An eddy-viscosity turbulence model employing three additional transport equations is presented and applied to a number of transitional flow test cases. The model is based on the k-ω framework and represents a substantial refinement to a transition-sensitive model that has been previously documented in the open literature. The third transport equation is included to predict the magnitude of low-frequency velocity fluctuations in the pretransitional boundary layer that have been identified as the precursors to transition. The closure of model terms is based on a phenomenological (i.e., physics-based) rather than a purely empirical approach and the rationale for the forms of these terms is discussed. The model has been implemented into a commercial computational fluid dynamics code and applied to a number of relevant test cases, including flat plate boundary layers with and without applied pressure gradients, as well as a variety of airfoil test cases with different geometries, Reynolds numbers, freestream turbulence conditions, and angles of attack. The test cases demonstrate the ability of the model to successfully reproduce transitional flow behavior with a reasonable degree of accuracy, particularly in comparison with commonly used models that exhibit no capability of predicting laminar-to-turbulent boundary layer development. While it is impossible to resolve all of the complex features of transitional and turbulent flows with a relatively simple Reynolds-averaged modeling approach, the results shown here demonstrate that the new model can provide a useful and practical tool for engineers addressing the simulation and prediction of transitional flow behavior in fluid systems.

A previously documented systematic computational methodology is implemented and applied to a jet-in-crossflow problem in order to document all of the pertinent flow physics associated with a film-cooling flowfield. Numerical results are compared to experimental data for the case of a row of three-dimensional, inclined jets with length-to-diameter ratios similar to a realistic film-cooling application. A novel vorticity-based approach is included in the analysis of the flow physics. Particular attention has been paid to the downstream coolant structures and to the source and influence of counterrotating vortices in the crossflow region. It is shown that the vorticity in the boundary layers within the film hole is primarily responsible for this secondary motion. Important aspects of the study include: (1) a systematic treatment of the key numerical issues, including accurate computational modeling of the physical problem, exact geometry and high-quality grid generation techniques, higher-order numerical discretization, and accurate evaluation of turbulence model performance; (2) vorticity-based analysis and documentation of the physical mechanisms of jet–crossflow interaction and their influence on film-cooling performance; (3) a comparison of computational results to experimental data; and (4) comparison of results using a two-layer model near-wall treatment versus generalized wall functions. Solution of the steady, time-averaged Navier–Stokes equations were obtained for all cases using an unstructured/adaptive grid, fully explicit, time-marching code with multigrid, local time stepping, and residual smoothing acceleration techniques. For the case using the two-layer model, the solution was obtained with an implicit, pressure-correction solver with multigrid. The three-dimensional test case was examined for two different film-hole length-to-diameter ratios of 1.75 and 3.5, and three different blowing ratios, from 0.5 to 2.0. All of the simulations had a density ratio of 2.0, and an injection angle of 35 deg. An improved understanding of the flow physics has provided insight into future advances to film-cooling configuration design. In addition, the advantages and disadvantages of the two-layer turbulence model are highlighted for this class of problems. [S0889-504X(00)01201-0]

This paper presents the development and implementation of a new model for bypass and natural transition prediction using Reynolds-averaged Navier-Stokes computational fluid dynamics (CFD), based on modification of two-equation, linear eddy-viscosity turbulence models. The new model is developed herein based on considerations of the universal character of transitional boundary layers that have recently been documented in the open literature, and implemented into a popular commercial CFD code (FLUENT) in order to assess its performance. Two transitional test cases are presented: (1) a boundary layer developing on a flat heated wall, with free-stream turbulence intensity Tu∞ ranging from 0.2 to 6%; and (2) flow over a turbine stator vane, with chord Reynolds number 2.3×105, and Tu∞ from 0.6 to 20%. Results are presented in terms of Stanton number, and compared to experimental data for both cases. Results show good agreement with the test cases and suggest that the new approach has potential as a predictive tool.

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The optical properties of core-shell CdSe-ZnS quantum dots (QDs) are characterized by complex photophysics leading to difficulties in interpreting quantitative measurements based on QD emission. By comparing the pH dependence of fluorescence of single QDs to that of an ensemble, we have been able to propose a molecular scale model of how QD surface chemical and physical processes are affected by protons and oxygen. We show that the connection between the ensemble fluorescence intensity and the single QD fluorescence properties such as dark fraction, blinking, particle brightness and a multi-exponential fluorescence lifetime decay is not trivial. The ensemble fluorescence intensity is more weakly dependent on pH than the single particle fluorescence which, together with fluorescence lifetime analysis, provided evidence that the dark fraction of QDs emits photons with low quantum efficiency and long lifetime. We uncovered two surface-dependent mechanisms that affected the fluorescence emission: an immediate physical effect of charges surrounding the QD and an irreversible chemical effect from reaction of the H+ and O2 with the QD shell surface. These results will have important implications for those using QD-based fluorescence lifetime imaging as well as for proper implementation of these probes for quantitative cellular imaging applications.