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Results for “Mechanistic Modeling”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Mechanistic model for catalytic recombination during aerobraking maneuvers

Several mechanistic models are developed to predict recombination coefficients for use in heat shield design for reusable surface insulation (RSI) on aerobraking vehicles such as space shuttles. The models are applied over a temperature range of 300 to 1800 K and a stagnation pressure range of 0 to 3,000 Pa. A four parameter model in temperature was found to work best; however, several models (including those with atom concentrations at the surface) were also investigated. Mechanistic models developed with atom concentration terms may be applicable when sufficient data becomes available. The requirement is shown for recombination experiments in the 300 to 1000 K and 1500 to 1850 K temperature range, with deliberate concentration variations.

Willey, Ronald J.↗
NASA NTRS1994

Tracer evolution in winds generated by a global spectral mechanistic model

The lower boundary of a spectral mechanistic model is prescribed with 100 hPa geopotentials, and its performance during a November 1989 through March 1990 integration is compared with National Meteorological Center observations. Although the stratopause temperatures quickly become biased near the pole in both hemispheres, the model develops a residual mean circulation which shows significant descent over the winter pole and ascent in the tropics and over the summer pole at pressures less than 10 hPa. The daily correspondence of observed to modeled features in the upper stratosphere and mesosphere degrades after one month. However, the long-term variability qualitatively follows the observations. The results of off-line transport experiments are also described. A passive tracer is instantaneously injected into the flow over the poles and evolves in a manner which is consistent with the residual mean circulation. It demonstrates a significant cross-equatorial flux in the mesosphere near solstice, and air which originates in the southern hemisphere polar mesosphere can be found descending deep into the nothern polar stratosphere at the end of the integration. Nitrous oxide is also transported, and its ability to act as a dynamical tracer is evaluated by comparison to the evolution of the passive tracer.

Nielsen, J. E.↗
NASA NTRS1999

Progress toward an explicit mechanistic model for the light-driven pump, bacteriorhodopsin

Recent crystallographic information about the structure of bacteriorhodopsin and some of its photointermediates, together with a large amount of spectroscopic and mutational data, suggest a mechanistic model for how this protein couples light energy to the translocation of protons across the membrane. Now nearing completion, this detailed molecular model will describe the nature of the steric and electrostatic conflicts at the photoisomerized retinal, as well as the means by which it induces proton transfers in the two half-channels leading to the two membrane surfaces, thereby causing unidirectional, uphill transport.

Review, Tutorial↗
NASA NTRS1994

Global QBO in circulation and ozone. Part 2: A simple mechanistic model

Although the phenomenon of equatorial quasi-biennial oscillation is relatively well understood, the problem of how the equatorially confined quasi-biennial oscillation (QBO) wave forcing can induce a signal in the extratropics of comparable or larger magnitude remains unsolved. A simple mechanistic model is constructed to provide a quantitative test of the hypothesis that the phenomenon of extratropical QBO is mainly caused by an anomalous seasonal circulation induced by an anomalous Eliassen-Palm (E-P) flux divergence. The anomaly in E-P flux divergence may be caused in turn by the relative poleward and downward shift of the region of irreversible mixing (breaking) of the extratropical planetary waves during the easterly phase of the equatorial QBO as compared to its westerly phase. The hemispheric nature of the anomaly wave forcing in solstice seasons (viz., no wave breaking in the summer hemisphere) induces a global circulation anomaly that projects predominantly into the first few zonal Hough modes of Plumb. Such a global QBO circulation pattern, although difficult to measure directly, is reflected in the distribution of stratospheric tracers transported by it. Our model produces a global pattern of QBO anomaly in column ozone that appears to account for much of the unfiltered interannual variability in the column ozone observed by the total ozone mapping spectrometer (TOMS) instrument aboard the Nimbus satellite. Furthermore, the model produces the characteristic spectrum of the observation with peaks at periods of 20 and 30 months.

Tung, K. K.↗
NASA NTRSDate not supplied

Potential of a Plant Gas Exchange Mechanistic Model to Predict Plant Transpiration in Veggie on ISS

Plants are an essential part of long-duration space travel, as they enable food production and contribute to air revitalization through photosynthesis, and water recycling through transpiration. Understanding their growth mechanisms is essential to use them to sustain human life in space. In particular, gas exchange – e.g., CO2 absorption and water transpiration – are modified in microgravity because of the lack of buoyancy-driven convection, and in the long run, this could result in impaired plant growth. Water absorbed by the plants mainly depends on their size and on environmental conditions (air temperature, humidity and ventilation), but in microgravity watering plants is a delicate operation – too much water results in flooded roots and too little water leads in a few hours to wilted plants. This is regularly experienced in the Veggie system on ISS, which enables small-scale food production in microgravity since 2014. This presentation explores how a mechanistic model of plant gas exchange can help predict plant transpiration in Veggie and thus better predict daily watering. For each plant, inputs on canopy leaf area (acquired with daily photos), air temperature and relative humidity in the plant compartment, as well as airspeed at the top of its canopy enable accurate predictions of transpiration in microgravity. This brings a better understanding of water movement through the plant in microgravity in relation to ventilation and plant size and would result in easier management of plant watering in Veggie. Ultimately, this work could be applied to any space crop production in microgravity and be used for water management and yield predictions. This work was funded by NASA Space Biology through NASA postdoctoral program / USRA.

Lucie Poulet↗
NASA NTRSDate not supplied

Potential of a Plant Gas Exchange Mechanistic Model to Predict Plant Transpiration in Veggie on ISS

Plants are an essential part of long-duration space travel, as they enable food production and contribute to air revitalization through photosynthesis, and water recycling through transpiration. Understanding their growth mechanisms is essential to use them to sustain human life in space. In particular, gas exchange – e.g., CO2 absorption and water transpiration – are modified in microgravity because of the lack of buoyancy-driven convection, and in the long run, this could result in impaired plant growth. Water absorbed by the plants mainly depends on their size and on environmental conditions (air temperature, humidity and ventilation), but in microgravity watering plants is a delicate operation – too much water results in flooded roots and too little water leads in a few hours to wilted plants. This is regularly experienced in the Veggie system on ISS, which enables small-scale food production in microgravity since 2014. This presentation explores how a mechanistic model of plant gas exchange can help predict plant transpiration in Veggie and thus better predict daily watering. For each plant, inputs on canopy leaf area (acquired with daily photos), air temperature and relative humidity in the plant compartment, as well as airspeed at the top of its canopy enable accurate predictions of transpiration in microgravity. This brings a better understanding of water movement through the plant in microgravity in relation to ventilation and plant size and would result in easier management of plant watering in Veggie. Ultimately, this work could be applied to any space crop production in microgravity and be used for water management and yield predictions. This work was funded by NASA Space Biology through NASA postdoctoral program / USRA.

Lucie Poulet↗
NASA NTRS1988

Irreversible wave-mean flow interactions in a mechanistic model of the stratosphere

A wave-1 minor warming is simulated in a mechanistic, global, primitive equation model. The modification of the zonal flow by the rearrangement of potential vorticity on a middle stratopheric isentrope is compared in a fully nonlinear model and in a model with only one wave and the zonal flow (a quasi-linear model). The permanent rearrangement of potential vorticity during the wave episode is more intense and more localized meridionally in the fully nonlinear model, which is able to capture the process of planetary wave breaking in some detail. Additional experiments reveal that the differences between the quasi-linear and nonlinear models persist for a broad range of wave amplitudes, and that the quasi-linear model can qualitatively reproduce the modification of the zonal flow by the wave when the diffusive dissipation of the wave is enhanced. These results are discussed in the context of the theory of barotropic Rossby waves in shear flows, and in comparison with recent numerical simulations of the middle atmosphere.

Robinson, Walter A.↗
NASA NTRS2013

Projected Climate Impacts to South African Maize and Wheat Production in 2055: A Comparison of Empirical and Mechanistic Modeling Approaches

Crop model-specific biases are a key uncertainty affecting our understanding of climate change impacts to agriculture. There is increasing research focus on intermodel variation, but comparisons between mechanistic (MMs) and empirical models (EMs) are rare despite both being used widely in this field. We combined MMs and EMs to project future (2055) changes in the potential distribution (suitability) and productivity of maize and spring wheat in South Africa under 18 downscaled climate scenarios (9 models run under 2 emissions scenarios). EMs projected larger yield losses or smaller gains than MMs. The EMs' median-projected maize and wheat yield changes were 3.6% and 6.2%, respectively, compared to 6.5% and 15.2% for the MM. The EM projected a 10% reduction in the potential maize growing area, where the MM projected a 9% gain. Both models showed increases in the potential spring wheat production region (EM = 48%, MM = 20%), but these results were more equivocal because both models (particularly the EM) substantially overestimated the extent of current suitability. The substantial water-use efficiency gains simulated by the MMs under elevated CO2 accounted for much of the EMMM difference, but EMs may have more accurately represented crop temperature sensitivities. Our results align with earlier studies showing that EMs may show larger climate change losses than MMs. Crop forecasting efforts should expand to include EMMM comparisons to provide a fuller picture of crop-climate response uncertainties.

DSSAT↗
NASA NTRS1983

A mechanistic model of Eulerian, Lagrangian mean, and Lagrangian ozone transport by steady planetary waves

Ozone transport is calculated for steady, dissipative planetary waves using the Eulerian, Lagrangian mean, and residual circulation. A Lagrangian model of parcel dynamics is used to interpret planetary wave-photochemistry interaction. In chemically active regions the mean field ozone changes are found to be significant only where there are large gradients in chemical sources and sinks along parcel trajectories. The largest changes in the mean field are found in the lower stratosphere and are due to the Lagrangian mean advection. When the Lagrangian mean advection is approximated by the residual circulation, errors in the transport velocities as large as 30 pct may occur.

Rood, R. B.↗
NASA NTRS1987

Proposed mechanistic model to simulate transfer line cool-down process using liquid helium

This paper proposes a mathematical model to simulate the cool-down behavior of a warm transfer line using liquid helium (including He II) in a microgravity environment. The thermal-hydraulic characteristics of different possible flow regimes which may occur during the cool-down process are discussed. Correlations for predicting the heat and momentum transfer rates in each flow regime are presented.

Ng, Y. S.↗
NASA NTRS1998

Mechanistic Models of Friction Stir Welding

Friction stir welding is a welding process developed at The Welding Institute (TWI) in England. The method uses very large strain plastic deformation of the material to join two pieces of metal together. The material is deformed using a tool which is forced between the two pieces which rotates causing a bond. Beyond this, very little is actually known although many people working in the field are willing to speculate on the detailed mechanisms involved. Some measurements made using sacrificial thermocouples at the weld joint indicate that the maximum temperature during the weld process is on the order of 370C - well below the melting temperature of the material. However, at this temperature, the material properties are highly temperature dependent, and the yield stress is approximately an order of magnitude less at this temperature than it is at room temperature. As expected, there are many interpretations of the physical mechanisms occurring during the weld process. Although there is very little published concerned with FSW, some of the anecdotal theories will be described. One describes the primary mechanism as frictional heating at the front of the tool caused by slip between the tool and the material. At elevated temperatures, the weld material becomes soft and deforms around the tool but not essentially altered by the tool rotation, similar to an extrusion. As the material meets again at the rear of the tool, the temperatures and pressures are sufficient to cause the material to bond. All other structures seen are secondary and unimportant. Another theory examined last summer at NASA's Marshall Space Flight Center (MSFC) was that there was no slip between the tool and the material resulting in a rotating mass of plastic weld material traveling at a variety of angular velocities - the greatest at the tool surface diminishing to zero at the outer edge of the plastic mass surrounding the tool. This conceptual model was followed by simplified calculations which showed that the balance of moments through the weld plug was not possible under steady state conditions and realistic temperature profiles. This led to some consideration of a quasi-steady oscillating process. Later when force measurements became available some models were modified and new ones were proposed.

Stewart, Michael B.↗