Impairments of manual tracking performance during spaceflight: more converging evidence from a 20-day space mission

Studies of human performance during spaceflight have consistently revealed degradations of manual tracking performance in space. The present investigation analysed these performance decrements in more detail by applying frequency response analyses of tracking performance. It was hypothesized that tracking impairments result from two factors: at an early adaptation phase in space they primarily reflect effects of microgravity on human visuo-motor processes, whereas later into the mission they are also caused by impairments of attentional processes induced by cumulative workload and fatigue. In order to investigate this hypothesis, performance of one cosmonaut in a first-order unstable tracking task was repeatedly assessed before, during and after a 20-day space mission. Singlecase statistical analyses revealed the following effects: tracking performance declined at the first assessment in space and in two later inflight sessions compared to pre-flight baseline. Whereas the early tracking decrement was mainly due to an increase of the effective time-delay during tracking and accompanied by only minor changes of mood or workload, one of the later inflight impairments was due to an increase of effective time-delay, a decreased tracking gain, and an increase of tracking remnant, and both were associated with considerably higher workload ratings. This pattern of effects supports the two-factor hypothesis.     

Mechanism of Gravity Effect on Solidification Microstructure of Eutectic Alloy

The influence of gravity on the solidification microstructure of Al-Al3Ni eutectic alloy was investigated by using a 50-m-long drop tube. It was found that at different growth rates the average inter-rod spacing was always larger under rnicrogravity （μg） than those under normal gravity （lg）. Moreover, with increasing growth rate, the spacing difference between lg samples and μg samples reduced progressively. Based on the experimental results and analysis, a physical model was proposed to describe the effect of gravity on the solidification process of eutectic alloy.     

Flame spread over thin fuels in actual and simulated microgravity conditions

Most previous research on flame spread over solid surfaces has involved flames in open areas. In this study, the flame spreads in a narrow gap, as occurs in fires behind walls or inside electronic equipment. This geometry leads to interesting flame behaviors not typically seen in open flame spread, and also reproduces some of the conditions experienced by microgravity flames.Two sets of experiments are described, one involving flame spread in a Narrow Channel Apparatus (NCA) in normal gravity, and the others taking place in actual microgravity. Three primary variables are considered: flow velocity, oxygen concentration, and gap size (or effect of heat loss). When the oxidizer flow is reduced at either gravity level, the initially uniform flame front becomes corrugated and breaks into separate flamelets. This breakup behavior allows the flame to keep propagating below standard extinction limits by increasing the oxidizer transport to the flame, but has not been observed in other microgravity experiments due to the narrow samples employed. Breakup cannot be studied in typical (i.e., “open”) normal gravity test facilities due to buoyancy-induced opposed flow velocities that are larger than the forced velocities in the flamelet regime.Flammability maps are constructed that delineate the uniform regime, the flamelet regime, and extinction limits for thin cellulose samples. Good agreement is found between flame and flamelet spread rate and flamelet size between the two facilities. Supporting calculations using FLUENT suggest that for small gaps buoyancy is suppressed and exerts a negligible influence on the flow pattern for inlet velocities ?5 cm/s. The experiments show that in normal gravity the flamelets are a fire hazard since they can persist in small gaps where they are hard to detect. The results also indicate that the NCA quantitatively captures the essential features of the microgravity tests for thin fuels in opposed flow.     

Flame-spread probability and local interactive effects in randomly arranged fuel-droplet arrays in microgravity

We investigated the flame-spread characteristics of randomly arranged fuel-droplet arrays in microgravity. Flame-spread probability was calculated based on a percolation model with the flame-spread-limit distance of evenly-spaced n-decane droplet arrays in microgravity. Flame-spread probability depends on the occupation fraction of droplets in a lattice and rapidly increases with the occupation fraction. The local flame-spread-limit distance of unevenly-spaced n-decane droplet arrays was experimentally investigated in microgravity. The droplets were arranged in a straight line at uneven intervals. The local flame-spread-limit distance of the unevenly-spaced droplet arrays depended on the droplet arrangement and increased from the flame-spread-limit distance of the evenly-spaced droplet arrays due to interactive effects. The flame-spread probability considering the increase in local flame-spread-limit distance is larger than that without it.     

Flow boiling in microchannels and microgravity

A critical review of the state of the art of research on internal forced convection boiling in microchannels and in microgravity conditions is the main object of the present paper.     

Distinctive combustion stages of single heavy oil droplet under microgravity

This report presents an investigation on the combustion of single droplets comprised of heavy oil and oil mixtures blending diesel light oil (LO) and a heavy oil residue (HOR). The tests were conducted in a microgravity facility that offered 10 s of free-fall time. Fine wire thermocouples supported the droplets, resulting in a measurement of droplet temperature history. Additional data were the droplet and flame size history. The results identified four distinctive burning stages between ignition and extinction for heavy oil (C class) and HOR-LO blends. They are, in succession, the start-up, inner evaporation, thermal decomposition (pyrolysis) and polymerization stages. The start-up stage denoted an initial transient period, where the LO components burned from the droplet surface and the droplet temperature increased rapidly. The latter three stages featured pronounced droplet swellings and contractions caused by fuel evaporation and decomposition inside the droplet. An evaporation temperature demarcated the start-up stage from the inner evaporation stage, and this temperature corresponded to a plateau in the temperature history of the droplet. Two additional temperatures, termed the decomposition and polymerization temperatures, indicated the ends of the evaporation and decomposition stages. These temperatures were similarly identified by plateaus or inflection points in the time-temperature diagram. The evaporation temperature gradually decreased with increasing the initial LO mass fraction in the droplet, whereas the other two temperatures were almost independent of the oil composition. All three temperatures increased with decreasing initial droplet diameter, but the dependence was very slight. Based on the results, the combustion of heavy oil droplets appears to be dominated by a distillation-like vaporization mechanism, because of the rapid mass transport within the droplets caused by the disruptive burning.     

Visualization of the surface tension and gravitational effects on flow injection in center-gated disks

Flow injection in center-gated disks is numerically studied in this paper for possible applications in the manufacturing of composite materials in microgravity environment. The numerical method, which combines the finite element method with a predictor/corrector scheme, is used to determine the transient flow field. The effects of gravitation and surface tension on the development of flow front profile and velocity field are examined for a wide range of the governing parameters (namely, the capillary and Bonds numbers). It has been found that surface tension tends to hold the flow front in symmetric shape while gravitation is to distort it. The balance of these two forces has significant effects on the front shape, front tip traveling speed and required injection pressure. Good agreement is found between the numerical prediction and the experimental results concerning the advancement of the flow front and the front shape. The present results provide useful information in the design of resin transfer molding process in space.     

Propagation speed of tribrachial (triple) flame of propane in laminar jets under normal and micro gravity conditions

The propagation speed of tribrachial (triple) flames in laminar propane jets has been investigated experimentally under normal and micro gravity conditions. We found in the present experiment that the displacement speed varied nonlinearly with axial distance because the flow velocity along the stoichiometric contour was comparable to the propagation speed of tribrachial flame. Approximate solutions for the velocity and concentration accounting density difference and virtual origins have been used in determining the propagation speed of tribrachial flame and the concentration field was validated from the measurement of Raman scattering. Under the microgravity condition, the results showed that the propagation speed of tribrachial flame decreased with the mixture fraction gradient, in agreement with previous studies. The limiting maximum propagation speed under the microgravity condition is in good agreement with the theoretical prediction, ie, the ratio of maximum propagation speed to the stoichiometric laminar burning velocity is proportional to the square root of the density ratio of unburned to burnt mixture.     

An experimental investigation of sootshell formation in microgravity droplet combustion

Spherically symmetric droplet combustion experiments were performed at the NASA Glenn Research Center (GRC) 2.2 second drop tower in Cleveland, OH in an effort to better understand the mechanism leading to sootshell formation. Rapid insertion of a blunt plunger was used to remove the symmetric sootshell that formed during the period of quasi-steady burning. This allowed for the observation of sootshell re-formation. Soot particles were formed near the flame front and migrated towards the droplet to ultimately reside at the sootshell location. These experiments helped to bring about a better understanding of soot transport in microgravity droplet combustion.