Free-fall Velocity Research Articles

Spotting ignition of plastic foam by a fast-moving hot metal particle

Spotting ignition involves dynamic interaction between fuel bed and hot particles, but the scientific understanding of the ignition by a fast-moving hot particle is still limited. Herein, a hot steel particle with various horizontal velocities, temperatures, and sizes is shot to ignite vertically oriented low-density expandable polystyrene foam. A high-speed particle can directly get embedded into the foam to achieve flash-point, fire-point, or no ignition, while a low-speed particle bounces away from the foam without ignition. Results show that for a particle of 1150 °C, its minimum velocity for embedding is 12.00 m/s. Such a critical velocity for hot-particle embedded or ignition slightly decreases as particle temperature increases. Minimum ignition temperature of these high-speed particles is 200 °C higher than that of near-static or with a low free-fall velocity, due to the shorter residence time and insufficient to produce a flammable mixture. Moreover, when the particle is neither too slow to bounce away nor too fast to get embedded, it will be partially embedded on the sample surface to burnout the fuel, posing the biggest fire hazard. It deepens our knowledge of the complex interaction between hot moving particles and insulation foam to reduce spotting fire risk for building façade.

Experimental study on dynamics of a free-falling finite-size sphere associated with its collision with a wall in a boundary layer

To reveal the physical mechanisms of particle transport in the pneumatic and hydraulic transports, this study experimentally investigated the dynamics of a free-falling finite-size sphere associated with its collision with a wall and the surrounding flow field in a boundary layer flow. The simultaneous motion of finite-size sphere and its inducing flow field near the wall were measured by using a time-resolved particle image velocimetry. The results revealed that the free-falling velocity of sphere increases with Reynolds number before the first impaction, and then is almost unrelated to Reynolds number. The effect of gravity on sphere motion becomes weak with increasing Reynolds number. As the sphere approaches to the wall, the vortex appeared at the front of sphere moves to left side near the wall and reduces the sphere falling speed. After the sphere collided on the wall, the upward motion of sphere induces another vortex pair, which generates strong rotation of sphere.

Source of the Fe Kalpha emission in T Tauri stars. Radiation induced by relativistic electrons during flares. An application to RY Tau

T Tauri stars (TTSs) are magnetically active stars that accrete matter from the inner border of the surrounding accretion disk; plasma becomes trapped into the large-scale magnetic structures and falls onto the star, heating the surface through the so-called accretion shocks. The X-ray spectra of the TTSs show prominent Fe Kalpha fluorescence emission at 6.4 keV (hereafter, Fe Kalpha emission) that cannot be explained in a pure accretion scenario because its excitation requires significantly more energy than the maximum available through the well-constrained free-fall velocity. Neither can it be produced by the hot coronal plasma. TTSs display all signs of magnetic activity, and magnetic reconnection events are expected to occur frequently. In these events, electrons may become accelerated to relativistic speeds, and their interaction with the environmental matter may result in Fe Kalpha emission. It is the aim of this work to evaluate the expected Fe Kalpha emission in the context of the TTS research and compare it with the actual Fe Kalpha measurements obtained during the flare detected while monitoring RY Tau with the XMM-Newton satellite. The propagation of high-energy electrons in dense gas generates a cascade of secondary particles that results in an electron shower of random nature, whose evolution and radiative throughput was simulated in this work using the Monte Carlo code PENELOPE. A set of conditions representing the environment of the TTSs where these showers may impinge was taken into account to generate a grid of models that can aid the interpretation of the data. The simulations show that the electron beams produce a hot spot at the point of impact; strong Fe Kalpha emission and X-ray continuum radiation are produced by the spot. This emission is compatible with RY Tau observations. The Fe Kalpha emission observed in TTSs could be produced by beams of relativistic electrons accelerated in magnetic reconnection events during flares.

Revisiting the Helium and Hydrogen Accretion Indicators at TWA 27B: Weak Mass Flow at Near-freefall Velocity

TWA 27B (2M1207b) is the first directly imaged planetary-mass (M p ≈ 5 M J) companion and was observed at 0.9–5.3 μm with ​​​JWST/NIRSpec. To understand the accretion properties of TWA 27B, we search for continuum-subtracted near-infrared helium and hydrogen emission lines and measure their widths and luminosities. We detect the He i triplet at 4.3σ and all Paschen-series lines covered by NIRSpec (Paα, Paβ, Paγ, Paδ) at 4σ–5σ. The three brightest Brackett-series lines (Brα, Brβ, Brγ) as well as Pfγ and Pfδ are tentative detections at 2σ–3σ. We provide upper limits on the other hydrogen lines, including on Hα through Hubble Space Telescope archival data. Three lines can be reliably deconvolved to reveal an intrinsic width Δv intrsc = (67 ± 9) km s−1, which is 60% of the surface freefall velocity. The line luminosities seem significantly too high to be due to chromospheric activity. Converting line luminosities to an accretion rate yields Ṁ≈5×10−9MJyr−1 when using scaling relationships for planetary masses, and Ṁ≈0.1×10−9MJyr−1 with extrapolated stellar scalings. Several of these lines represent the first detections at an accretor of such low mass. The weak accretion rate implies that formation is likely over. This analysis shows that JWST can be used to measure low line-emitting mass accretion rates onto planetary-mass objects, motivates deeper searches for the mass reservoir feeding TWA 27B, and hints that other young directly imaged objects might—hitherto unbeknownst—also be accreting.

Cloudlet capture model for the accretion streamer onto the disc of DG Tau

ABSTRACT DG Tau is a nearby T Tauri star associated with a collimated jet, a circumstellar disc, and a streamer a few hundred au long. The streamer connects to the disc at ∼50 au from DG Tau. At this location SO emission is observed, likely due to the release of sulphur from dust grains caused by the shock of the impact of the accretion streamer onto the disc. We investigate the possibility that the DG Tau streamer was produced via cloudlet capture on the basis of hydrodynamic simulations, considering a cloudlet initiating infall at 600 au from DG Tau with low angular momentum so that the centrifugal force is smaller than the gravitational force, even at 50 au. The elongation of the cloudlet into a streamer is caused by the tidal force when its initial velocity is much less than the free-fall velocity. The elongated cloudlet reaches the disc and forms a high-density gas clump. Our hydrodynamic model reproduces the morphology and line-of-sight velocity of CS (5 − 4) emission from the northern streamer observed with Atacama Large Millimeter/submillimeter Array. We discuss the conditions for forming a streamer based on the simulations. We also show that the streamer should perturb the disc after impact for several thousands of years.

Clump-scale Gas Infall in High-mass Star Formation: A Multitransition View with James Clerk Maxwell Telescope HCN (4–3) Mapping

Gas infall motions play a crucial role in high-mass star formation and are characterized by observable signatures of blueshifted asymmetric spectral line profiles (“blue profiles”). However, the connection between blue profiles and infall motions is unclear due to complex gas motions at parsec scales. In this study, we present the results of an HCN (4–3) mapping survey conducted with the James Clerk Maxwell Telescope, toward 38 massive clumps exhibiting blue profiles in HCO+ (3–2). We extract 34 HCN cores from the 38 observed fields. The core-averaged spectra show various line profiles, indicating that blue-profile HCO+ (3–2) does not guarantee the same in HCN (4–3). Through non-LTE radiative-transfer calculations, we attribute the low detection rate of high-J blue profiles to a combination of insufficient HCN (4–3) opacity and the intricate gas motion across different density layers. A comparison between the MALT90 and Bolocam Galactic Plane Survey line surveys highlights the importance of appropriate tracers, high spectral resolution, and column density thresholds when searching for blue profiles. We select 11 reliable infall candidates and adopt the Hill5 model to fit the infall velocity of 0.2–1.6 km s−1, corresponding to 5%–74% of freefall velocity. Assuming a spherically collapsing model, we estimate the median and mean mass infall rates to be 4.5 × 10−3 and 7.6 × 10−3 M ⊙ yr−1, respectively. The consistency of the mass infall rates among different transitions suggests a steady accretion process from the clump gas envelope to the inner region.

Possible episodic infall towards a compact disk in B335

Context. Previous observations of the isolated Class 0 source B335 have presented evidence of ongoing infall in various molecular lines, such as HCO+, HCN, and CO. There have been no confirmed observations of a rotationally supported disk on scales greater than ~12 au. Aims. The presence of an outflow in B335 suggests that a disk is also expected to be present or undergoing formation. To constrain the earliest stages of protostellar evolution and disk formation, we aim to map the region where gas falls inwards and observationally constrain its kinematics. Furthermore, we aim to put strong limits on the size and orientation of any disk-like structure in B335. Methods. We used high angular resolution 13CO data from the Atacama Large Millimeter/submillimeter Array (ALMA) and combined it with shorter-baseline archival data to produce a high-fidelity image of the infall in B335. We also revisited the imaging of high-angular resolution Band 6 continuum data to study the dust distribution in the immediate vicinity of B335. Results. Continuum emission shows an elliptical structure (10 by 7 au) with a position angle 5 degrees east of north, consistent with the expectation for a forming disk in B335. A map of the infall velocity (as estimated from the 13CO emission), shows evidence of asymmetric infall, predominantly from the north and south. Close to the protostar, infall velocities appear to exceed free-fall velocities. Three-dimensional (3D) radiative transfer models, where the infall velocity is allowed to vary within the infall region, may explain the observed kinematics. Conclusions. The data suggest that a disk has started to form in B335 and that gas is falling towards that disk. However, kinematically-resolved line data towards the disk itself is needed to confirm the presence of a rotationally supported disk around this young protostar. The high infall velocities we measured are not easily reconcilable with a magnetic braking scenario, suggesting that there is a pressure gradient that allows the infall velocity to vary in the region.

The capture of airborne particulates by rain

Rain cleans pollution out of the atmosphere, as droplets collide with airborne particles during free fall. Such particle–droplet collisions have been presumed to be capture events, but the details of such collisions lack thorough investigation. We show that rain droplets and pollution particles interact through multiple collision behaviours, including captures on the droplet surface, cavity-forming droplet entries and ricochets. Rain drop diameter and free fall velocity, in addition to pollution particle density, size, wettability and droplet impact location, determine which capture or escape behaviour occurs. Our findings reveal that rain does not capture all airborne pollutants equally even upon collision, and certain pollutants prove more difficult to remove from the air than others. Consequently, we must account for both rain and pollution characteristics to understand pollution capture mechanisms and pollution fluxes in the environment.

Global kinematics study of OH masers in W49N

Context. Star formation is underway in the W49N molecular cloud (MC) at a high level of efficiency, with almost twenty ultra-compact (UC) HII regions observed thus far, indicating a recent formation of massive stars. Previous works have suggested that this cloud is undergoing a global contraction. Aims. We analyse the data on OH masers in the molecular cloud W49N, observed with the VLBA at the 1612, 1665, and 1667 MHz transitions in left circular polarization (LCP) and right circular polarization (RCP) with an aim to study the global kinematics of the masers. Methods. We carried out our study based on the locations and observed velocities of the maser spots, Vobs. We found the location (α, δ)m of the maximum correlation between V = Vobs−Vsys (with Vsys the systemic velocity) and distance to it. The velocities were fitted to the straight line of Vobs−Vsys versus d(α,δ)m, resulting in Vftd. The difference between the fitted values and those obtained from observations is ∆ V = (Vobs−Vsys)-Vftd. The Vobs−Vsys velocity shows a gradient as a function of the distance to (α, δ)m, where the closer spots have the largest velocities. Spots with similar velocities are located in different sectors, with respect to (α, δ)m. Then, we assumed that the spots are moving towards a contraction centre (CCOH), which is at the apex of a CONUS. We also assumed that the distance of each spot to CCOH is dcc = √2 d(α,δ)m and that they fall with a velocity Vcc = √2Vftd, with the total velocity being VTot = Vcc + √2 Δ V. Using this velocity, we estimated the free-fall velocity. Results. The coordinates of (α, δ)m are effectively (α2000 = 19:10:13.1253, δ2000 = 9:6:13.570). The observed dispersion with respect to the global trend against dcc, shows a maximum at 0.12 pc, with a decay from 0.12 to 0.19 pc, which is faster than that taking place between 0.19 and 0.42 pc. Based on VTot, an inner mass of Minn = 2500 M⊙ was estimated. In addition, the estimated accretion rate is Ṁ = 1.4×10−3 M⊙yr−1, which requires a time of tinn = 1.8×106 yr to accumulate Minn. The free-fall time, assuming n = 1×10−4 cm−3, is tff = 3.4×105 yr. Performing the same procedure with published data that are of lower spatial resolution (than the VLBA data) produces similar results. For example, based on the available data, we find that (α, δ)m = (19:10:13.1392, 9:6:13.4387) J2000, which is at ≲ 0.3 asec from what has been calculated with the VLBA data, with an estimated inner mass of 2700 M⊙. A sub-collapse appears to be taking place in the region traced by the OH maser spots. Based on methanol maser cloudlets data, which lie in a smaller region, another possible centre of contraction is identified, which could be due to a sub-collapse towards a 75 M⊙ inner mass. Conclusions. The velocities of the OH spots at W49N, along with their positions with respect to (α, δ)m, make it possible to trace a global kinematics that is apparently due to a sub-collapse in the W49N MC.

Probing the Extent of Fe Kα Emission in Nearby Active Galactic Nuclei Using Multi-order Analysis of Chandra High Energy Transmission Grating Data

We present a study of the narrow Fe Kα line in seven bright, nearby active galactic nuclei (AGN) that have been observed extensively with the Chandra High Energy Transmission Grating (HETG). The HETG data reveal a wider Fe Kα line in the first-order spectrum than in the second- and third-order spectra, which we interpret as the result of spatially extended Fe Kα emission. We utilize these differences in narrow Fe Kα line widths in the multi-order Chandra HETG spectra to determine the spatial extent and intrinsic velocity width of the emitting material in each object. We find that there is modest evidence for spatially extended emission in each object, corresponding to extension of r ∼ 5–100 pc. These distances are significantly larger than those inferred from velocity widths assuming gravitational motions, which give r ∼ 0.01–1 pc. This implies either that the gas is emitting at a range of radii, with smaller radii dominating the velocity width and larger radii dominating the spatial extent, or that the gas is exhibiting nongravitational motions, which we suggest would be outflows due to slight excess redshift in the line and velocities that exceed the freefall velocity. We also use the spatial extent information to estimate the mass of the emitting gas by counting fluorescing iron atoms, finding masses on the order of M gas ∼ 105–108 M ⊙. Future work with observatories like XRISM will be able to extend this study to a larger number of AGN and decrease uncertainties that arise as a result of the low signal-to-noise ratio of the higher-order HETG data.

Planetary Accretion Shocks with a Realistic Equation of State

Abstract The final stage of gas giant formation involves accreting gas from the parent protoplanetary disk. In general, the infalling gas likely approaches a freefall velocity, creating an accretion shock, leading to strong shock heating and radiation. We investigate the kinematics and energetics of such accretion shocks using 1D radiation hydrodynamic simulations. Our simulations feature the first self-consistent treatment of hydrogen dissociation and ionization, radiation transport, and realistic gray opacity. By exploring a broad range of giant-planet masses (0.1–3M J) and accretion rates (10−3–10−2 M ⊕ yr−1), we focus on global shock efficiency and the final entropy of the accreted gas. We find that radiation from the accretion shock can fully disassociate the molecular hydrogen of the incoming gas when the shock luminosity is above a critical luminosity. Meanwhile, the post-shock entropy generally falls into “cold” (≲12k B/m H) and “hot” (≳16k B/m H) groups which depend on the extent of the endothermic process of H2 dissociation. While 2D or 3D simulations are needed for more realistic understandings of the accretion process, this distinction likely carries over and sheds light on the interpretation of young direct imaging planets.

Settling and clustering of particles of moderate mass density in turbulence

We present a numerical study of settling and clustering of small inertial particles in homogeneous and isotropic turbulence. Particles are denser than the fluid, but not in the limit of being much heavier than the displaced fluid. At fixed Reynolds and Stokes numbers we vary the fluid-to-particle mass ratio and the gravitational acceleration. The effect of varying one or the other is similar but not quite the same. We report non-monotonic behavior of the particles' velocity skewness and kurtosis with the second parameter, and an associated anomalous behavior of the settling velocity when compared to the free-fall Stokes velocity, including loitering cases. Clustering increases for increasing gravitational acceleration, and for decreasing fluid-to-particle mass ratio.

A SIMILARITY CRITERION FOR SPHERICAL FUEL ELEMENTS FREE FALL VELOCITY IN CYLINDRICAL CHANNELS WITH VISCOUSE LIQUID

The introduction of nuclear high-temperature gas-cooled reactors (HTGR) with an active zone based on spherical fuel elements (SFE) poses the task of determining the velocity of their free fall in cylindrical channels with a viscous liquid. To solve it, the experimental data of other researchers are generalized, and for a certain range of Reynolds numbers the criterion of similarity for the velocity of free fall of spheres in cylindrical channels with water is found. The criterion is formulated on the basis of the Freud number. It is shown that from the dependence of the velocity of falling of the model sphere in a cylindrical vessel with water on the dimensionless diameter of the sphere, it is possible to determine the velocity of falling of the sphere in water, arbitrary.

Multi-scale Dust Polarization and Spiral-like Stokes-I Residual in the Class I Protostellar System TMC-1A

Abstract We have observed the Class I protostar TMC-1A in the Taurus molecular cloud using the Submillimeter Array (SMA) and the Atacama Large Millimeter/submillimeter Array (ALMA) in the linearly polarized 1.3 mm continuum emission at angular resolutions of ∼3″ and ∼0.3″, respectively. The ALMA observations also include CO, 13CO, and C18O J = 2−1 spectral lines. The SMA observations trace magnetic fields on the 1000 au scale, the directions of which are neither parallel nor perpendicular to the outflow direction. Applying the Davis–Chandrasekhar–Fermi method to the SMA polarization angle dispersion, we estimate a field strength in the TMC-1A envelope of 1–5 mG. It is consistent with the field strength needed to reduce the radial infall velocity to the observed value, which is substantially less than the local freefall velocity. The ALMA polarization observations consist of two distinct components—a central component and a north/south component. The central component shows polarization directions in the disk minor axis to be azimuthal, suggesting dust self-scattering in the TMC-1A disk. The north/south component is located along the outflow axis and the polarization directions are aligned with the outflow direction. We discuss possible origins of this polarization structure, including grain alignment by a toroidal magnetic field and mechanical alignment by the gaseous outflow. In addition, we discover a spiral-like residual in the total intensity (Stokes I) for the first time. The C18O emission suggests that material in the spiral-like structure is infalling at a speed that is 20% of the local Keplerian speed.

Hydroelastic effects of slamming impact Loads During free-Fall water entry

ABSTRACT This paper examines the hydroelastic problems of a two-dimensional symmetric flexible wedge water entry through free-fall motion. Water entry is numerically investigated by coupled Finite Volume Method and Finite Element Method using a strong two-way coupling approach. The emphasis of this study is on numerical approach and the paper provides an accurate two-way FSI coupling method for the water entry of two-dimensional symmetric elastic wedge section in different conditions. The effect of freefall velocity is investigated by comparing the constant velocity and freefall impacts. It is shown that the bottom deflection is overestimated by using the constant velocity. In order to evaluate the accuracy of the numerical model, the numerical results are compared and validated against published experimental data and favourable agreement is reported. The vertical position, impact velocity, acceleration, pressure distribution, and deflection along the bottom plate of the elastic wedge are evaluated and compared to experimental data. For better understanding of the hydroelastic slamming, the results are presented for different deadrise angles and vertical velocities. The relation between the structural deflection and vertical velocity, deadrise angle, and pressure distribution is investigated. It is observed that the significance of hydroelasticity increases with decreasing deadrise angle and increasing impact velocity.

Gravitation near the Schwarzschild radius

The problem of gravitation near the Schwarzschild radius is addressed. The pressure due to free fall velocity is introduced. At the Schwarz-schild radius, this pressure produces the force balancing the gravity thus stopping the collapse of the matter. The minimum radius of the source of gravity is defined as a radius at which the proton reaches the Planck energy. The compact object as a thin shell at the minimum radius is considered. The proton is assumed to decay at the Planck scale into positron and hypothetical Planck neutrinos. Under accretion onto the compact object, half the protons decay, and the other protons retain at the minimum radius. Â

Free-fall velocities and heat transport enhancement in liquid metal magneto-convection

Abstract

Temperature effects on transient behaviour of a free-falling liquid gallium drop

The present investigation presents the results of an experimental study of a free-falling liquid gallium drop in a quiescent water. Experiments were performed with different drops size in the range of 2.67 mm to 5.56 mm. All experiments were carried out under isothermal conditions, where the liquid Gallium temperature equals the temperature of the quiescent water inside the column. The temperature of both phases was investigated in the range of 30 °C to 70 °C. The study focuses on the transient behaviour of the falling drops and their aspect ratio as well as the effect of size, viscosity ratio and temperature. The captured high speed images are studied to quantify the evolution of the aspect ratio of the falling Gallium drops in time. Transient oscillations of the aspect ratio are found to exhibit a decaying trend in time. Amplitude and frequency of these oscillation were analysed. The frequency of aspect ratio oscillations were found to decrease with Eötvös number following the fD-3/2 law according to Rayleigh theory The free fall velocity of the Gallium drop is found to be highly correlated to the aspect ratio oscillations. Furthermore, whilst the oscillations amplitude was found to be sensitive to viscosity ratio, the frequency of oscillations was not affected in any noticeable way. The trends reported are limited to the range of experimental conditions investigated in this study.

Explosion energies for core-collapse supernovae I: analytic, spherically symmetric solutions

ABSTRACT Recent multidimensional simulations of core-collapse supernovae are producing successful explosions and explosion-energy predictions. In general, the explosion-energy evolution is monotonic and relatively smooth, suggesting a possible analytic solution. We derive analytic solutions for the expansion of the gain region under the following assumptions: spherical symmetry, one-zone shell, and powered by neutrinos and α particle recombination. We consider two hypotheses: (I) explosion energy is powered by neutrinos and α recombination and (II) explosion energy is powered by neutrinos alone. Under these assumptions, we derive the fundamental dimensionless parameters and analytic scalings. For the neutrino-only hypothesis (II), the asymptotic explosion energy scales as $E_{\infty } \approx 1.5 M_\mathrm{ g}\nu _0^2 \eta ^{2/3}$, where Mg is the gain mass, $\nu _0$ is the free-fall velocity at the shock, and η is a ratio of the heating and dynamical time-scales. Including both neutrinos and recombination (hypothesis I), the asymptotic explosion energy is $E_{\infty } \approx M_g \nu _0^2 (1.5\eta ^{2/3} + \beta f(\rho _0))$, where β is the dimensionless recombination parameter. We use Bayesian inference to fit these analytic models to simulations. Both hypotheses fit the simulations of the lowest progenitor masses that tend to explode spherically. The fits do not prefer hypothesis I or II; however, prior investigations suggest that α recombination is important. As expected, neither hypothesis fits the higher mass simulations that exhibit aspherical explosions. In summary, this explosion energy theory is consistent with the spherical explosions of low progenitor masses; the inconsistency with higher progenitor-mass simulations suggests that a theory for them must include aspherical dynamics.

Status quo of Standard Penetration Test in India: A Review of Field Practices and Suggestions to Incorporate in IS 2131

SPT (Standard Penetration Test) being the most widely used field test has not been effectively regulated in India. Guidelines regarding hammer weight, the height of fall and use of liner are not followed correctly in the field, and guidelines for energy transfer efficiency and hammer blow rate are not addressed in the BIS (Bureau of Indian Standards) code for SPT. In this paper, some standard provisions on SPT in India and other countries are compared, and few observations on field SPT tests are presented. Operational and equipment-related variables, such as weights and dimensions of hammer, anvil, and drill rod, were noted in the field. Furthermore, hammer blow rate and inclination of guide rods during the field tests were also observed and are presented. It is found that IS 2131 (1981) does not provide standard dimensions of components of SPT set-up, and hence, there is a large variation in SPT set-up in practice and the whole set-up is usually an ensemble of locally manufactured components. This is a major defect in the SPT code and needs to be addressed. Sixteen different SPT set-ups were observed, which were employed on field SPT tests on 30 boreholes. Thirteen of the observed set-ups were used for the soil investigation for a large infrastructure project. Hammer blow rates are found to be very low as compared to those stated in IS 1893 (2016), DSO-98-17 (1999) and ASTM D6066 (2011). The inclination of guide rods in field was observed to be more than 5°, which causes friction between guide rod and drive weight and, hence, reduces the free fall velocity. These variables affect N values and international studies have reported correction factors to account for these. Because of the fact that large variety of set-up was observed, it will be prudent to standardise the dimensions of hammer and anvil, and hammer release mechanism to bring uniformity in SPT operations. Furthermore, normalising field N values to a standard energy ratio of 60% as per the international practice will facilitate reliability of N values obtained from different set-ups.