PENGUJIAN MODEL TURBULENSI U-RANS
DUA-PERSAMAAN YANG DIMODIFIKASI UNTUK MEMPREDIKSI TERJADINYA DYNAMIC STALL
Nama mahasiswa : Galih Senja Titah Aji Bangga NRP : 2111.202.901
Dosen pembimbing : Prof. Dr.-Ing. Herman Sasongko
ABSTRAK
Kebutuhan analisa pada sudu helikopter, kompresor, kincir angin dan struktur streamline lainya pada angle of attack yang tinggi yang melibatkan efek instasioner yang disebut dynamic stall menjadi semakin penting. Fenomena ini ditandai dengan naiknya dynamic lift melebihi static lift maksimum pada critical static stall
angle, vortex terbentuk pada leading edge yang mengakibatkan naiknya suction contribution yang kemudian terseparasi dan terkonveksi sepanjang permukaan
hingga mencapai trailling edge dan diikuti terbentuknya trailling edge vortex yang menunjukkan terjadinya lift stall. Prediksi numerik yang dilakukan pada fenomena ini sering menunjukkan ketidak-akuratan utamanya pada lift coefficient yang sering menunjukkan overprediction dengan penggunaan model turbulensi U-RANS dua-persamaan baik basis k-ω maupun k-ε, hal ini dikarenakan free stream turbulence pada model tersebut terlalu tinggi.
Metode numerik digunakan untuk memahami secara detail fenomena dynamic stall menggunakan CFD solver Ansys Fluent 13.0 untuk 2D geometrical configuration secara unsteady dan incompressible pada profil NACA 0012 dengan Reynolds
number sebesar 1.35x105. Model turbulensi standard k-ε dimodifikasi untuk
mereduksi tingkat turbulensi dengan menambahkan damping factor sebagai fungsi
y+ pada daerah buffer zone serta menambahkan koefisien redaman yang mengakomodasi efek rotasi airfoil. Model osilasi dan modifikasi model turbulensi disusun dengan bahasa pemrograman C dalam user defined function (UDF)
subroutine. Validasi dilakukan dengan hasil eksperimen serta model turbulensi original seperti standard k-ε, realizable k-ε, standard k-ω, serta SST k-ω.
Perubahan nilai parameter damping yang digunakan sangatlah mempengaruhi hasil simulasi. Hasil modifikasi memberikan peningkatan akurasi prediksi lift coefficient pada kasus dynamic stall. Reduksi overprediction ditunjukkan dengan menggunakan model ini dibandingkan dengan empat model turbulensi original U-RANS dua-persamaan meskipun pada angle of attack terendah nilai lift coefficient lebih mampu diprediksikan secara baik oleh model turbulensi standard k-ε dan SST
k-ω. Studi yang dilakukan pada daerah wake menunjukkan efek leading edge
vortex (LEV) dan trailling edge vortex (TEV) semakin berkurang dengan
bertambahnya jarak titik analisa dengan airfoil. Perkembangan aliran fluida pada kondisi dynamic stall yang dikarakterisasi oleh terbentuknya LEV dan TEV mampu diprediksikan secara baik pada simulasi yang dilakukan.
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ASSESSMENT OF MODIFIED TWO-EQUATIONS U-RANS TURBULENCE MODEL
TO PREDICT THE ONSET OF DYNAMIC STALL
Student Name : Galih Senja Titah Aji Bangga NRP : 2111.202.901
Supervisor : Prof. Dr.-Ing. Herman Sasongko
ABSTRACT
The necessity in the analysis of helicopter blade, compressor, wind turbine and other streamlined structure operating at high angle of incidence that included instationary effect the so called dynamic stall becomes increasingly important. This phenomenon is indicated by lift coefficient that continuously increase passed static lift on the critical static stall angle, vortex formed on the leading edge that increased suction contribution then it is separated and convected along airfoil surface to trailling edge and it is followed by trailling edge vortex formation that shows the beginning of lift stall. Numerical prediction on this phenomenon often shows inaccurate results primarily in the lift coefficient that usually shows overprediction using two-equations U-RANS turbulence model whether it is k-ω based or k-ε based, this is due to free stream turbulence on this models are too high. Numerical methods is used to understand in detail about dynamic stall phenomenon using Ansys Fluent 13.0 as CFD solver for 2D geometrical configuration in frame of unsteady and incompressible analysis using NACA 0012 profile by Reynolds number of 1.35x105. Standard k -ε turbulence model is modified to reduce turbulence intensity by the addition of damping factor as the function of y+ in the
buffer z one and damping coefficient that accommodate the effects of airfoil
rotation. Oscillation model and turbulence model modification is compiled in C programming language in frame of user defined function (UDF) subroutine. Experiment result and original turbulence model such as standard k-ε, realizable
k-ε, standard k-ω, and also SST k-ω are used as validation methods.
The changed in damping parameter value could highly affected simulation results. This modification increase the accuration of lift coefficient prediction in case of dynamic stall. This model could give reduction in overprediction value compared with four U-RANS original turbulence model even though the value of lift coefficient at lowest angle of attack is better predicted by standard k-ε and SST k-ω turbulence model. The effects of leading e dge v ortex (LEV) and trailling e dge
vortex (TEV) are decreased by the increased in distance of analysis point from
airfoil. Flow development of the fluid flow in dynamic stall case that is characterized by LEV and TEV formation is well predicted in this simulation.
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