Reduced Gravity Conditions Research Articles

Equilibrium sediment transport, grade and discharge for suspended-load-dominated flows on Earth, Mars and Titan

Recent surface observations have emphasised the importance of bedload transport by rivers and streams on Mars and Titan. Previous “hydraulic” analysis, however, has also shown that transport as suspended load should be possible, if not more prevalent due to lower critical shear stresses, under reduced gravity conditions compared with Earth; where the dominant mode of sediment flux by rivers is as suspended load. A new suspended-load equilibrium condition (zero net erosion and deposition) constraint is advanced, using a flux-balance model applicable to transport (capacity) limited alluvial channels. The theory unifies Gilbert's relation for slope and flow capacity model with the Shields criterion for incipient motion. Shields-stress analysis shows that sediment transport thresholds on Earth, Mars and Titan are dynamically similar. Thus, despite large differences in dimensional shear stresses, variations in the critical slopes and flow depths required for threshold and equilibrium conditions are comparatively modest. Under reduced gravity conditions, there exists a slightly higher potential for sediment transport as suspended load; except for systems involving high particle-fluid density ratios, as may apply to the transport of organic particles on Titan. Critically we show that graded suspended-load channels should develop gentler slopes under reduced gravity, and not steeper slopes as previously inferred based on generalized hydraulic geometry relationships of terrestrial river and submarine channels. We further argue that the use of submarine channels as analogues for channels formed under reduced gravity is not physically justified. Finally, compared with bedload channels, suspended-load channels should also have markedly higher discharges, with implications for mechanistic-based discharge predictions and precipitation models.

Impact of Sulfur on the melt dynamics of glass forming Ti75Ni25−xSx

Substitution of Nickel by Sulfur leads to bulk metallic glass formation in eutectic Ti75Ni25. In order to understand the improved glass forming ability, a detailed analysis of the melt dynamics was performed. We measured the density, the viscosity, and the self-diffusion of Ti75Ni25−xSx (x = 0, 5, 8) melts as a function of temperature. Moreover, the viscosity was measured under reduced gravity conditions to benchmark ground-based data. The self-diffusion coefficients were probed by incoherent, quasi-elastic neutron scattering. All measurements were combined with electrostatic or electromagnetic levitation, in order to process these highly reactive Titanium-based melts. From the measured density, the packing fraction was calculated as a function of temperature. In contrast to other bulk metallic glasses, the decrease in melt dynamics comes along with the decrease in packing density.

Reduced gravity promotes bacterially mediated anoxic hotspots in unsaturated porous media

Human endeavours into deep space exploration and the prospects of establishing colonies on nearby planets would invariably involve components of bioregenerative life support for food production, cabin atmosphere renewal, and waste recycling. Growing plants and their microbiomes in porous media under different gravitational fields may present new challenges due to effects of liquid distribution on gaseous exchange with roots and microorganisms. We provide the first direct evidence that capillary driven liquid reconfiguration in porous media under reduced gravity conditions reduces oxygen diffusion pathways and enhances anoxic conditions within bacterial hotspots. Parabolic flight experiments using model porous media inoculated with aerobic and facultative anaerobic bacteria reveal the systematic enhancement of anoxic conditions during the reduced gravity periods in the presence but not in the absence of bacterial activity. The promotion of anoxic conditions under reduced gravity may lead to higher nitrous oxide and methane emissions relative to Earth conditions, on the other hand, anoxic conditions could be beneficial for perchlorate bioremediation of Martian soil. The results highlight changes in soil bacterial microhabitats under reduced gravity and the challenges of managing bioregenerative life support systems in space.

Effect of thermocapillary convection on the melting of phase change materials in microgravity: Experiments and simulations

Experiments on the melting of n-octadecane in the presence of a free surface were performed under the reduced gravity conditions of a parabolic flight, where natural gravitational convection is negligible. The results constitute the first systematic experimental investigation of the capacity of thermocapillary (Marangoni) convection to enhance heat transfer during the melting of organic Phase Change Materials (PCMs) in microgravity, where their low thermal conductivity leads to significant lag (long response times) that, in some applications, constrains their utility as passive thermal control devices. Rectangular containers of three different aspect ratios holding n-octadecane and a layer of air were heated from one side in order to observe the melting process. During the relatively short microgravity periods, which lasted approximately 20 s, the effect of thermocapillary convection remained concentrated in a layer near the free surface that is thinner than the PCM height H for the aspect ratios considered. The weak dependence of this layer on the lower region, where heat transport is dominated by thermal diffusion, means that the relative contribution of thermocapillary convection decreases with H. The experimental results are supported and complemented by a series of numerical simulations modelling the phase change and liquid dynamics with an enthalpy-porosity-based formulation of the Navier-Stokes equations. Very good agreement with the experimental observations is found.

Control strategies of gecko’s toe in response to reduced gravity

Shear-induced adhesion is one of the key properties for the gecko moving safely and quickly in a three-dimensional environment. The control strategies of such locomotion strongly relying on adhesion are still not well understood. In this study, we measured foot alignment and three-dimensional reaction forces of the single toes of the Tokay gecko running on the ground freely (gravity condition) and running in a situation where the gravity force was counterbalanced (reduced gravity condition). The forelimb rotated from the outward position to the front-facing position and the hindlimb rotated from the outward position to the rear-facing position, when running with balanced force, which indicated that the adhesive system was employed behaviorally through the modulation of the foot alignment. The toe was compressed and pulled in the gravity condition, but it was tensed and pulled in the reduced gravity condition. There was an approximately linear relationship between peak normal forces and the corresponding shear forces in both the reduced gravity condition (FN = −0.40FS − 0.008) and the gravity condition (FN = 2.70FS − 0.12). The footpad was compressed and pushed in the gravity condition, whereas it was tensed and pulled in the reduced gravity condition. There was an approximately linear relationship between peak normal forces and the corresponding shear forces in both the reduced gravity condition (FN = −0.39FS − 0.001) and in the gravity condition (FN = −2.80FS − 0.08). The shear-induced adhesion of the gecko footpad is controlled by the coupling of the normal force and shear forces: that is why in this system adhesion was shear-sensitive and friction was load-sensitive. Our measurements of single toe reaction forces also show that geckos control their footpad attachment using ‘toe rolling-in and gripping’ motion in both gravity and reduced gravity conditions.

Vibroequilibria in microgravity: Comparison of experiments and theory.

Experiments on vibrated fluids confined in cylindrical and cuboidal containers were performed under the reduced gravity conditions of a parabolic flight. The results constitute a systematic quantitative investigation of the vibroequilibria effect, which refers to the reorientation of vibrated fluids in response to the inhomogeneous oscillatory velocity field and the accompanying dynamic pressure. This effect is amplified in microgravity where the restoring force of gravity is small or absent. Here the vibrations are transmitted via a pair of piezoelectric ceramics and a cantilever beam, excited in a resonant mode. The first and second resonances exhibit different types of motion and lead to different types of vibroequilibria surfaces, one with a dip or crater in the interior and the other flattened compared to the unforced reference experiment. The general tendency for interfaces to orient more perpendicular, on average, to the vibrational axis is confirmed. In the case of water in a cuboidal container, a quantitative comparison is made with vibroequilibria theory and with direct simulations of the Navier-Stokes equations. The good agreement confirms the predictions of vibroequilibria theory and suggests the capacity of this phenomenon to manipulate and position fluids in space environments through the choice of frequency and resonant mode.

Molecular response of Deinococcus radiodurans to simulated microgravity explored by proteometabolomic approach

Regarding future space exploration missions and long-term exposure experiments, a detailed investigation of all factors present in the outer space environment and their effects on organisms of all life kingdoms is advantageous. Influenced by the multiple factors of outer space, the extremophilic bacterium Deinococcus radiodurans has been long-termly exposed outside the International Space Station in frames of the Tanpopo orbital mission. The study presented here aims to elucidate molecular key components in D. radiodurans, which are responsible for recognition and adaptation to simulated microgravity. D. radiodurans cultures were grown for two days on plates in a fast-rotating 2-D clinostat to minimize sedimentation, thus simulating reduced gravity conditions. Subsequently, metabolites and proteins were extracted and measured with mass spectrometry-based techniques. Our results emphasize the importance of certain signal transducer proteins, which showed higher abundances in cells grown under reduced gravity. These proteins activate a cellular signal cascade, which leads to differences in gene expressions. Proteins involved in stress response, repair mechanisms and proteins connected to the extracellular milieu and the cell envelope showed an increased abundance under simulated microgravity. Focusing on the expression of these proteins might present a strategy of cells to adapt to microgravity conditions.

RNA-seq analyses of Arabidopsis thaliana seedlings after exposure to blue-light phototropic stimuli in microgravity.

Plants synthesize information from multiple environmental stimuli when determining their direction of growth. Gravity, being ubiquitous on Earth, plays a major role in determining the direction of growth and overall architecture of the plant. Here, we utilized the microgravity environment on board the International Space Station (ISS) to identify genes involved influencing growth and development of phototropically stimulated seedlings of Arabidopsis thaliana. Seedlings were grown on the ISS, and RNA was extracted from 7 samples (pools of 10-15 plants) grown in microgravity (μg) or Earth gravity conditions (1-g). Transcriptomic analyses via RNA sequencing (RNA-seq) of differential gene expression was performed using the HISAT2-Stringtie-DESeq2 RNASeq pipeline. Differentially expressed genes were further characterized by using Pathway Analysis and enrichment for Gene Ontology classifications. For 296 genes that were found significantly differentially expressed between plants in microgravity compared to 1-g controls, Pathway Analysis identified eight molecular pathways that were significantly affected by reduced gravity conditions. Specifically, light-associated pathways (e.g., photosynthesis-antenna proteins, photosynthesis, porphyrin, and chlorophyll metabolism) were significantly downregulated in microgravity. Gene expression in A. thaliana seedlings grown in microgravity was significantly altered compared to that of the 1-g control. Understanding how plants grow in conditions of microgravity not only aids in our understanding of how plants grow and respond to the environment but will also help to efficiently grow plants during long-range space missions.

Experimental Investigation of Capillary-Driven Two-Phase Flow in Water/Butanol under Reduced Gravity Conditions

The capillary flow of water/butanol mixture is investigated in a single groove heat pipe model on board the Airbus A310 ‘Zero-G’ of the European Space Agency. As working fluid for evaporation-based heat transfer devices like conventional or pulsating heat pipes, these kinds of mixtures give more stable behaviour and higher dry-out limit with respect to pure water because of an anomalous behaviour of the surface tension with temperature. The groove is embedded in a semi-transparent test cell that allows for the qualitative visualization of the liquid distribution along the channel and is heated and cooled at two opposite sides with an electrical resistance and a water circulation system. The evaporation/condensation process is regulated changing the power input in a range between 5 and 30 W and the liquid distribution is detected from a top window using a CCD camera and a LED illumination device. The results show that the liquid distribution is affected by the gravity level and this effect is normally masked on ground.

A comparative study on the migration of minor phase globule in different-sized droplets of Fe-58wt.%Sn immiscible alloy

The migration behaviors of minor-phase globules are comparatively studied inside (radius=) 200 μm and 400 μm droplets based on the model of Fe-58wt.%Sn alloy under reduced-gravity condition. It was found that the final relative positions of same-sized globules are not dependent on the droplet sizes, although the globule's migrating velocity and lifetime inside different-sized droplet alloys are different. This finding is contrary to the conventional understanding that the droplet size can exert great influence on the microstructures by controlling Marangoni motion, and moreover offers a new insight into how to design the desired structures in the droplet immiscible alloy.

Surface Tension, Viscosity, and Selected Thermophysical Properties of Ti48Al48Nb2Cr2, Ti46Al46Nb8, and Ti46Al46Ta8 from Microgravity Experiments

The surface tension and the viscosity of the alloys Ti48Al48Nb2Cr2, Ti46Al46Nb8, and Ti46Al46Ta8 are measured with the oscillating drop technique in an electromagnetic levitation device under reduced gravity conditions on board a parabolic flight airplane. The surface tensions as a function of temperature are obtained as σ(T) = (1.19 ± 0.02 − 3.80 × 10−4 (T − 1856 K)) Nm−1 for Ti46Al46Nb8, σ(T) = (1.21 ± 0.02 − 2.69 × 10−4 (T − 1879 K)) Nm−1 for Ti46Al46Ta8, and σ(T) = (1.20 ± 0.02 − 3.96 × 10−4 (T − 1776 K)) Nm−1 for Ti48Al48Nb2Cr2, concentrations in atomic per cent. The viscosities at the liquidus temperatures are obtained as η(Tl) = 6.62 mPa.s, η(Tl) = 6.9 mPa.s, and η(Tl) = 5.8 mPa.s in the same order. All processed specimens are solidified at an undercooling of ΔTu = 220 ± 30 K. In addition, the solidus and liquidus temperatures and the enthalpies of fusion are determined by calorimetry. From the large undercooling and the enthalpy of fusion, the average specific heat capacity at an undercooling of 120 K is obtained as cP(av) = 1.30 ± 0.20 J (kg)−1 for all alloys. The thermal expansion between the room and solidus temperatures is measured by dilatometry.

Containerless Solidification Processing and Phase-Field Simulation for Ternary Fe-Cu-Co Peritectic Alloy Under Reduced-Gravity Conditions

We experimentally and theoretically investigated the rapid solidification and microstructural evolution of freely falling droplets of the ternary Fe45Cu40Co15 peritectic alloy, with the critical undercooling temperature of metastable liquid phase-separation initiation being measured to be 38 K. We found that liquid phase separation occurs when the droplet diameter ranges from 80 to 980 μm, resulting in the formation of either microscopically or macroscopically segregated microstructures. The peritectic solidification microstructure was produced when the droplet diameter either exceeded 980 μm or was below 80 μm. The dispersed morphology of the large alloy droplets possessed nonuniformly dispersive characteristics, i.e., proximity to the droplet surface is negatively associated with the size of Cu-rich globules. With the further reduction of the droplet size, the phase-separated morphology first transformed into a core–shell structure and finally displayed a homogeneously dispersed structure. Our theoretical calculations showed that the residual Stokes motion, Marangoni convection, and surface segregation are the dominant dynamic mechanisms for the phase separation and microstructural evolution under reduced-gravity conditions inside the drop tube.

A simple model of mechanical effects to estimate metabolic cost of human walking

Since the advent of energy measurement devices, gait experiments have shown that energetic economy has a large influence on human walking behavior. However, few cost models have attempted to capture the major energy components under comprehensive walking conditions. Here we present a simple but unified model that uses walking mechanics to estimate metabolic cost at different speeds and step lengths and for six other biomechanically-relevant gait experiments in literature. This includes at various gait postures (e.g. extra foot lift), anthropometric dimensions (e.g. added mass), and reduced gravity conditions, without the need for parameter tuning to design new gait trajectories. Our results suggest that the metabolic cost of walking can largely be explained by the linear combination of four costs—swing and torso dynamics, center of mass velocity redirection, ground clearance, and body weight support. The overall energetic cost is a tradeoff among these separable components, shaped by how they manifest under different walking conditions.

Pattern selection in miscible liquids under periodic excitation in microgravity: Effect of interface width

We develop a connection between instabilities in a two-liquid miscible system and in a liquid mixture with a linear distribution of density. In both cases, the liquids are placed in a closed cell and subjected to horizontal oscillations at different frequencies and amplitudes in a microgravity environment. The study includes parabolic flight experiments and numerical simulations. We examine the transformation of the interfacial pattern when the diffusive interface widens from a thin transient zone occupying 4% of the cell height to a situation when it occupies the entire cell height. In the case of sharp concentration (density) difference between miscible liquids, under reduced gravity conditions, instability leads to the formation of rectangular columns of liquids of alternating densities with an amplitude nearly equal to the height of the cell. The increase of the interface width promotes the selection of a smaller wave number associated with columns. The experimental observations are confirmed by the numerical simulations. In order to quantify the pattern regimes, we suggested to use the averaged mass flux through the mid-height of the cell.

Pore-scale investigation of gravity effects on phase change heat transfer characteristics using lattice Boltzmann method

Latent heat thermal energy storage (LTES) has been recommended to complicated aerospace systems due to the excellent ability of storing thermal energy in recent years. Metal foam, as an effective additive to enhance the effective thermal conductive coefficient of phase change materials, has been wildly used in the LTES system. In support of the future space exploration, solid–liquid phase change within metal foam under different gravitational accelerations and corresponding heat transfer characteristics are investigated in this work to enhance understanding of heat transfer capability in reduced gravity conditions. A pore-scale lattice Boltzmann method with the double distribution functions is developed to study the solid–liquid phase change problems with the natural convection heat transfer in metal foam. Here, microstructure of metal foam is experimentally obtained by the use of X-ray computed tomography. Two numerical validation cases are performed, one for fluid–solid conjugate heat transfer problem, and the other for melting process in a square cavity without the porous media. The presented lattice Boltzmann results are in good agreement with previous analytical and numerical results. Based on the analysis of temperature and velocity fields during melting within metal foam, it is found that the clockwise circulation, caused by the temperature gradient, leads to the inhomogeneous distribution of liquid–solid interface along vertical direction. What’s more, gravity effects on the temperature and velocity fields together with the average Nusselt number and liquid fraction coefficient are investigated. Results show that with the decrease of gravitational accelerations from 1g to 0g, the maximum vertical velocity reduces and the thickness of velocity boundary layer thickens accordingly, and results in the pronounced reduction of average Nusselt number and melting rate. The natural convection appears to be even more tenuous as gravitational acceleration decreases, leading to the dominant heat transfer mechanism transited from convection to conduction, and finally alleviating the phase change process. The liquid fraction under 0g, 0.01g and 0.1g conditions are 54.6%, 58.9% and 67.9% respectively of that under 1g conditions at Fo = 0.06. In addition, porosity effects on phase change heat transfer characteristics are discussed by comparing the liquid fraction with porosity of 0.9, 0.94 and 1. It is found that metal foam with lower porosity shows higher effective thermal conductivity and outstanding heat transfer performance, while the restriction on natural convection should not be neglected, especially for melting region which is dominated by natural convection.

The Impact of Reduced Gravity on Free Convective Heat Transfer from a Finite, Flat, Vertical Plate

Convective heat transfer is governed by a number of factors including various fluid properties, the presence of a thermal gradient, geometric configuration, flow condition, and gravity. Empirically-derived analytical relationships can be used to estimate convection as a function of these governing parameters. Although it is relatively straightforward to experimentally quantify the contributions of the majority of these variables, it is logistically difficult to assess the influence of reduced-gravity due to practical limitations of establishing this environment. Therefore, in order to explore this regime, a series of tests was conducted to evaluate convection under reduced-gravity conditions averaging 0.45 m/sec2 (0.05 g) achieved aboard a parabolic aircraft. The results showed a reduction in net heat transfer of approximately 61% in flight relative to a 1g terrestrial baseline using the same setup. The average experimental Nusselt Number of 19.05 ± 1.41 statistically correlated with the predicted value of 18.90 ± 0.63 (N = 13), estimated using the Churchill-Chu correlation for free convective heat transfer from a finite, flat, vertical plate. Extrapolating this to similar performance in true microgravity (10−6 g) indicates that these conditions should yield a Nusselt Number of 1.27, which is 2.6% the magnitude of free convection at 1g, or a reduction of 97.4%. With advection essentially eliminated, heat transfer becomes limited to diffusion and radiation, which are gravity-independent and nearly equivalent in magnitude in this case. These results offer a general guideline for integrating components that utilize natural (free) convective gas cooling in a spacecraft habitat and properly sizing the thermal control system.

Microstructural Quantification of Rapidly Solidified Undercooled D2 Tool Steel

Rapid solidification of D2 tool steel is investigated experimentally using electromagnetic levitation (EML) under terrestrial and reduced gravity conditions and impulse atomization (IA), a drop tube type of apparatus. IA produces powders 300 to 1400 μm in size. This allows the investigation of a large range of cooling rates (~100 to 10,000 K/s) with a single experiment. On the other hand, EML allows direct measurements of the thermal history, including primary and eutectic nucleation undercoolings, for samples ~6 to 7 mm in diameter. The final microstructures at room temperature consist of retained supersaturated austenite surrounded by eutectic of austenite and M7C3 carbides. Rapid solidification effectively suppresses the formation of ferrite in IA, while a small amount of ferrite is detected in EML samples. High primary phase undercoolings and high cooling rates tend to refine the microstructure, which results in a better dispersion of the eutectic carbides. Evaluation of the cell spacing in EML and IA samples shows that the scale of the final microstructure is mainly governed by coarsening. Electron backscattered diffraction (EBSD) analysis of IA samples reveals that IA powders are polycrystalline, regardless of the solidification conditions. EBSD on EML samples reveals strong differences between the microstructure of droplets solidified on the ground and in microgravity conditions. While the former ones are polycrystalline with many different grains, the EML sample solidified in microgravity shows a strong texture with few much larger grains having twinning relationships. This indicates that fluid flow has a strong influence on grain refinement in this system.

Hamstring and calf muscle activation as a function of bodyweight support during treadmill running in ACL reconstructed athletes

Rehabilitation after injury and reconstruction to the anterior cruciate ligament is thought to require a gradual reintroduction of loading, particularly during resumption of running. One strategy to achieve this is via the use of a reduced-gravity treadmill but it is unknown, if and how muscle activity varies in the reduced gravity conditions compared to regular treadmill running. Nineteen healthy participants and 18 male patients at the end of their rehabilitation (8 with a bone-patellar-bone graft, 10 with a hamstring graft) participated in this multi-muscle surface electromyography (sEMG) running study. The hamstrings and triceps surae were evaluated during a 16km/h running while at 6 different relative bodyweight conditions from 50% (half weight-bearing) to 100% (full weight-bearing). Muscle activation was examined individually as well as normalized to a composite “entire” activation and considered across the entire gait cycle using Statistical Parametric Mapping. The healthy participants showed differences between the 50–100% BW and 60–100% conditions and in the hamstring graft group for 60–100% and 80–100% conditions. No differences were seen comparing all loading conditions in the bone-patellar-bone graft group. For the hamstrings, from 70% BW and above, there appear to be no difference in activation patterns for any of the groups.The activation patterns of the hamstrings was essentially the same from 70% indicated bodyweight through to full weight bearing when running at 16km/h. Accordingly, when running at this relatively high speed, we do not expect any adverse effects in terms of altered motor patterns during rehabilitation of these muscles.

Saturated film boiling at various gravity levels under the influence of electrohydrodynamic forces

The present work is focused on the analyses of the bubble growth and heat transfer characteristics in saturated film boiling at various levels of gravity. In addition to this, the occurrence of self-similarity in interface structures during the bubble growth is examined. The phenomenon of bubble growth is strongly influenced by the buoyant forces due to gravity and its dominant effect is found to be replaced by the electrohydrodynamic forces in reduced gravity conditions. The decrease in gravitational acceleration results in increasing the characteristic wavelength and time scale. The bubble volume and maximum height before pinch-off thus increase enormously as the gravity value is reduced. The bubble pinch-off velocity is found to be decreased significantly in the case of reduced gravity condition. Heat transfer rate deteriorates in reduced gravity conditions which can be recovered by the externally imposed electric field. The dominance of electric field on the heat transfer rate is found to be more in reduced gravity condition. However, as the value of imposed electric field is enhanced, the difference in the effect of increasing heat flux tends to reduce.

限定条件下における気液二相流モデルの進展

Gas-liquid two-phase flows exist in a wide range of applications and enabling technologies. If, however, “understanding” we mean that the phenomenon in two-phase flow system can be predicted in terms of governing parameter, it should be concluded that two-phase flow is poorly understood item, and many questions are still open. A thorough theoretical approach with such microscopic experimental results as three-dimensional and high spatiotemporal measurements is needed to assess the performance of gas-liquid two-phase flow. My study of gas-liquid two-phase flows at microgravity conditions and in small diameter pipe is outlined in this report, especially for which regarding to interfacial area transport and drift flux. Research on two-phase at micro- or reduced-gravity conditions and in small pipe is expected to be conducted more widely in the future, shedding light on two-phase flow phenomena in piping system.