Rough Surface and Volume Scattering of Soil Surfaces, Ocean Surfaces, Snow, and Vegetation Based on Numerical Maxwell Model of 3-D Simulations
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Leung Tsang↗
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Important considerations in a simple numerical plume model
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Winiarski, L. D.↗
An infrared radiation routine for use in numerical atmospheric models
Previous methods for calculating radiative fluxes due to water vapor and CO2 absorption bands are extended to take into consideration the entire water vapor and CO2 bands, including e-type absorption in the window region and the overlapping of different absorptions. Cooling rate profiles in the water vapor bands for a tropical atmosphere were computed by a detailed line-by-line method and by a far-wing approximation method, and the error of both methods is less than 0.2 C/day. Cooling rate profiles in the 15 micron band including overlapping of CO2 absorption with water vapor were calculated by a method in which flux transmittance is computed by means of a linear expansion and the multiplication rule, and maximum errors of 0.3 C/day were found in comparison with the exact line-by-line method.
Chow, M.-D.↗
Review of the metallographic cooling rates of meteorites and a new model for the planetesimals in which they formed
The cooling rates of meteorites through approximately 900 -650 K, as read from their metal alloy compositions, are reviewed. Metallographic cooling rates are compared with the cooling rates that appear to be required by the K/Ar and Ar-40/Ar-39 ages of five meteorite classes, and discrepancies are found in all cases. Either (1) the metallographic cooling rates (and also Pu-244 fission cooling rates) are systematically in error, being too slow by a factor of approximately 6; or (2) the traditional thermal model for parent meteorite planets (having constant dimension and uniform physical properties) is oversimplified and the Ar closure temperatures for chondrites derived by Turner et al. (1978) are too low. An alternative parent planet model is proposed and numerically modeled, in which accretion of thermally insulating particulate matter, heat generation by Al-26 decay, melting or sintering of the particulate matter into conductive rock, and establishment of the properties of the meteorites occurred concurrently. Meteorite chronologies are somewhat easier to understand in this context, since the initially small, hot (thus sintered and conductive) bodies would have cooled rapidly to isotopic closure, but later cooling might have been much slower as a result of the continued accretion of insulating particulate matter.
Wood, J. A.↗
Anomalous growth of single ice crystals in solution
It is shown that major discrepancies exist between experiments and theory for ice crystal growth from solution. Accurate data, taken in a microgravity environment, approximate analytical models, and exact (probably numerical) models all are needed to advance our understanding of ice crystal growth phenomena. A new approximate semi-empirical theory is presented which predicts that a relatively sharp transition from natural convection control to diffusion control for ice growth in pure water occurs at a subcooling of about 10 C (a reduced temperature difference of about 0.125). No reliable data exist to test this prediction. The theory also predicts qualitatively the growth of ice in NaCl solution in which maxima in the growth rates are observed at various levels of subcooling.
Gill, W. N.↗
Numerical model-based diagnostic study of the rapid development phase of the Presidents' Day cyclone
A mesoscale model simulation of the Presidents' Day cyclone at 1200 GMT 18 February 1979 is presented which captures the upper-tropospheric intrusion of stratospheric air upstream of the East Coast and subsequent development of the surface cyclone. The model simulation is then used to examine the descent of the stratospheric air mass and the interaction of this air mass with a lower-tropospheric potential vorticity maximum associated with an inverted trough and coastal front along the East Coast. The model is also used to examine the processes that contribute to the rapid decrease of sea-level pressure and increase in lower-tropospheric cyclonic vorticity during the explosive development phase of the cyclone.
Whitaker, Jeffrey S.↗
A parameterization of radiation heating at the surface in a numerical cloud model
The development of the parameterized radiation heat flux incorporated in a 2D time-dependent model that was tested for its predictive capabilities during the COHMEX project in 1986 is described, together with two experiments for using the model in the predictive mode. Using the 12 Z sounding during the North Dakota Thunderstorm Project in 1989, it was found that the COHMEX model was quite successful in simulating the convection and predicting the cloud characteristics.
Kopp, Fred J.↗
Response of a stably stratified atmosphere to low-level heating - An application to the heat island problem
Two-dimensional airflow characteristics past a heat island are investigated using both a linear analytic model and a nonlinear numerical model in the context of the response of a stably stratified atmosphere to specified low-level heating in a constant shear flow. Results from the steady-state, linear, analytic solutions exhibit typical flow response fields that gravity waves produce in response to the local heat source in the presence of environmental flow. The magnitude of the perturbation vertical velocity is shown to be much larger in the shear-flow case than in the uniform-flow case. Two distinct flow features are observed for larger heating amplitude (hence, larger nonlinearity factor): the gravity-wave-type response field on the upstream side of the heat island and the strong updraft circulation cell located on the downstream side. As the heating amplitude increases, the updraft circulation cell strengthens and shifts farther downwind. The strong updraft cell is believed to be partly responsible for precipitation enhancement observed on the downstream side of the heat island. It is found that the continuing downwind propagation of the updraft circulation cell is related to basic-state wind speed.
Baik, Jong-Jin↗
Parameterization of bulk condensation in numerical cloud models
The accuracy of the moist saturation adjustment scheme has been evaluated using a three-dimensional explicit microphysical cloud model. It was found that the error in saturation adjustment depends strongly on the Cloud Condensation Nucleii (CCN) concentration in the ambient atmosphere. The scheme provides rather accurate results in the case where a sufficiently large number of CCN (on the order of several hundred per cubic centimeter) is available. However, under conditions typical of marine stratocumulus cloud layers with low CCN concentration, the error in the amounts of condensed water vapor and released latent heat may be as large as 40%-50%. A revision of the saturation adjustment scheme is devised that employs the CCN concentration, dynamical supersaturation, and cloud water content as additional variables in the calculation of the condensation rate. The revised condensation model reduced the error in maximum updraft and cloud water content in the climatically significant case of marine stratocumulus cloud layers by an order of magnitude.
Kogan, Yefim L.↗
Toward a dynamic-thermodynamic assimilation of satellite surface temperature in numerical atmospheric models
An assimilation technique is described in which satellite-observed surface skin temperature tendencies are used in a model surface energy budget so that the predicted rate of temperature change in the model more closely agrees with the satellite observations. Both visible and infrared GOES satellite data are used in the assimilation. The technique is based on analytically recovering surface moisture from similarity expressions derived from an evapotranspiration residual obtained as a difference between the unadjusted model evapotranspiration and the satellite-inferred evapotranspiration. The technique has application in regional-scale models where surface parameters such as root zone moisture, soil moisture, etc., are unknown. It is assumed that the largest error in the surface energy budget is in the evapotranspiration term. Two tests are given for the technique, first, a one-dimensional test against FIFE data and, second, a three-dimensional test over Oklahoma. In these cases the technique appears to correctly adjust the model response to agree better with observations.
Mcnider, Richard T.↗
Modeling and Numerical Simulation of Microwave Pulse Propagation in Air Breakdown Environment
Numerical simulation is used to investigate the extent of the electron density at a distant altitude location which can be generated by a high-power ground-transmitted microwave pulse. This is done by varying the power, width, shape, and carrier frequency of the pulse. The results show that once the breakdown threshold field is exceeded in the region below the desired altitude location, electron density starts to build up in that region through cascading breakdown. The generated plasma attenuates the pulse energy (tail erosion) and thus deteriorates the energy transmission to the destined altitude. The electron density saturates at a level limited by the pulse width and the tail erosion process. As the pulse continues to travel upward, though the breakdown threshold field of the background air decreases, the pulse energy (width) is reduced more severely by the tail erosion process. Thus, the electron density grows more quickly at the higher altitude, but saturates at a lower level. Consequently, the maximum electron density produced by a single pulse at 50 km altitude, for instance, is limited to a value below 10(exp 6) cm(exp -3). Three different approaches are examined to determine if the ionization at the destined location can be improved: a repetitive pulse approach, a focused pulse approach, and two intersecting beams. Only the intersecting beam approach is found to be practical for generating the desired density level.
Kuo, S. P.↗
Analytical Study of the relationship Between an Absorber Cavity and Solar Fresnel Concentrator
This paper presents an analytical study of the relationship between an engine absorber cavity and a solar concentrator in a solar thermal propulsion system. The engine is a thermal storage engine, and the concentrator consists of a primary thin-film fresnel concentrator and a secondary Compound Parabolic Concentrator. Two thermal models of the engine are developed, a blackbody cavity model and a 2D numerical model. Results of the two models are compared, with attention to the limitations of the blackbody model 'Me fresnel concentrator model is based on a Circle of Least Confusion approach. The results of the thermal and optical models are combined to create a combined model with absorber temperature predictions. This combined model approach is used to predict an optimum engine aperture size.
Pearson, J. Boise↗
Trailing Vortex Measurements in the Wake of a Hovering Rotor Blade with Various Tip Shapes
This work examined the wake aerodynamics of a single helicopter rotor blade with several tip shapes operating on a hover test stand. Velocity field measurements were conducted using three-component laser Doppler velocimetry (LDV). The objective of these measurements was to document the vortex velocity profiles and then extract the core properties, such as the core radius, peak swirl velocity, and axial velocity. The measured test cases covered a wide range of wake-ages and several tip shapes, including rectangular, tapered, swept, and a subwing tip. One of the primary differences shown by the change in tip shape was the wake geometry. The effect of blade taper reduced the initial peak swirl velocity by a significant fraction. It appears that this is accomplished by decreasing the vortex strength for a given blade loading. The subwing measurements showed that the interaction and merging of the subwing and primary vortices created a less coherent vortical structure. A source of vortex core instability is shown to be the ratio of the peak swirl velocity to the axial velocity deficit. The results show that if there is a turbulence producing region of the vortex structure, it will be outside of the core boundary. The LDV measurements were supported by laser light-sheet flow visualization. The results provide several benchmark test cases for future validation of theoretical vortex models, numerical free-wake models, and computational fluid dynamics results.
Martin, Preston B.↗
Aging Wire Insulation Assessment by Phase Spectrum Examination of Ultrasonic Guided Waves
Wire integrity has become an area of concern to the aerospace community including DoD, NASA, FAA, and Industry. Over time and changing environmental conditions, wire insulation can become brittle and crack. The cracks expose the wire conductor and can be a source of equipment failure, short circuits, smoke, and fire. The technique of using the ultrasonic phase spectrum to extract material properties of the insulation is being examined. Ultrasonic guided waves will propagate in both the wire conductor and insulation. Assuming the condition of the conductor remains constant then the stiffness of the insulator can be determined by measuring the ultrasonic guided wave velocity. In the phase spectrum method the guided wave velocity is obtained by transforming the time base waveform to the frequency domain and taking the phase difference between two waveforms. The result can then be correlated with a database, derived by numerical model calculations, to extract material properties of the wire insulator. Initial laboratory tests were performed on a simple model consisting of a solid cylinder and then a solid cylinder with a polymer coating. For each sample the flexural mode waveform was identified. That waveform was then transformed to the frequency domain and a phase spectrum was calculated from a pair of waveforms. Experimental results on the simple model compared well to numerical calculations. Further tests were conducted on aircraft or mil-spec wire samples, to see if changes in wire insulation stiffness can be extracted using the phase spectrum technique.
Anastasi, Robert F.↗
Aerothermal Considerations for Entry, Descent, and Landing
Aerothermal modeling is inherently entwined with TPS design. Aerothermal, radiation, and TPS material response are coupled so it's important to check modeling assumptions. Validation of numerical models using ground and flight tests is important to quantify uncertainties. A margin policy based on statistical methods may provide greater insight in the key drivers and overall reliability of the design
Tang, Chun↗
Time Series Machine Learning Methods for Surface PM2.5 Estimations Using Geostationary Satellites and Numerical Weather Models
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George Priftis↗
Assessment of Numerical and Modeling Errors of RANS-based Transition Models for Low-Reynolds Number 2-D Flows based Transition Models
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CFD↗
Landslide Runout Analysis Using Remote Sensing for Lower and Upper Barun Lakes
Landslides are recognized as common triggers of glacial lake outburst floods in high mountain regions, but the remoteness of dangerous glacial lakes makes monitoring and disaster management very difficult. Using a very high resolution digital elevation model produced from a series of stereo satellite images, we simulate debris flows and rock avalanche runout around two very dangerous lakes in high mountain Nepal. We constrain particularly likely sources of these slope failures to simulate 79,800 potential debris flows and rock avalanches of volumes ranging from 1,000 m3 to 1,000,000 m3. This allowed us to identify zones in and around dangerous glacial lakes that are most exposed to landslide runout hazards, along with the most frequent source areas that produced landslides that may run into either of the two glacial lakes. Although our empirical-statistical model presented limitations that numerical models attempt to overcome, our probabilistic approach combined with a very high resolution DEM may shed new light on how to identify sources and impact areas of debris flows and rock avalanches that may cascade into downstream disasters.
landslides↗