This study investigates the linear instability of double-diffusive Hadley convection in a horizontal porous bed. Here, the flow is caused by the combined effects of inclined temperature and concentration gradients under constant gravity. The main flow has a straight velocity profile and steady mass flow, which is typical for Hadley-type circulation. The analysis of instability takes viscous heating into account. The system is described using six dimensionless parameters: horizontal and vertical thermal Rayleigh numbers (for heat input and output), temperature and concentration differences between the top and bottom surfaces, the Péclet number (which measures the base horizontal flow velocity), and the Gebhart number (which shows heat loss from viscous heating). For given values of the horizontal Rayleigh, Péclet, and Gebhart numbers, the study finds the critical vertical Rayleigh number and the related critical wave number. Numerical techniques such as the combined shooting method with Runge-Kutta integration are employed to analyze the system’s linear instability. The results identify the crucial parameters that cause instability in the Hadley flow and emphasise the major contribution of non-uniform, inclined gradients to lowering the convection thresholds.
Deorbiter systems based on micro-spacecraft presents a promising approach for the active removal of space debris. Stable in-orbit deployment is a prerequisite for the successful completion of the entire mission. To investigate the reliability and launch dynamics of in-orbit launch systems for micro-spacecraft, this study developed a ground-based test system using the drop-tower method and employed a high-precision measurement system to capture the dynamic response of the micro-spacecraft during launch. A numerical model of the micro-spacecraft’s in-orbit launch dynamics was developed employing multi-body dynamics. This model accounts for thrust eccentricity, motion coupling between the micro-spacecraft and the satellite platform, contact collisions during launch, and other perturbation factors. The accuracy of the numerical model was validated by comparing the simulation results with those from ground tests. Based on this, Monte Carlo simulations were conducted to analyze and discuss the factors influencing the separation angular velocity of the micro-spacecraft. The results indicate that, for specific structural and parameter configurations, controlling the eccentricity angle of the solid-propellant thruster and the clearance between the micro-spacecraft and the separation mechanism within certain limits can effectively reduce the micro-spacecraft’s separation angular velocity.
Slow fluctuations of electrostatic actuation gain couple commanded forces into the low-frequency acceleration observable of a space inertial sensor. This paper develops dual-pilot-tone (DPT) monitoring for the audio-frequency carrier actuation architecture of LISA Pathfinder-class gravitational reference sensors and establishes its observability limits. After carrier-period averaging, each low-frequency pilot modulates the squared carrier amplitude, and a drive-side observation contains a projected product of actuator, test-mass, controller, and readout responses. Two tones add a consistency test for frequency-selective anomalies but cannot separate common actuation drift from common sensing-chain drift or identify which tone is faulty after a mismatch. Metrological actuation calibration therefore requires an independent readout reference or a quantified sensing-stability bound. Across 24 stochastic seeds, the median residual-gain ASD over 0.001–0.1 Hz changes from 7.69 [7.55, 7.80] ppm/√Hz without compensation to 1.88 [1.85, 1.89] ppm/√Hz with a stable readout under the full-amplitude two-tone configuration, while readout drift gives 9.23 [9.10, 9.35] ppm/√Hz and a simulated independent monitor restores 2.07 [2.05, 2.09] ppm/√Hz. A matched total-injection comparison shows that single- and dual-tone nominal noise performance is similar; the practical value of the second tone is anomaly detection and guarded bypass rather than an intrinsic signal-to-noise advantage. The evidence is analytical and numerical, and hardware validation remains future work.
Shapes and dynamics of floating droplets under the action of an inclined temperature gradient in the presence of two-dimensional spatial modulation of the vertical component of the temperature gradient are investigated. The case when the vertical component of the temperature gradient is directed downwards is considered. The problem has been studied numerically by the finite difference method. The precursor model has been considered. It is shown that the shape of the droplet is strongly influenced by the thermocapillary stresses caused by a non-uniform substrate heating and the gravity force.
Countermeasures against the adverse effects of microgravity should be designed in such a way as to prevent the development of overexertion and pathological conditions. Changes in immune parameters serve as one of the sensitive indicators of the adequacy of physical activity and the state of the body’s adaptive capacity. The effectiveness and physiological adequacy of individualized training programs based on reaction of the cardiovascular and immune systems was assessed in a 366-day ground-based spaceflight analog study. Alternating 30- and 15-min aerobic exercise sessions individually tailored to each participant’s response to exercise was associated with maintenance of cardiorespiratory performance and, overall, was not accompanied by a statistically significant decrease in the studied cellular immunity parameters relative to baseline during the year-long isolation. A significant increase in the number of NK cells on days 67 and 160 of the experiment, compared to baseline values, was observed following training periods that involved 30‑minute protocols. Following the final training cycle, absolute peripheral blood levels of selected lymphocyte subpopulations were significantly increased, including CD3 + T cells, the CD3 + CD4+ T-cell subset, and CD3 − CD16+CD56 + NK cells.
Toroidal tanks offer geometric compatibility with launch vehicles and certain spacecraft configurations, making them suitable for liquid propellant storage in aerospace engineering. However, investigations into liquid behavior within such tanks under microgravity conditions remains relatively underexplored. This study addresses the static liquid interface generation in toroidal tanks under zero-gravity and microgravity environments, establishing a static equilibrium model based on surface free energy theory. Numerical simulations were conducted to investigate the static interface evolution under varying tank geometries, fill ratios, gravitational levels, and viscous models, thereby assessing the proposed liquid model. A comparative analysis was performed to evaluate the effects of these factors on the static interface morphology and the corresponding stabilization time. To improve propellant management, compartmentalization configurations with different numbers of compartments and perforations on baffle plates were implemented within the toroidal tank, and their effects on the static interface evolution process and final configuration were systematically analyzed. The findings of this research provide important insights for propellant management strategies in toroidal tanks.
Astronauts face multiple physiological challenges during spaceflight, including exposure to microgravity, ionizing radiation, altered circadian rhythms, and psychological stress due to confinement. Among these hazards, microgravity is a significant disruptor of gastrointestinal homeostasis. This review synthesizes evidence from human spaceflight missions, ground-based analogue studies, and preclinical models to characterize microgravity-induced gut dysbiosis and its downstream effects on central nervous system function via the gut–brain axis. Consistent findings across studies include shifts in the Firmicutes-to-Bacteroidetes ratio, depletion of beneficial commensal bacteria (notably Akkermansia muciniphila, Faecalibacterium prausnitzii, and Bifidobacterium species), and enrichment of potentially pathogenic taxa. These compositional changes correlate with reduced production of short-chain fatty acids and other neuroprotective metabolites, compromised intestinal barrier integrity, and systemic low-grade inflammation. Through immune, neuroendocrine, and vagal pathways, these peripheral perturbations propagate to the central nervous system, contributing to neuroinflammation, blood–brain barrier dysfunction, and altered neurotransmitter metabolism. We critically evaluate the mechanistic pathways linking microbial changes to neurocognitive outcomes, address methodological limitations and confounding factors in current research, and propose candidate countermeasures and priorities for future evidence based. Understanding these gut–brain interactions are essential for developing interventions to protect astronaut cognitive performance and psychological well-being during long-duration space exploration.
This study presents the engineering design, analysis, and experimental validation of a microgravity drop tower facility capable of providing approximately 3.26 s of microgravity under atmospheric conditions. Unlike conventional vacuum-based systems, the proposed approach employs a free-fall method within a 115 m vertical shaft, combined with a dual-capsule configuration to mitigate aerodynamic drag and enhance microgravity quality. A theoretical model describing the relative motion between the outer and inner capsules was developed based on drag-induced acceleration differences. The analysis demonstrated that minimizing the drag coefficient of the outer capsule is essential for maximizing the effective microgravity duration by delaying contact between the two capsules. Aerodynamic stability was achieved through appropriate center-of-gravity and center-of-pressure design, supported by a streamlined capsule geometry and rear-mounted fins. To enable safe and reusable operation, a multi-layered airbag recovery system was designed and validated through numerical simulations and repeated drop tests. Experimental results demonstrated a total drop time of 4.74 s and a microgravity duration of 3.26 s within the inner capsule. Acceleration measurements showed good agreement with simulation predictions, with peak residual accelerations below 10− 3g and an effective microgravity quality approaching the 10− 4g level during the stable free-fall period. High-speed video analysis was further performed to quantify the relative motion between the inner and outer capsules, revealing that the inner capsule advanced relative to the outer capsule more rapidly than predicted by the simplified analytical model. This observation provides a plausible explanation for the reduction in the experimentally obtained microgravity duration and identifies the release mechanism and capsule interaction as important subjects for future investigation. Additional qualitative experiments, including flame shape observation and object behavior tests, further confirmed the successful realization of microgravity conditions. The proposed facility provides a practical, low-cost platform capable of performing approximately six to seven experiments per working day, offering a viable alternative to conventional vacuum-based drop towers for repeated microgravity research.
The extravehicular material exposure experiments on the China Space Station (CSS) face a significant challenge: a mismatch between limited experimental resources and extensive scientific demands. This paper addresses the task planning problem of extravehicular material exposure experiments by constructing a unified modeling framework that covers experimental period, layout of exposure slots, and experiment scheduling. On this basis, a Mixed-Integer Linear Programming (MILP) model is formulated with the objective of maximizing the number of experimental projects, subject to primary constraints such as experimental period and the facility capacity. Furthermore, focusing on different engineering scheduling strategies, we develop and comparatively analyze two scheduling models: a fixed-frequency model and an optimizable-node model for payload installation and retrieval (PIR), examining their impact on experimental tasks. Numerical experiments demonstrate that the fixed-frequency PIR model, based on the current engineering practice, can provide decision-makers with selection recommendations among various options according to different planning priorities; however, it still faces a trade-off between the quantity and quality of experiments. Because fixed PIR nodes force actual exposure durations to passively adapt to engineering windows rather than strictly match scientifically required exposure periods, they may lead to an engineering-to-science constraint inversion in scheduling. To address this issue, the optimizable PIR-node model incorporates exposure-duration requirements as key constraints and optimizes PIR nodes while also considering the objective of maximizing the number of completed experiments. As a result, it can improve exposure-duration satisfaction while maintaining high experimental efficiency, thereby providing an effective decision-support method for task planning of extravehicular material exposure experiments on the space station.
In this study, a two-dimensional axisymmetric model is established using OpenFOAM. The bubble interface is captured using the volume of fluid method, where the liquid’s viscoelastic behavior is characterized by the Giesekus model. The authors systematically investigated bubble formation in viscoelastic liquids, focusing on the effects of rheological parameters (such as zero-shear viscosity, relaxation time, and mobility factor) and operating conditions (gas injection velocity and orifice radius) on bubble evolution (such as aspect ratio, contact line diameter, and duration of each stage) and departure behaviors (including departure time and volume). By analyzing the evolution of bubble contact line diameter and bubble height, bubble formation process can be divided into five stages: (1) nucleation, (2) expansion, (3) elongation, (4) retraction, and (5) departure. The present results indicate that mobility factor exerts a comparatively minor impact on bubble evolution and departure. In contrast, with increasing relaxation time, the apparent viscosity and the elastic stress of liquid decrease significantly, leading to a marked change in bubble morphology. Specifically, the bubble tail gradually changes from a pointed shape to a concave shape, and the gas line also presents at the bubble tail. Both the contact line diameter and the maximum pre-departure bubble height decrease with increasing relaxation time, but they increase with increasing zero-shear viscosity, injection velocity, and orifice radius. Departure time increases only with zero-shear viscosity and decreases with all other parameters studied. Bubble departure volume decreases solely with increasing relaxation time but it grows with an increase in the other parameters.
Understanding the effects of gravity on hydrogen-air flame propagation dynamics in straight and curved tubes is crucial for mitigating the fire and explosion risks associated with hydrogen. The impact of gravity on flame propagation is expected to depend on the angle between the gravity and flame-propagation directions, but this relationship remains poorly understood. In this study, we numerically investigated the propagation characteristics of hydrogen/air flames in straight and curved open-ended tubes, focusing on how tube placement angle affects flame propagation. The results reveal that variations in tube placement angle, together with the effects of gravity and curvature on pressure gradients, can significantly influence unburned-gas flow and thereby alter flame propagation, particularly in curved tubes. Flame propagation could be substantially influenced by gravity. The evolution of the flame front is analyzed using the vorticity equation and baroclinic torque, showing that in straight tubes, the flame tilt direction changes with tube placement angle. In contrast, the bend of curved tubes consistently directs the flame front toward the inner wall. Gravity induces two distinct flame structures in the transitional regime of curved tubes, thereby affecting flame-front evolution and propagation through the stretch rate and displacement velocity. These findings highlight the crucial role of tube orientation in flame propagation, underscoring the potential fire hazards associated with hydrogen utilization.
The ignition of solid materials is strongly influenced by environmental conditions, including flow velocity, oxygen concentration, and ambient pressure. Understanding ignition behaviors at low flow velocities is of paramount importance for spacecraft fire safety. In this study, the ignition process of thermally thick solid fuels is systematically investigated using a narrow-channel apparatus, with a specific focus on low-flow-velocity regimes. Within the tested parametric range, transient flame flashes are observed, the duration of which depends on the flow velocity, indicating a complex transport phenomenon driven by limited oxygen supply. Both the ignition delay time and the corresponding critical mass flux exhibit a non-monotonic dependence on flow velocity; that is, they decrease, reach minimum values at approximately 5 cm/s, and subsequently increase until ignition fails. This paper proposes an improved model for predicting the critical mass flux for ignition by explicitly accounting for the unique mass transport characteristics in low-flow-velocity environments. The model predictions demonstrate good agreement with both the present experimental results and existing literature data. Furthermore, by integrating this critical mass flux criterion with a comprehensive solid-phase degradation model, a one-dimensional transient ignition model is developed. By incorporating the additional heat feedback from the transient flashing flame at low velocities, this comprehensive model successfully reproduces the observed non-monotonic trend of the ignition delay time. The proposed modeling framework can also be extended to predict flame spread rates, thereby establishing a quantitative link between ignition and flame spread process.
Low-level vibration exposures are known to affect multiple tissues in living organisms. This study investigates the effects of vibration exposure on bone tissue in larval zebrafish (Danio rerio). Larvae were exposed to vibrations (44–50 Hz) for 24 h, with or without anaesthesia, and were compared to controls. Ossification levels of the larval skeleton together with the expression of colX (a marker of bone development) in the operculum bone was used to determine the morphological response to vibrations. Gene expression of bone-related (acp5a, bglap, rankl) and apoptosis-related markers (bax) was then evaluated via RT-qPCR after the 24-hour exposure as well as one week after treatment. Results showed no significant differences in overall ossification or colX expression due to vibration. However, gene expression analyses revealed transient changes: all genes were affected immediately after vibration in contrast to one week later, when only acp5a and bglap were affected. These findings suggest that while short-term low-frequency vibration does not visibly affect bone morphology, it can influence bone homeostasis at the molecular level. Our study also underscores the need for appropriate vibration controls in ground-based simulated microgravity studies in order to assess the impact of vibrations from equipment used (e.g. incubators, or other platforms), particularly in gene expression experiments.
We carried out an in situ observation of the feeding process of edible shrimp in a microgravity environment. To realize that condition, we developed a new type of clinostat with high-speed rotation, with a maximum rotation of 130 rpm. Under that rotation speed, aquatic organisms such as fish and crustaceans have a suppressed capability of recovering their proper swimming form, making it possible for them to be exposed to a pseudo-weightless condition. This method allowed us to successfully observe the feeding process of shrimp in that condition. As a supporting experiment, we conducted similar experiments using larval Artemia crustaceans, i.e. brine shrimp, and we confirmed that they, too, can exhibit feeding behavior under a weightless environment, even when exposed for the long period of four days. According to a gene ontology analysis of shrimp exposed to microgravity for 24 h, a significant difference was detected in the exposed shrimp as compared with control shrimp at 1 G. Those results suggested that exposure to weightlessness may have a biological impact on the shrimp, although the details are unknown yet. We believe that the high-speed clinostat will contribute greatly to the future progress of engineering applications in microgravity research.
We numerically investigate the motion of a Fluorinert FC-75 droplet suspended in silicone oil in a cylindrical container under a vertical temperature difference. The top and bottom walls are maintained at 343 and 283 K, the sidewall is adiabatic, and the droplet is initially placed at mid-height. Ansys Fluent with the Volume of Fluid method is used to solve the coupled momentum and energy equations. Container heights from 2.5 to 60 mm are examined, with droplet diameter scaled with container size, under zero gravity, normal gravity, and combined thermocapillary-buoyancy forcing. In zero gravity, the droplet migrates toward the hot upper wall, but its mean migration speed decreases as container height increases because the axial temperature gradient weakens and confinement distorts the isotherms. Under normal gravity, the denser FC-75 droplet settles toward the cold wall. Under combined forcing, containers with heights of 3 mm or less exhibit near cancellation between buoyancy and thermocapillary, whereas for heights of 15 mm or more the motion approaches the gravity dominated limit. At a fixed height of 2.75 mm, varying droplet diameter reverses the net drift. This transition is captured by a Marangoni-buoyancy balance criterion, which predicts a critical diameter of about 0.203 mm for quasi-equilibrium. Three-dimensional simulations show deformation only in the buoyancy dominated regime. These results identify confined configurations that approximate microgravity behavior and provide a practical criterion for droplet positioning and transport.
To examine growth patterns of a leafy vegetable under simulated microgravity, spinach seedlings were cultivated under clinorotation at 2 rpm in darkness for 5 d and compared with control seedlings grown under normal gravity. No significant differences were detected in root length and water content between the two growth conditions. However, interestingly, seedlings under clinorotation showed straight elongation, whereas curvature and waving appeared in control roots. Consequently, roots under clinorotation had significantly higher straightness, calculated as the straight line between the root base and tip divided by the actual root length. As the phytohormone auxin is involved in root bending, spinach seedlings were further cultivated in the presence of auxin inhibitors. Under inhibitor treatment, no significant difference in root straightness was observed between clinorotation and control, and the values were comparable to those of seedlings under clinorotation without an auxin inhibitor. These findings suggest that auxin contributes to the curvature of the control seedlings and clinorotation compensates for this. Therefore, spinach seedlings at the early developmental stage exhibited a unique growth pattern under clinorotation, unlike most other plant species.
While the thermocapillary migration of single and compound droplets has been extensively studied in unbounded media, the synchronized transport and interaction dynamics of multiple compound droplets within confined geometries remain poorly understood. This study addresses this gap by numerically investigating the thermocapillary migration of two axially aligned compound droplets inside a microchannel featuring a localized geometric constriction. Using the front-tracking method, we aim to delineate the transition between interacting and non-interacting regimes, a distinction critical for the precise control of multi-phase flows in microfluidic systems. The coupled thermal and hydrodynamic behaviors are systematically analyzed as a function of the Marangoni number (Ma), the axial positioning of the leading droplet, the inner-to-outer droplet size ratio (Rio), and the relative constriction depth (d/Rc). Our results reveal that inter-droplet interaction is primarily triggered when the trailing droplet catches up to the leading one near the constriction, where local velocity reaches its minimum. We identify Ma as a dominant control parameter; higher Ma values intensify thermal convection, which disrupts the local temperature field and delays the onset of interaction by reducing the trailing droplet’s velocity. Crucially, we find that geometric and internal configurations dictate the interaction mode: shallow constrictions (d/Rc < 0.58) or larger inner cores (Rio ≥ 0.7) promote independent passage by enabling droplet elongation or rapid escape. In contrast, deeper constrictions and smaller inner cores reduce interfacial deformation, thereby facilitating closer proximity and significant multi-body interaction. The study culminates in the establishment of a comprehensive phase diagram based on H0L/Ro, Rio, and Ma. This diagram serves as a predictive tool to define the boundaries between interacting (ID* = 0) and non-interacting (ID* > 0) regimes. By providing a mechanistic understanding of how geometry and thermal forces can be leveraged to synchronize or separate double emulsions, this work offers a framework for the optimized design of microfluidic devices for targeted drug delivery and advanced material synthesis.
Present work involves numerical CFD study of the flame structure of PMMA sphere at different velocities of the incident oxidizer flow under zero-gravity conditions. The performed modeling allowed us to establish the chemical processes to change significantly with a variation of velocity of the oxidizer flow. It was shown that both the thermal and chemical structure of the flame change, as well as the temperature gradient and the mechanism of heat release in the gas phase. It has been established that with an increase in the flow velocity, the combustion completeness decreases that is explained by an increase in the flame strain rate and, consequently, a decrease in the residence time in the chemical reaction zone. Calculation of the heat release rates in individual reactions at various oxidizer flow velocities allowed us to identify four key reactions determining the overall heat release rate. At average flow velocity values, the main contribution to the heat release is made by the HO2 + OH = H2O +O2 step, which is explained by a high concentration of HO2 in the flame. The remaining key steps in decreasing order of their contribution to the heat release are T-C3H5 + O2 = CH3 + CO + CH2O, HCO + O2 = CO + HO2 and CH3 + O = H + H2 + CO.
The ability of plants to adapt and grow in extraterrestrial environments is a crucial prerequisite for human spatial colonization of extraterrestrial environments. Sounding rockets allow biological payloads to experience real microgravity conditions for several minutes before returning to Earth. The present systematic review sought to gain new insights to guide the design of plant experiments for sounding rocket platforms. Most of the plant studies using sounding rocket technology have been conducted by Germany (Ten out of twelve launches). In the launches conducted, the minimum apogee was 86 km, and the maximum was 800 km for the Virgin Galactic Unity 22 and MAXUS, respectively. The microgravity quality was in the range of 10− 4 g to 10− 6 g. The plant payloads selected for the launches ranged from plant cells to young seedlings. Four (4) out of 10 studies deploy hardware capable of automatically fixing the plant material in different gravitational phases. All ten studies included 1 g ground controls in parallel with the flown samples. Two of the ten studies utilized another real microgravity platform in addition to 1 g ground controls, conducted in parallel with the sounding rocket flight. The biological responses to sounding rocket flight have been investigated at cellular, molecular, histological, and physiological levels. Sounding rockets and suborbital spacecraft, even though they have some limitations, can still be good options for plant biology researchers. They offer a way to do repeated observations and experiments in real microgravity conditions.
The success of lunar landing missions critically depends on the faithful reproduction of the target celestial body’s mechanical environment through ground simulation tests, particularly during the critical touchdown phase of the lunar lander (LL). To accurately simulate lunar gravity conditions during the dynamic descent of the LL on Earth, this paper proposes an adjustable-lever-arm constant spring support (ALA-CSS). This mechanism achieves dynamic compensation for potential energy changes. A dynamic model of the ALA-CSS is established, and its nonlinear dynamic characteristics are analyzed in detail. An inverse-based feedforward-feedback (IBFF) control strategy, integrated with a force feedback controller, is developed to simulate the entire LL touchdown process. This approach thereby ensures that an 83.3
