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Türbülanslı Rayleigh-Bénard Taşınım Probleminin Büyük Girdap Benzetimi: Ağaltı-ölçek Modellerinin Değerlendirilmesi

Bu çalışmada türbülanslı Rayleigh-Bénard ısıl taşınım problemi büyük girdap benzetimi metodu ile 6farklı ağaltı-ölçek modeli kullanılarak gerçekleştirilmiştir. Bu modeller; (i) Smagorinsky (duvarsönümleme fonksiyonu da içeren), (ii) Dinamik Smagorinsky, (iii) Wall-Adapting Local Eddy-Viscosity,(iv) Vreman, (v) Mixed-Scale, ve (vi) ilk defa bu çalışmada önerilen ve ağaltı ölçeklerden gelen türbülanslıkaldırma kuvveti etkilerini de içerecek şekilde, terimleri yeniden düzenlenmiş ve zenginleştirilmiş olan değiştirilmiş-Mixed-Scale modelidir. Benzetimlerde sönümleme içermeyen, kinetik enerjiyi koruyan vezamanda tamamıyla kapalı bir sayısal ayrıklaştırma algoritması kullanılmıştır. Modellerindeğerlendirilmesi için, akışın ortalama ve türbülanslı büyüklükleri (hem düşük hem de yüksek mertebeli)hesaplanmıştır. Ayrıca, asimetri, türbülanslı ısı akısı, ağaltı-ölçek kinetik enerjisi ve Nusselt sayısı gibiilave pek çok ileri seviye, türetilmiş türbülans parametresi de hesaplanmış ve karşılaştırmalardakullanılmıştır. Bunlara ilişkin detaylı analizlere bu kapsamlı çalışmada yer verilmiştir. Elde edilensonuçlar, her modelin zayıf ve güçlü yanlarını ortaya çıkarmıştır. Modeller arası farklılıkların özellikleduvara yakın bölgelerde kendini gösterdiği ortaya konmuştur. Genel olarak, Mixed-Scale ve ona dayalıolarak önerilen yeni modelin performanslarının, Doğrudan Sayısal Benzetim metodu ile daha iyi biruyum içerisinde olduğu gözlemlenmiştir. Bu iki model ayrıca daha kısa sürede sonuç vermesi sebebiylede sayısal hesaplama maliyeti açısından avantajlıdır. Kaldırma kuvveti etkilerini de içeren yeni modeleait sonuçlarının asıl modele oranla görece bir iyileşme içerdiği de görülmektedir.

Large Eddy Simulation of Turbulent Rayleigh-Bénard Convection: AnAssessment of Subgrid-Scale Models

Large eddy simulation of turbulent Rayleigh-Bénard convection was carried out to assess various algebraic eddy viscosity subgrid-scale models: (i) Smagorinsky with Wall-Damping, (ii) Dynamic Smagorinsky, (iii) Wall-Adapting Local Eddy-Viscosity, (iv) Vreman, (v) Mixed-Scale, and (vi) a buoyancymodified Mixed-Scale model that accounts for the buoyancy effects from subgrid-scales. The last model is proposed for the first time in this study. Non-dissipative, kinetic energy conserving, fully implicit method was employed for simulations. To evaluate the models, mean and turbulent (both low- and high-order) flow diagnostics were computed. Some advanced turbulent statistics such as skewness, turbulent heat flux, subgrid-scale kinetic energy and Nusselt number were also calculated and compared with each other and against a reference solution. Since models differ from each other by means of turbulent generation terms, they have their own strengths and weaknesses which are particularly observed in the near-wall treatments. Additionally, unlike the others, the Dynamic Smagorinsky model computes the subgrid-scale viscosity coefficient dynamically which has some effects on results. Overall, the Mixed-Scale and its new, buoyancy-modified variant show different characteristics and mostly the best agreement with Direct Numerical Simulation data. They are also found computationally less expensive. Moreover, buoyancy enhancement in the new model slightlyimproves the predictions of Mixed-Scale model. Although relatively poor performance by the Dynamic Smagorinsky model is observed especially in estimating the integrated Nusselt number, it captures the turbulent heat flux more accurately than the others. A more detailed discussion on the model's performance based on evaluations are also made.

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