Effective Gravitational Acceleration Research Articles

Attitude Calculation Method of Drilling Tools Based on Cross-Correlation Extraction and ASRUKF

As a key component of the measurement while drilling technology, the accuracy of attitude calculation is directly related to the efficiency of resource exploration. To reduce the influence of vibration, rotation, and other disturbances on the attitude sensor during drilling, a method based on cross-correlation extraction and the adaptive square-root unscented Kalman filter (ASRUKF) is proposed to solve the attitude of the drilling tool in this paper. Firstly, the error of the signal collected by the attitude sensor is compensated, and the unscented Kalman filter (UKF) is used for filtering. Then, the effective gravitational acceleration signal is extracted by the cross-correlation method. Finally, an experimental platform for simulating the fully rotating attitude measurement system is established, and the application effects of the UKF and ASRUKF in the attitude calculation are compared. Compared with the UKF, the root mean square error of the inclination angle calculated by the ASRUKF is reduced by 12.9%, and the variance is reduced by 27.3%; the root mean square error of the azimuth angle is reduced by 29.5%, and the variance is reduced by 39.9%. The experimental results show that the attitude calculation method proposed in this paper can stably and effectively improve the accuracy of the attitude calculation of drilling tools.

Inhomogeneous airflow and heat transfer characteristics of buoyancy-driven airflow in aircraft cockpits

ABSTRACT In-flight airflow and temperature distributions are key parameters for the thermal design of aircraft cockpit. In real working conditions, the effect of gravitational acceleration on flow and heat transfer characteristics is crucial but has long been ignored in the cockpit study. Gravitational acceleration also induces buoyant force and inhomogeneous thermal transport, which gives rise to thermal challenges of aircraft cockpit. To address this challenge, an SST k-ω turbulence model was well validated with 94% prediction accuracy to evaluate the inhomogeneous characteristics. The results indicated that the buoyancy-driven temperature distribution became progressively more inhomogeneous as gravitational acceleration increased. It was also found that the nonlinear variations in temperature were observed in the vertical direction from 0 to 1.5 m. Besides, a dimensionless parameter Gr* (∝107) was used to quantitatively assess the buoyant force. The results showed that the buoyancy force was considerably similar at 2.0 g and 4.0 g, and largely different from the trend at 1.0 g. Indeed, a common characteristic of these three conditions is the presence of a large inhomogeneity coefficient R in the top region of the cockpit. Therefore, an important limitation when designing the thermal structure of cockpit is the inhomogeneous feature of which the follow-up study needs to be carried out.

Interface instabilities in Faraday waves of two-layer liquids with free surface

For a fluid system with an interface between two-layer liquids and a free surface, there exist two series of natural frequencies. Under vertical excitations, the interface and the free surface can be excited separately or simultaneously. Laboratory experiments were conducted to excite the first- and second-mode Faraday waves on the free surface, which forced the in-phase motion of the interface. The experimental data were used to validate the extended numerical model NEWTANK. The secondary surface instability in the form of period tripling breaking occurred on the free surface when second-mode surface resonant response was large. Further numerical investigations were conducted to study the resonance of interface and in addition to the primary Faraday instability, the secondary interface instability occurred due to Rayleigh–Taylor (RT) instability. The analysis revealed that this was due to the contribution from the combined effects of vertical fluid acceleration and external excitation, which made the upper layer liquid essentially ‘heavier’ than the lower layer at certain instances, even when the amplitude of acceleration of the external excitation was lower than that of gravity at all times. A dimensionless parameter ‘normalised effective gravitational acceleration’ (NEGA) was proposed to quantify the occurrence criterion of RT instability in a dynamic fluid system. Furthermore, simultaneous resonances at both the interface and the free surface were studied and reverse flows could be formed across the interface as the upper Faraday waves and lower Faraday waves moved in opposite directions, which generated the secondary interface instability of Kelvin–Helmholtz (KH) type. When KH instability took place, more energy was transferred to higher frequency modes as time evolved.

The Preliminary Statistical Analysis of LAMOST DR8 Low Resolution AFGK Stars

Abstract We download the Large sky Area Multi-Object fiber Spectroscopic Telescope Data Release 8 (LAMOST DR8) low resolution catalog for 6,478,063 AFGK type stars and plot the figures of effective temperature, gravitational acceleration and metal abundance. Some small and medium mass stars are evolved from pre-main sequence or main sequence stage to planetary nebula stage or white dwarf stage by the stellar evolution code MESA. We analyze the observed statistical data and model calculation results, and then obtain some basic conclusions preliminarily. Most red giant and asymptotic giant branch stars with log g less than 0.85 have poor metal abundance. Most hot A type main-sequence stars are metal-rich stars with log g from 3.5 to 4.5. The conclusions are reasonable within a certain error range. The theory of a gap area in the Hertzsprung–Russell diagram for stellar evolutions of medium mass stars is reflected in the statistical plots. The central core hydrogen burning stage and the central core helium burning stage correspond to the peak structures in the statistical plots for gravitational acceleration. The metal abundances among A, F, G and K type stars have a wide distribution. We cannot simply replace the metal abundances of these stars with the metal abundance of the Sun when doing fine research work.

Experimental and Numerical Study of the Discharge Performance of Particle-Laden Turbulent Flow

In the marine fire suppression system, continuous delivery of dry chemical powder to the fire source with long powder discharge range and high dispersion concentration is essential. The work is devoted to experimental and numerical studies of the flow characteristics of the dry chemical powder jet from a horizontal injector with a wide range of Stokes numbers between 6 to 30 and Reynolds numbers between 4792 to 23,960 by considering the effect of gravitational acceleration. A CFD-based Eulerian–Eulerian multiphase model combined with Standard k-ω turbulence model was used to predict flow characteristics of particle-laden jet using dimensionless numbers, including the solid volume fraction, the normalized velocity magnitude, and the turbulent viscosity ratio. Experimental studies have been carried out for three different inflow velocities (2.06, 2.45, and 2.81 m/s). The results indicate that the particle density plays a significant role in the dispersion of the particles in the radial and axial directions. The transition from U-shaped to V-shaped solid dispersion structure on the ground can be captured with the increase of particle density. Moreover, the higher level turbulence intensity enhances the solid dispersion concentration. Finally, it was found that the Portland cement powder exhibits better discharge performance in terms of solid discharge range and dispersion concentration in comparison with other dry powders. These results have implications in the design of powder-based fire suppression system. Further studies should aim to the in-depth research on the fire extinguishing mechanism of the Portland cement powder, especially the fire suppression effectiveness and thermal decomposition process.

Investigation of granular capillary rising under vertical vibration

The phenomenon of granular capillary rising under vertical vibration provides a novel technical route for hoisting, transporting and collecting granular materials. However, there are still obvious deficiencies in the existing studies of the granular capillary rising behavior, especially the intensive investigation on the effects of gravitational acceleration, horizontal vibration component and particle size distribution are still lacking. To address these problems, the discrete element method is used to numerically simulate the granular capillary rising phenomenon under different operating conditions. The final capillary rising height and average capillary rising velocity of the granular matter are computed and analyzed based on the numerical simulations. The results show that the granular capillarity can also occur under low gravity conditions, and that the final capillary rising height and the average capillary rising velocity first increase and then decrease with the gravitational acceleration. It is also found that the final capillary rising height is insensitive to the variation of horizontal vibrational component, whereas the average capillary rising velocity increases with the augmentation of horizontal vibrational component. Compared with the mono-sized particles, the particles with the same mean size but having a Gaussian size distribution exhibit a maximal capillary rising height at a larger critical tube diameter. Meanwhile, the average capillary rising velocity of the particles having a Gaussian size distribution is faster in the tube diameter range where the granular capillary dynamics for both size distributions is dominated by the jamming effect.

The time-dependent Rayleigh–Taylor instability in interstellar shells and supershells, including the eROSITA bubbles

ABSTRACT The Rayleigh–Taylor (RT) instability is omnipresent in the physics of inversely density-stratified fluids subject to effective gravitational acceleration. In astrophysics, a steep stratification of the ambient medium can fragment a bubble shell faster due to a strongly time-dependent RT instability, causing the classical constant gravity models to fail. We derive the time-dependent instability criteria analytically for the cases of constant, exponential, and power-law accelerations, verifying them through high-resolution numerical simulations. Our results show that (1) even in the linear phase there is a term opposing exponential growth, (2) non-linear growth approaches asymptotically the solution found by Fermi and von Neumann, (3) the interpenetrating spikes and bubbles promote a significant mixing, with the fractal dimension of the interface approaching 1.6, only limited by numerical diffusion, and (4) the probability density function for the passive scalar to study mixing becomes increasingly sharper peaked for power-law and exponential accelerations. Applying our solutions to stellar wind bubbles, young supernova remnants (SNRs), and superbubbles (SBs), we find that the growth rate of the RT instability is generally higher in the shells of wind-blown bubbles in a power-law stratified medium than in those with power-law rising stellar mechanical luminosities, Tycho-like than Cas A-like SNRs, and one-sided than symmetric SBs. The recently observed eROSITA bubbles indicate smooth rim surfaces, implying that the outer shell has not been affected by RT instabilities. Therefore, the dynamical evolution of the bubbles suggests maximum final ages that are significantly above their current age, which we estimate to be about 20 Myr.

Partial similitude for dynamic characteristics of rotor systems considering gravitational acceleration

In practical engineering, gravitational acceleration cannot be scaled (λg = 1), which is not considered in the existing similitude studies of rotor systems. In this paper, a restriction in scaling procedure caused by λg = 1 is first demonstrated analytically. Aiming at this issue, a method to derive the scaling laws of rotor systems is proposed based on the generalized equations of subsystems, where gravitational acceleration, geometric and material distortion are considered. The derived scaling law for the vibration response is different from that of previous literatures because λg = 1 is considered. On this basis, this paper presents a method for designing scaled models that can reduce the difficulty and costs of the model test. The accuracy and effectiveness of the proposed methods are verified by predicting the dynamic characteristics of a rotor system. Moreover, the effect of gravitational acceleration is discussed and the predicted results are compared with the previous literature. The results show that neglecting the gravitational acceleration leads to a certain error when predicting the vibration responses in the vertical direction.

V723 Persei: A short-period Algol-like near-contact binary

Abstract Complete multi-color photometric observations in the BVRCIC bands and low-resolution spectra of the short-period binary V723 Per are presented for the first time. The stellar atmospheric parameters of effective temperature, gravitational acceleration, and metallicity are determined as Teff = 7407 ± 53 K, log g = 3.98 ± 0.10 cm s−2, and [Fe/H] = −0.25 ± 0.10 dex, respectively. Photometric solutions are obtained by analyzing the phased light curves with the Wilson–Devinney method. We found that V723 Per is an Algol-like semi-detached-type near-contact binary with a mass ratio of q = 0.3168 ± 0.0011. The secondary component is filling the critical Roche lobe, while the primary is nearly filling its Roche lobe (the filling degree is about 80.93%). The primary component contributes about 96% of the luminosity of the total system, and the temperature difference between the two component stars is about 3000 K. All available times of light minimum, including three new observed eclipse times, were used to construct the O − C curve. The period shows a continuous increase at a rate of dP/dt = +7.63(±0.75) × 10−6 d yr−1 superposed with a periodic oscillation of amplitude of 0.0392 d and period 17.4 yr. The period increase could be explained by mass transfer from the secondary component to the primary one on the thermal timescale, which is in agreement with the semi-detached configuration. The cyclic change may be caused by the light-time effect due to a massive ($M_3=3.03\, M_\odot$) but quite faint (L3/Ltotal ≃ 3%) tertiary companion orbiting the central binary. Such a third body might be a black hole or a binary with two nearly identical stars. More observations are required in the future to confirm our conclusion about the cycle variation and tertiary companion.

Prediction of interface states in liquid surface waves with one-dimensional modulation

We theoretically study the interface states of liquid surface waves propagating over the heterojunctions formed by a bottom with one-dimensional periodic undulations. By considering the periodic structure as a homogeneous one, our systematic study shows that the signs of the effective depth and gravitational acceleration are opposite within the band gaps whether the structure is symmetric or not. Those effective parameters can be used to predict the interface states which could amplify the amplitudes of liquid surface waves. These phenomena provide new opportunities to control the localization of water-wave energy.

Physical properties and catalog of EW-type eclipsing binaries observed by LAMOST

EW-type eclipsing binaries (hereafter called EWs) are strong interacting systems in which both component stars usually fill their critical Roche lobes and share a common envelope. Numerous EWs were discovered by several deep photometric surveys and there were about 40 785 EW-type binary systems listed in the international variable star index (VSX) by 2017 March 13. 7938 of them were observed with LAMOST by 2016 November 30 and their spectral types were identified. Stellar atmospheric parameters of 5363 EW-type binary stars were determined based on good spectroscopic observations. In the paper, those EWs are cataloged and their properties are analyzed. The distributions of orbital period (P), effective temperature (T), gravitational acceleration (log(g)), metallicity ([Fe/H]) and radial velocity (RV) are presented for these observed EW-type systems. It is shown that about 80.6% of sample stars have metallicity below zero, indicating that EW-type systems are old stellar populations. This is in agreement with the conclusion that EW binaries are formed from moderately close binaries through angular momentum loss via magnetic braking that takes a few hundred million to a few billion years. The unusually high metallicities of a few percent of EWs may be caused by contamination of material from the evolution of unseen neutron stars or black holes in the systems. The correlations between orbital period and effective temperature, gravitational acceleration and metallicity are presented and their scatters are mainly caused by (i) the presence of third bodies and (ii) sometimes wrongly determined periods. It is shown that some EWs contain evolved component stars and the physical properties of EWs mainly depend on their orbital periods. It is found that extremely short-period EWs may be older than their long-period cousins because they have lower metallicities. This reveals that they have a longer timescale of pre-contact evolution and their formation and evolution aremainly driven by angular momentum loss via magnetic braking.

The electrical conductance growth of a metallic granular packing

We report on measurements of the electrical conductivity on a two-dimensional packing of metallic disks when a stable current of ~1 mA flows through the system. At low applied currents, the conductance σ is found to increase by a pattern σ(t) = σ ∞ − Δσ E α [ − (t/τ) α ], where E α denotes the Mittag-Leffler function of order α ∈ (0,1). By changing the inclination angle θ of the granular bed from horizontal, we have studied the impact of the effective gravitational acceleration g eff = gsinθ on the relaxation features of the conductance σ(t). The characteristic timescale τ is found to grow when effective gravity g eff decreases. By changing both the distance between the electrodes and the number of grains in the packing, we have shown that the long term resistance decay observed in the experiment is related to local micro-contacts rearrangements at each disk. By focusing on the electro-mechanical processes that allow both creation and breakdown of micro-contacts between two disks, we present an approach to granular conduction based on subordination of stochastic processes. In order to imitate, in a very simplified way, the conduction dynamics of granular material at low currents, we impose that the micro-contacts at the interface switch stochastically between two possible states, “on” and “off”, characterizing the conductivity of the micro-contact. We assume that the time intervals between the consecutive changes of state are governed by a certain waiting-time distribution. It is demonstrated how the microscopic random dynamics regarding the micro-contacts leads to the macroscopic observation of slow conductance growth, described by an exact fractional kinetic equations.

Computational stability appraisal of rectangular natural circulation loop: Effect of loop inclination

Controlling stability behavior of single-phase natural circulation loops, without significantly affecting its steady-state characteristics, is a topic of wide research interest. Present study explores the role of loop inclination on a particular loop geometry. Accordingly a 3D computational model of a rectangular loop is developed and transient conservation equations are solved to obtain the temporal variation in flow parameters. Starting from the quiescent state, simulations are performed for selected sets of operating conditions and also with a few selected inclination angles. System experiences instability at higher heater powers and also with higher sink temperatures. Inclination is found to have a strong stabilizing influence owing to the reduction in the effective gravitational acceleration and subsequent decline in local buoyancy effects. The settling time and highest amplitude of oscillations substantially reduces for a stable system with a small inclination. Typically-unstable systems can also suppress the oscillations, when subjected to tilting, within a reasonable period of time. It is possible to stabilize the loop within shorter time span by increasing the tilt angle, but at the expense of reduction in steady-state flow rate. Overall a tilt angle of 15° is suggested for the selected geometry. Results from the 3D model is compared with the predictions from an indigenous 1D code. While similar qualitative influence of inclination is observed, the 1D model predicts early appearance of the stability threshold and hence hints towards larger instability. Accordingly the limitations of 1D approach in terms of the dependence on correlations is highlighted.

The structure of critically-rotating accreting stars

Context. The structure characteristics of the critically-rotating accretor in binaries are investigated in this paper, on the basis of the potential function including rotational and tidal distortions.Aims. Our aim is to investigate the structure of the accretor when the accreting star reaches the critical velocity.Methods. In this paper, we have implemented the prescription described by Kippenhahn & Thomas (, Proc. IAU Colloq., 4, 20) and Landin et al. (, A&A, 494, 209).Results. The traditional model merely included the hydrodynamical effect of rotation. When comparing this model with ours, we find that it is very necessary for the rapidly rotating accreting star to include the gravitational potentials from tides Ψtide , and the distortions of the star resulting from rotation Ψdis,rot . Furthermore, we find that the mean effective gravitational acceleration can be decreased in the distort model, and the star shifts towards low temperature and low luminosity. Rotation and tides can extend the convection zone below the surface, and reduce the convective core in the center of stars due to the Solberg-Hoiland criterion. Rotational distortions derived from Ψdis,rot can intensify the critical velocity whereas the tide force derived from Ψtide tends to reduce the critical velocity. Rapid rotation induced by mass transfer also causes the central temperature to decrease, and triggers efficient mixing which can significantly modify the H-profile.

Detail investigation of the inclined pressure structure and gravity darkening in critical rotating star Achernar

Rotation and tide are two important factors that have very important impacts on the stellar structure and evolution. Based on the observational data of Achernar, we have derived the inclined pressure structure in a single rotating star or as a member in the binaries. We have given the distributions of the physical quantities on the isobaric surface and these distributions are derived from the Legendre series of expansions. We have also found the relationship between all levels of perturbation potential functions (including rotational and tidal distortions) and the distributions of density and pressure under the condition of inclined pressure structure. In particular, the gravitational darkening with the models including the effects of rotation and tide is investigated. We have found that the critical ratio of equatorial radius to the polar radius is consistent with the observations in rotating binaries better than that in single rotating model. The reason is that the tidal force can make the polar radius shortened because the tidal force exerts an inward force to the two polar points. However, the theoretical angular velocity in binaries is smaller than that observed. It is also shown that the positive shear enhances the centrifugal force and decreases the mean effective gravitational acceleration and effective temperatures whereas the negative shear plays a role to strengthen the effective gravitational acceleration. Moreover, the solid body rotation has not been supported inside Achernar because magnetic fields have not been detected through observations. Furthermore, the theoretical angular velocity in rigid rotation is higher than the angular velocity observed. Achernar has a periodic variation of light curves due to mass outburst, which also supports differential rotation. A positive shear indicates that the mass in accretion disks is falling to Achernar and the Achernar is spun up to critical rotation according to current observations. By comparing the theoretical results with observations, it can be seen that when the theoretical spin angular velocity of Achernar is 4.65 10-5 s-1 and the positive shears / s are 0.7851, the temperature of the polar points is 16041 K and that of equatorial sphere is 12073 K. Relative errors between the theoretical values and observations are less than 3% and are listed in the text. This model is the best and is the most possible one for Achernar.

The effect of gravitational acceleration in the streaming potential on the surface of a planetary body and in orbit around it

A corrected gravitational acceleration has been used for the study of the electric streaming potential. The acceleration has been corrected for the oblateness of a spherical body, as well as the rotational velocity of the spherical body. For higher viscosity values we find that on the surface of a spherical body the electric streaming field is slightly higher at the poles when compared to that of the equator. Aboard an orbiting spacecraft, we find that equatorial circular orbits that they carry experiments which use high viscosity fluids result to a higher streaming field values when compared to circular polar orbits. In a similar way, we find that the electric streaming field as well as the electrophoretic velocity of the particle in the fluid considered increases significantly in elliptical orbits as the orbital eccentricity increases, being higher in equatorial orbits and smaller in polar orbits.

Structural characterization of submerged granular packings.

We consider the impact of the effective gravitational acceleration on microstructural properties of granular packings through experimental studies of spherical granular materials saturated within fluids of varying density. We characterize the local organization of spheres in terms of contact connectivity, distribution of the Delaunay free volumes, and the shape factor (parameter of nonsphericity) of the Voronoï polygons. The shape factor gives a clear physical picture of the competition between less and more ordered domains of particles in experimentally obtained packings. As the effective gravity increases, the probability distribution of the shape factor becomes narrower and more localized around the lowest values of the shape factor corresponding to regular hexagon. It is found that curves of the pore distributions are asymmetric with a long tail on the right-hand side, which progressively reduces while the effective gravity gets stronger for lower densities of interstitial fluid. We show that the distribution of local areas (Voronoï cells) broadens with decreasing value of the effective gravity due to the formation of lose structures such as large pores and chainlike structures (arches or bridges). Our results should be particularly helpful in testing the newly developed simulation techniques involving liquid-related forces associated with immersed granular particles.

Transport characteristics of thermal plume driven by turbulent mixing in stairwell

A set of experiments was conducted to study the transport characteristics of thermal plume driven by turbulent mixing in a 1/3 scale stairwell with single opening. The opening location was varied at the stairwell height direction to obtain different air supply conditions. When the thermal plume moves upwardly in the vertical stairwell, the interface between the upper cold layer and the lower hot layer is unstable. The gravitational instability leads to the turbulent mixing in the two layers. The location of the thermal plume front is determined using the temperature increases at different measuring points. Based on the dimensional analysis, the plume front location can be expressed as a function of time and effective gravitational acceleration. The velocity of thermal plume in stairwell is lower than those in hollow shafts, resulting from the block and cooling effects of stair steps. The comparison of the experimental and calculated plume front locations shows good agreement with the maximum error less than 20%. The results could improve the understanding of thermal plume movement driven by turbulent mixing in stairwells.

UNIVERSALITY OF THE ACCELERATION DUE TO GRAVITY ON THE SURFACE OF A RAPIDLY ROTATING NEUTRON STAR

On the surface of a rapidly rotating neutron star, the effective centrifugal force decreases the effective acceleration due to gravity (as measured in the rotating frame) at the equator while increasing the acceleration at the poles due to the centrifugal flattening of the star into an oblate spheroid. We compute the effective gravitational acceleration for relativistic rapidly rotating neutron stars and show that for a star with mass $M$, equatorial radius $R_e$, and angular velocity $\Omega$, the deviations of the effective acceleration due to gravity from the nonrotating case take on a universal form that depends only on the compactness ratio $M/R_e$, the dimensionless square of the angular velocity $\Omega^2R_e^3/GM$, and the latitude on the star's surface. This dependence is universal, in that it has very little dependence on the neutron star's equation of state. The effective gravity is expanded in the slow rotation limit to show the dependence on the effective centrifugal force, oblate shape of the star and the quadrupole moment of the gravitational field. In addition, an empirical fit and simple formula for the effective gravity is found. We find that the increase in the acceleration due to gravity at the poles is of the same order of magnitude as the decrease in the effective acceleration due to gravity at the equator for all realistic value of mass, radius and spin. For neutron stars that spin with frequencies near 600 Hz the difference between the effective gravity at the poles and the equator is about 20%.

The stochastic mode of the Faraday instability of shallow fluid layers

The instability of a vertically oscillated layer of fluid is a classical problem whose history dates back to Faraday in the 19th century. We consider the stability of a shallow layer for which the oscillation, as expressed through the effective gravitational acceleration, is a random function of time. Using both theoretical linear stability analysis and high-resolution numerical simulations, including both individual realizations and ensemble calculations, of the nonlinear system of equations, we find that two different stochastic modes of instability exist. Both modes find their expression in finite amplitude oscillations of the free surface that exhibit sharp crests and broad troughs, or in other words, that resemble the classical Stokes wave. We demonstrate that a necessary condition for the first type of instability is snapshot, or instantaneous, instability. The subdominant instability resembles classical parametric resonance that can exist in a harmonically oscillated layer of fluid, and occurs even when the flow is always snapshot stable (or the gravitational acceleration is non-negative).