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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 NTRS2019

Turbulent Transport in the Gray Zone: A Large Eddy Model Intercomparison Study of the CONSTRAIN Cold Air Outbreak Case

To quantify the turbulent transport at gray zone length scales between 1 and 10 km the Lagrangian evolution of the CONSTRAIN cold air outbreak (CAO) case was simulated with seven large eddy models. The case is characterized by rather large latent and sensible heat fluxes, and a rapid deepening rate of the boundary layer. In some models the entrainment velocity exceeds 4 cm/s. A significant fraction of this growth is attributed to a strong longwave radiative cooling of the inversion layer. The evolution and the timing of the breakup of the stratocumulus cloud deck differ significantly among the models. Sensitivity experiments demonstrate that a decrease in the prescribed cloud droplet number concentration, and the inclusion of ice microphysics, both act to speed up the thinning of the stratocumulus by enhancing the production of precipitation. In all models the formation of mesoscale fluctuations is clearly evident in the cloud fields but also in the horizontal wind velocity. Resolved vertical fluxes remain important for scales up to 10 km. The simulation results show that the resolved vertical velocity variance gradually diminishes with a coarsening of the horizontal mesh, but the total vertical fluxes of heat, moisture, and momentum are only weakly affected. This is a promising result as it demonstrates the potential use of a mesh size dependent turbulent length scale for convective boundary layers at gray zone model resolutions.

gray zone↗
NASA NTRS1993

A normal stress subgrid-scale eddy viscosity model in large eddy simulation

The Smagorinsky subgrid-scale eddy viscosity model (SGS-EVM) is commonly used in large eddy simulations (LES) to represent the effects of the unresolved scales on the resolved scales. This model is known to be limited because its constant must be optimized in different flows, and it must be modified with a damping function to account for near-wall effects. The recent dynamic model is designed to overcome these limitations but is compositionally intensive as compared to the traditional SGS-EVM. In a recent study using direct numerical simulation data, Horiuti has shown that these drawbacks are due mainly to the use of an improper velocity scale in the SGS-EVM. He also proposed the use of the subgrid-scale normal stress as a new velocity scale that was inspired by a high-order anisotropic representation model. The testing of Horiuti, however, was conducted using DNS data from a low Reynolds number channel flow simulation. It was felt that further testing at higher Reynolds numbers and also using different flows (other than wall-bounded shear flows) were necessary steps needed to establish the validity of the new model. This is the primary motivation of the present study. The objective is to test the new model using DNS databases of high Reynolds number channel and fully developed turbulent mixing layer flows. The use of both channel (wall-bounded) and mixing layer flows is important for the development of accurate LES models because these two flows encompass many characteristic features of complex turbulent flows.

Horiuti, K.↗
NASA NTRSDate not supplied

Transonic Lift and Drag Predictions using Wall Modelled Large Eddy Simulations

Wall-modelled Large Eddy Simulations (WMLES) of the NASA Common Research Model (CRM) at transonic conditions and various angles of attacks leading up to and including shock-induced flow separation are performed using the LAVA computational framework. The simulations are shown to accurately predict the lift curve slope and the onset of separation characterized by the break in the pitching moment. Furthermore, careful assessment of skin-friction drag at cruise condition and its subsequent decrease with increasing angles of attack are shown to be in agreement with viscous sublayer resolving Reynolds Averaged Navier Stokes(RANS) simulations. The small differences between WMLES and RANS appear to be of the same order as differences seen between two RANS models at the cruise-point. Some sensitivity is observed to the coefficient used in the subgrid scale model, although this can be reconciled by noting the low chord- and shock-incidence Reynolds numbers (Rex≈106) seen in the outboard regions of the wing, along with the uncertainties associated with tripping and the numerical transition that occurs near the leading edge. Among the primary drawbacks seen in the WMLES predictions is the subdued increase in wave-drag with increasing angle-of-attack and the shock intensity when compared with experimental data. Preliminary assessment suggests that additional span- and stream-wise grid resolution is likely needed in mid- and out-board portions of the wing to better resolve the shock-induced separated flow and to further investigate prediction accuracy of unsteady temporal characteristics of the problem.

ARMD↗
NASA NTRS1972

Turbulent-wake calculations with an eddy-viscosity model.

A simple eddy-viscosity model is shown to make it possible to calculate numerically the mean properties of a turbulent wake. Although the structure of the Reynolds stress terms is not resolved, the results obtained are adequate for predicting velocity profiles and displacement thicknesses.

Inouye, M.↗
NASA NTRS1990

A dynamic subgrid-scale eddy viscosity model

One major drawback of the eddy viscosity subgrid-scale stress models used in large-eddy simulations is their inability to represent correctly with a single universal constant different turbulent field in rotating or sheared flows, near solid walls, or in transitional regimes. In the present work, a new eddy viscosity model is presented which alleviates many of these drawbacks. The model coefficient is computed dynamically as the calculation progresses rather than input a priori. The model is based on an algebraic identity (Germano 1990) between the subgrid-scale stresses at two different filtered levels and the resolved turbulent stresses. The subgrid-scale stresses obtained using the proposed model vanish in laminar flow and at a solid boundary, and have the correct asymptotic behavior in the near-wall region of a turbulent boundary layer. The results of large-eddy simulations of transitional and turbulent channel flow that use the proposed model are in good agreement with the direct simulation data.

Germano, Massimo↗
NASA NTRS1994

A New K-epsilon Eddy Viscosity Model for High Reynolds Number Turbulent Flows: Model Development and Validation

A new k-epsilon eddy viscosity model, which consists of a new model dissipation rate equation and a new realizable eddy viscosity formulation, is proposed. The new model dissipation rate equation is based on the dynamic equation of the mean-square vorticity fluctuation at large turbulent Reynolds number. The new eddy viscosity formulation is based on the realizability constraints: the positivity of normal Reynolds stresses and Schwarz' inequality for turbulent shear stresses. We find that the present model with a set of unified model coefficients can perform well for a variety of flows. The flows that are examined include: (1) rotating homogeneous shear flows; (2) boundary-free shear flows including a mixing layer, planar and round jets; (3) a channel flow, and flat plate boundary layers with and without a pressure gradient; and (4) backward facing step separated flows. The model predictions are compared with available experimental data. The results from the standard k-epsilon eddy viscosity model are also included for comparison. It is shown that the present model is a significant improvement over the standard k-epsilon eddy viscosity model.

Shih, T.-H.↗
NASA NTRS1989

Application of large eddy interaction model to a mixing layer

The large eddy interaction model (LEIM) is a statistical model of turbulence based on the interaction of selected eddies with the mean flow and all of the eddies in a turbulent shear flow. It can be utilized as the starting point for obtaining physical structures in the flow. The possible application of the LEIM to a mixing layer formed between two parallel, incompressible flows with a small temperature difference is developed by invoking a detailed similarity between the spectra of velocity and temperature.

Murthy, S. N. B.↗
NASA NTRS1999

Modeling Mesoscale Eddies in the North Atlantic Ocean

Ocean modeling plays an important role in understanding the current climatic conditions and predicting the future climate change. Modeling the ocean at eddy-permitting and/or eddy resolving resolutions (1/3 degree or higher) has a two-fold objective. One part is to represent the ocean as realistically as possible, because mesoscale eddies have an impact on the large-scale circulation. The second objective is to learn how to represent effects of mesoscale eddies without explicitly resolving them. This is particularly important for climate models which cannot be run at eddy-resolving resolutions because of the computational constraints. At JPL, a 1/6 degree latitude by 1/6 degree longitude with 37 vertical levels Atlantic Ocean model has been developed. The model is based on the Parallel Ocean Program (POP) developed at Los Alamos National Laboratory (LANL). Using the 256-processor Cray T3D, we have conducted a 40-year integration of this Atlantic eddy-resolving ocean model. A regional analysis demonstrate that many observed features associated with the Caribbean Sea eddies can be realistically simulated by this model. Analysis of this Atlantic eddy-resolving ocean model further suggests that these Caribbean Sea eddies are connected with eddies formed outside the Caribbean Sea at the confluence of the North Brazil Current (NBC) and the North Equatorial Countercurrent. The diagram of the model simulated surface current shows that the Caribbean eddies ultimately originate in the NBC retroflection region, traveling more than a year from the North Brazil coast through the Lesser Antilles into the Caribbean Sea and eventually into the Gulf of Mexico. Additional information is contained in the original.

Chao, Yi↗
NASA NTRS1989

Turbulent eddy viscosity modeling in transonic shock/boundary-layer interactions

The treatment of turbulence effects on transonic shock/turbulent boundary layer interaction is addressed within the context of a triple deck approach valid for arbitrary practical Reynolds numbers between 1000 and 10 billion. The modeling of the eddy viscosity and basic turbulent boundary profile effects in each deck is examined in detail using Law-of-the-Wall/Law-of-the-Wake concepts as the foundation. Results of parametric studies showing how each of these turbulence model aspects influences typical interaction zone property distributions (wall pressure, displacement thickness and local skin friction) are presented and discussed.

Inger, G. R.↗
NASA NTRS1973

A local eddy viscosity model for turbulent shear flow

In the model described, the eddy viscosity is assumed to be a fluid property dependent on the state of the fluid locally, namely the local density, turbulent kinetic energy, turbulence scale, and Mach number. An empirical law was found which related eddy viscosity to these properties satisfactorily for free jets. This law is used without modification for a set of test cases in free shear layers, free-jet decay, coaxial mixing, and wakes. The scale of turbulence is taken as a constant at any axial location equal to the width of the shear layer. By utilizing the boundary-layer order-of-magnitude analysis, a coupled set of fluid dynamic equations is formulated, which of necessity includes the equation for the production of turbulent kinetic energy.

Ortwerth, P. J.↗
NASA NTRS2000

Prediction of Transonic Vortex Flows Using Linear and Nonlinear Turbulent Eddy Viscosity Models

Three-dimensional transonic flow over a delta wing is investigated with a focus on the effect of transition and influence of turbulence stress anisotropies. The performance of linear eddy viscosity models and an explicit algebraic stress model is assessed at the start of vortex flow, and the results compared with experimental data. To assess the effect of transition location, computations that either fix transition or are fully turbulent are performed. To assess the effect of the turbulent stress anisotropy, comparisons are made between predictions from the algebraic stress model and the linear eddy viscosity models. Both transition location and turbulent stress anisotropy significantly affect the 3D flow field. The most significant effect is found to be the modeling of transition location. At a Mach number of 0.90, the computed solution changes character from steady to unsteady depending on transition onset. Accounting for the anisotropies in the turbulent stresses also considerably impacts the flow, most notably in the outboard region of flow separation.

Bartels, Robert E.↗
NASA NTRS1992

The present state and the future direction of eddy viscosity models

Information is given in viewgraph form on the present state and future direction of eddy viscosity models. Topics covered include the eddy viscosity dilemma, two-equation models, equations of motion, free shear flows, incompressible free shear flows, model-predicted boundary layer structure, defect layer analysis, the effects of pressure gradients, viscous sublayer structure, wall functions and viscous damping, viscous damping for kappa-omega, the effects of compressibility, perturbation analysis of the wall layer, an alternative compressibility term, unsteady boundary layers, incompressible separation, backstep results, and compressible separation.

Wilcox, David C.↗
NASA NTRS1973

A kinematic eddy viscosity model including the influence of density variations and preturbulence

A model for the kinematic eddy viscosity was developed which accounts for the turbulence produced as a result of jet interactions between adjacent streams as well as the turbulence initially present in the streams. In order to describe the turbulence contribution from jet interaction, the eddy viscosity suggested by Prandtl was adopted, and a modification was introduced to account for the effect of density variation through the mixing layer. The form of the modification was ascertained from a study of the compressible turbulent boundary layer on a flat plate. A kinematic eddy viscosity relation which corresponds to the initial turbulence contribution was derived by employing arguments used by Prandtl in his mixing length hypothesis. The resulting expression for self-preserving flow is similar to that which describes the mixing of a submerged jet. Application of the model has led to analytical predictions which are in good agreement with available turbulent mixing experimental data.

Cohen, L. S.↗
NASA NTRS1992

Application of algebraic-RNG eddy viscosity model to simulation of transitional boundary layer flow

An algebraic eddy-viscosity model is derived from the renormalization group (RNG) theory of turbulence. A new length scale, based on boundary layer characteristics (displacement thickness, shape factor), is proposed. The model was applied to transitional boundary layer flow over a flat plate. Integral characteristics, such as the total skin friction coefficient, and mean velocity profile across the boundary layer, are found to be in good agreement with experimental data.

Yakhot, Alexander↗
NASA NTRS1994

Modelling of eddy currents related to large angle magnetic suspension test fixture

This report presents a preliminary analysis of the mathematical modelling of eddy current effects in a large-gap magnetic suspension system. It is shown that eddy currents can significantly affect the dynamic behavior and control of these systems, but are amenable to measurement and modelling. A theoretical framework is presented, together with a comparison of computed and experimental data related to the Large Angle Magnetic Suspension Test Fixture at NASA Langley Research Center.

Britcher, Colin P.↗
NASA NTRS1987

Computation of turbulent channel flow using Large-Eddy Interaction Model

The objective of the paper is to investigate the nature and values of closure parameters appearing in the proposed Large-Eddy Interaction Model for prediction of turbulent flow field. Effects of two closure parameters on predicted Reynolds stresses and other turbulence structural quantities are examined for channel flows at two Reynolds numbers.

Hong, S. K.↗
NASA NTRS1987

Development of Large-Eddy Interaction Model for inhomogeneous turbulent flows

The objective of this paper is to demonstrate the applicability of a currently proposed model, with minimum empiricism, for calculation of the Reynolds stresses and other turbulence structural quantities in a channel. The current Large-Eddy Interaction Model not only yields Reynolds stresses but also presents an opportunity to illuminate typical characteristic motions of large-scale turbulence and the phenomenological aspects of engineering models for two Reynolds numbers.

Hong, S. K.↗
NASA NTRS1996

Anisotropic eddy viscosity models

A general discussion on the structure of the eddy viscosity tensor in anisotropic flows is presented. The systematic use of tensor symmetries and flow symmetries is shown to reduce drastically the number of independent parameters needed to describe the rank 4 eddy viscosity tensor. The possibility of using Onsager symmetries for simplifying further the eddy viscosity is discussed explicitly for the axisymmetric geometry.

Carati, D.↗