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LES of flow around a car

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Old   November 26, 2018, 13:17
Default LES of flow around a car
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Terrance Charles
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Hello,

I am trying to simulate a LES of flow around a car. I am not able to compute Turbulent Kinetic Energy. It shows the same value for every iteration. I am not sure what i am missing in setting up the simulation. Kindly help me.
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Old   November 26, 2018, 13:36
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Hi,
Which model are you using? Which software? "Standard" les model like smagorinky, wale or dynamic model do not use tke and does not solve for this quantity.
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Old   November 26, 2018, 13:36
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There is no turbulent kinetic energy (a time-averaged variable) for a time-accurate simulation (LES is time-accurate). How do you even have any values to look at?
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Old   November 26, 2018, 13:40
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Same answer at the same time up to my knowledge there is a les model with an improvede eddy viscosity formulation which solves for tke, but I don't know if this is the case.
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Old   November 26, 2018, 13:42
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Im using Star CCM+. Im using WallE model.
Could i get an idea of how i should set up LES for flow around a car pls. The user guide and star ccm+ help have steps only for Aeroacoustics and transient.
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Old   November 26, 2018, 13:52
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Ok, wale model does not solve for tke. You can look at this model here in the form to convince yourself. It calculates an effective viscosity using the strain tensor and the flow gradients.

First of all, LES is always transient, are you trying to do a steady state simulation?

Apart this, a LES is not so easy to setup, grid requirements are much larger than a urans and you need some testing to have a nice mesh. Then you shuold specify the filter to use (usually cubic root of cell volume).

If you use star, than there is an ad hoc forum where you can post you question.
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Old   November 28, 2018, 08:46
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I am trying to setup an LES on Star CCM+. I am not sure how i will know when the simulation has converged or not, because Star CCM+ does show that the simulation has converged like Ansys. I will have to do some more indepth study on Star CCM+ before i could start doing LES. Thank you very much.
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Old   November 28, 2018, 08:52
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Ok, but please note that a LES is always an unsteady simulation. Thus, what do you mean with "convergence"? You can have a convergence only from a statistical point of view!
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Old   November 28, 2018, 09:21
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I'm aware LES is unsteady. When i say converged, i mean the difference in Mean averaged data for time steps.
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Old   November 28, 2018, 12:02
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Quote:
Originally Posted by AliE View Post
Same answer at the same time up to my knowledge there is a les model with an improvede eddy viscosity formulation which solves for tke, but I don't know if this is the case.

Just wanted to add that these models solve for the subgrid-scale tke, not the full tke. Even with Smagorinsky models you can calculate the sgs tke by using a cute formulas (the square of turbulent viscosity divided by length scale).


To calculate the full tke, you need the time-accurate filtered velocity (the instantaneous x-velocity, y-velocity, z-velocity from Star), take the rms, and add to it the sgs tke calculated using the same formula. The rms of the velocities you can get from Star tools built-in. You have to do some work to get the sgs tke. Maybe you have to define a field function if it's not built-in (I'm not sure if it is built-in or not in Star-CCM, I assume probably not).
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Old   November 29, 2018, 07:32
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Thank you everyone. I have a a clearer idea of what i need to do now.
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