We study isotropic turbulence decay in the context of the k-epsilon model, which solves the dissipation and kinetic energy equations. In modeling the dissipation equation, the coefficient C_epsilon2, suggested by Hanjalic and Launder [Journal of Fluid Mechanics, 1972] [1], is related to the temporal decay power-law by n = 1/(C_epsilon2 -1 )) and is assumed to be a constant value. In this work, we perform high-fidelity numerical simulations to examine the mathematical terms responsible for the decay of isotropic turbulence, considering both scenarios of forced and decaying turbulence. Our data suggest that the instantaneous C_epsilon2 not only depends on the instantaneous Reynolds number but is also sensitive to the history of energy injection in turbulence. We attribute these observations to the finite time required for the cascade from energetic to dissipative scales. Considering data from both decaying and growing forced turbulence, we develop an evolution equation for C_epsilon2 with Reynolds-dependent coefficients. We demonstrate that this model accurately captures the time evolution of dissipation and kinetic energy over a wide range of Reynolds numbers under a wide range of forced and decay scenarios.

We report on the status of a variable density transition model suitable for shear and buoyancy driven flows. Although current variable density turbulence models behave relatively well for high Reynolds numbers asymptotic turbulent flows, in the early stages of the disturbance growth, such models generally perform poorly. Therefore, there is a crucial need for a new generation of variable density turbulence models able to provide physics based prediction in the linear and transitional regimes. The proposed physics-based approach uses a complete set of second order moment equations (Reynolds stress, density self-correlation and mass-flux equations) for which the production terms are exact, while also limiting the number of unclosed correlations requiring modeling. We perform both linear stability theory and direct numerical simulations for a Rayleigh-Taylor instability dominated flow. The data is then used for a priori validation of the new model terms.

For more than a century, linear stability theory has been used extensively to get physics-based insights into the early stages of the laminar-turbulent transition process. Besides the prediction of the growth rate and instability frequency (temporal analysis) or wave-number (spatial analysis), the solution of the (generalized) eigenvalue problem associated with the linear stability equations and boundary-conditions also provides the mode shape in the form of the eigenfunctions of the (generalized) eigenvalue problem. In this context, the disturbance energy balance identify the main mechanisms that determine the growth and decay of instability waves. Although the absolute amplitude remains unknown, the eigenfunctions of the stability operator can nevertheless be used to assess the relative contributions of production, dissipation, and transport, hence providing useful information for the design and validation of transition models. In the context of turbulence modeling, most efforts have focused on the fully turbulent flow and therefore such models have generally a poor behavior in the transitional region. From a physics point of view, it is therefore important that the development of new transition models take into account the information provided by linear stability theory.

This work investigates the importance of verification and validation (V&V) to achieve predictive scale-resolving simulations (SRS) of turbulence, i.e., computations capable of resolving a fraction of the turbulent flow scales. Toward this end, we propose a novel but simple V&V strategy based on grid and physical resolution refinement studies that can be used even when the exact initial flow conditions are unknown, or reference data are unavailable. This is particularly relevant for transient and transitional flow problems, as well as for the improvement of turbulence models. We start by presenting a literature survey of results obtained with distinct SRS models for flows past circular cylinders. It confirms the importance of V&V by illustrating a large variability of results, which is independent of the selected mathematical model and Reynolds number. The proposed V&V strategy is then used on three representative problems of practical interest. The results illustrate that it is possible to conduct reliable verification and validation exercises with SRS models, and evidence the importance of V&V to predictive SRS of turbulence. Most notably, the data also confirm the advantages and potential of the proposed V&V strategy: separate assessment of numerical and modeling errors, enhanced flow physics analysis, identification of key flow phenomena, and ability to operate when the exact flow conditions are unknown or reference data are unavailable.

Earth and Space Science Open Archive PosterOpen AccessYou are viewing the latest version by default [v1]Partially-Averaged Navier-Stokes Equations Model for Prediction of Turbulent Ocean FlowsAuthorsFilipePereiraiDDanielIsraelLukevan RoekelSee all authors Filipe PereiraiDCorresponding Author• Submitting AuthorLos Alamos National LaboratoryiDhttps://orcid.org/0000-0003-0715-0194view email addressThe email was not providedcopy email addressDaniel IsraelLos Alamos National Laboratoryview email addressThe email was not providedcopy email addressLuke van RoekelLos Alamos National Laboratoryview email addressThe email was not providedcopy email address

For real flows, whether it be a supernova or a wing, the initial state is generally laminar and steady. In the inevitable presence of background disturbances, instabilities lead to the development of structures, first linear, then non-linear, and finally these structures “break down” to turbulence. The complex interplay of these small initial disturbances with the instability mechanisms generate structures which leave a lasting imprint on the subsequent turbulent flow. The aerodynamics community has made considerable progress in modeling the transition process for boundary-layers, although the current state-of-the-art still relies heavily on empirical correlations and may not apply to all modes of transition. However, transition modeling for other types of flows is still more rudimentary. This paper is intended to give an overview of current transition modeling efforts at Los Alamos National Laboratory. These are focused on Rayleigh-Taylor, Richtmeyer-Meshkov, and Kelvin-Helmholtz instabilities that are of importance for material mixing, particularly at extremely high temperatures and pressures. The modal model, which is a spectral model for second-order coupled non-linear mode growth, is one such approach. Our current research project, “Local Transition Modeling for Mixing,” has shown very promising results using a rigorous second-order moment closure, and may have potential application for aerospace applications as well. In addition, we describe the range of exciting high-energy experiments the Laboratory is performing specifically to validate future transition models.

and neutron transport. The computer science issues are concerned with matching numerical algorithms to emerging architectures and maintaining the quality of extremely large codes built to perform multi-physics calculations. Although graduate programs associated with computational physics are emerging, it is apparent that the pool of U.S. citizens in this multi-disciplinary field is relatively small and is typically not focused on the aspects that are of primary interest to LANL. Furthermore, more structured foundations for LANL interaction with universities in computational physics is needed; historically interactions rely heavily on individuals’ personalities and personal contacts. Thus a tertiary purpose of the Summer Workshop is to build an educational network of LANL researchers, university professors, and emerging students to advance the field and LANL’s involvement in it.