Numerical Simulation of Bubble Dynamics in Microgravity

A numerical method for the simulation of two-phase flows under microgravity conditions is presented in this paper. The level set method is combined with the moving mesh method in a collocated grid to capture the moving interfaces of the two-phase flow, and a SIMPLER-based method is employed to numerically solve the complete incompressible Navier-Stokes equations, and the surface tension force is modeled by a continuum surface force approximation. Based on the numerical results, the coalescence process of two bubbles under microgravity conditions (10^???2×g) is compared to that under normal gravity, and the effect of gravities on the bubbles coalescence dynamics is analyzed. It is showed that the velocity fields inside and around the bubbles under different gravity conditions are quite similar, but the strength of vortices behind the bubbles in the normal gravity is much stronger than that under microgravity conditions. It is also found that under microgravity conditions, the time for two bubbles coalescence is much longer, and the deformation of bubbles is much less, than that under the normal gravity.     

Dynamics for Droplets in Normal Gravity and Microgravity

A numerical study has been carried out to investigate the deformation dynamics of droplets under normal gravity and the thermocapillary migration of droplets under microgravity. The Navier–Stokes equations coupled with the energy conservation equation are solved on a staggered grid by the method of lines, and the mass conserving level set method is used to predict the surface deformation of the droplet. The simulation for the falling droplet in the air under normal gravity shows that the value of Weber number affects mainly the deformation of the droplet, while the value of Reynolds number has direct impact on the falling velocity of the droplet. From the simulation for the droplet thermocapillary migration and lateral oscillation under microgravity, it is found that the value of Marangoni number has obvious effects on the moving velocity and temperature distribution of the droplet.     

Numerical Investigation on Turbulence and Bubbles Distribution in Bubbly Flow Under Normal Gravity and Microgravity Conditions

Two-phase flows of gas and liquid are increasingly paid much attention to space application due to excellent properties of heat and mass transfer, so it is very meaningful to develop studies on them in microgravity. In this paper, gas-phase distribution and turbulence characteristics of bubbly flow in normal gravity and microgravity were investigated in detail by using Euler–Lagrange two-way model. The liquid-phase velocity field was solved by using direct numerical simulations (DNS) in Euler frame of reference, and the bubble motion was tracked by using Newtonian motion equations that took into account interphase interaction forces including drag force, shear lift force, wall lift force, virtual mass force and inertia force, etc. in Lagrange frame of reference. The coupling between gas–liquid phases was made with regarding interphase forces as a momentum source term in the momentum equation of the liquid phase. Under the normal gravity condition, a great number of bubbles accumulate near the walls under the influence of the shear lift force, and addition of bubbles reduces turbulence of the liquid phase. Different from the normal gravity condition, in microgravity, an overwhelming majority of bubbles migrate towards the centre of the channel driven by the pressure gradient force, and bubbles have little effect on the turbulence of the liquid phase.     

Flow physics of upward cocurrent gas-liquid annular flow in a vertical small diameter tube

In this paper, a steady RNG k-ε model, in conjunction with enhanced wall treatment method, was applied to the gas core in order to simulate the flow physics of annular two-phase flow. The model incorporated a physical model of wave characteristics and included the liquid entrainment influence on the flow. Based on the simulation results, flow features in the gas core were quantitatively presented and a model of the liquid entrainment mechanism was proposed. In addition, a parametric study was conducted to determine the impact of changing wave velocity, pressure, and gravitational force on the liquid film flow. The results were validated using a large set of experimental data at normal and microgravity conditions. Also, the law of the wall was applied to previously-collected experimental data. Analysis yielded different flow features of the liquid film at microgravity and normal gravity conditions.     

Simultaneous measurements of thermal conductivity and thermal diffusivity of liquids under microgravity conditions

A transient short-hot-wire technique is proposed and used to measure the thermal conductivity and thermal diffusivity of liquids simultaneously. The method is based on the numerical evaluation of unsteady heat conduction from a wire with the same length diameter ratio and boundary conditions as those in the experiments. To confirm the applicability and accuracy of this method. Measurements were made for five sample liquids with known thermophysical properties and were performed under both normal gravity and microgravity conditions. The results reveal that the present method determines both the thermal conductivity and the diffusivity within 2 and 5%. respectively. The microgravity experiments clearly indicate that even under normal gravity conditions, natural-convection effects are negligible for at least l s after the start of heating. This method would be particularly suitable for a valuable and expensive liquid, and has a potential for application to electrically conducting and or corrosive liquids when the probe is effectively coated with an insulating and anticorrosive material. Paper presented at the Fourth Asian Thermophysical Properties Conference, September 5–8, 1995, Tokyo, Japan.     

A Six-DOF Buoyancy Tank Microgravity Test Bed with Active Drag Compensation

Ground experiment under microgravity is very essential because it can verify the space enabling technologies before applied in space missions. In this paper, a novel ground experiment system that can provide long duration, large scale and high microgravity level for the six degree of freedom (DOF) spacecraft trajectory tracking is presented. In which, the most gravity of the test body is balanced by the buoyancy, and the small residual gravity is offset by the electromagnetic force. Because the electromagnetic force on the test body can be adjusted in the electromagnetic system, it can significantly simplify the balancing process using the proposed microgravity test bed compared to the neutral buoyance system. Besides, a novel compensation control system based on the active disturbance rejection control (ADRC) method is developed to estimate and compensate the water resistance online, in order to improve the fidelity of the ground experiment. A six-DOF trajectory tracking in the microgravity system is applied to testify the efficiency of the proposed compensation controller, and the experimental simulation results are compared to that obtained using the classic proportional-integral-derivative (PID) method. The simulation results demonstrated that, for the six-DOF motion ground experiment, the microgravity level can reach to 5 × 10^?4 g. And, because the water resistance has been estimated and compensated, the performance of the presented controller is much better than the PID controller. The presented ground microgravity system can be applied in on-orbit service and other related technologies in future.     

The Influence of the Gravity Force on Longwave Marangoni Patterns in Two-Layer Films

An Euler–Euler two-fluid model based on the second-order-moment closure approach and the granular kinetic theory of dense gas-particle flows was presented. Anisotropy of gas-solid two-phase stress and the interaction between two-phase stresses are fully considered by two-phase Reynolds stress model and the transport equation of two-phase stress correlation. Under the microgravity space environments, hydrodynamic characters and particle dispersion behaviors of dense gas-particle turbulence flows are numerically simulated. Simulation results of particle concentration and particle velocity are in good agreement with measurement data under earth gravity environment. Decreased gravity can decrease the particle dispersion and can weaken the particle–particle collision as well as it is in favor of producing isotropic flow structures. Moreover, axial–axial fluctuation velocity correlation of gas and particle in earth gravity is approximately 3.0 times greater than those of microgravity and it is smaller than axial particle velocity fluctuation due to larger particle inertia and the larger particle turbulence diffusions.     

Effects of Gravity on Soot Formation in a Coflow Laminar Methane/Air Diffusion Flame

Simulations of a laminar coflow methane/air diffusion flame at atmospheric pressure are conducted to gain better understanding of the effects of gravity on soot formation by using detailed gas-phase chemistry, complex thermal and transport properties coupled with a semiempirical two-equation soot model and a nongray radiation model. Soot oxidation by O₂, OH and O was considered. Thermal radiation was calculated using the discrete ordinate method coupled with a statistical narrow-band correlated-K model. The spectral absorption coefficient of soot was obtained by Rayleigh’s theory for small particles. The results show that the peak temperature decreases with the decrease of the gravity level. The peak soot volume fraction in microgravity is about twice of that in normal gravity under the present conditions. The numerical results agree very well with available experimental results. The predicted results also show that gravity affects the location and intensity for soot nucleation and surface growth.     

Iron Burning in Pressurised Oxygen Under Microgravity Conditions

An investigation of cylindrical iron rods burning in pressurised oxygen under microgravity conditions is presented. It has been shown that, under similar experimental conditions, the melting rate of a burning, cylindrical iron rod is higher in microgravity than in normal gravity by a factor of 1.8 ± 0.3. This paper presents microanalysis of quenched samples obtained in a microgravity environment in a 2.0 s duration drop tower facility in Brisbane, Australia. These images indicate that the solid/liquid interface is highly convex in reduced gravity, compared to the planar geometry typically observed in normal gravity, which increases the contact area between liquid and solid phases by a factor of 1.7 ± 0.1. Thus, there is good agreement between the proportional increase in solid/liquid interface surface area and melting rate in microgravity. This indicates that the cause of the increased melting rates for cylindrical iron rods burning in microgravity is altered interfacial geometry at the solid/liquid interface.     

Bubble Motion near a Wall Under Microgravity: Existence of Attractive and Repulsive Forces

A numerical simulation for a bubble motion near a wall under microgravity, relevant to material processing such as crystal growth in space, is presented based on a mass conservation level set algorithm to predict the bubble behavior affected by the near wall. The simulation for the wall effect on the bubble driven by an external acceleration parallel with the near wall referred to as g-jitter confirms for the first time the existence of the wall attractive force to the bubble near the wall under certain conditions such as the initial distance between the bubble and the wall, density and viscosity ratios between the bubble and surrounding liquid under microgravity. The wall effect mechanism is explained, and the results show that the wall attractive force increases with the increasing of density ratio. Moreover, the simulation for the wall repulsive effect on the bubble near the wall under microgravity has been carried out as well.     

Sessile Drop in Microgravity: Creation, Contact Angle and Interface

We present in this paper the results obtained from a parabolic flight campaign regarding the contact angle and the drop interface behavior of sessile drops created under terrestrial gravity (1g) or in microgravity (μg). This is a preliminary study before further investigations on sessile drops evaporation under microgravity. In this study, drops are created by the mean of a syringe pump by injection through the substrate. The created drops are recorded using a video camera to extract the drops contact angles. Three fluids have been used in this study : de-ionized water, HFE-7100 and FC-72 and two heating surfaces: aluminum and PTFE. The results obtained evidence the feasibility of sessile drop creation in microgravity even for low surface tension liquids (below 15 mN m^− 1) such as FC-72 and HFE-7100. We also evidence the contact angle behavior depending of the drop diameter and the gravity level. A second objective of this study is to analyze the drop interface shape in microgravity. The goal of the these experiments is to obtain reference data on the sessile drop behavior in microgravity for future experiments to be performed in an French-Chinese scientific instrument (IMPACHT).     

Microgravity conditions and electrical resistivity of liquid alloys with critical mixing

The phenomena of two-liquid phase separations are significantly influenced by the gravity on the ground because of the difference in the densities of the constituent components, particularly, in the case of liquid alloys with critical mixing. In this paper, experimental techniques and results are reported for the measurements of the electrical resistivity for typical liquid alloys with critical mixing, such as Bi−Ga, under microgravity by the use of a rocket S520-19 belonging to ISAS (Institute of Space and Astronautical Science, Japan). It was found that the temperature coefficient of the electrical resistivity, on cooling of the homogeneous liquid phase, increases with the approach to the critical temperature. This trend under microgravity by the rocket experiment is more pronounced compared to the trend of the reference experiment on the ground. In addition, the supercooling of homogeneous liquids under microgravity is larger than that on the ground. These differences are explained by the difference in the degree of the growth of concentration fluctuations; the concentration fluctuations are far greater under microgravity than on the ground. Therefore, it is found to be very important to study the process and the critical phenomena of two-liquid phase separations under microgravity. Measurement of electrical resistivity is an effective method to obtain informations about the process, the critical phenomena, and the supercooling of two-liquid phase separations in liquid alloys with critical mixing. Paper presented at the Fourth Asian Thermophysical Properties Conference, September 5–8, 1995, Tokyo, Japan.     

Effect of rotation on surface tension driven flow during protein crystallization

The effect of rotation on surface tension gradient driven flow, also known as Marangoni convective flow, during protein crystallization is modeled and studied computationally under microgravity conditions, where the surface tension gradient force is the main significant driving force. The main parameters are the solutal Marangoni number M_c, representing the surface tension gradient force and the Taylor number Ta representing the rotational effect. The numerical computations for various values of the parameters and low gravity levels indicated nontrivial competing effects, due to surface tension gradient, centrifugal and Coriolis forces on the flow adjacent to the protein crystal interface and the associated solute flux. In particular, for given values of M_c, certain values of Ta were detected where the Sherwood number (Sh), representing the convective solute flux, and the convective flow effects are noticeably reduced. These results can provide conditions under which convective flow transport during the protein crystallization approaches the diffusion limited transport, which is desirable for the production of higher quality protein crystals.     

Computation of free-surface flows and fluid–object interactions with the CIP method based on adaptive meshless soroban grids

The CIP Method [J comput phys 61:261–268 1985; J comput phys 70:355–372, 1987; Comput phys commun 66:219–232 1991; J comput phys 169:556–593, 2001] and adaptive Soroban grid [J comput phys 194:57–77, 2004] are combined for computation of three- dimensional fluid–object and fluid–structure interactions, while maintaining high-order accuracy. For the robust computation of free-surface and multi-fluid flows, we adopt the CCUP method [Phys Soc Japan J 60:2105–2108 1991]. In most of the earlier computations, the CCUP method was used with a staggered-grid approach. Here, because of the meshless nature of the Soroban grid, we use the CCUP method with a collocated-grid approach. We propose an algorithm that is stable, robust and accurate even with such collocated grids. By adopting the CIP interpolation, the accuracy is largely enhanced compared to linear interpolation. Although this grid system is unstructured, it still has a very simple data structure.     

Flow Structure and Surface Deformation of High Prandtl Number Fluid Under Reduced Gravity and Microgravity

The numerical simulation has been conducted to investigate the flow structure and surface deformation in a liquid bridge of high Prandtl number fluid under reduced gravity and microgravity. The Navier–Stokes equations coupled with the energy conservation equation are solved on a staggered grid, and the mass conserving level set approach is used to capture the free surface deformation of the liquid bridge. The effect of reduced gravity and thermocapillary convection on the surface deformation of the liquid bridge is investigated, and the results show that the amplitude of the surface horizontal vibration decreases gradually, and the thermocapillary convection inside the liquid bridge starts to turn into a steady state after the initial period. Moreover, the shift of the center of the recirculating flow inside the liquid bridge under horizontal external acceleration and zero gravity is also studied, and the results indicate that the vortex centers move initially toward the cold disk and reach an equilibrium position, and then the vortex centers vibrate around the equilibrium position periodically.     

Effect of convective disturbances induced by g-jitter on the periodic precipitation of lysozyme

Numerical simulations are carried out to investigate the crystallization process of a protein macromolecular substance under two different conditions: pure diffusive regime and microgravity conditions present on space laboratories. The configuration under investigation consists of a protein reactor and a salt chamber separated by an “interface”. The interface is strictly related to the presence of agarose gel in one of the two chambers. Sedimentation and convection under normal gravity conditions are prevented by the use of gel in the protein chamber (pure diffusive regime). Under microgravity conditions periodic time-dependent accelerations (g-jitter) are taken into account. Novel mathematical models are introduced to simulate the complex phenomena related to protein nucleation and further precipitation (or resolution) according to the concentration distribution and in particular to simulate the motion of the crystals due to g-jitter in the microgravity environment. The numerical results show that gellified lysozyme (crystals “locked” on the matrix of agarose gel) precipitates to produce “spaced deposits”. The crystal formation results modulated in time and in space (Liesegang patterns), due to the non-linear interplay among transport, crystal nucleation and growth. The propagation of the nucleation front is characterized by a wavelike behaviour. In microgravity conditions (without gel), g-jitter effects act modifying the phenomena with respect to the on ground gellified configuration. The role played by the direction of the applied sinusoidal acceleration with respect to the imposed concentration gradient (parallel or perpendicular) is investigated. It has a strong influence on the dynamic behaviour of the depletion zones and on the spatial distribution of the crystals. Accordingly the possibility to obtain better crystals for diffraction analyses is discussed.     

Numerical Simulation of Transient Development of Flame,Temperature and Velocity under Reduced Gravity in a Methane Air Diffusion Flame

A methane air co flow diffusion flame has been numerically simulated with the help of an in-house developed code at normal gravity, 0.5 G, and 0.0001 G (microgravity) for the study of transient behavior of the flame in terms of flame shape, temperature profile and velocity (streamlines). The study indicates that lower is the gravity level, the higher is the time of early transience. The flame developments during transience are marked by the formation of a secondary flamelet at different heights above the primary flame at all gravity levels. The development of temperature profile at microgravity takes a much longer time to stabilize than the flame development. At normal gravity and 0.5 G gravity level, streamlines, during transience, show intermediate vortices which are finally replaced by recirculation of ambient air from the exit plane. At microgravity, neither any vortex nor any recirculation at any stage is observed. Centerline temperature plots, at all gravity levels during transience, demonstrate a secondary peak at some instants as a consequence of the secondary flamelet formation. The centerline velocity at microgravity decreases gradually during transience, unlike at other two gravity levels where the fall is very sharp and is indicative of negligible buoyancy at microgravity.     

Gravity Effect on the Axisymmetric Drop Spreading

The flattening (spreading) of the axisymmetrical drop on a plane horizontal surface under action of gravity force at zero tangential force (no shear at the gas–liquid interface) is investigated analytically and numerically. We determine the exact profile of compressed drop assuming the condition of drop volume conservation. 2D time dependant numerical model, based on a finite difference method, has been developed to describe the hydrodynamics inside the drop. The energy and Navier–Stokes equations are solved within the drop’s analytical profile. Effects of surface tension and thermocapillarity are taken into account. The effect of gravity has been studied to define main features of the drop dynamics. In calculations vector of gravitational acceleration is oriented perpendicularly to the surface, the Bond number is changed in the range from Bo = 0 to Bo = 151.6. Our results show that the gravity has a significant effect on the drop spreading.     

Particle conveying under microgravity in a vibrating vessel

This paper presents a numerical study on the conveying of particles in a vibrating vessel under microgravity. Such a vessel is composed of parallel plates with sawtooth wavy surfaces, which are specifically designed to convey particles using simple vibration. The numerical model was validated by good agreement between the simulated and experimental results. Then the effects of key variables, including the vessel geometry, vibration amplitude and frequency and gravity level, were systematically investigated by a series of controlled simulations. The results confirm the optimised design from the previous experiments, and numerically demonstrate that using such a system a steady conveying operation can be achieved under microgravity. The convey rate is positively affected by the vibration amplitude and frequency in a complicated way, which cannot be simply described by the commonly used vibration intensity or velocity amplitude. The gravity level also has a significant effect on the convey rate when it is over 0.001g. The convey rate can be estimated by the product of the average solid fraction and velocity. And the effects of the variables can be better understood through the analyses on these two parameters. Finally, a predictive model is proposed to estimate the convey rate under different operational conditions. The findings are useful for the design of particle conveying techniques for outer space applications.     

Mathematical and Numerical Modelling of Microconvection in the Long Rectangular Domains

The thermal gravitational convection of liquids under conditions of microgravity is studied in a 2D long rectangular domain elongated in the direction of the gravity force. The liquid is located between two solid regions of equal thickness. The solid parts are heat conducting. The mathematical modelling of the coupled problem is presented. Two mathematical models of convection are used to describe a motion of liquid: the classical Oberbeck-Boussinesq model and the microconvection model of isothermally incompressible liquid. The numerical experiments of convection are performed and demonstrate the qualitative and quantitative differences in the flow characteristics.