Simulation of Jet-Induced Geysers in Reduced Gravity

Control of cryogenic propellant tank pressure during tank refueling and expulsion in low gravity is an important technical challenge to overcome for future long duration missions in space. One method proposed to control tank pressurization involves the use of jet-induced geysers. Two-dimensional computational models have been developed and used with limited success in previous efforts to predict geyser heights in microgravity. A three-dimensional flow simulation is used to model jet-induced geysers in reduced gravity. Geyser flows are commonly characterized by the presence of turbulent jets, transient flow, deforming free surfaces, and surface tension effects. As is the case for many turbulent flow applications, accuracy in simulating complex turbulent flows is critically dependent on the selection of a suitable turbulence model. The sensitivity of the simulation geyser predictions to a suite of popular turbulence models is assessed. Simulation results are compared to available experiment results. By expanding upon the work already completed, the model is used to simulate a broad range of cases within the experiment test matrix. Simulation results suggest the two dimensional simulation using the k-ε turbulence model provides the most accurate results for jet-induced geysers in reduced gravity when compared to available experiment data.     

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.     

Sessile Drop Wettability in Normal and Reduced Gravity

This paper presents initial work performed to develop a database of contact angles of sessile drops in reduced gravity. Currently, there is no database of wettability of sessile drops in reduced gravity. The creation of such a database is imperative for continued investigations of heat and/or mass transfer in reduced gravity and future engineering designs. In this research, liquid drops of water and ethanol were created on aluminum and PTFE substrates. The formed drops were characterized by their dimensions including contact angle, wetted perimeter and droplet shape in both normal gravity and reduced gravity. The droplets were recorded during testing with high definition video and the images obtained digitally analyzed, post-test, to determine their characteristics as a function of the experimental parameters. The Queensland University of Technology (QUT) Drop Tower Facility was utilized for the reduced gravity experimentation. For droplets with diameters above their capillary length, the changes in drop dimensions and/or wettability was observed. The Young-Laplace equation was validated to accurately predict the contact angle in reduced gravity for small droplets, however it was not adequate to describe the contact angle for larger drops (above the drops associated capillary length).     

Surface Waves on Superfluid <Superscript>4</Superscript>He Under Reduced Gravity

Superfluid ⁴He was produced on a small jet plane for the first time using a small GM-refrigerator to condense the liquid and a scroll pump to get the superfluid by evaporation. The surface wave on superfluid under 0.5g _E, 0.1g _E and 0.05g _E, together with 2g _E and 1g _E, was successfully examined by an optical method utilizing parabolic flight. Here, g _E is the gravitational constant on the ground. Assuming that only the fundamental mode was excited as determined by the sample cell width, the resonance peak in the frequency domain was well reproduced by the gravity wave with corresponding gravity constant.     

Diffusion Coefficient Measurements Under Reduced Gravity Conditions by Means of the Shear Cell Technique. The Impact of Free Surfaces

A recent computational methodology is applied here in order to analyze the impact of free surfaces on the measurement of the interdiffusion coefficients if shear cells are used. Calculations indicate that the above-mentioned impact depends on the intensity of the solutal Marangoni convection but mainly on the number of active Marangoni segments, that is to say, on the extent of the free surface considered. Bulk solutal convection due to reduced gravity levels is not relevant if the gravity vector acts orthogonally to the gradient of concentration.     

Particle Dynamics in a Fluid Under High Frequency Vibrations of Linear Polarization

The paper deals with the investigation of a behavior of a system of particles suspended in a fluid in a container subjected to high frequency translational vibrations of linear polarization. Pair interaction forces act on the particles under these conditions. These forces decrease with the distance and depend on the interacting particles orientation with respect to the vibration direction. The presence of these forces leads to the formation of structures in space. The problem is solved numerically using molecular dynamics method with pair interaction approximation. It is shown that the process of the structures formation consists of the fast stage of compact cluster formation and slow evolution of these clusters. It is found that for vibrations of linear polarization the particles form the chains oriented perpendicular to the direction of vibrations. At long time-scales these chains form the layers perpendicular to the direction of the vibrations and located almost periodically all over the fluid volume.     

Premixed Flames Under Microgravity and Normal Gravity Conditions

Premixed conical CH₄-air flames were studied experimentally and numerically under normal straight, reversed gravity conditions and microgravity. Low-gravity experiments were performed in Drop tower. Classical Bunsen-type burner was used to find out features of gravity influence on the combustion processes. Mixture equivalence ratio was varied from 0.8 to 1.3. Wide range of flow velocity allows to study both laminar and weakly turbulized flames. High-speed flame chemoluminescence video-recording was used as diagnostic. The investigations were performed at atmospheric pressure. As results normalized flame height, laminar flame speed were measured, also features of flame instabilities were shown. Low- and high-frequency flame-instabilities (oscillations) have a various nature as velocity fluctuations, preferential diffusion instability, hydrodynamic and Rayleigh-Taylor ones etc., that was explored and demonstrated.     

Cryogenic Boiling and Two-Phase Flow during Pipe Chilldown in Earth and Reduced Gravity

For many industrial, medical and space technologies, cryogenic fluids play indispensable roles. An integral part of the cryogenic transport processes is the chilldown of the system components during initial applications. In this paper, we report experimental results for a chilldown process that is involved with the unsteady two-phase vapor-liquid flow and boiling heat transfer of the cryogen coupled with the transient heat conduction inside pipe walls. We have provided fundamental understanding on the physics of the two-phase flow and boiling heat transfer during cryogenic quenching through experimental observation, measurement and analysis. Based on the temperature measurement of the tube wall, the terrestrial cryogenic chilldown process is divided into three stages of film boiling, nucleate boiling and single-phase convection that bears a close similarity to the conventional pool boiling process. In earth gravity, cooling rate is non-uniform circumferentially due to a stratified flow pattern that gives rise to more cooling on the bottom wall by liquid filaments. In microgravity, there is no stratified flow and the absence of the gravitational force sends liquid filaments to the central core and replaces them by low thermal conductivity vapor that significantly reduces the heat transfer from the wall. Thus, the chilldown process is axisymmetric, but longer in microgravity.      

Effects of Reduced Gravity Conditions on Bubble Dispersion Characteristics in the Bubble Column

Bubble-liquid turbulent flow has an excellent heat and mass transfer behaviors than single gas or liquid flow. In order to analyze the effects of normal and reduced gravity on cold bubble-liquid two-phase turbulent flow in bubble column a second-order moment cold bubble-liquid two-phase turbulent model was developed to disclose the bubble dispersion characteristics. Under the reduced gravity condition, volume fraction caused by the decrease of buoyance force is larger than normal gravity level due to bigger bubble solid volume. In addition, bubble frequency is also decreased by in decrease of buoyance force. Normal and shear stresses have strongly anisotropic characteristics at every directions and have larger values under normal gravity than reduced gravity. The liquid turbulent kinetic energy has the two-peak bimodal distribution and weaker than bubble turbulent kinetic energy with one peak unimodal, which is caused by vigorous wake fluctuations. The correlation of fluctuation velocities between bubble and liquid has clearly anisotropic behaviors Under reduced gravity, the bubble motion has a little impact on liquid turbulent flow caused by slight buoyancy force, however, it will greatly reduce the liquid turbulent intensity due to energy cascade transport, which was transformed into bubbles or dissipated by interface friction. Bubble formation and detachment mechanisms affected by gravity conditions lead to the different levels of bubble dispersion distributions.     

On the Convection of a Binary Mixture in a Horizontal Layer Under High-frequency Vibrations

The convective instability and non-linear flows are considered in a horizontal, binary-mixture layer with negative Soret coupling, subjected to the high-frequency vibration whose axis is directed at an arbitrary angle to the layer boundaries. The limiting case of long-wave disturbances is studied using the perturbation method. The influence of the intensity and direction of vibration on the spatially-periodic traveling wave solution is analyzed. It is shown that the shift in the Rayleigh number range, in which the traveling wave regime exists, toward higher values is a response to a horizontal-to-vertical transition in the vibration axis orientation. The characteristics of amplitude- and phase-modulated traveling waves are obtained and discussed.     

Changes in Gene Expression of E. coli under Conditions of Modeled Reduced Gravity

Relatively few studies have examined bacterial responses to the reduced gravity conditions that are experienced by bacteria grown in space. In this study, whole genome expression of Escherichia coli K12 under clinorotation (which models some of the conditions found under reduced gravity) was analyzed. We hypothesized that phenotypic differences at cellular and population levels under clinorotation (hereafter referred to as modeled reduced gravity) are directly coupled to changes in gene expression. Further, we hypothesized that these responses may be due to indirect effects of these environmental conditions on nutrient accessibility for bacteria. Overall, 430 genes were identified as significantly different between modeled reduced gravity conditions and controls. Up-regulated genes included those involved in the starvation response (csiD, cspD, ygaF, gabDTP, ygiG, fliY, cysK) and redirecting metabolism under starvation (ddpX, acs, actP, gdhA); responses to multiple stresses, such as acid stress (asr, yhiW), osmotic stress (yehZYW), oxidative stress (katE, btuDE); biofilm formation (lldR, lamB, yneA, fadB, ydeY); curli biosynthesis (csgDEF), and lipid biosynthesis (yfbEFG). Our results support the previously proposed hypothesis that under conditions of modeled reduced gravity, zones of nutrient depletion develop around bacteria eliciting responses similar to entrance into stationary phase which is generally characterized by expression of starvation inducible genes and genes associated with multiple stress responses.     

Presynaptic Release of Glutamate by Heteroexchange Under Altered Gravity Conditions

The study focused on the release of glutamate by reversal of Na^?+?-dependent glutamate transporters that was investigated in cortical synaptosomes isolated from Wistar rats subjected to centrifuge-induced hypergravity of 10×g for 1 h. Transportable competitive inhibitor of glutamate transporters—DL-threo-beta-hydroxyaspartate (DL-THA) induced the release of L-[¹⁴C]glutamate via heteroexchange that consisted of 22.5 ± 1.7% of total label in normal gravity and 23.7 ± 1.7% in altered gravity conditions. Inhibition of the ability of synaptic vesicles to accumulate the neuromediator by preliminary treatment of synaptosomes with the protonophore (FCCP) augmented the release of cytosolic L-[¹⁴C]glutamate evoked by DL-THA from 44.0 ± 2.0% to 52.0 ± 2.3% of total label for G-loaded animals as compared to controls. Thus, combined application of DL-THA and FCCP revealed the increase in transporter-mediated release of glutamate that might be evidence for hypoxic injury of neurons.     

Long-wave Marangoni Convection in Two-layer Films Under the Action of Gravity Modulation

The influence of the gravity on the long-wave Marangoni patterns in two-layer films is considered. The numerical analysis is carried out in the lubrication approximation. Periodic boundary conditions have been applied on the boundaries of the computational region. The development of instabilities is investigated by means of nonlinear simulations. The cases of a constant gravity and a time-periodic gravity modulation are considered. In the case of a constant gravity, non-stationary three-dimensional and two-dimensional structures have been found. It is shown that the periodic modulations of the gravity force lead to the development of new three-dimensional spatially-periodic patterns. Outside the region of parameters, corresponding to three-dimensional structures, dynamical regime of two-dimensional traveling waves have been observed.     

Convection in a Two-Layer System with a Deformable Interface Under Low Gravity Conditions

The onset of thermal convection in a system of two horizontal layers of immiscible liquids of similar densities is studied under low gravity conditions. A constant heat flux is prescribed at both rigid boundaries. A generalized Boussinesq approach that allows correct accounting for the interface deformation is used. The long-wave perturbations emerge under low-gravity conditions; either monotonic or oscillatory modes are critical depending on the problem. Moreover, two different modes of the monotonic instability exist. For the first instability mode, the convection dominates, whereas the interface remains nearly undeformable. The second monotonic instability mode is substantially related to interface deformations. The system of non-linear amplitude equations describing the behavior of long-wave regimes at finite-amplitude interface deflection and finite supercriticalities is obtained. The analytical and numerical investigations of these equations show that the stable non-trivial stationary solutions are absent, and after a transient at least one of the layers is split into the areas not connected to each other. The nonlinear regimes of cellular convection are studied numerically by the Level Set method.