Gravity Variations Research Articles

Stability analysis of nonlinear synchronous vibration in an inclined rotor system supported by journal bearing with variational gravity

In vertical rotating shaft-bearing systems, synchronous vibration undergoes destabilization and stabilization owing to the self-excited vibration. Recently, these phenomena have been clarified numerically, experimentally, and analytically based on a newly proposed analytical method. In this method, a two-step linearization of the nonlinear journal bearing (JB) force and generalized eigenvalue analysis are proposed to directly determine the stability of the synchronous orbit. However, this method cannot be used directly for inclined-rotor systems with a large gravitational effect. Moreover, stability changes caused by gravity variations in inclined-rotor systems have not been fully investigated. This study extended the analytical method to one with weighted average dynamic coefficients to efficiently predict the stability changes of synchronous whirling vibrations in inclined rotor systems. The extended analytical method clearly clarifies gravity-induced stability changes in comparison with the numerical (shooting) method. Our analytical method enhances the ability of rotor dynamics software to calculate the stability thresholds of inclined rotor systems with gravity variations.

Supplementation of Chlorella vulgaris Extracts During Brewing: The Effects on Fermentation Properties, Phytochemical Activity and the Abundance of Volatile Organic Compounds

Chlorella vulgaris, a microalga rich in secondary metabolites and nutrients, offers a promising alternative for promoting microbial growth in food fermentation processes. This study aimed to evaluate the effects of C. vulgaris extracts on fermentation kinetics, sensory characteristics, phytochemical composition, antioxidant activity, and the abundance of volatile organic compounds (VOCs) in treated versus control beers. The bioactive compounds from C. vulgaris were extracted using an ultrasound-assisted method with water as the solvent. A German Pilsner-style lager beer (GPB) was brewed and supplemented with 0.5, 1, and 5 g/L of C. vulgaris extracts prior to primary fermentation. Yeast viability, °Brix, and pH levels were monitored to assess fermentation progress. Phytochemical composition was analyzed by quantifying total polyphenols and flavonoids. The antioxidant activity of the beer was evaluated using both the 2,2-diphenyl-1-picrylhydrazyl (DPPH●) and hydrogen peroxide assays. The addition of C. vulgaris extracts resulted in increased yeast viability and slight variations in gravity during the 7-day fermentation period. Moreover, the beers supplemented with C. vulgaris extracts demonstrated higher levels of total polyphenols, flavonoids, and antioxidant activity compared to the GPB. Specific volatile organic compounds, including 2-methyl-1-propanol, 1-hexanol, isopentyl hexanoate, 2-methylpropyl octanoate, β-myrcene, and geranyl acetate, were significantly more abundant (p < 0.05) in the treated beers than in the control. Sensory evaluations revealed a favorable impact of the treatment on aroma scores compared to the GPB. Overall, the findings indicate that C. vulgaris extracts could be a valuable ingredient for developing functional beers with enhanced health benefits, particularly regarding antioxidant activity. Additionally, the results underscore the importance of exploring innovative approaches that utilize natural sources like Chlorella to enrich the nutritional profile and sensory qualities of fermented products.

Variation in specific gravity and shrinkage of tapped rubberwood

Hevea brasiliensis (rubber tree), a major source of natural rubber, could also be an important source of lumber as senescence occurs. However, latex collection is known to affect Hevea brasiliensis (rubber tree) wood formation and consequently, wood properties. The impact tapping (cutting made in the bark of the tree for latex harvest) has on the tree and the way the tree responds after tapping is often overlooked. Knowledge on wood properties of tapped rubber trees in Nigeria would enhance its sustainable utilization which is especially important in developing countries where lumber is limited. Variation in specific gravity and shrinkage of rubberwood wood due to tapping duration was examined. Tapping duration had significant effect in specific gravity and longitudinal shrinkage of rubberwood but had no effect on tangential and radial shrinkage. The specific gravity (SG) of rubberwood ranged from 0,55 - 0,59. Longitudinal shrinkage (average 1,42 %) was higher than typically observed for mature wood. Average tangential shrinkage for rubberwood of all the ages was 5,37 % while radial shrinkage ranged from 2,87 % to 3,84 %. decrease in SG observed in trees tapped for 20 years may indicate the initiation of senescence. Tapped rubberwood could be used in areas not exposed to high moisture as well as in other wood composites.

Seafloor Topography Recovery Using the Observation Data of Tiangong-2 Interferometric Imaging Radar Altimeter

Tens of years of altimetry sea surface height (SSH) data has continuously contributed to the development of global seafloor topography models because it can be used to precisely retrieve the gravity variation caused by seafloor topography. The development and application of altimeter has come to a new era from conventional nadir-looking altimeters to interferometric wide-swath altimeters as the successful launch of the Interferometric Imaging Radar Altimeter on board Chinese Tiangong 2 space laboratory (TG2 InIRA). Compared with conventional altimeters, the observation efficiency of TG2 InIRA has been greatly improved. This paper presents for the first time the results of seafloor topography derived from TG2 InIRA data in west Pacific Ocean and South China Sea regions. The root mean squared errors (RMSE) of the recovered topographies by gravity-geologic method (GGM) referring to the shipborne measurements in these two regions are 69.882 and 50.110 m, respectively. The recovered topographies are also evaluated by comparing with five well-known topography models. Furthermore, two subareas in west Pacific Ocean region are selected to conduct a supplemental experiment showing that more revisits can help achieve better recovery. The RMSE of the more revisited subarea is 66.808 m, while the RMSE of the less revisited subarea is 79.273 m.

Data-Driven Fault Detection and Isolation for Multirotor System Using Koopman Operator

This paper presents a data-driven fault detection and isolation (FDI) for a multirotor system using Koopman operator and Luenberger observer. Koopman operator is an infinite-dimensional linear operator that can transform nonlinear dynamical systems into linear ones. Using this transformation, our aim is to apply the linear fault detection method to the nonlinear system. Initially, a Koopman operator-based linear model is derived to represent the multirotor system, considering factors like non-diagonal inertial tensor, center of gravity variations, aerodynamic effects, and actuator dynamics. Various candidate lifting functions are evaluated for prediction performance and compared using the root mean square error to identify the most suitable one. Subsequently, a Koopman operator-based Luenberger observer is proposed using the lifted linear model to generate residuals for identifying faulty actuators. Simulation and experimental results demonstrate the effectiveness of the proposed observer in detecting actuator faults such as bias and loss of effectiveness, without the need for an explicitly defined fault dataset.

Gravity Investigations Applied to the Geological Framework Study of the Mambasa Territory in Democratic Republic of Congo

This study concerns the gravity survey carried out in the territory of Mambasa in the province of Ituri in DR Congo, with the aim of studying its subsurface framework. We applied Free-Air and Bouguer corrections to the gravity measurements in order to eliminate gravity variations of non-geological origin. As for the graphical representation of the results, the spatial interpolation tools were useful to us in the elaboration of Bouguer anomaly maps. Then, we classified the study zone into Bouguer anomaly zones based on intensity. Gravity highs could indicate basement uplifts due to compressional movements. These uplifts would have led to the formation of granite domes which could be associated with mineralization. The enormous gravity depression would, for its part, be synonymous with a ditch or the presence of very low-density geological formations. The gravity profiles drawn support the analyzes of the maps: the anomaly curves are very fluctuating and uneven, highlighting several gravity highs and depressions. These fluctuations are probably induced by the presence of faults which separate the different anomaly zones.

Неотектоника зоны перехода океан-континент в районе Кот д’Ивуара (Западная Африка)

Structural and geophysical indicators of neotectonics in the area of the continental margin of Côte d'Ivoire are represented in three interacting zones: oceanic, continental and transitional. Together with the time-varying gravitational field, they form a system in which neotectonic processes are enhanced due to their equatorial position. Seismicity is represented by dense clusters of events in the areas of junction of the Strakhov, Sao Paulo and Romansh oceanic transform faults with the continent and their overland continuation. Seismicity and gravity variations show the relationship of deep density changes to stress discharge in the area and the effect of volumetric forces on lithospheric blocks. Free air and isostasy anomalies along the shelf are segmented by the effects of vertical motions. Pull-apart depressions near the passive parts of transform faults indicate basement deformations in the interfault blocks and activation of shear displacements of dextral kinematics near the bend of the passive parts of the faults to the northeast from main oceanic azimuth. Segments with isostasy maxima on the shelf correspond to areas with coarser grain size granulometric composition, indicating leaching of fine sedimentary fraction from the bottom material.

Subsurface tidal gravity variation and gravimetric factor

SUMMARY Taking advantage of the simultaneous recording during 471 d between 2019 and 2021 by two superconducting gravimeters installed at the surface and 520 m under the surface at the Low Noise Underground Laboratory (LSBB) in Rustrel, France, we investigate whether a difference between the tidal gravity signals at the two locations can be detected. First, we model the periodical variations of the Earth’s gravity owing to the tidal influence from the Sun and Moon, at the Earth’s surface and at shallow depths. We provide analytical formulae for the Love numbers, gravimetric factor and gravity variation of simple spherical planetary models. We also numerically compute those parameters and function for a realistic spherical Earth model. We find that the fractional difference between the semi-diurnal tidal gravity variations at the surface and 520 m below is as small as 8.5 × 10$^{-5}$. We next evaluate the effect on the amplitude of the recorded gravity signal due to the calibration factors of the two superconducting gravimeters at LSBB. Finally, we compute the spectra of the difference between the gravity variations measured on and under the surface in the semi-diurnal band of the M$_2$ tidal wave. We find that the uncertainties associated to the calibration factors are larger than the theoretical or observational difference between the tidal gravity variations on the surface and at a 520-m depth.

Effects of variable gravity on stability in couple-stress fluids across various conducting boundaries

This work aims to inspect the effect of variable gravity field on the convective stability of couple stress fluid for different combinations of bounding surfaces. Both linear and nonlinear analyses are conducted to obtain eigenvalue problems. For this analysis, three different combinations of bounding surfaces have been considered. Normal mode analysis is used for linear analysis, while the energy method is used for nonlinear analysis. The numerical values of critical Rayleigh numbers are calculated using the single-term Galerkin technique. It is found that the Rayleigh numbers for the two analyses coincide, suggesting global stability. It is found that the increase in the value of couple stresses stabilize the system. The variable gravity can enhance or reduce the stability of the system depending on the direction of gravity variation which can be easily controlled by its parameter values. It is also observed that fluid confined in rigid-rigid bounding surface is more thermally stable, which is suitable for convection in couple stress fluid.

SDUST2020MGCR: a global marine gravity change rate model determined from multi-satellite altimeter data

Abstract. Investigating the global time-varying gravity field mainly depends on GRACE/GRACE-FO gravity data. However, satellite gravity data exhibit low spatial resolution and signal distortion. Satellite altimetry is an important technique for observing the global ocean and provides many consecutive years of data, which enables the study of high-resolution marine gravity variations. This study aims to construct a high-resolution marine gravity change rate (MGCR) model using multi-satellite altimetry data. Initially, multi-satellite altimetry data and ocean temperature–salinity data from 1993 to 2019 are utilized to estimate the altimetry sea level change rate (SLCR) and steric SLCR, respectively. Subsequently, the mass-term SLCR is calculated. Finally, based on the mass-term SLCR, the global MGCR model on 5′ × 5′ grids (SDUST2020MGCR) is constructed by applying the spherical harmonic function method and mass load theory. Comparisons and analyses are conducted between SDUST2020MGCR and GRACE2020MGCR resolved from GRACE/GRACE-FO gravity data. The spatial distribution characteristics of SDUST2020MGCR and GRACE2020MGCR are similar in the sea areas where gravity changes significantly, such as the eastern seas of Japan, the western seas of the Nicobar Islands, and the southern seas of Greenland. The statistical mean values of SDUST2020MGCR and GRACE2020MGCR in global and local oceans are all positive, indicating that MGCR is rising. Nonetheless, differences in spatial distribution and statistical results exist between SDUST2020MGCR and GRACE2020MGCR, primarily attributable to spatial resolution disparities among altimetry data, ocean temperature–salinity data, and GRACE/GRACE-FO data. Compared with GRACE2020MGCR, SDUST2020MGCR has higher spatial resolution and excludes stripe noise and leakage errors. The high-resolution MGCR model constructed using altimetry data can reflect the long-term marine gravity change in more detail, which is helpful in studying seawater mass migration and its associated geophysical processes. The SDUST2020MGCR model data are available at https://doi.org/10.5281/zenodo.10701641 (Zhu et al., 2024).

Towards Intercity Mobility System – Insights into the Spatial Interaction Gravity Model and Determination Approach

The current development of urban agglomeration greatly promotes the intercity connection and elevates the significance of intercity mobility system. However, intercity mobility often exhibits extreme spatiotemporal imbalances due to the diverse urban characteristics. This poses a huge challenge for traffic management and reveals the necessity on understanding the urban attractiveness for intercity mobility, which is represented as spatial interaction gravity in this study. While recent works have explored relevant aspects, they failed to provide insights into temporal variations in spatial interaction gravity or capture the determining factors from multiple perspectives. To fill this gap, this study proposed a two-phase framework to measure the urban spatial interaction gravity and developed determination approaches utilising the large-scale location-based services (LBS) dataset. Specifically, the inverse gravity model was adopted for the measure within multiple urban agglomerations and city sets during weekdays, weekends and holidays. Then, we developed the fitting equations of spatial interaction gravity by incorporating the correlated features associated with social, economic, network accessibility and land use. The findings present spatial interaction gravity across different periods and substantiate the distinct determination effects of features, with a high fitting accuracy. They provide promising supports for the intercity mobility prediction and pre-emptive traffic management.

Detectability of gravity changes on the sea surface due to magma accumulation beneath submarine volcanoes

SUMMARY This study estimated the gravity change expected on the sea surface due to magma accumulation beneath submarine volcanoes. For calculation, a semi-analytical model describing the deformation of an infinite elastic cone was considered, in which a point spherical source was embedded on the axis. The expected gravity change exceeds 0.2 mGal in certain scenarios when $\delta V = 10^8\, \mathrm{m^3}$ of source volume change occurs 2 km below the summit, and the summit is shallower than 300 m below the sea surface. The steeper the slope of a submarine volcano, the greater the expected gravity change. 0.2 mGal of gravity variation can be detected with recent marine gravimeters such as cold atom interferometers. The computation method for the gravity change based on the integral transform (Mellin type) is elaborated in this paper.

A disturbance rejection control for urban air mobility using artificial sensor-based Gaussian process regression

This paper presents a learning-based disturbance rejection control strategy for Urban Air Mobility (UAM) with vertical take-off and landing capability, which is subject to uncertainties in system parameters. The two primary sources of uncertainty during UAM operation, specifically moment of inertia uncertainty and center of gravity variation, are thoroughly analyzed as they negatively impact control performance. Building upon the analysis outcomes, a novel adaptive scheme is proposed that employs the modeling capabilities of Gaussian process regression for online learning to estimate model uncertainties. To ensure the collection of high-quality training data without relying on state derivatives, a nonlinear disturbance observer is employed as an artificial sensor. The suggested control algorithm is formulated by integrating Gaussian process regression with a baseline control derived using the feedback linearization control technique. Theoretical analysis grounded in the Lyapunov theorem reveals that the tracking error of the closed-loop system is semi-globally uniformly and ultimately bounded. Numerical simulations are conducted to validate the effectiveness of the proposed approach. The results obtained confirm that the proposed method can achieve superior tracking performance, even in the presence of model uncertainties and time-varying disturbances, surpassing existing approaches.

Influence of microgravity on spontaneous calcium activity of primary hippocampal neurons grown in microfluidic chips

The influence of variations of gravity, either hypergravity or microgravity, on the brain of astronauts is a major concern for long journeys in space, to the Moon or to Mars, or simply long-duration missions on the ISS (International Space Station). Monitoring brain activity, before and after ISS missions already demonstrated important and long term effects on the brains of astronauts. In this study, we focus on the influence of gravity variations at the cellular level on primary hippocampal neurons. A dedicated setup has been designed and built to perform live calcium imaging during parabolic flights. During a CNES (Centre National d’Etudes Spatiales) parabolic flight campaign, we were able to observe and monitor the calcium activity of 2D networks of neurons inside microfluidic devices during gravity changes over different parabolas. Our preliminary results clearly indicate a modification of the calcium activity associated to variations of gravity.

Identification of Variations in Gravity Anomaly Values in the Geothermal Distribution Area of the Rantau Dedap Region, South Sumatra

Identification of Variations in Gravity Anomaly Values in the Geothermal Distribution Area of the Rantau Dedap Region, South Sumatra

High‐Resolution Terrestrial Water Storage Estimates From GRACE and Land Surface Models

AbstractTerrestrial Water Storage (TWS) changes have been estimated at basin to continental scales from gravity variations using data from the Gravity Recovery and Climate Experiment (GRACE) satellites since 2002. The relatively low spatial resolution (∼300 km) of GRACE observations has been a main limitation in such studies. Various data processing strategies, including mascons, forward modeling, and constrained linear deconvolution (CLD), have been employed to address this limitation. Here we develop a revised CLD method to obtain a TWS estimate that combines GRACE observations with much higher spatial resolution land surface models. The revised CLD constrains model estimates to agree with GRACE TWS when smoothed. As an example, we apply the method to obtain a high spatial resolution TWS estimate in Australia. We assess the accuracy of the approach using synthetic GRACE data.

CAUSAL PHYSICS: CROSS-SECTIONAL AREA INSTEAD OF MASS

A review and analysis on the topic of mass and physical interaction within the framework of causal physics is presented. Mass is considered as a special case of a quantity that is proportional to the cross-sectional area of physical objects. Replacing the mass with this quantity allows further development of the kinetic theory of gravity by Le Sage. Physical interaction is considered as a result of mutual shielding of elementary mass–forming elements - nucleons and systems consisting of them - atomic nuclei in the path of movement of cosmic ether particles. It is shown that the applied approach makes it possible to create a visual and causal picture of physical interaction, both in the microcosm and in the macrocosm, leading to completely new results. The existence of gravitational shielding is substantiated, a visual picture of the mass defect is created, the presence of hidden mass in massive celestial bodies is revealed, the reason for the precession of the perihelion of Mercury and artificial Earth satellites is indicated, semi-daily behavior in the diurnal variation of gravity is described and interpreted, features in the manifestation of tidal processes are described. The applied approach allows to consider the issues of interaction in the micro and macrocosm from a single mechanism.

EFFECT OF VARIABLE GRAVITY ON THERMAL CONVECTION IN ROTATING JEFFREY NANOFLUID: DARCY-BRINKMAN MODEL

In this study, we explore the influence of varying gravity on the beginning of thermal instability in a rotating layer of Jeffrey nanofluid with Darcy-Brinkman porous media. The solution of the fluid layer retained between two free-free barriers is determined using linear stability analysis based on the normal mode approach. In this study, we used the Galerkin approach to find the eigenvalue. The influence of exponential and cubic variable gravity on the start of stationary and oscillatory convection is calculated analytically and graphically. The graphs are made with the help of MATLAB R2021b software. For both stationary and oscillatory convection, we've examined how variations in gravity affect the Jeffrey parameter, rotation (Taylor number), Darcy-Brinkman number, Lewis number, moderated diffusivity ratio, porosity of porous media, and nanoparticle Rayleigh number. It is discovered that stationary as well as oscillatory convections are more stabilized by negative exponential variable gravity parameter.

Helium pool boiling critical heat flux under various magnetically controlled gravity levels

The characterization of Critical Heat Flux (CHF) under magnetic compensation of gravity in liquid helium (LHe) is particularly important for the design of the future very high magnetic field superconducting magnets. Through a series of experimental sessions, the critical heat flux in LHe pool boiling has been determined under various gravity conditions ranging from 0.03g to 2.2g using magnetic forces. It is the largest range of gravity variation, from reduced to hyper-gravity, ever studied in a single facility so far. The study was conducted in a very high magnetic field up to 30 T. The magnetic compensation of gravity does not allow for perfect resulting gravity homogeneity. Nevertheless, for the first time, the magnetic field configuration is considered in the experimental results analysis. The relative resulting gravity inclination with respect to the heat exchange surface is also taken into account. The experimental results are compared with the well-known CHF correlation in LHe pool-boiling and show a very good behavior agreement. For our experimental gravity level, the CHF is ranging from 3000 W/m2 to 12000 W/m2.

Gravitational Constraints on the Earth's Inner Core Differential Rotation

AbstractThe differential axial rotation of the solid inner core (IC) is suggested by seismic observations and expected from core dynamics models. A rotation of the IC by an angle α takes its degree 2, order 2 topography (peak‐to‐peak amplitude δh) out of its gravitational alignment with the mantle. This creates a gravity variation of degree 2, order 2 proportional to δh and to α. Here, we use gravity observations from Satellite Laser Ranging, the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow‐On to reconstruct the time‐variable S2,2 Stokes coefficient. We show that for δh = 90 m, S2,2 provides upper bounds on α of 0.09°, 0.3°, and 0.4° at periods of ∼4, ∼6, and ∼12 years, respectively. These are overestimates, as our reconstructed S2,2 signal likely remains polluted by hydrology, although viscous relaxation of the IC can permit larger amplitudes.