima, Zongo Yacouba and Jean de Dieu Zabsonré (2017) and we use a better transport equation than that used by Zabsonré (2012).

Quantizing viscous transport in bilayer graphene

The momentum transport in ultraclean bilayer graphene is characterized by the viscous transport. In quantizing magnetic field the momentum current passes through the guiding center of the cyclotron orbit. In this study we derive the formula of the quantized Hall viscosity for bilayer graphene. This can be detected in the non-local magnetoresistivity measurements that varies with the quantized step. For weak magnetic field the Landau levels start overlapping and lead to the Shubnikov–de-Haas oscillations, superimposed on the classical formulae, reference Steinberg (1958 Phys. Rev. 109 1486). These oscillations are present in the longitudinal and Hall viscosities.

CFD-DEM simulation of proppant transport by supercritical CO2 in a vertical planar fracture

The great potential of supercritical CO2 fracturing technique in the stimulation of unconventional reservoirs requires further insight into the mechanism of proppant transport in the fracture. In this paper, we adopted the computational fluid dynamics (CFD) coupled discrete element method (DEM) scheme to study the proppant transport in supercritical CO2 fluid, and verified this method via previous experiment data. Based on the visual analysis at the particle-level, transport mechanisms of proppant in the fracture were revealed. The difference in the proppant transport capacity between supercritical CO2 and commonly used fracturing fluids were demonstrated, and effects of slurry injection velocity, proppant concentration, injection temperature, and proppant diameter were discussed. Results indicate that the transport and placement process of the proppant can be divided into three stages, i.e., flat laying stage, proppant dune development stage, and proppant bank development stage. The supercritical CO2 slurry is characterized by obvious stratified flow in the fracture: the static bed at the bottom, the fluidization layer above it, and the pure fluid flow region at the top. Viscous drag transport, lifting transport, and fluidization transport are three proppant transport mechanisms for supercritical CO2 slurry flow, among which the fluidization transport plays a more dominant role. This work is expected to provide an in-depth understanding of the transport mechanism of supercritical CO2 slurry in a vertical planar fracture.

OpenWorkstation: A modular open-source technology for automated in vitro workflows

Automation liberates scientific staff from repetitive tasks, decreases the probability of human error and consequently enhances the reproducibility of lab experiments. However, the use of laboratory automation in academic laboratories is limited due to high acquisition costs and the inability to customize off-the-shelf hardware. To address these challenges, we present an Open Source Hardware concept, referred to as OpenWorkstation, to build an assembly line-inspired platform consisting of ready-to-use and customizable modules. In contrast to current standalone solutions, the OpenWorkstation concept enables the combination of single hardware modules – each with a specific set of functionalities – to a modular workstation to provide a fully automated setup. The base setup consists of a pipetting and transport module and is designed to execute basic protocol steps for in vitro research applications, including pipetting operations for liquids and viscous substances and transportation of cell culture vessels between the modules. We demonstrate the successful application of this concept within a case study by the development of a storage module to facilitate high-throughput studies and a photo-crosslinker module to initiate photo-induced polymerization of hydrogel solutions. We present a Systems Engineering framework for customized module development, guidance for the design and assembly of the presented modules, and operational instructions on the usage of the workstation. By combining capabilities from various open source instrumentations into a modular technology platform, the OpenWorkstation concept will facilitate efficient and reliable experimentation for in vitro research. Ultimately, this concept will allow academic groups to improve replicability and reproducibility in cell culture process operations towards more economical and innovative research in the future.

Imaging viscous flow of the Dirac fluid in graphene

The electron-hole plasma in charge-neutral graphene is predicted to realize a quantum critical system in which electrical transport features a universal hydrodynamic description, even at room temperature1,2. This quantum critical 'Dirac fluid' is expected to have a shear viscosity close to a minimum bound3,4, with an interparticle scattering rate saturating1 at the Planckian time,the shortest possible timescale for particles to relax. Although electrical transport measurements at finite carrier density are consistent with hydrodynamic electron flow in graphene5-8, a clear demonstration of viscous flow at the charge-neutrality point remains elusive. Here we directly image viscous Dirac fluid flow in graphene at room temperature by measuring the associated stray magnetic field. Nanoscale magnetic imaging is performed using quantum spin magnetometers realized with nitrogen vacancy centres in diamond. Scanning single-spin and wide-field magnetometry reveal a parabolic Poiseuille profile for electron flow in a high-mobility graphene channel near the charge-neutrality point, establishing the viscous transport of the Dirac fluid. This measurement is in contrast to the conventional uniform flow profile imaged in a metallic conductor and also in a low-mobility graphene channel. Via combined imaging and transport measurements, we obtain viscosity and scattering rates, and observe that these quantities are comparable to the universal values expected at quantum criticality. This finding establishes a nearly ideal electron fluid in charge-neutral, high-mobility graphene at room temperature4. Our results will enable the study of hydrodynamic transport in quantum critical fluids relevant to strongly correlated electrons in high-temperature superconductors9. This work also highlights the capability of quantum spin magnetometers to probe correlated electronic phenomena at the nanoscale.

Mapping the local viscosity of non-Newtonian fluids flowing through disordered porous structures

Flow of non-Newtonian fluids through topologically complex structures is ubiquitous in most biological, industrial and environmental settings. The interplay between local hydrodynamics and the fluid’s constitutive law determines the distribution of flow paths. Consequently the spatial heterogeneity of the viscous resistance controls mass and solute transport from the micron to the meter scale. Examples range from oil recovery and groundwater engineering to drug delivery, filters and catalysts. Here we present a new methodology to map the spatial variation of the local viscosity of a non-Newtonian fluid flowing through a complex pore geometry. We use high resolution image velocimetry to determine local shear rates. Knowing the local shear rate in combination with a separate measurement of the fluid’s constitutive law allows to quantitatively map the local viscosity at the pore scale. Our experimental results—which closely match with three-dimensional numerical simulations—demonstrate that the exponential decay of the longitudinal velocity distributions, previously observed for Newtonian fluids, is a function of the spatial heterogeneity of the local viscosity. This work sheds light on the relationship between hydraulic properties and the viscosity at the pore scale, which is of fundamental importance for predicting transport properties, mixing, and chemical reactions in many porous systems.

Homogenization of a stochastic viscous transport equation

In the present paper we prove an homogenisation result for a locally perturbed transport stochastic equation. The model is similar to the stochastic Burgers' equation and it is inspired by the LWR model. Therefore, the interest in studying this equation comes from it's application for traffic flow modelling. In the first part of paper we study the inhomogeneous equation. More precisely we give an existence and uniqueness result for the solution. The technical difficulties of this part come from the presence of the function $ \varphi $ under assumptions coherent for the model, which is giving the inhomogeneity with respect to the space variable, not present in the classical results. The second part of the paper is the homogenisation result in space.

Scattering mechanisms, relaxation times, and shear viscosity in universal anomalous transport of unitary Fermi gases

In this paper we investigate the scattering mechanisms of viscous momentum transport in unitary Fermi gases below the pseudogap temperature T* by opening almost all the elastic and inelastic scattering channels. For a given system Hamiltonian, we adopt a microscopic t-matrix approximation to determine the thermodynamical quantities, and we approach an equivalent three-fluid dynamics to calculate the viscous relaxation time τ by two kinds of elementary excitations. The exotic scattering processes raised by uncondensed Fermi pairs greatly decay τ and lead to a universal behavior of τ with temperature, which is closely related to the anomalous transport in a wide range of strongly correlated systems. For linking our findings of τ to the shear viscosity η, we present a Kubo-based expression of η by the stress-tensor correlation function with conserved vertex corrections, and we give an approximate relation for a possible experimental determination of τ. Our results fit well with measurements and are comparable with other theories. Moreover, we verify the newly proposed universal upper bound of the ratio of η and entropy density s in unitary Fermi gases.

Viscous transport in eroding porous media

Transport of viscous fluid through porous media is a direct consequence of the pore structure. Here we investigate transport through a specific class of two-dimensional porous geometries, namely those formed by fluid-mechanical erosion. We investigate the tortuosity and dispersion by analyzing the first two statistical moments of tracer trajectories. For most initial configurations, tortuosity decreases in time as a result of erosion increasing the porosity. However, we find that tortuosity can also increase transiently in certain cases. The porosity-tortuosity relationships that result from our simulations are compared with models available in the literature. Asymptotic dispersion rates are also strongly affected by the erosion process, as well as by the number and distribution of the eroding bodies. Finally, we analyze the pore size distribution of an eroding geometry. The simulations are performed by combining a high-fidelity boundary integral equation solver for the fluid equations, a second-order stable time stepping method to simulate erosion, and new numerical methods to stably and accurately resolve nearly-touching eroded bodies and particle trajectories near the eroding bodies.

Surface wave effects on energy transfer in overlying turbulent flow

Phase-resolved wave simulation and direct numerical simulation of turbulence are performed to investigate the surface wave effects on the energy transfer in overlying turbulent flow. The JONSWAP spectrum is used to initialize a broadband wave field. The nonlinear wave field is simulated using a high-order spectral method, and the resultant wave surface provides the bottom boundary conditions for direct numerical simulation of the overlying turbulent flow. Two wave ages of and 25 are considered, corresponding to slow and fast wave fields, respectively, where denotes the celerity of the peak wave and denotes the friction velocity. The energy transfer of turbulent motions in the presence of surface waves is investigated through the spectral analysis of the two-point correlation transport equation. It is found that the production term has an extra peak at the dominant wavelength scale in the vicinity of the surface, and the energy transported to the surface via viscous and spatial turbulent transport is enhanced in the region of . The presence of surface waves results in an inverse turbulent energy cascade in the near-surface region, where small-scale wave-related motions transfer energy back to the dominant wavelength scale. Pressure-related terms reflecting the spatial and inter-component energy transfer are strongly dependent on the wave age. Furthermore, triadic interaction analysis reveals that the energy influx at the dominant wavelength scale is due to the contribution of the neighbouring streamwise turbulent motions, and those at the harmonic wavelength scales contribute the most.

Aggregation equations with gradient potential as Radon measure and initial data in Besov‐Morrey spaces

In this work, we present conditions to obtain a global‐in‐time existence of solutions to a class of nonlinear viscous transport equations describing aggregation phenomena in biology with sufficiently small initial data in Besov‐Morrey spaces and gradient potential as a Radon measure. We also study the self‐similarity and asymptotic stability of solutions at large times.

Numerical simulation of solute removal from a cross-flow past a row of parallel hollow-fiber membranes

A multi-parametric simulation of the 2-D steady transverse flow of viscous incompressible fluid and diffusion transport of a dissolved solute past a model fibrous medium – a row of parallel sorbent fibers or hollow-fiber membranes is performed in a wide range of Schmidt (Sc) and Peclet (Pe) numbers for fiber Reynolds numbers up to Re = 100. The Navier-Stokes and convection-diffusion equations are solved numerically and the fiber capture efficiency for solute molecules is calculated as a function of Re, Sc and inter-fiber distance. The field of solute concentration and the fiber capture efficiency are found with zero concentration-fixed and diffusion flux boundary conditions set at the streamlined fiber surface, and the difference between the results of simulations with the two boundary conditions is shown for high Peclet numbers. It is also shown that the fiber capture efficiencies estimated with both boundary conditions tend to a constant maximum value for low Peclet numbers. The results of simulations for the fiber capture efficiency are validated by comparison with existing analytical asymptotic solutions for low and high Peclet numbers, and the agreement is shown.

A model of angular momentum transport between a planet and accretion disk

This paper presents a simplified model of the viscous transport of angular momentum and sets up a simplified expression of the rate of specific angular momentum transfer from the planet to the disk. Thanks to Whittaker’s functions, we have found an analytic solution for the disk surface density.