State Of Free Fall Research Articles

Impact dynamics of a free-falling reference test mass in space

Outer space may constitute a privileged environment for experiments involving impacts. In fact, disturbing phenomena like drag or weight forces are negligible and a nearly pure free-fall state may be achieved for the impacting bodies. The case analysed in this manuscript comes from the initialization of the science phase of a space mission (LISA Pathfinder, flown in 2015), in which an extended metallic body – starting from a caged configuration – was released to free-fall inside a spacecraft by a dedicated mechanism, the grabbing positioning and release mechanism (GPRM). During the release phase, the test mass (TM) experienced some impacts with the GPRM end effector, before being electrostatically controlled to its nominal position, i.e. still in the centre of the hosting electrode housing. The impact dynamics of the TM therefore resulted critical, since it helped reduce its kinetic energy with respect to the spacecraft. In this paper, the TM position and attitude telemetry signals are analysed, and the impacts coefficient of restitution is calculated, providing reference values for similar space instruments, where a free-falling reference body is used (for instance, for geodesy and/or spacecraft navigation).

Drag-free control methods for space-basedgravitational-wave detection

In order to meet the requirements of “high precision” for space-based gravitational-wave detection, the design of control systems for a drag-free satellite is researched from the point of view of dynamics and control theory. Based on the LISA Pathfinder mission, the drag-free satellite contains two test masses, and its control system includes an attitude control loop, a drag-free control loop, and an electrostatic-suspension control loop. Drag-free control technology is employed for the drag-free control loop, where a test mass is in a state of free fall, and the satellite platform is controlled to follow the test mass “strongly” in order to achieve a high-level microgravity of the satellite platform and the test mass. Active vibration-isolation control technology is applied in the electrostatic-suspension control loop, where the other test mass is controlled to follow the satellite platform “weakly” and is isolated from vibrations of the satellite platform, so as to achieve high-level microgravity of the test mass and avoid collisions between the test mass and the satellite platform. The frequency-domain characteristics of the two control technologies are analyzed, assuming them to be based on the PID control algorithm. A set of formulas for calculating the parameters for the PID controllers are established, so that the parameters can be determined quickly according to the performance requirements for these controllers. On this basis, a control strategy is proposed and the PID controllers are designed. The effectiveness of the control strategy and the accuracy of the controller parameters are verified through numerical simulations.

Prandtl-number Effects in High-Rayleigh-number Spherical Convection

Abstract Convection is the predominant mechanism by which energy and angular momentum are transported in the outer portion of the Sun. The resulting overturning motions are also the primary energy source for the solar magnetic field. An accurate solar dynamo model therefore requires a complete description of the convective motions, but these motions remain poorly understood. Studying stellar convection numerically remains challenging; it occurs within a parameter regime that is extreme by computational standards. The fluid properties of the convection zone are characterized in part by the Prandtl number = ν/κ, where ν is the kinematic viscosity and κ is the thermal diffusion; in stars, is extremely low, ≈ 10−7. The influence of on the convective motions at the heart of the dynamo is not well understood since most numerical studies are limited to using ≈ 1. We systematically vary and the degree of thermal forcing, characterized through a Rayleigh number, to explore its influence on the convective dynamics. For sufficiently large thermal driving, the simulations reach a so-called convective free-fall state where diffusion no longer plays an important role in the interior dynamics. Simulations with a lower generate faster convective flows and broader ranges of scales for equivalent levels of thermal forcing. Characteristics of the spectral distribution of the velocity remain largely insensitive to changes in . Importantly, we find that plays a key role in determining when the free-fall regime is reached by controlling the thickness of the thermal boundary layer.

Effect of Rotation on Scaffold Motion and Cell Growth in Rotating Bioreactors.

Efficient use of different bioreactor designs to improve cell growth in three-dimensional scaffolds requires an understanding of their mechanism of action. To address this for rotating wall vessel bioreactors, fluid and scaffold motion were investigated experimentally at different rotation speeds and vessel fill volumes. Low cost bioreactors with single and dual axis rotation were developed to investigate the effect of these systems on human osteoblast proliferation in free floating and constrained collagen-glycosaminoglycan porous scaffolds. A range of scaffold motions (free fall, periodic oscillation, and orbital motion) were observed at the rotation speeds and vessel fluid/air ratios used, with 85% fluid fill and an outer vessel wall velocity of ∼14 mm s−1 producing a scaffold in a free fall state. The cell proliferation results showed that after 14 and 21 days of culture, this combination of fluid fill and speed of rotation produced significantly greater cell numbers in the scaffolds than when lower or higher rotation speeds (p < 0.002) or when the chamber was 60% or 100% full (p < 0.01). The fluid flow and scaffold motion experiments show that biaxial rotation would not improve the mass transfer of medium into the scaffold as the second axis of rotation can only transition the scaffold toward oscillatory or orbital motion and, hence, reduce mass transport to the scaffold. The cell culture results confirmed that there was no benefit to the second axis of rotation with no significant difference in cell proliferation either when the scaffolds were free floating or constrained (p > 0.05).

Quantum Gravity as Higher Dimensional Perspective

Although highly predictive in their respective macroscopic and microscopic domains of applicability, General Relativity and quantum mechanics are mathematically incompatible, perhaps most markedly in assumptions in their formalisms concerning the nature of space and time. In &lt;em&gt;perspective&lt;/em&gt; we already have a conceptual structure that links the local, macroscopic frame and the remote, apparently microscopic frame. A mathematical principle is invoked as a natural limit on D(n), so that effects which are clearly perspectival at D=3 become ‘more real’ (&lt;em&gt;effectively&lt;/em&gt; observer-independent) with each D(n) increment. For instance, the apparently microscopic becomes the effectively microscopic and &lt;em&gt;scale extremes are juxtaposed&lt;/em&gt;, so that black holes are local, macroscopic vanishing-points, in a similar way to that in which in projective geometry the point at infinity is incorporated into the foreground. (In other words, &lt;em&gt;a black hole is a blown-up ‘Planck-scale’ singularity&lt;/em&gt;.) Characteristics of the earthbound frame are applied to D&amp;gt;3, suggesting a physical basis for entanglement, and perspectival interpretations of quantum gravity, dimensional reduction and the information paradox. We claim that the familiar processes whereby multiple physical states become describable by a single state in which composition information appears to be lost (e.g., ‘falling into a black hole’, the state of quantum linearity, and the state of freefall) are all examples of effective convergence of a space or &lt;em&gt;n&lt;/em&gt;-surface to a single point of perspective.

Modelling supermassive black hole growth: towards an improved sub-grid prescription

Accretion onto supermassive black holes (SMBHs) in galaxy formation simulations is frequently modelled by the Bondi-Hoyle formalism. Here we examine the validity of this approach analytically and numerically. We argue that the character of the flow where one evaluates the gas properties is unlikely to satisfy the simple Bondi-Hoyle model. Only in the specific case of hot virialised gas with zero angular momentum and negligible radiative cooling is the Bondi-Hoyle solution relevant. In the opposite extreme, where the gas is in a state of free-fall at the evaluation radius due to efficient cooling and the dominant gravity of the surrounding halo, the Bondi-Hoyle formalism can be erroneous by orders of magnitude in either direction. This may impose artificial trends with halo mass in cosmological simulations by being wrong by different factors for different halo masses. We propose an expression for the sub-grid accretion rate which interpolates between the free-fall regime and the Bondi-Hoyle regime, therefore taking account of the contribution of the halo to the gas dynamics.

Experimental study of the shock–bubble interaction with reshock

The interaction of a planar shock wave with a spherical density inhomogeneity is studied experimentally under reshock conditions. Reshock occurs when the incident shock wave, which has already accelerated the spherical bubble, reflects off the tube end wall and reaccelerates the inhomogeneity for a second time. These experiments are performed at the Wisconsin Shock Tube Laboratory, in a 9m-long vertical shock tube with a large square cross section (25.4×25.4 cm2). The bubble is prepared on a pneumatically retracted injector and released into a state of free fall. Planar diagnostic methods are used to study the bubble morphology after reshock. Data are presented for experiments involving two Atwood numbers (A = 0.17 and 0.68) and three Mach numbers (1.35 < M < 2.33). For the low Atwood number case, a secondary vortex ring appears immediately after reshock which is not observed for the larger Atwood number. The post-reshock vortex velocity is shown to be proportional to the incident Mach number, M, the initial Atwood number, A, and the incident shock wave speed, W i.

Reacting shock bubble interaction

In this work, experimental results are presented for the first time on the shock-initiated combustion of a spherical density inhomogeneity. A converging lens geometry, in which a heavy bubble free-falls in a lighter fluid, is used to focus a planar shock wave resulting in the ignition of a mixture of H 2, O 2 and Xe (55%, 30% and 15%, respectively) in N 2. The experiments are performed at the Wisconsin Shock Tube Laboratory in a 9 m vertical shock tube with a 25.4 cm square cross-section. A pneumatic injector is used to generate a 4 cm diameter soap film bubble filled with the combustible mixture. The injector retracts flush into the side of the tube releasing the bubble into a state of free-fall. The initial conditions are captured with two high speed CMOS cameras orientated perpendicular to one another. Simultaneous Mie scattering and chemiluminescence diagnostics are used to image the post-shock bubble morphology and combustion, respectively. The ignition delay times are measured for several incident shock strengths (1.3 < M < 2.9, where M is the Mach number) and range between 5 μs and 400 μs. The ignition delay times are used in combination with 1D gas dynamics to estimate the effect of shock-focusing. It is observed that two limiting cases emerge, with the combustion characteristics depending heavily on the incident shock strength. A comparison to non-combusting mixtures is made by matching the density of the bubble mixture without the fuel (49% Xe, 51% O 2).

Motion of a tiny tool thrown by an astronaut towards another astronaut inside a spinning space vehicle in a state of free fall revisited

A space vehicle rotating with a uniform angular velocity about a vertical axis fixed to it is falling freely vertically downwards, say, with its engine shut off. It carries two astronauts inside it. One astronaut throws a tiny tool towards the other astronaut. The motion of the tiny tool with reference to a rotating frame rigidly fixed to the space vehicle, i.e., relative to the astronauts, is considered. Second-order differential equations of its motion instead of fourth order [1] are obtained and are completely solved with exposure of a subtle method of solution, from pedagogic viewpoint utilizing the relevant initial conditions of its projection. Under certain conditions of projection of the tiny tool, it is established that it begins to float in the space inside the vehicle after describing an analytical maximum distance. The position and velocity of the tiny tool at any instant of time with reference to an inertial frame defined by the non-rotating frame initially coincident with the rotating frame at the time of projection of the tiny tool, are also found out. Finally two numerical examples are given.

Motion of a tiny tool thrown by an astronaut inside a spinning space vehicle in a state of free fall

One of two astronauts, while conducting an experiment inside a spinning space vehicle in a state of free fall neglecting all gravitational effects, throws a tiny tool at the other with a velocity perpendicular to its axis of rotation. Considering the motion of the tool with respect to the rotating frame rigidly fixed to the vehicle, we can obtain the position of the tool at a subsequent time by use of rotational kinematics and geometry without involving the concepts of coriolis and centripetal forces.

Axial rotation, orbital revolution and solar spin-orbit coupling

The orbital motion of the Sun has been linked with solar variability, but the underlying physics remains unknown. A coupling of the solar axial rotation and the barycentric orbital revolution might account for the relationships found. Some recent published studies addressing the physics of this problem have made use of equations from rotational physics in order to model particle motions. However, our standard equations for rotational velocity do not accurately describe particle motions due to orbital revolution. The Sun’s orbital motion is a state of free fall; in consequence, aside from very small tidal motions, the associated particle velocities do not vary as a function of position on or within the body of the Sun. In this note, I describe and illustrate the fundamental difference between particle motions in rotation and revolution, in order to dispel some part of the confusion that has arisen in the past and that which may yet arise in the future. This discussion highlights the principal physical difficulty that must be addressed and overcome by future dynamical spin‐orbit coupling hypotheses.

Shocked Self-similar Collapses and Flows in Star Formation Processes

We propose self-similar shocked flow models for certain dynamical evolution phases of young stellar objects, champagne of H II regions surrounding OB stars, and shaping processes of planetary nebulae. We analyze an isothermal fluid of spherical symmetry and construct families of self-similar shocked flow solutions that feature (1) either a core expansion with a finite central density or a core accretion at a constant rate with a density scaling proportional to r-3/2 (a free-fall state); (2) a shock moving outward at a constant speed (a wind, a breeze, or an accretion flow); and (3) a preshock gas approaching a constant speed at large r with a density scaling proportional to r-2. In addition to testing numerical codes, our models can accommodate diverse shocked flows with or without a core collapse or outflow and with or without an envelope expansion or contraction. As an application, we introduce our model analysis to observations of Bok globule B335.

Envelope expansion with core collapse - I. Spherical isothermal similarity solutions

We investigate self-similar dynamical processes in an isothermal self-gravitational fluid with spherical symmetry. With reference to the earlier complementary solution results of Larson, Penston, Shu, Hunter and Whitworth & Summers, we further explore the ‘semi-complete solution space’ from an initial instant t→ 0+ to a final stage t→+∞. These similarity solutions can describe and accommodate physical processes of radial inflow, core collapse, oscillations and envelope expansion (namely, outflow or wind) or contraction as well as shocks. In particular, we present new classes of self-similar solutions, referred to as ‘envelope expansion with core collapse’ (EECC) solutions, that feature concurrent interior core collapse and exterior envelope expansion. The interior collapse towards the central core approaches a free-fall state as the radius r→ 0, while the exterior envelope expansion gradually approaches a constant radial flow speed as r→+∞. There exists at least one spherical stagnation surface of zero flow speed that separates the core collapse and the envelope outflow and that travels outward at constant speed, either subsonically or supersonically, in a self-similar manner. Without crossing the sonic critical line the travel speed of non-linear disturbances relative to the radial flow is equal to the sound speed, there exists a continuous band of infinitely many EECC solutions with only one supersonic stagnation point as well as a continuous band of infinitely many similarity solutions for ‘envelope contraction with core collapse’ (ECCC) without stagnation point. Crossing the sonic critical line twice analytically, there are infinitely many discrete EECC solutions with one or more subsonic stagnation points. Such discrete EECC similarity solutions generally allow radial oscillations in the subsonic region between the central core collapse and the outer envelope expansion. In addition, we obtained complementary discrete ECCC similarity solutions that cross the sonic critical line twice with subsonic oscillations. In all these discrete solutions, subsonic spherical stagnation surfaces resulting from similarity oscillations travel outward at constant yet different speeds in a self-similar manner. With specified initial boundary or shock conditions, it is possible to construct an infinite number of such EECC similarity solutions, which are conceptually applicable to various astrophysical problems involving gravitational collapses and outflows. We mention potential applications of EECC similarity solutions to the formation process of protoplanetary nebulae connecting the asymptotic giant branch phase and the planetary nebula phase to H ii clouds surrounding star formation regions, and to a certain evolution phase of galaxy clusters.

高圧気体及び低圧気体中に於ける氷晶の成長

The habit and the growth feature of small ice crystals formed in a free fall state in high- and low-pressure helium and argon gases at the temperatures of -7 and -15°C were studied experimentally. It is found that the habit and the growth feature of small ice crystals in the range of about 20 to 50μm depend not only on the temperature and the degree of supersaturation but also on the kind and the pressure of the carrier gas and the size of crystals when small ice crystals were formed at water saturation. It is found that the dependence of the habit of small ice crystals on temperature is more emphasized under a high pressure but has a tendency to vanish under a low pressure. In high-pressure gas, ice crystal shapes which have never been observed in the earth's atmospheres are found. Such ice crystals may form in the atmospheres of Jupiter and Saturn where pressures are higher than that of the earth.

水の自然凍結温度附近で急凍結する微水滴の荷電について

The electric charge on freezing water droplets with sizes of 20 to 100μ was measured in a state of free fall at temperatures of -25°C to -40°C. As a result of the measurements, it was noted that the number of negatively charged droplets was greater than that of positively charged ones. It was also found that the mean value of the negative charge was much greater than that of the positive charge in a temperature region colder than -35°C, especially at water homogeneous nucleation temperature; -40°C.This result suggests that the freezing water droplets obtained a negative charge by some mecha- nism, when they were frozen very rapidly. However it appears that the electrification mechanism of the freezing water droplets around -40°C was different from that presented by Mason and Maybank, or Stott and Hutchinson.

Dynamics of Self-Gravitating Gaseous Spheres--II: Collapses of Gas Spheres with Cooling and the Behaviour of Polytropic Gas Spheres

Numerical integrations of the full non-linear hydrodynamic equations are given for a number of different self-gravitating flows with spherical symmetry. The collapse of a sphere of gas to form a galaxy is studied under the effects of cooling by bremsstrahlung and recombination radiation and heating by cosmic rays and the results show that a state of free fall soon develops. The collapse of a Bok globule to form a star is also computed under the effects of cosmic ray heating and ionic cooling. In this case it is found that the flow is essentially similar to that obtained assuming isothermality. The behaviour of adiabatic spheres with different values of γ are explored and here the results show that over-reliance on energy arguments leads to a false picture of the behaviour of |$\gamma = 4/3$| spheres. It is also suggested that the hydrodynamics of the centre of a variable star may be of more importance than hitherto supposed. Finally and of most general significance, self-gravitating flows are classified according to various dimensionless ratios governing their behaviour. The effects of these ratios are discussed and a ‘ dimensionless cooling time ’ and a ‘Jeans number’ are defined. The effect on thermal instability of self-gravitation is also discussed and a criterion for instability is given.

Galaxy Formation: the Fragmentation Problem.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Galaxy Formation: the Fragmentation Problem. Michie, Richard W. Abstract This study of fragmentation uses the moment technique reported previously (Astron. J. 70, 327, 1965), and includes the following physics: (a) a nonlinear, nonhomologous fluid flow, (b) the full radiation of energy modified by the self-absorption of the Lyman lines and continua, (c) the (2s- 1s) two photon emission, (d) the conduction of heat into or out of the fragment, and (e) the tidal force exerted on the nonspherical fragment by the surrounding gas in the cloud. In one example, a cloud forms from a small perturbation in an expanding universe and initially shares the cosmic motion. The gas temperature at the start is l0~ 0K. Before the outer parts of the cloud cool very much and begin to collapse it is shown that a perturbation can grow, even if it initially shares the motion of the surrounding (cloud) gas. This example can lead to a cluster of galaxies. A second example illustrates the fragmentation of gas clouds when a cloud is in a state of free fall and is developing a large central concentration. In both examples, as the fragment grows, it loses pressure support and starts to move inward toward the center faster than the surrounding (cloud) gas. The role of the opacity and the tidal force is to delay the growth of fragments and to help determine the mass spectrum. The nonhomologous collapse is of first importance for the tidal force affects the outer parts of the fragment, and when the fragment develops some central concentration the tidal force becomes much less important. The opacity always delays the collapse by maintaining the internal pressure forces for a long time, and the fragmentation studies made so far indicate the thermal instability is of little or no importance due to the lengthened cooling time scales. Publication: The Astronomical Journal Pub Date: 1965 DOI: 10.1086/109630 Bibcode: 1965AJ.....70R.685M full text sources ADS |