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Open Source Computational Fluid Dynamics

An MSc course to gain extended knowledge in Computational Fluid Dynamics (CFD) using open source software.

Zoltán Hernádi

Department of Fluid Mechanics

Budapest University of Technology and Economics

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Turbulent flows

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Turbulence modeling

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Some of the RAS turbulence models

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Some of the LES turbulence models

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Wall functions for RAS turbulence

Standard wall functions: y+ < 5 or 30 < y+ < 300.

→UserGuide 7.2.2

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Compressible flows

• Incompressible model: ρ = constant → kinematic equations, e.g. Pa → m2/s2

• Compressible model: ρ is based on equationOfState, e.g. perfectGas: ρ = p/RT

• Constant temperature air:

– If Δρ/ρ = 0.05 → Δp/p = 0.05 → Δp = 5000 Pa

– 5000 Pa dynamic pressure → 92 m/s air speed

– If Ma < 0.27, then incompressible model for air is 5% accurate!

• Historically: pEqn for incompressible, rhoEqn for compressible.

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Example: supersonic forward facing step