Simulated Low Gravity Research Articles

Stabilization of a capillary bridge far beyond the Rayleigh–Plateau limit using active feedback and acoustic radiation pressure

A liquid bridge between two solid surfaces is known as a capillary bridge. Long bridges naturally become unstable to a symmetric mode by bulging near one end while the opposite end thins. Suppression of this mode is important for managing fluids in low gravity. For a cylindrical bridge in low gravity of radius R and length L, the slenderness S=L/2R has a natural (Rayleigh–Plateau) limit of π beyond which the bridge breaks. Using acoustic radiation pressure to control the bridge’s shape, and an optical sensor to detect the bridge’s shape, an active feedback system in simulated low gravity was constructed. This system stabilized bridges significantly beyond the Rayleigh–Plateau limit, with S as large as 4.3 [Marr-Lyon et al., J. Fluid Mech., accepted for publication]. To be useful in low gravity, this technique will have to be modified to work on liquid bridges in air. In preparation for an experiment to be performed on NASA’s low gravity KC-135 aircraft, acoustic resonators in air that have the required property that the sound amplitude can be spatially redistributed rapidly are being investigated and tested in normal gravity using soap-film bridges filled with sulfur hexafluoride. [Work supported by NASA.]

4HE EXPERIMENTS NEAR TLAMBDA IN A LOW-GRAVITY SIMULATOR

We report on our latest measurements of gravity cancellation in the low-gravity simulator using a magnetostrictive force. We made these measurements using a new thermal conductivity cell design that is 0.5cm in diameter and 0.5cm in height. Gravity cancellation was verified by measuring both the reduction in the Tλvariation across the cell and the suppression of thermal convection as a function of the magnetic field. Full gravity cancellation was achieved in the simulator with B(dB/dz) ≈ 21 T2/cm, agreeing well with the calculated value and the valve found from levitating drops of helium.

The shape, stability and breakage of pendant liquid bridges

Pendant liquid bridges are defined as pendant drops supporting a solid axisymmetric endplate at their lower end. The stability and shape properties of such bridges are defined in terms of the capillary properties of the system and of the mass and radius of the lower free-floating endplate. The forces acting in the pendant liquid bridge are defined exactly and expressed in dimensionless form. Numerical analysis has been used to derive the properties of a given bridge and it is shown that as the bridge grows by adding more liquid to the system a maximum volume is reached. At this maximum volume, the pendant bridge becomes unstable with the length of the bridge increasing spontaneously and irreversibly at constant volume. Finally the bridge breaks with the formation of a satellite drop or an extended thread. The bifurcation and breakage processes have been recorded using a high-speed video camera with a digital recording rate of up to 6000 frames per second. The details of the shape of the bridge bifurcation and breakage for many pendant bridge systems have been recorded and it is shown that satellite drop formation after rupture is not always viscosity dependent. Bifurcation and breakage in simulated low gravity demonstrated that breakage was very nearly symmetrical about a plane through the middle of the pendant bridge.

Stable magnetic field gradient levitation of Xenopus laevis: toward low-gravity simulation

We have levitated, for the first time, living biological specimens, embryos of the frog Xenopus laevis, using a large inhomogeneous magnetic field. The magnetic field/field gradient product required for levitation was 1430 kG2/cm, consistent with the embryo's susceptibility being dominated by the diamagnetism of water and protein. We show that unlike any other earth-based technique, magnetic field gradient levitation of embryos reduces the body forces and gravity-induced stresses on them. We discuss the use of large inhomogeneous magnetic fields as a probe for gravitationally sensitive phenomena in biological specimens.

Improved active stabilization of capillary bridges using acoustic radiation pressure and dynamics of a breaking bridge

Liquid bridges between two solid surfaces have applications in low gravity, such as the solidification of floating zones. Long bridges naturally become unstable to a symmetric mode by bulging near one end while the opposite end thins. For a cylindrical bridge of radius R and length L, the slenderness S=L/2R has a natural (Rayleigh) limit of π beyond which the bridge breaks. Using acoustic radiation pressure in simulated low gravity as described previously [J. Acoust. Soc. Am. 99, 2540 (1996)], stable bridges significantly beyond the Rayleigh limit can be produced. Improvements in the acoustics, optics, and electronics have enabled stabilization of bridges with S as large as 4.3. To be useful in low gravity, this technique will have to be modified to work on liquid bridges in air. Currently, acoustic resonators in air that have the required property that the sound amplitude can be spatially redistributed rapidly are being investigated. The laboratory experiments with simulated low gravity have made it possible to observe the dynamics of a breaking super-long bridge. [Work supported by NASA.]

Active acoustic stabilization of capillary bridges significantly beyond the Rayleigh limit: Experimental confirmation.

Liquid bridges between two solid surfaces have applications in low gravity such as the solidification of floating zones. Long bridges naturally become unstable to a symmetric mode by bulging near one end while the opposite end thins. For a cylindrical bridge in low gravity of radius R and length L, the slenderness S=L/2R has a natural (Rayleigh) limit of π beyond which the bridge breaks. It has been demonstrated that acoustic radiation pressure may be used in simulated low gravity to produce stable bridges significantly beyond the Rayleigh limit with S as large as 3.6. The bridge (PDMS mixed with a dense liquid) has the same density as the surrounding water bath containing an ultrasonic standing wave. Modulation can be used to excite specific bridge modes [Morse et al., Phys. Fluids 8, 3–5 (1996)]. The shape of our bridge is optically sensed and the ultrasonic drive is electronically adjusted such that the radiation stress distribution dynamically quenches the most unstable mode. This active control simulates passive stabilization suggested for low gravity [J. Acoust. Soc. Am. 97, 3377(A) (1995)]. Feedback increases the mode frequency in the naturally stable region. [Work supported by NASA.]

Reducing gravity at the superfluid transition in helium-4

There are two intrinsic experimental limitations in measurements near Tλ for traditional ground based experiments: the gravitationally induced pressure variations present in any macroscopic helium sample limit how closely the transition can be approached, and the onset of convection in a sample with T>Tλ limits the range of heat values that can be used. To overcome these limitations, we have built a low gravity simulator consisting of a superconducting magnet with a magnetic field profile shaped to provide a magnetic force opposite that of gravity. Preliminary measurements have shown a decrease in the range of reduced temperatures across a sample but not the expected suppression of the onset of convection.

Dynamic analysis of space-based inflated beam structures

This paper examines the dynamic behaviors of inflated beam aerospace structures. The principal foci of this investigation are the determination of the damping mechanisms active in structures constructed from inflated cylindrical beams, development of a practical modeling method for complex structures, and an examination of the difficulties in predicting on-orbit dynamic behavior from ground tests. A Euler–Bernoulli model of the inflated beam is used to determine distributed damping coefficients from modal tests. The results show that the viscous damping in the inflated beam is independent of beam pressure, but that the pressurization stress levels in the beam fabric affect the strain-rate damping. The Euler–Bernoulli inflated beam model is used in conjunction with a conventional finite element package to model the dynamic behavior of a complex inflated beam structure, a 1.7-m-diam inflated dish antenna mockup. The model accurately predicts the lower natural frequencies of the dish structure. A comparison of modal tests of inflated beam structures performed both in the near weightlessness of the NASA KC-135 low gravity simulator aircraft and in a ground laboratory demonstrates that gravity level can have a dramatic effect on system damping.

Dynamic measurements near the lambda-point in a low-gravity simulator on the ground

The properties of liquid helium very near the lambda-transition in the presence of a heat current has received recent theoretical and experimental attention. In this regime, gravity induced pressure effects place severe constraints on the types of experiments that can be performed. A new experiment is described which largely overcomes these difficulties by magnetostrictively cancelling gravity influences in the helium sample with a suitable magnetic coil. Design limitations of the technique and a discussion of proposed experiments is presented.

Lymphocytes on sounding rockets.

The effect of gravity on human lymphocytes in-vitro has been extensively studied by our team in true microgravity in space /1-3/, in simulated lowgravity in the clinostat /4,5/ and in hypergravity in the centrifuge /5-7/. Lymphocytes are easily purified from peripheral blood and can be ictivated either chemically or by exposure to mitogens in culture. Thereby, cells start to proliferate, T—lymphocytes secrete lymphokines (IL-2, IL-3, IL—4 as well as interferon). The mechanism of the in-vitro activation is analogous to that occurring in—vivo when the immune system is challenged by antigens.

Mechanics of running under simulated low gravity

Using a linear mass-spring model of the body and leg (T. A. McMahon and G. C. Cheng. J. Biomech. 23: 65-78, 1990), we present experimental observations of human running under simulated low gravity and an analysis of these experiments. The purpose of the study was to investigate how the spring properties of the leg are adjusted to different levels of gravity. We hypothesized that leg spring stiffness would not change under simulated low-gravity conditions. To simulate low gravity, a nearly constant vertical force was applied to human subjects via a bicycle seat. The force was obtained by stretching long steel springs via a hand-operated winch. Subjects ran on a motorized treadmill that had been modified to include a force platform under the tread. Four subjects ran at one speed (3.0 m/s) under conditions of normal gravity and six simulated fractions of normal gravity from 0.2 to 0.7 G. For comparison, subjects also ran under normal gravity at five speeds from 2.0 to 6.0 m/s. Two basic principles emerged from all comparisons: both the stiffness of the leg, considered as a linear spring, and the vertical excursion of the center of mass during the flight phase did not change with forward speed or gravity. With these results as inputs, the mathematical model is able to account correctly for many of the changes in dynamic parameters that do take place, including the increasing vertical stiffness with speed at normal gravity and the decreasing peak force observed under conditions simulating low gravity.

Methods of low gravity simulation

The dependence on gravity forces of both static and dynamic fluid behaviour is explained, and dimensionless numbers are defined which express the relative influence of gravitational and various other forces. Methods of reducing gravitational effects are outlined, including neutral buoyancy, free fall and orbital flight and their relative merits, in studies of various types of fluid behaviour, are compared.

Optical Measurements And Tests Performed In A Low-Gravity Environment

Shadowgraph and schlieren techniques have been used on board the NASA KC-1 35 low-gravity simulation aircraft to measure gravity-related fluid flow occurring in various experimental configurations. These configurations included cooled and solidified ammonium chloride and water, electrodeposited cobalt in CoSO4 solution, and immiscible and crystal melt solutions. Hardware tests have also been performed for such critical optical system components as lasers and spatial filters to determine their performance and operation under low-gravity conditions. These test results will be used in the design of the various future Shuttle experiments which will utilize advanced optical mea-surement techniques. The design and configuration of the optical systems used to make these measurements and tests are presented, and the performance and operation of these systems and their components under low-gravity conditions are discussed. Some preliminary results are included.

Laser shadowgraph and schlieren studies of gravity-related flow during solidification

Shadowgraph and schlieren techniques were used to observe gravity-related flow arising during solidification. The purpose of these studies was to elucidate the results of previous low gravity solidification experiments. Shadowgraph and schlieren techniques were selected as most suitable for operation in the anticipated experimental environment. A laser shadowgraph/schlieren system was built and flown on KC-135 low gravity simulation flights. A solution of ammonium chloride and water was cooled during the experiment, causing solidification. Growth plumes in the solution were observed and photographed using shadowgraph and schlieren techniques. Results are presented and related to previous work.

Experimental and Theoretical Studies of Liquid Sloshing at Simulated Low Gravity

Analyses and experimental comparisons are given for liquid sloshing in a rigid cylindrical tank under conditions of moderately small axial accelerations; in particular, the theory is valid for Bond numbers larger than 10. The analytical results are put in the form of an equivalent mechanical model, and it is shown that the sloshing mass and the natural frequency of the first mode, for a liquid having a 0 deg contact angle at the tank walls, are smaller than for high-g conditions. The experimental data, obtained by using several small-diameter tanks and three different liquids, are compared to the predictions of the mechanical model; good correlation is found in most cases for the sloshing forces and natural frequency as a function of Bond number.

Scale model testing of land vehicles in a simulated low gravity field: D. Schuring.S.A.E. automot. engng. congress, January, 1966.

Scale model testing of land vehicles in a simulated low gravity field: D. Schuring.S.A.E. automot. engng. congress, January, 1966.