# 4 - 1 Turbulence mixing Turbulence mixing 4th lecture FACE...

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1 Turbulence & mixing Turbulence & mixing 4 th lecture FACE 8 2007

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2 Turbulence & mixing ·P r o g r a m 08:15-08:45 Lecture: Derivation of RANS 08:45-09:00 Exercise: Questions about turbulence 09:00-09:15 Break 09:15-10:00 Lecture: Turbulence models 10:00-12:00 Exercise: Test case in Fluent ·L i t e r a t u r e Steve Bergs lecture notes – Chapter 3 Suggested reading: Fluent user guide Best practice guidelines L. Davidsons – introduction to turbulence models
3 Turbulence & mixing · Turbulence structure · How to simulate this?

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4 Turbulence & mixing · DNS (Direct Numerical Simulation) Idea: Just use the NS-equations… Problem: Need to resolve the time and length scales of turbulence / Conclusion: Not practical possible for most problems ·L E S (Large Eddy Simulation) Idea: Okay, let just resolve the larger time and length scales then Problem: Still very demanding + difficult to setup Conclusion: We only want to use LES for specific problems ·R A N S (Reynolds Averaged Navier Stokes) Idea: Lets forget about eddies and whirls and try to find some way to model the effect of turbulence on the overall flow Problem: This is an oversimplification of the problem which yields erroneous results and require an insight to use Conclusion: This is the fast solution and we must be skeptical of the result; but we’ll probably be using RANS models most of the time! More time More computational demanding
5 Turbulence & mixing · Starting point: NS ·R A N S · Closure problem How to approximate τ T ? We need an universal model! (no a priori knowledge needed) ij i j i i jj j uu u P S tx x x ρ τ −∂ += + + ∂∂ () i ij i j i j u P S x x ρρ −∂ ′′ +− + Turbulent stress or Reynolds stresses: τ T

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6 Turbulence & mixing · Many different solutions to the closure problem Zero and one equation models: Spalart-Allmaras Two equation models: Standard k- ε RNG k- ε Realisable k- ε k- ω SST k- ω Reynolds Stress Model (RSM) Why do we need to know about different turbulence models ? Just mark the k- ε and press go!
7 Turbulence & mixing ·W h y k - ε and why not k - ε is the most (”only”) used in the industry It is stable and fast It is very well documented Since it has been in use for so long time people know what they get; other factors are more important!

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4 - 1 Turbulence mixing Turbulence mixing 4th lecture FACE...

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