GAMMA VE BC GEÇİŞ MODELLERİNİN DIŞ AKIŞLAR İÇİN DEĞERLENDİRİLMESİ VE KARŞILAŞTIRILMASI

Abstract

Laminer akıştan türbülanslı akışa geçişin modelleri, yenilenebilir enerji, İHA teknolojileri ve benzeri havacılık uygulamaları alanındaki son gelişmeler nedeniyle yeniden popüler haline gelmiştir. Laminer akıştan türbülansa geçişin HAD analizlerinde modellenmesi oldukça zor bir konudur. Geçiş bölgesi ihmal edilirek HAD çözümlerinde akışın tamamen türbülanslı olduğu varsayılırsa sürükleme kuvveti gerçeğinden fazla tahmin edilir. Bu durum, akışın temel özelliklerinin gözden kaçırılmasına ve akış alanının yanlış tahmin edilmesine neden olmaktadır. En popüler geçiş modelleri SST türbülans modeline uygulanan Menter modelleri ve Spalart-Almaras modeline uygulanan Baş-Çakmakçıoğlu (BC) modelidir. Bu çalışmada, Menter'in daha sade ama daha popüler olan γ modeli ve Baş Çakmakçıoğlu modellerinin dış akışlaradaki performansına odaklanılmıştır. γ modeli, harici akışlarda uygulanmasını zorlaştıran yerel türbülans yoğunluğuna dayanmaktadır. Bu zorluk, türbülans azalması ile geçiş başlangıcı arasındaki karmaşık ilişkiden kaynaklanmaktadır. BC geçiş modeli ise serbest akış türbülans yoğunluğunu kullanmaktadır. Her iki model de Klebanoff ve ERCOFTAC düz levha deney verileri ve iki boyutlu harici akış deney verisi kullanılarak doğrulanmıştır. Yüzey sürtünme katsayısı sonuçları deneysel verilerle karşılaştırılır. Sonuçlar, her iki modelin de türbülans geçişini çok benzer şekilde tahmin ettiğini göstermektedir. BC geçiş modeli hesaplama açısından γ modelinden daha ucuz ve uygulaması daha kolaydır. Ayrıca, γ modeli sınır koşullarının pratik olarak belirlenmesindeki belirsizlikten muzdariptir.

Keywords

• Geçişli Akış
• Geçiş Modeli
• Kesiklilik
• HAD
• Düz Levha

ASSESSMENT AND COMPARISON OF THE GAMMA AND BC TRANSITION MODELS FOR EXTERNAL FLOWS

Abstract

Modelling of transition from the laminar to turbulent flow became a hot topic due to recent developments in renewable energy, UAV technologies and similar aerospace applications. The transition from laminar flow to turbulence is challenging to model in CFD analysis. The drag is overestimated if the transition is neglected in CFD solutions by assuming the flow is fully turbulent. This results in missing the fundamental characteristics of the flow and inaccurate predictions of the flow field. The most popular transition models are Menter's models applied to the SST turbulence model and the Baş-Çakmakçıoğlu (BC) transition model applied to the Spalart-Almaras model. We have focused on Menter's simpler but more popular γ model and Baş Çakmakçıoğlu models. The γ model relies on the local turbulence intensity, which makes applying the model challenging in external flows. This difficulty stems from the complex relationship between turbulence decay and transition onset. BC transition model utilizes the free stream turbulence intensity. Both models are verified using the Klebanoff and ERCOFTAC flat plate cases and several 2D external flow cases. Skin friction coefficient results are compared to experimental data. Results show that both models predict transition very similarly. BC model is computationally cheaper and easier to implement than the γ model. Also, γ model suffers from boundary conditions ambiguity.

Keywords

• Transition Flow
• Transition Model
• Intermittency
• CFD
• Flat Plate

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