Earth Gravity Research Articles

Runoff concentration decline for Tarim river due to a dramatic increasing of runoff in cold season and hydro-junction regulation: Past and future

Study regionThe three headwaters of Tarim river, south of the Xinjiang Province, China. Study focusTemporal runoff concentration variations are crucial for water resources utilization and management, particularly in arid regions. In this study, the runoff concentration in the Tarim River Basin is studied based on analysis of the trend, abrupt changes, and periodicity, identifying key extreme climate indices influencing the Gini-runoff coefficient and predicting its future changes under three Shared Socioeconomic Pathways (SSPs). New hydrological insights for the regionFrom 1962–2020, the Gini-runoff coefficient decreased by 8.3 %, while annual runoff increased by 14.2 %. This is attributed to temperature changes, escalating hydrological risk uncertainty. The anomaly of low geopotential high fields and Central Asian anticyclones have caused an increase in total cloud cover (TCC) and precipitation, and a decrease in the Daily Temperature Range (DTR). Increasing of Warm nights (TN90), Maximum Tmax (TXx), and Hot nights (TR) have enhanced the warm-season runoff. However, the climatic impact on warm-season runoff is less pronounced than that in the cold season, which led to the runoff concentration decline. Changes in hydro-junction regulation led to decrease of Gini-runoff coefficient by 12.11 % from 2001 to 2020 along the Hotan river. Under the three SSP scenarios, the average Gini-runoff coefficient for the Aksu and Hotan rivers are expected to decrease by 3.3 % and 2.6 %, respectively.

A Supposed Direction of Gravity

A Supposed Direction of Gravity

The Benefits of Future Quantum Accelerometers for Satellite Gravimetry

AbstractWe investigate the benefits of future quantum accelerometers based on cold atom interferometry (CAI) on current and upcoming satellite gravity mission concepts. These mission concepts include satellite‐to‐satellite tracking (SST) in a single‐pair (GRACE‐like) and double‐pair constellation as well as satellite gravity gradiometry (SGG, single satellite, GOCE‐like). Regarding instruments, four scenarios are considered: current‐generation electrostatic (GRACE‐, GOCE‐like), next‐generation electrostatic, conservative hybrid/CAI and optimistic hybrid/CAI. For SST, it is shown that temporal aliasing poses currently the dominating error source in simulated global gravity field solutions independent of the investigated instrument and constellation. To still quantify the advantages of CAI instruments on the gravity functional itself, additional simulations are performed where the impact of temporal aliasing is synthetically reduced. When neglecting temporal aliasing, future accelerometers in conjunction with future ranging instruments can substantially improve the retrieval performance of the Earth's gravity field (depending on instrument and constellation). These simulation results are further investigated regarding possible benefit for hydrological use cases where these improvements can also be observed (when omitting temporal aliasing). For SGG, it is demonstrated that, with realistic instrument assumptions, one is still mostly insensitive to time‐variable gravity and not competitive with the SST principle. However, due to the improved instrument sensitivity of quantum gradiometers compared to the GOCE mission, static gravity field solutions can be improved significantly.

Computational account for the naturalness perception of others' jumping motion based on a vertical projectile motion model.

The visual naturalness of a rendered character's motion is an important factor in computer graphics work, and the rendering of jumping motions is no exception to this. However, the computational mechanism that underlies the observer's judgement of the naturalness of a jumping motion has not yet been fully elucidated. We hypothesized that observers would perceive a jumping motion as more natural when the jump trajectory was consistent with the trajectory of a vertical projectile motion based on Earth's gravity. We asked human participants to evaluate the naturalness of point-light jumping motions whose height and duration were modulated. The results showed that the observers' naturalness rating varied with the modulation ratios of the jump height and duration. Interestingly, the ratings were high even when the height and duration differed from the actual jump. To explain this tendency, we constructed computational models that predicted the theoretical trajectory of a jump based on the projectile motion formula and calculated the errors between the theoretical and observed trajectories. The pattern of the errors correlated closely with the participants' ratings. Our results suggest that observers judge the naturalness of observed jumping motion based on the error between observed and predicted jump trajectories.

Outer spiral drill rod lunar soil sampling resistance moment simulation study

Abstract The task of unmanned automated sampling of China’s lunar exploration is to bring back lunar soil samples with a certain depth, and sampling of external auger drill pipe is the best way. The power and moment size of the lunar surface is limited, so a special design is required. In this paper, a unique structural model of a double-auger drilling tool is designed. It is a movement analysis of lunar soil and drill pipe. We use simulated lunar soil simulations and experiments. The Hertz-Mindlin slip-free model in the discrete element software EDEM was used for numerical simulation. Given the disadvantage of the large amount of computation of discrete elements, the “embedding simulation method” proposed in this paper reduces the amount of computation. When drilling 0.4m and 1m under lunar and earth gravity, the magnitude and influence of drill pipe rotation and vertical velocity on the drag moment were analyzed. We provide the relevant technical parameters of the real unmanned drilling activities on the lunar surface. Through simulation, the rationality parameters of several groups of drill pipe drilling are obtained to ensure the smooth progress of drilling.

Computational fluid dynamics simulation of cryogenic vertical upflow boiling under Earth gravity

This study presents numerical simulations and their validation for flow boiling of liquid nitrogen (LN2) in a vertical upflow orientation, with a primary aim to understand the complex two-phase flow and heat transfer phenomena important to space applications. The computational fluid dynamics (CFD) model utilized the coupled level set volume of fluid (CLSVOF) method, incorporating additional source terms for bubble collision dispersion force and shear lift force in the momentum conservation equation to enhance simulation accuracy. The simulations were conducted for two mass velocities (G = 526 and 804 kg/m2s) and three different heat flux levels (approximately 10%, 30%, and 70% of critical heat flux (CHF) under Earth gravity. The model was validated against measured wall temperature data acquired from the authors’ previous experimental studies, demonstrating average deviations of less than 2.8 K across all operating conditions. The simulated two-phase flow contours illustrated various flow patterns, including bubbly, slug, churn, and annular. Both mass velocity and heat flux were observed to impact the onset of nucleate boiling (ONB), bubble nucleation, growth, and coalescence, and overall vapor structure. The simulations also offered insight into axial and radial void fraction and velocity profiles, revealing local flow acceleration trends synchronized with void fraction development. A comparison between predicted and measured bulk fluid temperature profiles showed excellent agreement, further validating the CFD model’s accuracy and practical usefulness for two-phase cryogenic flow boiling simulations in space applications.

Digital bead modeling for wire-arc directed energy deposition

Prediction of 2D cross-section and full 3D geometry for stacked weld beads is critical for the outcome of wire-arc directed energy deposition (DED) parts; however, most additive path planning software packages model beads as extrusions of a rectangle. Weld beads are not rectangular, and the resulting shape is dependent upon physics effects at the moment of deposition. Physics phenomena such as the geometry of the underlying surface, the heat input of the welding mode, and the direction of gravity contribute to bead shape. This paper presents a novel implicit modeling method that discretizes a 2D area or 3D volume of space into pixels or voxels and constructs fields based on these physics phenomena. The fields are combined using a weighting scheme trained on 3D scan measurements of welds and wire-arc DED prints. Pixels or voxels are added until the known amount of deposited volume has been achieved. Thereby, a strong conservation of mass principle is applied to the process. Utilizing machine learning techniques, the present model can be trained on a database of scans allowing for the representation of a wide variety of prints. Results show that this method can produce predictions with realistic bead morphology and sub-millimeter form error.

An Improved Average Acceleration Approach of Modelling Earth Gravity Field Based on K-Band Range-Rate Observations

The conventional average acceleration approach relies on K-band range observation, containing an unknown bias, which leads to possible degradation of the precision of Earth’s gravity field modelling. It also suffers from correlated errors caused by three-point numerical differentiation. In this study, an improved approach is proposed that makes use of K-band range-rate observations instead and overcoming the influence of correlated errors by introducing a whitening filter. GRACE-Follow On data spanning the period from January 2019 to December 2022 were processed by the proposed approach and a series of time-varying gravity field models was derived, referred to as SSM-AAA-GFO in this paper. This model series is compared comprehensively with three official model series. Results demonstrate that all model series are highly coincident below degree 30 and reflect similar time-varying gravity field signals in both large and small basins. After filtering, SSM-AAA-GFO shows uncertainty, in the form of equivalent water height below 2.5 cm, which is comparable with three official model series. The comparison results confirm the effectiveness of the proposed approach for precisely modelling a time-varying gravity field based on K-band range-rate observations.

Two-phase flow instabilities during microgravity flow boiling onboard the International Space Station

This study is an elaboration on flow instabilities observed during flow boiling experiments conducted onboard the International Space Station (ISS) as part of the Flow Boiling and Condensation Experiment (FBCE). During highly subcooled flow boiling, liquid backflow into the channel that rapidly condenses vapor within the channel was observed. Experiments were conducted with an enhanced sampling frequency of 30 Hz and an extended image sequence recording duration of at least 4 seconds at 500 frames per second to further investigate the instability. Identical experiments were performed both in microgravity onboard the International Space Station (ISS) and in Earth gravity during vertical upflow. Instabilities in microgravity are more severe than those in Earth gravity and, at their most severe, propagate to the channel's upstream region. Instabilities are observed within the channel when the intensity, I, which is dependent on inlet pressure fluctuations and mass velocity, exceeds 1.8 × 105 W/m2. Parametric trends of the frequency and amplitude of inlet pressure fluctuations during instability are examined, which reveal instabilities are most severe at low flow rates, high inlet subcoolings, and high heat fluxes. Various stability maps proposed in the literature are evaluated against the present database, and instabilities only manifest for subcooling numbers greater than 14. A subset of the database containing the Onset of Flow Instability (OFI) point is extracted to first evaluate correlations available in the literature and then develop a new correlation. The new correlation is applicable in both microgravity and Earth gravity conditions and predicts the database with a Mean Absolute Error (MAE) of 1.3%.

Localization and circumnavigation of an unknown target using distance measurements in 3D space

This paper investigates the problem of localization and circumnavigation of an unknown stationary target in 3D space using distance measurements. An estimator is first designed in 2D space to locate the target, with which a control protocol is proposed to force the agent to travel along a circular trajectory around the target. Then the algorithm is extended to 3D space to circumnavigate the target under the condition of obtaining gravity direction additionally. In particular, the agent moves alternately in the X-Y horizontal planes and the X-Z vertical planes in different patterns under different switching conditions, which is beneficial to monitor the target from a broader perspective. Meanwhile, the rigorous stability analyses of two cases are given in terms of the Lyapunov method. Finally, the effectiveness of the proposed algorithms is verified through numerical simulations and a real experiment.

Ground-based simulation of laser link acquisition for inter-satellite laser interferometry.

Laser link acquisition and pointing technique is one of the essential techniques for the inter-satellite laser interferometry for space-based gravitational waves detection and next-generation Earth gravity measurement missions. The first step of building up inter-satellite laser link is using an acquisition camera to capture the inter-satellite laser beam signals within a pre-scanning uncertain cone. Subsequently, high-precision angle measurement technology, namely differential wavefront sensing, is used to achieve a high pointing precision required. Due to the distance constraint of a ground-based simulation experiment, it is difficult to verify directly the feasibility of an inter-satellite laser link acquisition and pointing control scheme. By means of controlling the optical properties of the received laser beam, the long-distance beam propagation is simulated with two optical benches of an inter-satellite interferometer, and the process of a laser link acquisition experiment has been demonstrated. The experimental results show that the inter-satellite laser beams could establish the dual-way locking successfully. The fluctuation of the laser beam pointing direction (in-loop) can be suppressed to about 5 µrad in atmospheric environment. The results verify the feasibility of the laser link acquisition scheme. The experimental setup can be extended to conduct experiments with various parameters, providing technical support for further testing of the inter-satellite laser link acquisition and pointing control methods.

Vibration-induced wall–bubble interactions under zero-gravity conditions

This work is devoted to a theoretical and numerical study of the dynamics of a two-phase system vapour bubble in equilibrium with its liquid phase under translational vibrations in the absence of gravity. The bubble is initially located in the container centre. The liquid and vapour phases are considered as viscous and incompressible. Analysis focuses on the vibrational conditions used in experiments with the two-phase system SF $_6$ in the MIR space station and with the two-phase system para-Hydrogen (p-H $_2$ ) under magnetic compensation of Earth's gravity. These conditions correspond to small-amplitude high-frequency vibrations. Under vibrations, additionally to the forced oscillations, an average displacement of the bubble to the wall is observed due to an average vibrational attraction force related to the Bernoulli effect. Vibrational conditions for SF $_6$ correspond to much smaller average vibrational force (weak vibrations) than for p-H $_2$ (strong vibrations). For weak vibrations, the role of the initial vibration phase is crucial. The difference in the behaviour at different initial phases is explained using a simple mechanical model. For strong vibrations, the average displacement to the wall stops when the bubble reaches a quasi-equilibrium position where the resulting average force is zero. At large vibration velocity amplitudes this position is near the wall where the bubble performs only forced oscillations. At moderate vibration velocity amplitudes the bubble average displacement stops at a finite distance from the wall, then large-scale damped oscillations around this position accompanied by forced oscillations are observed. Bubble shape oscillations and the parametric resonance of forced oscillations are also studied.

Decreased human sperm motility and vitality after fast gravity load changes in a parabolic flight

Little is known about the effects of low gravity on human gametes. The aim of this study was to analyze if fresh human sperm samples after fast gravity load changes suffered any detrimental effect in comparison to the splits maintained in Earth's gravity. Fifteen fresh samples from normozoospermic donors were analyzed. Statistically significant differences in vitality (69.7 ± 9.9 % vs 72.4 ± 9.7 %, [95 % CI: 0.002,0.07]); motile sperm concentration (23.7 ± 15.3 M/ml vs 31.5 ± 25.1 M/ml, [95 % CI: 1.03,14.65]); grade “a” sperm concentration (8.7 ± 6.5 M/ml vs 11.7 ± 9.9 M/ml, [95 % CI: 0.71,5.28]); percentage of progressive motility sperm (30 ± 12.9 % vs 36 ± 14.3 %, [95 % CI: 0.10,0.37]) and curvilinear velocity VCL: 45.7 ± 12.8 μm/s vs 47.7 ± 13.3 μm/s, [95 % CI: 0.79,3.22]) were observed. No statistical differences were observed in other sperm kinematic parameters, morphology, DNA fragmentation, apoptosis, and oxidative stress. In conclusion, even though it did not result in a total loss, heavy gravity load changes including microgravity causes a significant decrease in sperm vitality and motility suggesting that negative consequences would be even higher if the exposure were longer. The results obtained indicate that further research is really needed before Assisted Reproduction will be considered for the future human reproduction outside the Earth.

Development of gravitationally aligned pendant IMU frailty identifier

This study focused on the practical development of a device for the early detection of frailty by leveraging the advanced Inertial Measurement Unit (IMU) technique. A five-time sit-to-stand test (FTSST) was conducted with people simply wearing a devised IMU pendant to evaluate the degree of frailty. As anticipated, the pendant sensor was recommended to be loosely strapped on older people's bodies for a comfortable outfit. Hence, the measurements from the IMU device would vary with the strap angle of the pedant. A calibration of gravitational alignment, which aligns the measurements to the gravity direction exactly with the FTSST motion signals consistently with an identical principal axis, would benefit the straightforward successive signal analysis and assessment. A prototype was thus developed with the designated calibration by using an open and systematic principle. This study analyzed the signals of accelerometer and gyroscope from IMU, and the subject's demographic profiles comprised a spectrum of features for identification analysis. Pearson correlation and principal component analysis for relevance ranking were used to reduce the feature dimensionality. Classifiers, including Support Vector Machine (SVM), Random Forest (RF), and Artificial Neural Network (ANN), were introduced to infer the frailty level of the subject. An innovative signal calibration and alignment process with solid mathematical support was imposed in the scheme. As deployed, the designated frailty identifier arrived at its generalized accuracy of 95.45 % with real clinic subjects in prediction. The alignment calibration process is the key for not only resolving the loose-attachment problem tolerantly but also enhancing the discriminability of the features. The prototype revealed the better integrity in developing a cheap, tiny, wearable device to identify the frailty.

Resolution evaluation of the satellite altimetric gravity anomaly models with shipborne gravity data over the Cosmonaut Sea, East Antarctica (44–52° E)

AbstractSeveral satellite gravity anomaly models are freely available to calculate the free‐air gravity anomaly in areas where shipborne gravity measurements are scarce. Two models produced by the Technical University of Denmark (DTU17) and the Scripps Institution of Oceanography (SIOv32.1), respectively, were selected to compute the free‐air anomalies over the Cosmonaut Sea, East Antarctica. A statistical comparison analysis was performed to evaluate the resolution of satellite gravity anomaly models by comparing them with the shipborne surveying date. The radially averaged energy spectra of free‐air anomaly from different sources were calculated and compared over two selected regions to further evaluate the reliability of the data derived from satellite gravity anomaly models. The satellite gravity anomaly models have a better resolution in the ocean basin than in the area near the continental shelf. The comparison analysis revealed that the precision of both DTU17 and SIOv32.1 is close to the shipborne gravity data, but on average, SIOv32.1 is a little bit better than DTU17. The spectral analysis showed that the shipborne measurements may provide higher resolution than the satellite gravity anomaly model at wavelengths shorter than 20 km, and the free‐air data derived from SIOv32.1 have better resolution than the one from DTU17. These shipborne datasets will provide contributions for the updates of the Antarctic gravity anomaly and enable new high‐resolution combined Earth gravity models to be derived in Antarctica.

Numerical analysis of blood flow in the abdominal aorta under simulated weightlessness and earth conditions

Blood flow through the abdominal aorta and iliac arteries is a crucial area of research in hemodynamics and cardiovascular diseases. To get in to the problem, this study presents detailed analyses of blood flow through the abdominal aorta, together with left and right iliac arteries, under Earth gravity and weightless conditions, both at the rest stage, and during physical activity. The analysis were conducted using ANSYS Fluent software. The results indicate, that there is significantly less variation in blood flow velocity under weightless conditions, compared to measurement taken under Earth Gravity conditions. Study presents, that the maximum and minimum blood flow velocities decrease and increase, respectively, under weightless conditions. Our model for the left iliac artery revealed higher blood flow velocities during the peak of the systolic phase (systole) and lower velocities during the early diastolic phase (diastole). Furthermore, we analyzed the shear stress of the vessel wall and the mean shear stress over time. Additionally, the distribution of oscillatory shear rate, commonly used in hemodynamic analyses, was examined to assess the effects of blood flow on the blood vessels. Countermeasures to mitigate the negative effects of weightlessness on astronauts health are discussed, including exercises performed on the equipment aboard the space station. These exercises aim to maintain optimal blood flow, prevent the formation of atherosclerotic plaques, and reduce the risk of cardiovascular complications.

Motion extrapolation in sport expertise: Representational momentum and representational gravity in volleyball athletes

When people indicate the vanishing location of a moving target that suddenly disappears, systematic errors forward (in the direction of motion) and downward (in the direction of gravity) emerge. These spatial displacements were coined, respectively, Representational Momentum and Representational Gravity, and are believed to reflect internalized ecologically relevant physical invariants useful for the anticipation of future states of an event. Previous research has shown that sports athletes exhibit increased Representational Momentum, indicating enhanced motion extrapolation and anticipation, albeit it is still not clear up to what degree this effect is specific for the expertise context or if it generalizes to other dynamic events. Furthermore, the influence of expertise on Representational Gravity, particularly in contexts where anticipation of vertically moving objects is crucial, remains understudied. This study aimed to address these gaps by focusing on Volleyball as a context of expertise due to the prevalence of fast vertically moving balls. Volleyball athletes and non-athletes indicated the perceived offset location of a smoothly moving target, which moved at a constant speed or was subjected to acceleration/deceleration, embedded either in a Volleyball or neutral context. Outcomes revealed that for the Volleyball context, athletes, but not non-athletes, revealed a significant trend to misperceive targets moving along the left diagonal to be further displaced forward beyond what would be expected due to Representational Momentum alone. This finding is discussed in relation to the natural statistics of Volleyball games, where crossed ball trajectories, particularly by the outside hitter, are more prevalent, fast, and offensive, requiring better anticipation to be efficiently dealt with.

Improved gravity field estimation by incorporating the Tiangong Space Station and next-generation CAI satellite gravity mission

The next-generation gravity satellite mission equipped with the Cold Atom Interferometry (CAI) gradiometer has great potential for the Earth's gravity field estimation. Deploying a CAI gradiometer on the Chinese Tiangong Space Station launched for long-term Earth science research not only reduces the cost compared to a dual-satellite constellation but also enhances interdisciplinary collaboration in the Earth's gravity field detection. In this study, we conducted gravity gradient-based simulations to assess the contribution of deploying a CAI gradiometer on the Tiangong Space Station to collaboratively observe the Earth's gravity field with a polar-orbit gravity satellite. The simulation results demonstrate that whether utilizing Vyy component, three diagonal components or full components, the derived gravity field models show significant improvements within 100 degree and above 200 degree after incorporating Tiangong Space Station. In particular, the gravity field solution recovered from three diagonal components achieves the best accuracy. In the case of using diagonal components, the collaboration observation scheme effectively reduced the cumulative geoid height error by approximately 5.3cm (300 d/o). In the spatial domain, the incorporation of the Tiangong Space Station primarily impacts the estimated gravity field within the orbital coverage area of the space station, and this effect is particularly pronounced when just employing Vyy component. However, due to the limitation of angular velocity observation inaccuracy associated with the CAI gradiometer in nadir mode, there is no substantial accuracy improvement observed above 200 degree when adding gradient components.

Simulation on solidification process of molten salt-based phase change material as thermal energy storage medium for application in Stirling engine

Abstract Thermal energy storage technologies have been widely used to mitigate intermittency from renewable energy such as solar energy. Phase change material (PCM) is a certain material that can be used as a heat storage medium and is available in a wide range of operating temperatures. Molten salt is one of the PCMs that has the advantage of a very high operating temperature. The PCM solidification simulation based on HitecXL molten salt using COMSOL Multiphysics software will be carried out with variations in heat absorption of 1 - 5 kW/m2, assuming constant heat absorption. The results show that the PCM solidification process starts from the surface of the Stirling engine heat exchanger pipe. The part of the PCM that has been solidified will fall following the direction of gravity and cause a phenomenon such as a droplet. The flow that occurs is a natural flow caused by the buoyancy force due to changes in density due to temperature gradients in the solidification process. The time required for the PCM to completely solidify is closely related to the amount of heat absorption. The greater the heat absorption from the pipe, the faster the PCM to fully solidified.

Absolute Pose Estimation With a Known Direction by Motion Decoupling

This paper develops an extremely robust solution for absolute pose estimation with known prior gravity direction by motion decoupling. Absolute pose estimation is a fundamental problem in computer vision, and recently the prior known vertical direction is commonly applied to help solve the pose estimation problem. In this paper, we explore the geometrical constraints of the absolute pose estimation with a known direction. We find that the rigid pose can be decoupled with the help of the known direction. Thereby, absolute pose estimation algorithms, which decouple rigid motion, are proposed. Notably, in real applications, there may be imperfect inputs, i.e., outliers, due to incorrect 2D-3D matches. Unfortunately, these outliers may lead to unacceptable results. To suppress the outliers, the decoupled absolute pose estimation problem is solved by branch-and-bound algorithm and globally voting, which can provide the optimal solution with provable guarantees. Moreover, in extreme case, the proposed method can solve absolute pose estimation problem without knowing the 2D-3D correspondences, which is also known as simultaneous camera pose correspondence estimation. To demonstrate the feasibility and the superiority of the proposed methods, comprehensive comparison experiment are conduced. The source code is available at https://github.com/Liu-Yinlong/algorithm-for-PnP-with-known-vertical-direction.