Yazar "Yilmaz, Ilyas" seçeneğine göre listele

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    A novel buoyancy-modified subgrid-scale model for large-eddy simulation of turbulent convection
    (Emerald Group Publishing Ltd, 2021) Yilmaz, Ilyas
    Purpose - The purpose of this paper is to develop a subgrid-scale (SGS) model for large eddy simulation (LES) of buoyancy- and thermally driven transitional and turbulent flows and further examine its performance. Design/methodology/approach - Favre-filtered, non-dimensional LES equations are solved using non-dissipative, fully implicit, kinetic energy conserving, finite-volume algorithm which uses an iterative predictor-corrector approach based on pressure correction. Also, to develop a new SGS model which accounts for buoyancy, turbulent generation term in SGS viscosity is properly modified and enhanced by buoyancy production. Findings - The proposed model has been successfully applied to turbulent Rayleigh-Benard convection. The results show that the model is able to reproduce the complex physics of turbulent thermal convection. In comparison with the original wall-adapting local eddy-viscosity (WALE) and buoyancy-modified (BM) Smagorinsky models, turbulent diagnostics predicted by the new model are in better agreement with direct numerical simulation. Originality/value - A BM variant of the WALE SGS model is newly developed and analyzed.
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    Application of a parallel solver to the LES modelling of turbulent buoyant flows with heat transfer
    (Inderscience Enterprises Ltd, 2018) Yilmaz, Ilyas; Saygin, Hasan; Davidson, Lars
    An existing fully implicit, non-dissipative direct numerical simulation (DNS) algorithm is reformulated to utilise the sub-grid scale (SGS) models in large eddy simulation (LES). The Favre-filtered equations with low-Mach number scaling are derived. The wall-adapting local eddy-viscosity (WALE) is used as SGS model. A fully parallel, finite volume solver is developed based on the resulting LES algorithm using PETSc library and applied to buoyancy-and thermally-driven transitional/turbulent flows in Rayleigh-Taylor instability and turbulent Rayleigh-Benard convection. Results verify that the proposed low-Mach number LES approach, which is physically more accurate than pure incompressible methods for flows with variable properties, perfectly captures the evolution and complex physics of turbulent buoyant flows with or without heat transfer by taking the effects of density and viscosity changes into account without the Oberbeck-Boussinesq (OB) assumption even at large temperature differences with uniform accuracy and efficiency.
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    Development and Validation of a High-Order Fully-Implicit DNS/LES Algorithm for Transitional and Turbulent Buoyant Flows with Heat Transfer
    (Springer International Publishing Ag, 2019) Yilmaz, Ilyas
    A high-order, finite-volume algorithm specially designed for simulating low-Mach number, variable-density, buoyancy- and thermally-driven, transitional and turbulent flows with or without heat transfer is proposed. For this purpose, the fully-implicit, non-dissipative, discrete kinetic energy-conserving Direct Numerical Simulation (DNS) algorithm is combined with high-order, symmetric/central-differencing finite-volume approximations. The Wall-Adapting Local Eddy-viscosity (WALE) model is also utilized for subgrid-scale (SGS) modeling. To validate the proposed algorithm, two types of flows are considered; the turbulent Rayleigh-Benard Convection (RBC) and the Rayleigh-Taylor Instability (RTI). The selected problems include various mechanisms and multiple scales such as baroclinic vorticity, diffusion, mixing, interface interactions, density gradients, buoyancy and thermal forces. All those effects drive the flows into a transitional regime that eventually results in a relative turbulent state. As the first aim of this ongoing study, the proposed algorithm is successfully validated against the two challenging test cases. The results show its efficiency on coarse grids. Additionally, the wall-clock time of the computations are only 10-15% higher than the lower-order ones and unlike the many other high-order methods such as spectral, compact, WENO-type or DG, the proposed one is easy to implement into an existing code, relatively low-cost, robust, extendable to complex geometries and not seriously limited by flow physics or numerical constraints, due to inherently advanced properties of the base algorithm. The very small discrepancies observed near walls in RBC may point out that a more careful treatment of boundaries with walls might be required with the higher-order scheme.
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    Parallel direct numerical simulation and analysis of turbulent Rayleigh-Benard convection at moderate Rayleigh numbers using an efficient algorithm
    (Pergamon-Elsevier Science Ltd, 2020) Yilmaz, Ilyas
    Direct numerical simulation of turbulent Rayleigh-Benard convection up to Rayleigh number 10(8) is performed using a fully-implicit, non-dissipative, discrete kinetic energy-conserving algorithm and a parallel flow solver based on it. The algorithm is especially suitable for simulating low-Mach number, variable density/viscosity, transitional and turbulent flows with or without heat transfer. Furthermore, since it does not rely on the Boussinesq assumption, large temperature differences and high Rayleigh numbers can be handled without loss of accuracy, unlike the pure incompressible ones. It is first shown that the algorithm is able to predict the evolution of thermally-driven instability to turbulent regime and all the characteristics of turbulent convection accurately, using low- and high-order turbulent statistics and various secondary diagnostics derived. Then, effects of increasing Rayleigh numbers on the development of the instability are analyzed in detail. Additionally, Nusselt-Rayleigh scaling properties are studied and a scaling relation is provided. Results show that Rayleigh-Benard convection at relatively high Rayleigh numbers, corresponding to a boundary layer-dominated regime and little beyond it to a bulk-dominated regime, is characterized by weakening thermal fluctuations, thinning thermal boundary layers, increasing vertical velocity fluctuations and decreasing skewness. It is also observed that the turbulent heat flux dominates the heat transfer. Finally, the corresponding Nusselt-Rayleigh scaling relation is predicted as Nu = 0.132Ra(0.297). (C) 2020 Elsevier Ltd. All rights reserved.