Viscous effects in Rayleigh-Taylor instability

A simple, physical approximation is developed for the effect of viscosity for stable interfacial waves and for the unstable interfacial waves which correspond to Rayleigh‐Taylor instability. The approximate picture is rigorously justified for the interface between a heavy fluid (e.g., water) and a light fluid (e.g., air) with negligible dynamic effect. The approximate picture may also be rigorously justified for the case of two fluids for which the differences in density and viscosity are small. The treatment of the interfacial waves may easily be extended to the case where one of the fluids has a small thickness; that is, the case in which one of the fluids is bounded by a free surface or by a rigid wall. The theory is used to give an explanation of the bioconvective patterns which have been observed with cultures of microorganisms which have negative geotaxis. Since such organisms tend to collect at the surface of a culture and since they are heavier than water, the conditions for Rayleigh‐Taylor instability are met. It is shown that the observed patterns are quite accurately explained by the theory. Similar observations with a viscous liquid loaded with small glass spheres are described. A behavior similar to the bioconvective patterns with microorganisms is found and the results are also explained quantitatively by Rayleigh‐Taylor instability theory for a continuous medium with viscosity

Task Persistence: A Potential Mediator of the Income-Achievement Gap

Background: The pervasive gap in achievement among impoverished children has been investigated primarily in terms of parental investments, specifically parent to child speech and other forms of cognitive stimulation (e.g., toys, print materials). This research extends that literature by considering the role of a non-cognitive factor, namely task persistence, in the income-achievement gap. Using task persistence as the hypothesized mediator, duration of childhood in poverty is used to predict two educational variables - perceived academic competence and educational attainment. Although bivariate relationships between each of the variables have been demonstrated in past research, a full model linking task persistence with the income-achievement gap has not been investigated thus far. Methods: Using multiple waves of longitudinal data, duration of childhood poverty (ages 0-9) is used to predict both perceived academic competence (age 17) and educational attainment (age 23) with task persistence (average of ages 9, 13, 17) as a mediator. Results: With task persistence included in each model, the relationships between duration of childhood in poverty and both perceived academic competence and educational attainment are significantly reduced, confirming a mediational influence of task persistence. Conclusions: As hypothesized, task persistence statistically mediates the relationship between duration of childhood in poverty and educational outcomes. The implications of these findings on school success and intergenerational poverty are addressed, as well as suggestions for future research

Riparian Resilience in the Face of Interacting Disturbances

Riparian systems of the dryland western United States provide critical ecosystem functions such as diverse habitat for numerous species, flood attenuation and essential water storage in water-limited environments. These systems have experienced long term disturbance from anthropogenic activities including the grazing of livestock in dryland riparian systems and near extirpation of a keystone riparian species, Castor canadensis (beaver). However, increasing frequency of large-scale wildfires and climate change driven weather is altering the severity and scale of riparian disturbance. Beaver restoration has been gaining use to address long term riparian disturbances, yet little is understood regarding the impact of restored beaver activity on recently burned riparian systems. The objective of this study is to determine the interactions between beaver populations, large scale wildfire and subsequent erosion events in grazed riparian systems of the Methow River watershed, north central Washington. A fully factorial study comparing vegetation, water quality and channel cross-sections of burned and not burned stream reaches, with beaver presence and without, was conducted in 2018. Our research indicates increased vegetation diversity and nutrient storage and greater connectivity between stream and floodplain in beaver occupied, burned riparian systems. By studying interacting variables of fire, stream channel erosion and beaver activity in grazed systems, more effective and holistic approaches to adaptive ecological and economic management will emerge. Results from this research will be used by public land managers to improve and restore riparian function and management after wildfire for ecological and economical resilience in dryland western ecosystems

Anomalously high potentials observed on ISEE

Data from two electric field experiments and from the plasma composition experiment on ISEE-1 are used to show that the spacecraft charged to close to -70 V in sunlight at 0700 UT on March 17, 1978. Data from the electron spectrometer experiment show that there was a potential barrier of -10 to -20 V about the spacecraft during this event. The potential barrier was effective in turning back emitted photoelectrons to the spacecraft. The stringent electrostatic cleanliness specifications imposed on ISEE make the presence of differential charging unlikely. Modeling of this event is required to determine if the barrier was produced by the presence of space charge

Comments on "Rayleigh–Taylor instability of thin viscous layers"

In a paper by Craik, (1) frequent references are made to our paper (2) which we believe are incorrect. It may also be pointed out that quite unusual circumstances would be required to provide a physical basis for Craik’s analysis; the experiments described in his paper are not appropriately explained by his analysis

Manned Venus Flyby

This study is one of several being conducted at Bellcomm and in Manned Space Flight whose purpose is to give guidance to the Apollo Applications Program's technical objectives by focusing on a longer range goal. The assumed mission in this case is a three-man flyby of Venus launched in November, 1973 on a single standard Saturn V. The selected flight configuration includes a Command and Service Module similar in some respects to Apollo, an Environmental Support Module which occupies the adapter area and a spent S-IVB stage which is utilized for habitable volume and structural support of a solar cell electrical power system. The total injected weight, 106,775 lbs., is within the capability of a single Saturn V of the early 1970's. The study is focused on the selection of subsystem technologies appropriate to long duration flight. The conclusions are reported in terms of the technical characteristics to be achieved as part of the Apollo Applications Program's long duration objectives