Annual research briefs, 1993

The 1993 annual progress reports of the Research Fellow and students of the Center for Turbulence Research are included. The first group of reports are directed towards the theory and application of active control in turbulent flows including the development of a systematic mathematical procedure based on the Navier Stokes equations for flow control. The second group of reports are concerned with the prediction of turbulent flows. The remaining articles are devoted to turbulent reacting flows, turbulence physics, experiments, and simulations

Annual Research Briefs: 1995

This report contains the 1995 annual progress reports of the Research Fellows and students of the Center for Turbulence Research (CTR). In 1995 CTR continued its concentration on the development and application of large-eddy simulation to complex flows, development of novel modeling concepts for engineering computations in the Reynolds averaged framework, and turbulent combustion. In large-eddy simulation, a number of numerical and experimental issues have surfaced which are being addressed. The first group of reports in this volume are on large-eddy simulation. A key finding in this area was the revelation of possibly significant numerical errors that may overwhelm the effects of the subgrid-scale model. We also commissioned a new experiment to support the LES validation studies. The remaining articles in this report are concerned with Reynolds averaged modeling, studies of turbulence physics and flow generated sound, combustion, and simulation techniques. Fundamental studies of turbulent combustion using direct numerical simulations which started at CTR will continue to be emphasized. These studies and their counterparts carried out during the summer programs have had a noticeable impact on combustion research world wide

Studying Turbulence Using Numerical Simulation Databases. 4: Proceedings of the 1992 Summer Program

Papers are presented under the following subject areas: small scales; turbulence physics; compressible flow and modeling; and reacting flows and combustion

Tackling viscoelastic turbulence

Turbulence in viscoelastic flows is a fascinating phenomenon with important technological implications, e.g. drag reduction at high Reynolds numbers and increased mixing efficiencies at low Reynolds numbers. The dynamics of these flows have been extensively studied experimentally over the last seventy years and more recently, in direct numerical simulations (DNS). However, theoretical progress in viscoelastic turbulence has been hindered by the fundamental challenges posed by the need to account for both the velocity as well as the elastic deformation history, encapsulated in the positive-definite conformation tensor. Due to the positivity constraint, the latter tensor is not a vector space quantity and thus classical approaches used to quantitatively analyze turbulence in Newtonian flows cannot be directly extended to viscoelastic flows. This fundamental issue is addressed in the present thesis in two parts. Firstly, we develop a decomposition of the conformation tensor about a given base-state that respects the mathematical and physical nature of this quantity. Scalar measures to quantify the resulting fluctuating conformation tensor are developed based on the non-Euclidean Riemannian geometry of the set of positive-definite tensors. The three measures are (a) the logarithmic volume ratio of the conformation tensor with respect to the base--state conformation tensor (b) the squared geodesic distance of the conformation tensor from the base--state, (c) the geodesic distance of the fluctuating conformation tensor from the closest isotropic tensor. Secondly, we develop an approach to perturb the conformation tensor in a physically consistent manner. This approach is an alternative to the classical weakly nonlinear expansion of vector space quantities, and is thus termed the weakly nonlinear deformation. When specialized to linear perturbations, this approach reveals the correct Hilbert space structure for the linearized problem. Viscoelastic (FENE-P) channel flow DNS are developed and used to illustrate the theoretical framework: fully turbulent flow is used for the first part, and the nonlinear evolution of Tollmien-Schlichting waves are considered for the second part. Several important insights are gleaned from these simulations, demonstrating the efficacy of the proposed approach. The fundamental contributions in the present thesis pave the road for theoretical modelling and analysis of viscoelastic turbulence

Finite-Volume Filtering in Large-Eddy Simulations Using a Minimum-Dissipation Model

Large-eddy simulation (LES) seeks to predict the dynamics of the larger eddies in turbulent flow by applying a spatial filter to the Navier-Stokes equations and by modeling the unclosed terms resulting from the convective non-linearity. Thus the (explicit) calculation of all small-scale turbulence can be avoided. This paper is about LES-models that truncate the small scales of motion for which numerical resolution is not available by making sure that they do not get energy from the larger, resolved, eddies. To identify the resolved eddies, we apply Schumann’s filter to the (incompressible) Navier-Stokes equations, that is the turbulent velocity field is filtered as in a finite-volume method. The spatial discretization effectively act as a filter; hence we define the resolved eddies for a finite-volume discretization. The interpolation rule for approximating the convective flux through the faces of the finite volumes determines the smallest resolved length scale δ. The resolved length δ is twice as large as the grid spacing h for an usual interpolation rule. Thus, the resolved scales are defined with the help of box filter having diameter δ= 2 h. The closure model is to be chosen such that the solution of the resulting LES-equations is confined to length scales that have at least the size δ. This condition is worked out with the help of Poincarés inequality to determine the amount of dissipation that is to be generated by the closure model in order to counterbalance the nonlinear production of too small, unresolved scales. The procedure is applied to an eddy-viscosity model using a uniform mesh

Annual Research Briefs, 1992

This report contains the 1992 annual progress reports of the Research Fellows and students of the Center for Turbulence Research. Considerable effort was focused on the large eddy simulation technique for computing turbulent flows. This increased activity has been inspired by the recent predictive successes of the dynamic subgrid scale modeling procedure which was introduced during the 1990 Summer Program. Several Research Fellows and students are presently engaged in both the development of subgrid scale models and their applications to complex flows. The first group of papers in this report contain the findings of these studies. They are followed by reports grouped in the general areas of modeling, turbulence physics, and turbulent reacting flows. The last contribution in this report outlines the progress made on the development of the CTR post-processing facility

Numerical Simulation

Nowadays mathematical modeling and numerical simulations play an important role in life and natural science. Numerous researchers are working in developing different methods and techniques to help understand the behavior of very complex systems, from the brain activity with real importance in medicine to the turbulent flows with important applications in physics and engineering. This book presents an overview of some models, methods, and numerical computations that are useful for the applied research scientists and mathematicians, fluid tech engineers, and postgraduate students

Annual Research Briefs - 1996

This report contains the 1996 annual progress reports of the research fellows and students supported by the Center for Turbulence Research. Last year, CTR hosted twelve resident Postdoctoral Fellows, three Research Associates, four Senior Research Fellows, and supported one doctoral student and ten short term visitors