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Regarding initialization of LES Channel395
Hello Foamers,
I was trying to run the LES channel395 with Ubar=10.5 instead of 0.1355 which was the default. Whenever I do so I'm getting very unexpected result and its obvious because I kept the 0 folder unchanged where the U field had non-uniform list initialization with velocities around 0.1335. Quote:
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Just open the U file and change internalField uniform (0.1335 0 0); to internalField uniform (10.5 0 0);
Yes you have to run it for a long time, but you have to do for any velocity. The tutorial starts with a uniform field of 0.1335 unless you have taken a case already run from someone else. |
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As my general understanding, you are simulating a plane channel flow and you have periodic condition in streamwise direction driven by a forcing term, right? No matter how you initialize the velocity field, the mass flow rate is determined by the forcing term. You can start from any intial condition as you have to run the solution until the initial condition is totally disregarded.
In your case you can simply use the initial list using a multiplier of the entries C= 10.5/0.1355. But if the forcing term is not changed accordingly, I am quite sure that the solution is the same of the tutorial. Check that. |
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The reason is the the transient starting from the prescribed initial condition is only numerics, has no physical correlation. Therefore, you need to wait enough time until the flow is in statistically energy equilibrium. Only after that the velocity field is physically correlated and can be sampled. |
The confusion comes from an unmentioned, important, fact: he is using OpenFOAM.
So, for Filippo: OF settings for this case are based on fixed mass flow rate (i.e. average velocity) instead of pressure gradient. For Arijit: come on, don't be so Fluent user. This is the main forum so, you either, at least, mention what you are using (I don't even want to get into the general utility of the discussion) or go to the OF forums (there are, literally, tens here). |
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If this is so then isn't it simply better to specify uniform field(10.5,0,0) because in anyway we have to wait until the flow becomes develop? Note:- According to my understanding we used initial non-uniform field in the tutorial channel so that the flow is in developed stage from 0th sec. |
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Paolo, I remember you can set in OF both mass flow and pressure forcing, isn't that?
At least in the version I worked on with Franco and Andrea during LESinItaly benchmark. In that case the full set of equations implied also a forcing in the energy equation, maybe this is not the set of equations adopted in the tutorial... However, even using the mass flow rate, the forcing is determined dinamically and does not depend on the initial velocity field, isn't it?? |
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You can set a uniform field but this way the onset of the departure from the Poiseulle solution is due to the perturbation introduced by the numerical errors. That can produce a very long transient but also a false convergence to a steady laminar solution if the method produced too artificial dissipation. |
I guess the issue here is just how to multiply a given initial field in OF, one given cell by cell and not as uniform initial field, by the constant 10.5/0.1355. So it should not regard the forcing or LES at all, that's why I wrote my previous post.
My suggestion, write a small OF program that just reads it, makes the multiplication and saves it again. It is a one shot need that you have and the whole point of using OF is that you can do this stuff easily. |
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Keeping in mind we cant use uniform field as it will lead to immense dissipation error and long transient time. If the solution to all these problems is just scale up the initial field then Python will be a good choice isnt it? |
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Paolo suggested the way previously. Just read and scale each value of the list. |
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