Variable Gravity Effects Research Articles

Effects of variable gravity on stability in couple-stress fluids across various conducting boundaries

This work aims to inspect the effect of variable gravity field on the convective stability of couple stress fluid for different combinations of bounding surfaces. Both linear and nonlinear analyses are conducted to obtain eigenvalue problems. For this analysis, three different combinations of bounding surfaces have been considered. Normal mode analysis is used for linear analysis, while the energy method is used for nonlinear analysis. The numerical values of critical Rayleigh numbers are calculated using the single-term Galerkin technique. It is found that the Rayleigh numbers for the two analyses coincide, suggesting global stability. It is found that the increase in the value of couple stresses stabilize the system. The variable gravity can enhance or reduce the stability of the system depending on the direction of gravity variation which can be easily controlled by its parameter values. It is also observed that fluid confined in rigid-rigid bounding surface is more thermally stable, which is suitable for convection in couple stress fluid.

Variable gravity effects on penetrative porous convection

The effect of variable gravity on the onset of penetrative convection in a horizontal fluid-saturated porous layer is studied, performing the linear instability analysis and the nonlinear stability analysis (via the weighted energy method) of the thermal conduction solution. The linear and nonlinear critical Rayleigh numbers for the onset of penetrative porous convection are determined and their behaviour with respect to the fundamental physical parameters of the model is studied. In particular, we found that there exists a region of subcritical instabilities and that the increase of the upper bounding plane temperature and the decrease of gravity have a stabilizing effect on the onset of convection.

Effect of variable gravity field on the onset of convection in a Brinkman porous medium under convective boundary conditions

The effect of variable gravity on the onset of convection in a horizontal porous layer has been analyzed due to external heating. The third kind of thermal boundary conditions is considered on both the boundaries which are connected by Biot numbers B0 and B1. Also, the Brinkman model has been employed in the momentum equation. Further, linear and quadratic varying downward gravity fields depending on height have been taken to investigate the above observation. The bvp4c scheme is used in MATLAB to solve the eigenvalue problem for neutral stability mode. It is observed that the system's stability increases monotonically due to the rising effect of gravity variation parameter, Darcy number, and Biot numbers. Further, the onset of convection has been discussed for the two limiting cases of external heating on the boundaries where one boundary is associated with constant heat flux while another one is associated with an isothermal state. Finally, it is observed that the convection comes earlier for quadratic varying gravity field compared to linear varying gravity field when the external heat supplies from one boundary or both boundaries.

Thermal instability analysis in magneto-hybrid nanofluid layer between rough surfaces with variable gravity and space-dependent heat source

The onset of thermal instability with hybrid nanoparticles Al2O3–Cu in nanofluid is investigated with the combined effects of variable gravity, variable heat source and Lorentz force in a porous medium. Its applications are in many areas like chemical engineering, geophysics, astrophysics, etc. Based on literature, many gravity variations are assumed in the present analysis, along with a space-dependent heat source/sink parameter that varies along the width of the channel. The finite difference-based Lobatto IIIa method has been applied to solve the system under no nanoparticle flux and rough boundary conditions, and approximate analytical results are generated for some special cases. The roughness parameters ([Formula: see text]), Darcy number (Da), gravity variation function ([Formula: see text]), and Chandrasekhar number (Q) delay the onset of convection and thus stabilize the system. It is also observed that an increase in the Lewis number Le, power index in variable heat source, and the nanoparticle Rayleigh concentration number Rn decreases the critical Rayleigh number [Formula: see text] which destabilizes the system, and increases the critical wave number, which enlarges the convection cell size. In the case of exponential variation ([Formula: see text]) in the gravity variation parameter, the system becomes stabilized due to a delay in the onset of convection. In addition, we have considered multilayer perceptron-artificial neural network (MLP-ANN) computation to predict the critical Rayleigh number as per function of important controlling parameters. The data set of 625 observations is chosen keeping 70% for testing, 15% for training and 15% for validation using efficient Levenberg−Marquardt back propagation algorithm with optimal accuracy measures i.e., root mean square deviation (RMSE), root mean relative error (RMRE) and [Formula: see text] (coefficient of determination). Finally, the regression plots are drawn that correlate, target and output data.

The Effect of Variable Gravity and Chemical Reaction on Double Diffusive Convection in a Sparsely Packed Porous Layer

The Effect of Variable Gravity and Chemical Reaction on Double Diffusive Convection in a Sparsely Packed Porous Layer

The effect of variable gravity on rotating Rayleigh–Bénard convection in a sparsely packed porous layer

AbstractThe effect of variable gravity on rotating convection in a sparsely packed porous layer is studied numerically. For gravity force variation, the linear, parabolic, cubic, and exponential functions are considered. Boundary conditions for the governing equations are considered to be either free or rigid. The normal mode method is employed, and the resulting eigenvalue problem is solved numerically for free–free and rigid–rigid boundaries using bvp4c in MATLAB R2020b. The influence of Darcy number (), gravity variation parameter (), and Taylor number () on the stability of the system is investigated graphically. It is confirmed that the gravity variation parameter and Taylor number delay the onset of convection and Darcy number enhances the onset of convection. The size of convection cells decreases on raising the variable gravity parameter and rotation parameter, while the Darcy number amplifies the size of convection cells. It is also observed that the system is stable for exponential gravity function and less stable for the cubic gravity function.

The onset of convection in a magnetic nanofluid layer with variable gravity effects

This paper deals with the study of the onset of convection in a thin magnetic nanofluid layer which is heated from below under the influence of an applied magnetic field and a variable gravity field. The model used in this study includes the effect of Brownian diffusion, thermophoresis, and magnetophoresis. We applied the Chebyshev pseudospectral method to determine the numerical solutions and discussed the results for Rigid–Rigid, Rigid–Free and Free–Free boundary conditions for water and ester based magnetic nanofluids. The effect of significant parameters affecting the instability of the system has been investigated at the onset of convection.

Nanopore sequencing in microgravity.

Rapid DNA sequencing and analysis has been a long-sought goal in remote research and point-of-care medicine. In microgravity, DNA sequencing can facilitate novel astrobiological research and close monitoring of crew health, but spaceflight places stringent restrictions on the mass and volume of instruments, crew operation time, and instrument functionality. The recent emergence of portable, nanopore-based tools with streamlined sample preparation protocols finally enables DNA sequencing on missions in microgravity. As a first step toward sequencing in space and aboard the International Space Station (ISS), we tested the Oxford Nanopore Technologies MinION during a parabolic flight to understand the effects of variable gravity on the instrument and data. In a successful proof-of-principle experiment, we found that the instrument generated DNA reads over the course of the flight, including the first ever sequenced in microgravity, and additional reads measured after the flight concluded its parabolas. Here we detail modifications to the sample-loading procedures to facilitate nanopore sequencing aboard the ISS and in other microgravity environments. We also evaluate existing analysis methods and outline two new approaches, the first based on a wave-fingerprint method and the second on entropy signal mapping. Computationally light analysis methods offer the potential for in situ species identification, but are limited by the error profiles (stays, skips, and mismatches) of older nanopore data. Higher accuracies attainable with modified sample processing methods and the latest version of flow cells will further enable the use of nanopore sequencers for diagnostics and research in space.

Erratum to: Three-Dimensional Simulations for Convection in a Porous Medium with Internal Heat Source and Variable Gravity Effects

The problem of convection in a fluid-saturated porous layer which is heated internally and where the gravitational field varies with distance through the layer is studied. The accuracy of both the linear instability and global nonlinear energy stability thresholds is tested using a three-dimensional simulation. Our results support the assertion that the linear theory is very accurate in predicting the onset of convective motion, and thus, regions of stability.

Effect of Variable Gravity on Darcy Flow with Impressed Horizontal Gradient and Viscous Dissipation

Effect of Variable Gravity on Darcy Flow with Impressed Horizontal Gradient and Viscous Dissipation

Effects of Vertical Throughflow and Variable Gravity on Hadley–Prats Flow in a Porous Medium

The stability of convection in a horizontal porous layer which is saturated with fluid and induced by horizontal temperature gradients subjected to horizontal mass flow is investigated by means of linear and nonlinear stability analysis. The effects of variable gravity field and vertical throughflow are also considered in this analysis. The nonlinear stability analysis part has been developed via energy functional. Shooting and Runge–Kutta methods have been used to solve eigenvalue problem in both cases. Critical vertical thermal Rayleigh numbers for both linear and nonlinear analyses \(R_\mathrm{L}\) and \(R_\mathrm{E}\) are evaluated for different values of horizontal Rayleigh number \(R_x\), horizontal Peclet number Pe, vertical Peclet number \(Q_\mathrm{v}\) and variable gravity parameter \(\eta \). Comparison is made between linear and nonlinear stability results. It has been observed that linear stability results are overpredicting the onset of convection compared with nonlinear theory, and hence subcritical instabilities would arise before one gets the onset of linear stability threshold.

Chemical reaction effect on double diffusive convection in porous media with magnetic and variable gravity effects

We study the problem of double diffusive convective movement of a reacting solute in a viscous incompressible occupying a plane layer in a saturated porous medium and subjected to a vertical magnetic field. The thresholds for linear instability are found and compared to those derived by a global nonlinear energy stability analysis. Then, the accuracy of both the linear instability and global nonlinear energy stability thresholds are tested using a three dimensional simulation. The strong stabilizing effect of gravity field and magnetic field is shown. Moreover, the results support the assertion that the linear theory, in general, is accurate in predicting the onset of convective motion, and thus, regions of stability.

Effect of variable gravity on thermal instability of rotating nanofluid in porous medium

Effect of variable gravity on the onset of thermal convection in a horizontal layer of rotating nanofluid in a porous medium is investigated. For the porous medium Darcy model is used. A linear stability analysis based upon normal mode technique is used to find solution of the fluid layer confined between two free-free boundaries. Rayleigh numbers on the onset of stationary convection and oscillatory convection have been derived by using Galerkin method. Graphs have been plotted to study the effect of Taylor number (rotation), Lewis number and variable gravity parameter on the stationary convection.

Three-Dimensional Simulations for Convection Problem in Anisotropic Porous Media with Nonhomogeneous Porosity, Thermal Diffusivity, and Variable Gravity Effects

A convection problem in anisotropic and inhomogeneous porous media has been analyzed. In particular, the effect of variable permeability, thermal diffusivity, and variable gravity with respect to the vertical direction, has been studied. A linear and nonlinear stability analysis of the conduction solution has been performed. The validity of both the linear instability and global nonlinear energy stability thresholds are tested using a three- dimensional simulation. Our results show that the linear threshold accurately predicts on the onset of instability in the basic steady state. However, the required time to arrive at the basic steady state increases significantly as the Rayleigh number tends to the linear threshold.

Three-Dimensional Simulations for Convection in a Porous Medium with Internal Heat Source and Variable Gravity Effects

Three-Dimensional Simulations for Convection in a Porous Medium with Internal Heat Source and Variable Gravity Effects

Variable Gravity Effects on Thermal Instability of Nanofluid in Anisotropic Porous Medium

In this paper, we study the effects of variable gravity on thermal instability in a horizontal layer of a nanofluid in an anisotropic porous medium. Darcy model been used for the porous medium. Also, it incorporates the effect of Brownian motion along with thermophoresis. The normal mode technique is used to find the confinement between two free boundaries. The expression of the Rayleigh number has been derived, and the effects of variable gravity and anisotropic parameters on the Rayleigh number have been presented graphically

Measurements and Modeling of Variable Gravity Effects on Water Distribution and Flow in Unsaturated Porous Media

Liquid behavior under reduced gravity conditions is of considerable interest for various components of life‐support systems required for manned space missions. High costs and limited opportunities for spaceflight experiments hinder advances in reliable design and operation of elements involving fluids in unsaturated porous media such as plant growth facilities. We used parabolic flight experiments to characterize hydraulic properties under variable gravity conditions deduced from variations in matric potential over a range of water contents. We designed and tested novel measurement cells that allowed dynamic control of water content. Embedded time domain reflectometry probes and fast‐responding tensiometers measured changes in water content and matric potential. For near‐saturated conditions, we observed rapid establishment of equilibrium matric potentials during the recurring 20‐s periods of microgravity. As media water content decreased, the concurrent decrease in hydraulic diffusivity resulted in limited attainment of equilibrium distributions of water content and matric potential in microgravity, and water content heterogeneity within the sample was influenced by the preceding hypergravity phase. For steady fluxes through saturated columns, we observed linear and constant hydraulic gradients during variable gravity, yielding saturated hydraulic conductivities similar to values measured under terrestrial gravity. Our results suggest that water distribution and retention behavior are sensitive to varied gravitational forces, whereas saturated hydraulic conductivity appears to be unaffected. Comparisons between measurements and simulations based on the Richards equation were in reasonable agreement, suggesting that fundamental laws of fluid flow and distribution for macroscopic transport derived on Earth are also applicable in microgravity.

Variable gravity effects on thermal instability in a porous medium with internal heat source and inclined temperature gradient

The onset of convection in a horizontal fluid saturated isotropic porous layer, induced by inclined temperature gradient and internal heat source, subject to a gravity field varying linearly with location along the gravitational acceleration direction, is investigated using Galerkin technique. The boundaries are assumed to be impermeable and perfectly conducting. It is seen that the value of the variable gravity parameter plays a decisive role on the onset of convection. When the variable gravity parameter is zero and positive, an increase in the heat generation due to internal heat source advances the onset of convection in both the presence and absence of inclined temperature gradient. On the other hand, when the variable gravity parameter is negative, the opposite effect is seen. Further, it is observed that at the onset of convection the favourable mode is always the stationary longitudinal mode.

Non-linear convection in a porous medium with inclined temperature gradient and variable gravity effects

The energy method is developed to discuss the non-linear stability of convection in a horizontal porous layer subjected to an inclined temperature gradient and a variable gravity field. Both linear and non-linear stability analyses are carried out for a large number of parameter values. The eigenvalue problems in both cases are solved by the Chebyshev tau-QZ method with optimization routine. It is found that the preferred mode at the onset of convection is a longitudinal mode and that a decrease in gravity variation has a stabilizing effect on the system. Comparisons between the linear and energy stability results show that as the horizontal Rayleigh number increases the difference between the two results increases and thus indicates the possibility of subcritical instability.

Convection in a porous medium with internal heat source and variable gravity effects

The problem is studied of convection in a fluid saturated porous layer which is heated internally and where the gravitational field varies with distance through the layer. For this problem it is not known whether exchange of stabilities holds. We here find critical Rayleigh numbers for both linear instability and nonlinear energy stability, and numerical findings indicate that for the heat sources and gravity fields chosen, the growth rate of linear theory is real at criticality.