Siva Thangam

Development of a Continuous Model for Simulation of Turbulent Flows

The development of a continuous turbulence model that is suitable for representing both the subgrid scale stresses in large eddy simulation and the Reynolds stresses in the Reynolds averaged Navier-Stokes formulation is described. A recursion approach is used to bridge the length scale disparity from the cutoff wave number to those in the energy-containing range. The proposed model is analyzed in conjunction with direct numerical simulations of Kolmogorov flows.

Development of a turbulence model based on the energy spectrum for flows involving rotation

A generalized eddy viscosity model is formulated by using the rotation modified energy spectrum. Rotation and mean shear effects are directly included in the eddy viscosity without the use of the local equilibrium assumption. The formulation also includes the modeling of vortex stretching and viscous destruction terms of the dissipation rate equation based on the limit of rotating isotropic turbulence at high Reynolds numbers. The rotation modified energy spectrum includes the contribution of rotation effects on the dissipation and structure of turbulence. The model is shown to reproduce the dominant effects of rotation on turbulence in rotating homogeneous shear flows and turbulent channel flows subject to spanwise rotation. The general applicability of the model and its implications are also addressed.

Simulation and Analysis of Rigid/Foil Electrolytic In-Process Dressing (ELID) Systems for Grinding

The fluid flow problem in a traditional electrolytic in-process dressing (ELID) system is analyzed and solved numerically. The predicted mean velocity profiles in the dressing zone show flow patterns that are in good agreement with the mean velocity distributions for plane laminar/turbulent Couette flows observed in the experiments. The computational results reveal that insufficient electrolyte supply rate is the cause of the failure of the traditional ELID system for high-speed grinding. Results also show that to obtain effective high-speed ELID grinding, a consistent high inlet electrolyte velocity or supply rate is required. For the foil ELID system, governing equations describing the fluid flow in the dressing zone and the foil elastic deformation are formulated. Analytical solution based on unidirectional flow model for the problem is presented and effects of wheel surface speed and foil tension on the performance of the dressing system are discussed. It is shown that the foil ELID system has the potential to be effective for high-speed grinding with low electrolyte supply rates. The results will be useful to the development of new machine systems and processes for high-speed grinding.

Development and Analysis of Foil Electrodes for High Speed Electrolytic In‐Process Wheel Dressing

This paper discusses the development of an effective electrolytic in‐process dressing technique for high speed grinding. An innovative foil electrode has been designed and tested. The performance of a hydrodynamic foil electrode is discussed. Experimental investigations confirm that foil electrodes show significant improvement on electrolytic in‐process dressing even when the electrolyte supply rate is low.

The Role of Vortex Stretching In Turbulence Modeling

Traditional models for the turbulent dissipation rate assume an equilibrium in which the production by vortex stretching is exactly balanced by the leading order part of the viscous destruction term. In the present study, the effect of allowing for unbalanced vortex stretching is explored in an effort to describe departures from equilibrium. It is found that the presence of a small unbalanced vortex stretching term has a number of profound consequences for the calculation of isotropic decay, homogeneous shear flow, and more complex turbulent shear flows with separation. In the case of isotropic decay it accounts for enstrophy blow-up in the limit of zero viscosity, while for homogeneous shear flow it predicts a production-equals- dissipation equilibrium at large times instead of an unbounded exponential growth of turbulent kinetic energy. Preliminary calculations for turbulent flow over a backward facing step indicate that even a minute imbalance in vortex stretching can have a major influence on the reattachment length.

Development of A Turbulence Model for Flows with Rotation and Curvature

A generalized eddy viscosity model is formulated by using the rotation modified energy spectrum. Rotation and mean shear effects are directly included in the eddy viscosity without the use of the local equilibrium assumption. Since the model is of a general form, additional modifications to the eddy viscosity for non-equilibrium effects are not needed. The formulation also includes modeling vortex stretching and viscous destruction terms of the dissipation rate equation based on the limit of rotating isotropic turbulence at high Reynolds numbers. Since the rotation modified energy spectrum includes the contribution of rotation effects on the dissipation and structure of turbulence, the model coefficients in the dissipation rate equation are also modified. The model is shown to reproduce the rotation effects in isotropic turbulence. The implications of the model and its general applicability for turbulent flows are also addressed.

Development and Validation of Continuous Models for Simulation of Turbulent Flows

The development of a continuous turbulence model that is suitable for representing both the subgrid scale stresses in large eddy simulation and the Reynolds stresses in the Reynolds averaged Navier-Stokes formulation is described. A recursion approach is used to bridge the length scale disparity from the cutoff wavenumber to those in the energy-containing range. The proposed model is analyzed in conjunction with direct numerical simulations of Kolmogorov flows.Copyright

Turbulence Modeling: A Brief Overview of Charles G. Speziale’s Contributions

Charles Speziale was, throughout his career, associated almost exclusively with the academic community. His affiliations included Princeton, Stevens, Georgia Tech, ICASE, CTR, and Boston University. His collaborators during his career constitute a sizable portion of the “Movers and Shakers” in the Turbulence Modeling and Turbulence Computation Arenas. However and perhaps atypically for the era of the late 70’s to early 90’s he was interested in and focused on the exigencies/requirements arising from “Practical Applications”. The Research areas to which he contributed throughout his career are exceedingly broad in scope — allowing him to bring to the Turbulence Modeling problem a rich/continually-enriched background in continuum mechanics. These areas included non-Newtonian fluid dynamics, kinetic theory of gases, vortex dynamics and non-linear transition flow dynamics. This intellectual experience base enabled him to make serious and lasting contributions to the turbulence modeling areas of streamline curvature, rotational influences, renormalization group theory, large eddy simulation, dissipation rate equation development, wall region modeling, direct numerical simulation, compressibility influence, second order closure and a plethora of canonical criteria for modeling, calibration and validation.Copyright

Current Status and Future Needs in Turbulence Modeling: An Aeronautical Perspective

An overview of current issues in turbulence modeling is presented along with a brief description of the current and future needs of NASA, especially from the point of advancing the state-of-the-art in aircraft design and air transportation.Copyright

Development and Application of an Anisotropic Two-Equation Model for Flows With Swirl and Curvature

An anisotropic two-equation Reynolds-stress model is developed by modeling the energy spectrum and through invariance based scaling. In this approach the effect of rotation is used to modify the energy spectrum, while the influence of swirl is modeled based on scaling laws. The resulting generalized model is validated for benchmark turbulent flows with swirl and curvature.Copyright