Study of fluid behaviour under gravity compensated by a magnetic field

Fluids, and especially cryogenic fluids like hydrogen and oxygen, are widely used in space technology for propulsion and cooling. The knowledge of fluid behaviour during the acceleration variation and under reduced gravity is necessary for an efficient management of fluids in space. Such a management also rises fundamental questions about thermo-hydrodynamics and phase change once buoyancy forces are cancelled. For security reasons, it is nearly impossible to use the classical microgravity means to experiment with such cryofluids. However, it is possible to counterbalance gravity by using the paramagnetic (O₂) or diamagnetic (H₂) properties of fluids. By applying a magnetic field gradient on these materials, a volume force is created that is able to impose to the fluid a varying effective gravity, including microgravity. We have set up a magnetic levitation facility for H₂ in which numerous experiments have been performed. A new facility for O₂ is under construction. It will enable fast change in the effective gravity by quenching down the magnetic field. The facilities and some particularly representative experimental results are presented.     

Magnetic Gravity Compensation

Magnetic gravity compensation in fluids is increasingly popular as a means to achieve low-gravity for physical and life sciences studies. We explain the basics of the magnetic gravity compensation and analyze its advantages and drawbacks. The main drawback is the spatial heterogeneity of the residual gravity field. We discuss its causes. Some new results concerning the heterogeneity estimation and measurement are presented. A review of the existing experimental installations and works involving the magnetic gravity compensation is given for both physical and life sciences.     

Using Superconducting Magnet to Reproduce Quick Variations of Gravity in Liquid Oxygen

A ground based facility (OLGA), providing magnetic compensation of gravity in oxygen, has been developed. A 2-T superconducting magnetic solenoid is used to create the required magnetic field. A novel electrical supply permits to quickly vary the magnetic field, leading to rapid variation of the acceleration forces applied to oxygen. These variations can be made from overcompensation of gravity (−0.5g) to zero gravity or from zero gravity to reduced gravity (0.4g) with a time constant of 340 ms. This time is typical of the cutoff or reignition of spacecraft engines. Preliminary results on the transient flows induced by these acceleration variations in a reservoir filled with liquid and gaseous oxygen are presented.     

A magnet system design for reduced gravity environment

Simulating a reduced gravity environment experienced in spaceships in a laboratory setting for studying different technical aspects (fluid transfer or propellant behavior, for example) is a primordial step prior to extraterrestrial explorations. We first present some results on boiling heat transfer in helium under reduced gravity using a commercial magnet and point out the limitations in volume and magnetic force homogeneity to perform reduced gravity experiments with such a non ad hoc magnet. Then, we present a new magnetic design to create a reduced gravity environment in large volume suitable for boiling test experiments in oxygen. Based on a modified design we present the magnetic configurations that allow compensating gravity for different elements such as hydrogen, water or helium but in smaller volumes. We detail the different aspects of winding techniques to achieve the requirements on magnetic force.     

Design of a Large Oxygen Magnetic Levitation Facility

The present magnetic levitation facility offers an alternative to space means for studying fluids and especially liquid oxygen that exhibits important safety requirements. The station has great use flexibility. Thus it enables us to study hydrodynamic instabilities (acceleration and deceleration phase) and also to focus on thermal exchanges (nuclear and film boiling) and transition phase of fluids under reduced gravity such as Lunar, Martian or micro-gravity. The volume of liquid oxygen levitated is about two litres with a resulting acceleration less than 0.1 m/s². Besides the aspect ratio of the working cell can be changed.     

The onset of steady Bénard-Marangoni convection in a two-layer system of conducting fluid in the presence of a uniform magnetic field

The onset of steady Bénard-Marangoni convection in two horizontal liquid layers of electrically conducting immiscible fluids subjected to a uniform vertical magnetic field and temperature gradient is analysed by means of a combination of analytical and numerical techniques. The free surface can be either deformable or nondeformable and the interface between the fluids is always assumed to be flat. The effect of the lower layer on the critical values of Rayleigh, Marangoni and wave numbers for the onset of steady convection is investigated. When the free surface is nondeformable, the critical parameters for the onset of pure Marangoni convection are increased, whereas for the onset of pure Bénard convection they are decreased compared to the single-layer model. The results for a single-layer and for two-layers are qualitatively similar for Bénard-Marangoni convection when the free surface is deformable. All disturbances can be stabilized with sufficiently strong magnetic field when the free surface is nondeformable. If the free surface is allowed to deform and gravity waves are excluded, then the layers are always unstable to disturbances with sufficiently small wave number with magnetic field. Inclusion of gravity waves has a stabilizing effect on certain disturbances of small wave number in the presence of weak or moderate magnetic field.     

Effect of magnetically simulated zero-gravity and enhanced gravity on the walk of the common fruitfly

Understanding the effects of gravity on biological organisms is vital to the success of future space missions. Previous studies in Earth orbit have shown that the common fruitfly (Drosophila melanogaster) walks more quickly and more frequently in microgravity, compared with its motion on Earth. However, flight preparation procedures and forces endured on launch made it difficult to implement on the Earth''s surface a control that exposed flies to the same sequence of major physical and environmental changes. To address the uncertainties concerning these behavioural anomalies, we have studied the walking paths of D. melanogaster in a pseudo-weightless environment (0g*) in our Earth-based laboratory. We used a strong magnetic field, produced by a superconducting solenoid, to induce a diamagnetic force on the flies that balanced the force of gravity. Simultaneously, two other groups of flies were exposed to a pseudo-hypergravity environment (2g*) and a normal gravity environment (1g*) within the spatially varying field. The flies had a larger mean speed in 0g* than in 1g*, and smaller in 2g*. The mean square distance travelled by the flies grew more rapidly with time in 0g* than in 1g*, and slower in 2g*. We observed no other clear effects of the magnetic field, up to 16.5 T, on the walks of the flies. We compare the effect of diamagnetically simulated weightlessness with that of weightlessness in an orbiting spacecraft, and identify the cause of the anomalous behaviour as the altered effective gravity.     

Magnetic-Field Modulation of Gravity: Martian,Lunar, and Time-Varying Gravity

Magnetic micro-gravity is used to simulate on Earth gravity conditions that occur on the moon, Mars or in interstellar space. The possibilities offered by this magnetic method are theoretically studied so as to develop ground-based devices enabling one to simulate various gravity conditions. The results of this theoretical study focus on perfect gravity compensation (micro-gravity), partial gravity compensation (lunar or Martian gravity) and also transient gravity compensation (acceleration or deceleration phases of spaceships).     

Magnetic Compensation of Gravity: Experiments with Oxygen

The CEA Grenoble, through the ESEME/SBT team, has developed a new ground based facility providing magnetic compensation of gravity in oxygen. A 2T superconducting magnetic coil has been used to create the magnetic field. The installation is described. Well adapted to the heat and mass transfer studies, for example of the various boiling regimes, it permits to enhance the understanding of these phenomena in reduced gravity and gives a convenient way to reproduce space conditions on the ground. The first experimental results are presented.     

磁性液体在均匀磁场中的瞬态双热线导热系数的测试

将封有聚α-烯烃合成油基磁性液体的两玻璃管放置于磁场中,为消除磁场力、重力所引起的磁性液体自然对流的影响,消除端部效应,研制了磁性液体在均匀磁场中瞬态双热线导热系数的实验测量系统,经与蒸馏水、乙醇标准样品的导热系数测量比较,实验装置有较高的测量精度。实验测量了不同方向的均匀磁场对不同体积浓度的磁性液体导热系数的影响。结果显示,当磁场方向与热通量方向一致时,磁场显著强化磁性液体的导热系数,其导热系数随磁场强度的增加而近似线性增加,且体积浓度越大增加量越大;当磁场方向与热通量方向垂直时,磁性液体的导热系数随磁场强度的变化不明显。     

含间隙铰的机械多体系统动力学模型

对四类间隙铰模型(连续接触模型、有限元模型、经典碰撞模型和接触变形模型)进行分析比较，给出了经典碰撞和接触变形模式的含间隙多体系统动力学模型。经典碰撞模式的动力学模型着重考察系统动力学的整体行为，不能预测碰撞过程中铰关节局部接触力的变化历程。接触变形模式的多体系统动力学模型考虑了碰撞体间的法向和切向接触力，以及阻力矩，能够求解系统运动过程中间隙铰轴销与孔体的碰撞力。采用间隙铰的非线性弹簧阻尼模型，研究了重力场及其方向性对空间可展机构动力学性能的影响。计算结果表明，无重力与有重力环境时含间隙可展机构动力学行为有较大地差别，重力作用使间隙铰内碰撞减弱。为了保证在地面试验中可靠地预测在太空展开性能，可展机构在地面试验时应采取重力补偿措施。     

Gravity Force in Capillary Flow Through Open Channels

Certain conditions of superficial properties can provoke spontaneous movement of liquids at corners such as liquid filaments. The understanding of this type of fluid dynamics phenomena is very important in many areas, for example, oil recovery or design of gathering systems of condensed fluids with capillary forces. In previous studies, several models were developed for formation and advancing of filaments at horizontal corners neglecting the gravity force. The objective of this investigation was to provide a mathematical tool to estimate the influence of gravity and to establish a clear approach for taking this effect into account or not. The proposed model is developed from a differential equation applied to an open corner for Poiseuille flow. It provides good agreement with experimental values with a maximum deviation of 3.3%.     

Critical flow of high-speed ions in a high-current vacuum-arc discharge

A physical model of vacuum arc is developed, which is used to construct a quasi-one-dimensional model and a two-dimensional two-fluid mathematical model. The two-dimensional magnetohydrodynamic model is based on the method of trajectories, according to which a set of partial equations is reduced to a set of ordinary differential equations written for derivatives along the lines of current. The quasi-one-dimensional model is based on the principle of compensation of radial magnetic forces, which is valid for a short high-current vacuum arc in axial magnetic field. A criterion is obtained, which defines the range of validity of the quasi-one-dimensional model in external axial magnetic field B ₀. Calculations are performed of the dependence of critical current on B ₀ for discharge gaps of different geometries. The calculation results agree with experiment.     

Magnetooptics of single and microresonator iron-garnet films at low temperatures

We have investigated the low-temperature behavior of the optical and magneto-optical properties of (Bi, Gd, Al)-substituted yttrium iron-garnet films that are either single or microresonator, i.e. sandwiched between two dielectric Bragg mirrors. It was shown that the magneto-optical properties of the microresonators with a magnetic film core are mainly determined by the properties of the constituent magnetic films. Special attention was paid to the compositions possessing magnetic compensation temperatures. The phenomenon of the temperature hysteresis was found and discussed for several samples. This testifies the fact that the magnetic moment reorientation in a magnetic field occurs by the full cycle of the first-order phase transitions “collinear phase – non-collinear phase – collinear phase”. The Faraday hysteresis curves at around magnetic compensation temperatures are demonstrated to be very informative concerning composition of a sample. In particular, the hysteresis curves measured for the magnetic films on the garnet substrates showed bursts that indicates formation of a transition layer.     

Visualization in cryogenic environment: Application to two-phase studies

This paper reviews recent technical developments devoted to the study of cryogenic two-phase fluids. These techniques span from simple flow visualization to quantitative measurements of light scattering. It is shown that simple flow pattern configurations are obtained using classical optical tools (CCD cameras, endoscopes), even in most severe environments (high vacuum, high magnetic field). Quantitative measurements include laser velocimetry, particle sizing, and light scattering analysis. In the case of magnetically compensated gravity boiling oxygen, optical access is used to control the poistioning of a bubble subject to buoyancy forces in an experimental cell. Flow visualization on a two-phase superfluid helium pipe-flow, performed as a support of LHC cooldown studies, leads to flow pattern characterization. Visualization includes stratified and atomized flows. Thanks to the low refractive index contrast between the liquid and its vapor, quantitative results on droplet densities can be obtained even in a multiple scattering regime.     

Modifications of gravity

General relativity (GR) is a phenomenologically successful theory that rests on firm foundations, but has not been tested on cosmological scales. The deep mystery of dark energy (and possibly even the requirement of cold dark matter (CDM)) has increased the need for testing modifications to GR, as the inference of such otherwise undetected fluids depends crucially on the theory of gravity. Here, I discuss a general scheme for constructing consistent and covariant modifications to the Einstein equations. This framework is such that there is a clear connection between the modification and the underlying field content that produces it. I argue that this is mandatory for distinguishing modifications of gravity from conventional fluids. I give a non-trivial example, a simple metric-based modification of the fluctuation equations for which the background is exact ΛCDM, but differs from it in the perturbations. I show how this can be generalized and solved in terms of two arbitrary functions. Finally, I discuss future prospects and directions of research.     

Mechanomics and Physicomics in Gravisensing

Sensing gravity by ‘non-specialized’ cells is still puzzling. We don’t know where or by which mechanism such cells sense gravity. These questions in ‘gravisensing’ are not much different from questions in general mechanobiology. Numerous studies have been reported in this field in the last couple of decades. What are the mechanical properties of a cell? Are there differences in mechanical properties between cell types and if so why? How are forces perceived and transduced to a meaningful biological event. Novel techniques such as optical and magnetic tweezers, atomic force microscopy, magnetophoresis and computer modeling make the field of mechano-sensing or perhaps physicomics accessible. A similar approach should also be applied for gravity-related research. This paper addresses the current techniques used in mechanosensing and exemplifies how a cell could sense the relatively weak force of gravity.     

In search of the pitching momentum that enables some lizards to sustain bipedal running at constant speeds

The forelimbs of lizards are often lifted from the ground when they start sprinting. Previous research pointed out that this is a consequence of the propulsive forces from the hindlimbs. However, despite forward acceleration being hypothesized as necessary to lift the head, trunk and forelimbs, some species of agamids, teiids and basilisks sustain running in a bipedal posture at a constant speed for a relatively long time. Biomechanical modelling of steady bipedal running in the agamid Ctenophorus cristatus now shows that a combination of three mechanisms must be present to generate the angular impulse needed to cancel or oppose the effect of gravity. First, the trunk must be lifted significantly to displace the centre of mass more towards the hip joint. Second, the nose-up pitching moment resulting from aerodynamic forces exerted at the lizard''s surface must be taken into account. Third, the vertical ground-reaction forces at the hindlimb must show a certain degree of temporal asymmetry with higher forces closer to the instant of initial foot contact. Such asymmetrical vertical ground-reaction force profiles, which differ from the classical spring-mass model of bipedal running, seem inherent to the windmilling, splayed-legged running style of lizards.     

Experimental study on the fluidization discharging characteristics of Geldart-C kaolin powders in a blow tank with pulsed gas

Kaolin powders have been suggested to be able to adsorb heavy metal vapor from coal-fired flue gas. However, due to the influence of inter particle forces, such as liquid bridge force, it is difficult to realize stable pneumatic conveying. In the present work, the fluidization characteristics of kaolin powders were investigated. A series of unstable flow phenomena such as agglomeration, channeling, and slugging occurred during the fluidization process. Also, the fluidization discharging characteristics of kaolin powder in an optimized blow tank were experimentally studied. The results indicated that the introduction of pulsed gas can effectively destroy agglomeration and thus improving the stability of discharging. Visual experiments in pseudo-2D fluidized bed were also confirmed the destructive effect of pulsed gas on agglomeration. With an increase in either fluidization gas velocity U_f or pulsed gas velocity v_pulsed, the mass flow rate of kaolin powder G first increased and then decreased. Finally, drying experiments demonstrated that there is free water on the surfaces of the kaolin powders. The analysis of forces indicated that the liquid bridge force F_lb between particles is much larger than the particle gravity F_g. The liquid bridge force might be one of key reasons for kaolin powder agglomerating.     

Magneto-optical effects in temperature-sensitive ferrofluids

A temperature-dependent magneto-optical study of three different temperature-sensitive fluids was carried out. The changes in transmitted intensity as a function of applied magnetic field and temperature are recorded. The study provides an alternative technique for determining the Curie temperature of such fluids, and it is also feasible for the monitoring of temperature changes of such fluids optically within a limited time span.