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Öğe An algebraic hybrid RANS-LES model with application to turbulent heat transfer(Springer Verlag, 2018) Yilmaz, I.An algebraic hybrid RANS-LES model is proposed. The RANS mode employs a modified mixing-length approach near walls. Instead of using the magnitude of the resolved strain-rate tensor, the advanced spatial operator used by the WALE subgrid-scale model is embedded into the formulation due to its better scaling properties for the turbulent/eddy-viscosity. The LES mode incorporates the well-known WALE model to resolve flow in the rest of the domain. Similar to the robust, well-calibrated, low-cost, algebraic hybrid RANS-LES model (HYB0) which combines simple mixing length-type RANS near wall with Smagorinsky model in the off-wall region, the same turbulent length-scale adaptation approach for the RANS-LES interface is utilized for proper interaction between the modes. In addition to correct near-wall behaviour, the use of the advanced WALE differential operator should theoretically ensure smooth transition at the interface, detection of turbulent structures inside the eddies and better prediction of the log-layer velocity profile. Turbulent Rayleigh-Bénard problem is studied using both models to examine and to compare their performances without the Boussinesq approximation. It is found that HYB0 is slightly dissipative compared to mHYB0 and predicts fewer interactions among thermal plumes. However, HYB0 has interestingly better near-wall behavior. No interface issues are observed with the usage of the WALE operator. Further applications are required to characterize the behavior of the both models. © 2018, Springer International Publishing AG.Öğe Analysis of Rayleigh-Taylor instability at high Atwood numbers using fully implicit, non-dissipative, energy-conserving large eddy simulation algorithm(Amer Inst Physics, 2020) Yilmaz, I.Large eddy simulation of three-dimensional, multi-mode Rayleigh-Taylor instability at high Atwood numbers is performed using a recently developed, kinetic energy-conserving, non-dissipative, fully implicit, finite volume algorithm. The algorithm was especially designed for simulating low-Mach number, variable density/viscosity, transitional, and turbulent flows. No interface capturing mechanism is required. Buoyancy and heat transfer effects can be handled without relying on the Boussinesq assumption. Because of this feature, unlike the pure incompressible ones, it does not suffer from the loss of physical accuracy at high Atwood and Rayleigh numbers. In this study, the mixing phenomenon in Rayleigh-Taylor instability and the effects of high Atwood numbers on the development of the flow are investigated using various diagnostics such as local mole fractions, bubble and spike penetration lengths and growth rates, mixing efficiencies, Taylor micro-scales, and corresponding Reynolds numbers and energy ratios. Additionally, some important terms of the Reynolds stress transport equation are also introduced, such as Reynolds stresses (and their anisotropies) and turbulent production. Results show that Rayleigh-Taylor instability at high Atwood numbers is characterized by rapid development of instability due to the increasing growth rates and higher velocities of spike fronts, larger asymmetry in the mixing region, denser interactions in the non-linear phase, and changes in bubble and spike morphologies. It is also found that interactions of spike-fronts with their surroundings are the primary mechanisms of turbulent production and transition to turbulence. However, late time mean flow measures such as energy ratio and mixedness are not significantly affected. A scaling relation between the spike to bubble penetration ratio and the heavy to light density ratio is also provided.