Thickness Of Fluid Layer Research Articles

Heat and Mass Transfer in Three-Dimensional Flow of an Oldroyd-B Nanofluid with Gyrotactic Micro-Organisms

This paper discusses the three-dimensional flow of the gyrotactic bioconvection of an Oldroyd-B nanofluid over a stretching surface. Theory of microorganisms is utilized to stabilize the suspended nanoparticles through bioconvection induced by the effects of buoyancy forces. Analytic solution for the governing nonlinear equations is obtained by using homotopy analysis method (HAM). The effects of involved parameters on velocity, temperature, nanoparticles concentration, and density of motile microorganisms are discussed graphically. The local Nusselt, Sherwood, and motile microorganisms numbers are also analyzed graphically. Several known results have been pointed out as the particular cases of the present analysis. It is found that the non-Newtonian fluid parameters, i.e., relaxation time parameterβ1and retardation time parameterβ2, have opposite effects on the velocity profile. The velocity of the fluid and boundary layer thickness decreases for increasing relaxation time while it decreases for increasing retardation time effects.

Sensitivity of lateral heat transfer on the convection onset in a transient Rayleigh-Bénard-Marangoni flow

The convection onset in a evaporation thermal model of a solute/solvent solution is numerically studied. The transient nature of the diffusion temperature profile requires to control the perturbations which will lead to the transition to a convective flow. Contrary to previous papers, we adopted here a perturbation mode, which is physical and experimentally reproducible, by accounting for a heat transfer at the lateral walls. Based on the Péclet number evaluated in the core flow field only, the critical viscosities and the corresponding times for the transition between a diffusion and convection regimes were calculated as a function of the fluid layer thickness, the aspect ratio and the convective heat transfer coefficient. The study of the spatio-temporal diagrams allowed us to characterize the way the convection propagates in the fluid domain as well as the evolution of the mean cell size and wavenumber.

Potential of Solar Cells Performance Enhancement Using Liquid Absorption Filters

This work aims to assess the effect of using “Durathem G” as a liquid absorption fluid filter on the performance of PV cells since its spectral profile matches the spectral profile of silicon cells to a large extent. Also, it aims to evaluate the heat gained by the fluid and its potential to be employed in hybrid PV systems. The experiment was carried out by installing three identical 20 W PV modules. The first one was the standard panel without any modifications. The other two were modified using identical sheet-and-tube heat exchangers attached to the back surface of each panel as a cooling assembly and employing a thermal receiver above each panel with a 2-cm air gap in between. The effect of fluid layer thickness on the system performance was investigated by adopting two different thicknesses 5 and 8 mm. The main outcomes found that 5-mm filtered panel generates 87.3% of the electrical energy generated by the standard panel, while the 8-mm filtered panel generates only 71.8%. The 5-mm filtered panel was found to generate thermal energy gain of about 9.6 times its generated electrical energy, while the 8-mm filtered panel generates thermal energy gain of about 14.5 times its generated electrical energy.

Natural Convection Flow of Nanofluids in a Composite System with Variable-Porosity Media

A numerical study is performed to examine the effects of nonuniform porosity of a porous medium and thermal dispersion on the natural convection flow and heat transfer of a nanofluid inside a rectangular enclosure containing a fluid and porous layers using a single-phase thermal dispersion model. Copper is considered as nanoparticles and is suspended in a base fluid (water) and the Brinkman–Forchheimer-extended Darcy model is assumed to represent the flow inside the porous layer. The finite volume method with the semi-implicit method for pressure-linked equations (SIMPLE) algorithm is used to solve the dimensionless governing equations, and validation tests of the obtained results are conducted. It is found that the increase in the fluid layer thickness enhances the heat transfer rate at the right wall. Also, an improvement with rate greater than 10% is obtained in the heat transfer rate when the nanoparticle volume fraction is increased to 4%.

The Onset of Double-Diffusive Convection in a Superposed Fluid and Porous Layer Under High-Frequency and Small-Amplitude Vibrations

We numerically simulate the initiation of an average convective flow in a system composed of a horizontal binary fluid layer overlying a homogeneous porous layer saturated with the same fluid under gravitational field and vibration. In the layers, fixed equilibrium temperature and concentration gradients are set. The layers execute high-frequency oscillations in the vertical direction. The vibration period is small compared with characteristic timescales of the problem. The averaging method is applied to obtain vibrational convection equations. Using for computation the shooting method, a numerical investigation is carried out for an aqueous ammonium chloride solution and packed glass spheres saturated with the solution. The instability threshold is determined under two heating conditions—on heating from below and from above. When the solution is heated from below, the instability character changes abruptly with increasing solutal Rayleigh number, i.e., there is a jump-wise transition from the most dangerous shortwave perturbations localized in the fluid layer to the long-wave perturbations covering both layers. The perturbation wavelength increases by almost 10 times. Vibrations significantly stabilize the fluid equilibrium state and lead to an increase in the wavelength of its perturbations. When the fluid with the stabilizing concentration gradient is heated from below, convection can occur not only in a monotonous manner but also in an oscillatory manner. The frequency of critical oscillatory perturbations decreases by 10 times, when the long-wave instability replaces the shortwave instability. When the fluid is heated from above, only stationary convection is excited over the entire range of the examined parameters. A lower monotonic instability level is associated with the development of perturbations with longer wavelength even at a relatively large fluid layer thickness. Vibrations speed up the stationary convection onset and lead to a decrease in the wavelength of most dangerous perturbations of the motionless equilibrium state. In this case, high enough amplitudes of vibration are needed for a remarkable change in the stability threshold. The results of numerical simulation show good agreement with the data of earlier works in the limiting case of zero fluid layer thickness.

Absolute and convective instabilities in combined Couette-Poiseuille flow past a neo-Hookean solid

Temporal and spatio-temporal stability analyses are carried out to characterize the occurrence of convective and absolute instabilities in combined Couette-Poiseuille flow of a Newtonian fluid past a deformable, neo-Hookean solid layer in the creeping-flow limit. Plane Couette flow of a Newtonian fluid past a neo-Hookean solid becomes temporally unstable in the inertia-less limit when the parameter Γ = V η/(GR) exceeds a critical value. Here, V is the velocity of the top plate, η is the fluid viscosity, G is the shear modulus of the solid layer, and R is the fluid layer thickness. The Kupfer-Bers method is employed to demarcate regions of absolute and convective instabilities in the Γ-H parameter space, where H is the ratio of solid to fluid thickness in the system. For certain ranges of the thickness ratio H, we find that the flow could be absolutely unstable, and the critical Γ required for absolute instability is very close to that for temporal instability, thus making the flow absolutely unstable at the onset of temporal instability. In some cases, there is a gap in the parameter Γ between the temporal and absolute instability boundaries. The present study thus shows that absolute instabilities are possible, even at very low Reynolds numbers in flow past deformable solid surfaces. The presence of absolute instabilities could potentially be exploited in the enhancement of mixing at low Reynolds numbers in flow through channels with deformable solid walls.

(Invited) In Situ Electrochemical STEM As a Platform for Interpreting Electrochemical Phenomena

In situ electrochemical STEM (ec-STEM) is a multi-modal characterization technique that allows for the direct interpretation of nanoscale mechanisms and kinetics during electrochemical operation. The major advantage of this method is the ability to perform quantitative electrochemical measurements while simultaneously characterizing nanoscale structural and chemical changes that occur at site-specific electrode/electrolyte interfaces using high spatial resolution imaging, spectroscopy, and/or diffraction; all within a scanning transmission electron microscope (STEM). ec-STEM is being used for battery research by studying the formation mechanisms of the solid electrolyte interphase (SEI) and growth mechanisms of lithium metal dendrites within an organic electrolyte (1.2M LiPF6 EC:DMC). The initial stage of lithium metal dendrite nucleation and growth is observed on the surface and edge of the glassy carbon working electrode during cyclic voltammetry experiments below -3.0V vs. Ptpseudo reference electrode. The Li dendrites grow with a globular morphology during a sequence of bursts, whereby newly formed lithium metal deposits are passivated by the SEI followed by additional growth events associated with localized regions of SEI breakdown. Electron energy loss spectroscopy (EELS) confirms that the deposits are lithium metal with surfaces composed of inorganic compounds commonly found in the SEI. The EEL spectra reveal a Li metal plasmon peak at 7.5eV and a characteristic LiF spectral feature on the Li K-edge at 55eV. Furthermore, a framework for obtaining and analyzing quantitative EELS measurements is demonstrated to directly determine the oxidation state of battery electrodes (LiMn2O4 and Li4Ti5O12) using the “white-lines” of the L2,3 core loss ionization edges of Mn and Ti. By simultaneously acquiring low-loss and core-loss EEL spectra, plural scattering effects caused by scattering through the electrolyte can be removed to quantify the oxidation state of Mn and Ti using the white-line intensity ratio method. RF magnetron sputtering was used to deposit a thin layer of LiMn2O4, a commonly used battery cathode material, directly onto the silicon nitride membrane of the ec-STEM cell. Simultaneous low-loss and core-loss EEL spectra are acquired with and without the presence of dimethyl carbonate (DMC) in the liquid cell (Figure 3c). The low-loss portion of the EEL spectra confirms the presence of the DMC within the liquid cell and aides in the quantification of fluid layer thickness. If the fluid layer thickness is small, it is possible to extract and analyze the Mn L2,3-edge for quantification of the Mn oxidation state using the “white-line” intensity ratio method. Mixed Mn3+/4+ oxidation is measured in the as-deposited LiMn2O4, which shifts to a lower oxidation state when DMC is present in the cell, suggesting that LiMn2O4 may be prone to electron beam induced chemical reduction when immersed in a battery electrolyte during in situ ec-STEM experiments. The information obtained from these studies can help provide a deeper understanding of how batteries function at the nanoscale. Research supported by the Fluid Interface Reactions Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the Department of Energy’s Office of Basic Energy Sciences Division and by the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

Determination of the stress jump coefficient, the interstitial heat transfer coefficient and the interface heat transfer coefficient in a porous composite system

In this work, the microscopic and macroscopic models of the porous composite system are established to study the stress jump coefficient, the interstitial heat transfer coefficient and the interface heat transfer coefficient under the local thermal non-equilibrium condition. The influences of the free fluid layer thickness, the flow types (including the Poiseuille flow, the Couette flow and the free boundary flow), the porosity and the pore diameter on the stress jump coefficient, the Biot number and the interface Biot number are discussed. In addition, an innovative method is presented to calculate the interstitial heat transfer coefficient inside the porous medium when the phenomenon of heat flux bifurcation occurs.

Aeroelastic stability of cylindrical shells interacting with internal annular fluid flow

This paper is devoted to the analysis of the dynamic behavior of cylindrical shells, containing an internal annular layer of ideal fluid and subject to the external supersonic gas flow. The aerodynamic pressure is calculated based on the quasi-static aerodynamic theory. The behavior of the compressible fluid is described in terms of the perturbation velocity potential. A mathematical formulation of the problem is developed based on the classical theory of shells and virtual displacement principle. A solution of the problem involves computation of complex eigenvalues of the coupled system of equations. The paper presents the results of numerical experiments, which were performed to estimate the influence of the fluid flow velocity on the value of the static pressure in the unperturbed gas flow for shells, interacting with fluid layers of different thicknesses. The numerical simulation shows that a reduction of the fluid layer thickness and increase of the fluid velocity produce a stabilizing effect by virtue of increasing the threshold of aerodynamic stability. However, an essential reduction of the layer thickness can lead, depending on the preset combinations of boundary conditions, to a considerable growth of the stability threshold or to the onset of instability.

Investigation of the momentum transfer conditions at the porous/free fluid interface: A benchmark solution

ABSTRACTIn this work, the conformal mapping method is used to obtain the microscopic solution for the velocity distribution in the porous composite system. The corresponding macroscopic solution is determined by using a two-domain approach, in which the momentum transfer at the porous/free fluid interface is governed by the Beavers-Joseph condition or the stress-jump condition. The boundary parameters in the macroscopic solution, including the velocity slip and the stress jump coefficients, are determined by comparing the microscopic and macroscopic solutions. The impacts of the porous structure, flow type and the thickness of the free fluid layer on the velocity slip coefficient and the stress jump coefficient are discussed. In addition, the errors for the interface velocity, predicted by the velocity slip coefficient and the stress jump coefficient, are investigated. The analytical solution obtained in this work can be used as a benchmark for relevant numerical simulation.

Squeeze-film flow between a curved impermeable bearing and a flat porous bed

Axisymmetric squeeze-film flow in the thin gap between a stationary flat thin porous bed and a curved impermeable bearing moving under a prescribed constant load is analysed. The unsteady Reynolds equation is formulated and solved for the fluid pressure. This solution is used to obtain the time for the minimum fluid layer thickness to reduce to a given value, and, in particular, the finite time for the bearing and the bed to come into contact. The effect of varying the shape of the bearing and the permeability of the layer is investigated, and, in particular, it is found that both the contact time and the fluid pressure behave qualitatively differently for beds with small and large permeabilities. In addition, the paths of fluid particles initially situated in both the fluid layer and the porous bed are calculated. In particular, it is shown that, unlike in the case of a flat bearing, for a curved bearing there are fluid particles, initially situated in the fluid layer, that flow from the fluid layer into the porous bed and then re-emerge into the fluid layer, and the region in which these fluid particles are initially situated is determined.

Improved thermal effusivity measurements of solids using the photopyroelectric technique in the front configuration

The photopyroelectric (PPE) technique in the front configuration consists of illuminating one surface of a pyroelectric slab while the other surface is in contact with the test sample. This method has been widely used to measure the thermal effusivity of liquids. However, in the case of solid samples the PPE signal is affected by the thin coupling fluid layer used to guarantee the thermal contact. Recently, a thickness scan of the coupling fluid layer has been proposed to extract the thermal effusivity of the solid. However, this method is delicate since it needs absolute values of the fluid thickness. In this work, we propose a classical frequency scan of the PPE signal in the front configuration, which only depends on two parameters: the effusivity of the solid and the thickness of the coupling fluid layer. It is demonstrated that both quantities are not correlated and that the sensitivity to the thermal effusivity of the solid is very high. PPE measurements performed on a set of calibrated solids covering a wide range of effusivity values (from polymers to metals) confirm the ability of the method to obtain the thermal effusivity of solids accurately. Moreover, this method is specially suited to measure the temperature dependence of the thermal effusivity across phase transitions.

Hydroelastic stability of multi‐plate structures interacting with flowing fluid

AbstractThe paper is concerned with the finite‐element analysis of natural vibrations and hydroelastic stability of two parallel plates interacting with liquid. The influence of the fluid layer thickness on the eigenfrequencies, vibration modes and hydroelastic stability boundary are estimated based on the results of numerical solution of some example problems. The numerical investigation substantiates the existence of various instability modes, the onset of which depends on the combinations of kinematic boundary conditions specified at the plate ends. (© 2015 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Volumetric bounds on subsurface fluid substitution using 4D seismic time shifts with an application at Sleipner, North Sea

A method is presented for the volumetric estimation of subsurface fluid substitution based on the analysis of 4D seismic time shifts. Since time shifts cannot resolve for fluid saturation and layer thickness simultaneously without additional constraints, mass estimates are derived from the complete set of possible fluid saturations and layer thicknesses. The method considers velocity-saturation relationships that range from uniform fluid mixing to patchy fluid mixing. Based on a generalized velocity-saturation relationship that is parameterized by the degree of patchiness, explicit upper and lower fluid mass bounds are provided. We show that the inherent ambiguity between fluid saturation, fluid mixing, and layer thickness has a severe impact on the convergence of these mass bounds. That is, fluid substitution scenarios with patchy fluid mixing and roughly linear velocity-saturation relationships allow for more accurate fluid quantification than scenarios associated with uniform mixing. Application of the method to two 4D seismic monitor data sets from Sleipner results in [Formula: see text] mass bounds that are consistent with the true injected masses of [Formula: see text]. Moreover, a linear relationship between progressively developing 4D time shifts and known injected [Formula: see text] mass is observed, suggesting that the evolving patterns of fluid saturation and fluid mixing in the [Formula: see text] plume have remained roughly constant with time.

Propagation regimes of interfacial solitary waves in a three-layer fluid

Abstract. Long weakly nonlinear finite-amplitude internal waves in a fluid consisting of three inviscid layers of arbitrary thickness and constant densities (stable configuration, Boussinesq approximation) bounded by a horizontal rigid bottom from below and by a rigid lid at the surface are described up to the second order of perturbation theory in small parameters of nonlinearity and dispersion. First, a pair of alternatives of appropriate KdV-type equations with the coefficients depending on the parameters of the fluid (layer positions and thickness, density jumps) are derived for the displacements of both modes of internal waves and for each interface between the layers. These equations are integrable for a very limited set of coefficients and do not allow for proper description of several near-critical cases when certain coefficients vanish. A more specific equation allowing for a variety of solitonic solutions and capable of resolving most near-critical situations is derived by means of the introduction of another small parameter that describes the properties of the medium and rescaling of the ratio of small parameters. This procedure leads to a pair of implicitly interrelated alternatives of Gardner equations (KdV-type equations with combined nonlinearity) for the two interfaces. We present a detailed analysis of the relationships for the solutions for the disturbances at both interfaces and various regimes of the appearance and propagation properties of soliton solutions to these equations depending on the combinations of the parameters of the fluid. It is shown that both the quadratic and the cubic nonlinear terms vanish for several realistic configurations of such a fluid.

Nonlinear stable steady solutions to the Ostroumov problem

We investigate the linear and weakly nonlinear solutions to a convection problem that was first studied by Ostroumov in 1947. The problem pertains to the stability of the equations governing convective motion in an infinite vertical fluid layer that is heated from below. Ostroumov’s linear stability analysis yields instability threshold conditions that are characterized by zero wavenumber for the Fourier mode in the vertical direction and by eigenfunctions that are independent of the vertical coordinate. Thus, any undertaking at determining the supercritical nonlinear solutions and their stability through a small amplitude expansion fails. This failure is attributed to the fact that the nonlinear interaction of the linear modes vanish identically. In this paper, we put forth exact and stable similarity type solutions to the Ostroumov problem. These solutions are characterized by the same linear threshold conditions as Ostroumov’s solutions. Moreover, we are able to extend the analysis to the supercritical regime through a small amplitude analysis to obtain steady two-dimensional solutions for a small range of Prandtl numbers. These solutions are found to be stable to general two-dimensional, time-dependent disturbances. Furthermore, when the analysis is extended to the case where the fluid layer thickness is also allowed to be infinite, we find that the infinite two-dimensional fluid region becomes linearly unstable through a Batchelor–Nitsche (BN) instability mechanism. Thus, the nonlinear solutions are obtained through a long wavelength expansion, and consequently our analysis provides the nonlinear development of the BN instability for the case of an unstable and uniform density profile. Finally, numerical solutions of the nonlinear problem are also presented which shed light on the flow patterns and temperature distributions for both the vertical channel and the infinitely extended stratified region problems.

Instability of viscous flow over a deformable two-layered gel: experiments and theory.

The instability of the flow of a viscous fluid past a soft, two-layered gel is probed using experiments, and the observations are compared with results from a linear stability analysis. The experimental system consists of the rotating top plate of a rheometer and its stationary bottom plate on which the two-layer gel is placed. When the flow between the top plate and the two-layer gel is viscometric (i.e., laminar), the viscosity obtained from the rheometer is a measure of the material property of the fluid. However, after a critical shear stress, there is a sudden increase in apparent viscosity, indicating that the flow has undergone an instability due to the deformable nature of the two-layer gel. Experiments are carried out to quantify how the critical value of fluid shear stress required to destabilize the flow varies as a function of ratio of solid to fluid layer thickness, and the ratio of the shear moduli of the two gels. A linear stability analysis is carried out for plane Couette flow of a Newtonian fluid past the two-layered gel, by assuming the two solid layers to be elastic neo-Hookean materials. In order to compare the experimental and theoretical results, the effective shear modulus (Geff, defined by H/Geff=H1/G1+H2/G2) of the two-layer gel is found to be useful, where H=H1+H2. Here, Hi and Gi (i=1,2), respectively, denote the thickness and shear modulus of each layer. Results for the nondimensional parameter Γeff=ηV/(dGeff) (V is the velocity of the top plate; η is fluid viscosity, d is the fluid thickness) as a function of solid to fluid thickness H/d obtained from the stability analysis agree well with experimental observations, without any fitting parameters. In general, we find that the flow is more unstable if the softer gel is adjacent to the fluid flow compared to the case when it is not. This suggests that the instability is more interfacial in nature and is crucially dependent on the relative placement of the two layers, and not just on the effective modulus of the two-layer gel. We further show that the theoretical and experimental data for two-layer gels can be suitably collapsed onto the results obtained for a single-gel layer.

Hydrodynamic enhancement of the diffusion rate in the region between two fluctuating membranes in close opposition: a theoretical and computational study.

Periodic variation of the distance between two weakly adhering bodies gives rise to a huge tangential motions of the sandwiched solvent layer (squeezing flow). Oscillations either can be induced by an external applied field or can spontaneously arise from the coupling with the solvent heat bath. First we calculated by the Navier-Stokes equation the components of the fluid velocity near two oscillating juxtaposed plates. Then we evaluated the influence of plate oscillations on the transport properties of a trace diffusant dissolved at t = 0 in the outer medium for both deterministic and stochastic excitations. By employing both analytical (Fokker-Planck) and coarse-grained molecular dynamics (MD) simulations, we proved that the entry and migration rates of the diffusant sharply increases with the oscillation amplitudes. Enhancement was related to relevant parameters like oscillation frequency, fluid layer thickness, fluid viscosity, and temperature. An extension to the case of oscillating multistacked lamellae has been also made. Theoretical and MD results suggest a significant enhancement of the diffusant flux even in the worse situation of thermally excited small amplitude fluctuations. Excitation arising from other sources (e.g., microwave or ultrasound irradiation of solid-fluid layered systems) could have a dramatic effect on the transport phenomena. Possible implications to relevant biological problems have been discussed.

Correlation between skin, bone, and cerebrospinal fluid layer thickness and optical coefficients measured by multidistance frequency-domain near-infrared spectroscopy in term and preterm infants

Near-infrared spectroscopy (NIRS) is increasingly used in neonatal intensive care. We investigated the impact of skin, bone, and cerebrospinal fluid (CSF) layer thickness in term and preterm infants on absorption-(μa) and/or reduced scattering coefficients (μs') measured by multidistance frequency-domain (FD)-NIRS. Transcranial ultrasound was performed to measure the layer thicknesses. Correlations were only statistically significant for μa at 692 nm with bone thickness and μs' at 834 nm with skin thickness. There is no evidence that skin, bone, or CSF thickness have an important effect on μa and μs'. Layer thicknesses of skin, bone, and CSF in the range studied do not seem to affect cerebral oxygenation measurements by multidistance FD-NIRS significantly.

Dynamic characterization of a laminated composite magnetorheological fluid sandwich plate

This study investigates the dynamic properties of a laminated composite magnetorheological (MR) fluid sandwich plate. The governing differential equations of motion of a sandwich plate embedding a MR fluid layer as the core layer and laminated composite plates as the face layers are presented in a finite element formulation. The validity of the developed finite element formulation is demonstrated by comparing the results in terms of the natural frequencies derived from the present finite element formulation with those in the available literature. Various parametric studies are also performed to investigate the effect of a magnetic field on the variation of the natural frequencies and loss factors of the MR fluid composite sandwich plate under various boundary conditions. Furthermore, the effect of the thickness of the MR fluid layer and the ply orientation of the composite face layers on the variation of the natural frequencies and loss factors are studied. The free vibration mode shapes under various boundary conditions of a MR fluid laminated composite sandwich plate are also presented. The forced vibration response of a MR fluid composite plate is investigated to study the dynamic response of the sandwich plate under harmonic force excitations in various magnetic fields. The study suggests that the natural frequency increases with increasing magnetic field, irrespective of the boundary conditions. The reduction in peak deflection at each mode under a harmonic excitation force with variation of the applied magnetic field shows the effectiveness of the MR fluid layer in reducing the vibration amplitude of the composite sandwich plate.