Assessment of closure coefficients for compressible-flow turbulence models

A critical assessment is made of the closure coefficients used for turbulence length scale in existing models of the transport equation, with reference to the extension of these models to compressible flow. It is shown that to satisfy the compressible 'law of the wall', the model coefficients must actually be functions of density gradients. The magnitude of the errors that result from neglecting this dependence on density varies with the variable used to specify the length scale. Among the models investigated, the k-omega model yields the best performance, although it is not completely free from errors associated with density terms. Models designed to reduce the density-gradient effect to an insignificant level are proposed

Discharge characteristics of dielectric materials examined in mono-, dual-, and spectral energy electron charging environments

The effects of midenergy electrons on the charge and discharge characteristics of spacecraft dielectric materials and the data base from which basic discharge models can be formulated is expanded. Thin dielectric materials were exposed to low, mid combined low and mid, and spectral energy electron environments. Three important results are presented: (1) it determined electron environments that lead to dielectric discharges at potentials less negative than -5 kV; (2) two types of discharges were identified that dominate the kinds of discharges seen; and (3) it is shown that, for the thin dielectric materials tested, the worst-case discharges observed in the various environments are similar

Reflectivities of uniform and broken stratiform clouds: An update

The reflectivities of uniform and broken stratiform clouds obtained from the NOAA-9 and NOAA-10 overpasses collected during the FIRE Marine Stratocumulus Intensive Field Observations (IFO) were compared, and these reflectivities were compared with those obtained through radiative transfer calculation performed for plane-parallel cloud models. The objective was to determine the extent to which plane-parallel radiative transfer calculations could reproduce the reflectivities observed for uniform clouds and to determine the extent to which finite cloud effects cause broken clouds to reflect differently than uniform clouds. The latter study is to provide guidance in the parameterization of finite cloud effects in general circulation climate models as well as to assess the ability of plane-parallel theory, which is used by ISCCP to retrieve cloud properties, to treat the reflectivities of broken clouds. Some results from this study were reported at the last FIRE Science Team meeting and some were reported elsewhere (Coakley and Briegleb, 1989). Improvements since the previous reports include: (1) the analysis of additional satellite passes, and (2) a modification to the analysis which helps to show the significance of the differences in reflectivities for uniform and broken clouds

The Origin and Evolution of a Complex Cuspate Foreland: Pointe-aux-Pins, Lake Erie, Ontario

The origin of Pointe-aux-Pins, a large, rounded, cuspate foreland protruding from the north shore of Lake Erie, is difficult to explain by conventional spit formation processes. Stratigraphic evidence from boreholes, the distribution of nearshore sediments, surface geomorphology, and previously published interpretations of Lake Erie water levels were combined to produce an hypothetical model of the development of the foreland from approximately 12,000 years BP to now. According to the model, the ancestral Pointe-aux-Pins began as a promontory caused by the intersection of the cross-lake Erieau moraine with the original lake shoreline, then located tens of kilometres lakeward of its present position. Lake levels at the time were about 30 m below present datum (173.3 m a.s.l.). Modern Pointe-aux-Pins dates from after the Nipissing "flood", at about 3500 BP, when the thereto-submerged sandy spit platform was again subjected to wave action, leading to beach ridge and dune formation. The age of the foreland of 3500 to 4000 years compares well with estimates based on the annual sand supply rate and the present sand volume in Pointe-aux-Pins.L'origine de la pointe aux Pins, vaste avancée triangulaire arquée sur la rive septentrionale du lac Érié, est difficile à expliquer par les processus courants de formation des flèches littorales. Les éléments de preuves d'ordre stratigraphique trouvés dans les trous de forage, la répartition des sédiments sur le littoral, la géomorphologie des formations superficielles et les interprétations déjà publiées sur les niveaux du lac Érié ont été combinés afin de créer un modèle hypothétique de l'évolution de cette pointe de terre, depuis environ 12 000 ans BP. Selon le modèle, l'ancienne pointe aux Pins a d'abord été un promontoire qui s'est formé à l'intersection de la moraine d'Erieau et du rivage originel du lac, alors situé à des dizaines de kilomètres au large. Le niveau du lac était alors à 30 m au-dessous de la surface actuelle (173,3 m a.n.m.). L'actuelle pointe aux Pins est postérieure à « l'inondation » de Nipissing, survenue il y a environ 3500 ans BP, lorsque la plate-forme constituée par la flèche de sable a de nouveau été soumise à l'action des vagues, ce qui a entraîné la formation de crêtes de plage et de dunes. L'âge de 3500-4000 ans attribué à la pointe de terre correspond bien aux estimations fondées sur le taux annuel d'apport en sable et le volume actuel de sable à la pointe aux Pins.Der Ursprung von Pointe-aux-Pins, einem breiten, geschweiften dreieckigen Strandvorsprung, der aus dem Nordufer des Erie-Sees herausragt, ist nicht einfach aus den ùblichen Prozessen der Landspitzenbildung zu erklâren. Stratigraphische Nachweise aus Bohrlôchern, die Verteilung von kùstennahen Sedimenten, die Oberflàchengeomorphologie und frûher verôffentlichte Interpretationen der Wasserspiegel des Erie-Sees wurden kombiniert, um ein hypothetisches Modell der Entwicklung des Strandvorsprungs von etwa 12 000 v.u.Z. bis heute zu entwickeln. Aus dem Modell geht hervor, dass die Ur-Pointe-aux-Pins-Landzunge als ein Vorgebirge begann, das sich am Schnittpunkt der quer durch den See verlaufenden Erieau-Moràne mit der ursprùnglichen Seekùste bildete, die sich damais 10 km seeeinwàrts von ihrer heutigen Position befand. Der Seewasserspiegel war damais etwa 30 m unter dem gegenwàrtigen (173.3 m ù.M.). Die heutige Pointe-aux-Pins-Landzunge stammt aus derZeit nach der "Ùberflutung" von Nipissing um ungefahr 3500 v.u.Z., als die hoch dazu untergetauchte Sandlandspitze wieder dem Einfluss der Wellen ausgesetzt war, was zur Bildung von Strandkâmmen und Dùnen fùhrte. Das Alter des Strandvorsprungs von 3550 bis 4000 Jahren stimmt gut ùberein mit den auf die jahrliche Sandzufuhrrate und das gegenwârtige Sandvolumen gestùtzten Schatzungen in Pointe-aux-Pins

An assessment and application of turbulence models for hypersonic flows

The current approach to the Accurate Computation of Complex high-speed flows is to solve the Reynolds averaged Navier-Stokes equations using finite difference methods. An integral part of this approach consists of development and applications of mathematical turbulence models which are necessary in predicting the aerothermodynamic loads on the vehicle and the performance of the propulsion plant. Computations of several high speed turbulent flows using various turbulence models are described and the models are evaluated by comparing computations with the results of experimental measurements. The cases investigated include flows over insulated and cooled flat plates with Mach numbers ranging from 2 to 8 and wall temperature ratios ranging from 0.2 to 1.0. The turbulence models investigated include zero-equation, two-equation, and Reynolds-stress transport models

Calculations of Supersonic and Hypersonic Flows using Compressible Wall Functions

The present paper presents a numerical procedure to calculate supersonic and hypersonic flows using the compressible law of the wall. The turbulence models under consideration include the Launder-Reece-Rodi-Gibson Reynolds-stress model and the k-epsilon model. The models coupled with the proposed wall function technique have been tested in both separated and unseparated flows. The flows include (1) an insulated flat plate flow over a range of Mach numbers, (2) a Mach 5 flat plate flow with cold wall conditions, (3) a two dimensional supersonic compression corner flow, (4) a hypersonic flow over an axisymmetric flare, and (5) a hypersonic flow over a 2-D compression corner. Results indicate that the wall function technique gives improved predictions of skin friction and heat transfer in separated flows compared with models using wall dampers. Predictions of the extent of separation are not improved over the wall damper models except with the Reynolds-stress model for the supersonic compression corner flow case

A method to quantitatively evaluate Hamaker constant using the jump-into-contact effect in Atomic Force microscopy

We find that the jump-into-contact of the cantilever in the atomic force microscope (AFM) is caused by an inherent instability in the motion of the AFM cantilever. The analysis is based on a simple model of the cantilever moving in a nonlinear force field. We show that the jump-into-contact distance can be used to find the interaction of the cantilever tip with the surface. In the specific context of the attractive van der Waals interaction, this method can be realized as a new method of measuring the Hamaker constant for materials. The Hamaker constant is determined from the deflection of the cantilever at the jump-into-contact using the force constant of the cantilever and the tip radius of curvature, all of which can be obtained by measurements. The results have been verified experimentally on a sample of cleaved mica, a sample of Si wafer with natural oxide and a silver film, using a number of cantilevers with different spring constants. We emphasize that the method described here is applicable only to surfaces that have van der Waals interaction as the tip-sample interaction. We also find that the tip to sample separation at the jump-into-contact is simply related to the cantilever deflection at this point, and this provides a method to exactly locate the surface.Comment: 11 pages, 4 figures, 1 tabl

Turbulence Modeling Validation, Testing, and Development

The primary objective of this work is to provide accurate numerical solutions for selected flow fields and to compare and evaluate the performance of selected turbulence models with experimental results. Four popular turbulence models have been tested and validated against experimental data often turbulent flows. The models are: (1) the two-equation k-epsilon model of Wilcox, (2) the two-equation k-epsilon model of Launder and Sharma, (3) the two-equation k-omega/k-epsilon SST model of Menter, and (4) the one-equation model of Spalart and Allmaras. The flows investigated are five free shear flows consisting of a mixing layer, a round jet, a plane jet, a plane wake, and a compressible mixing layer; and five boundary layer flows consisting of an incompressible flat plate, a Mach 5 adiabatic flat plate, a separated boundary layer, an axisymmetric shock-wave/boundary layer interaction, and an RAE 2822 transonic airfoil. The experimental data for these flows are well established and have been extensively used in model developments. The results are shown in the following four sections: Part A describes the equations of motion and boundary conditions; Part B describes the model equations, constants, parameters, boundary conditions, and numerical implementation; and Parts C and D describe the experimental data and the performance of the models in the free-shear flows and the boundary layer flows, respectively