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Trying to validate simulation of flow over a tractor-trailer

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Old   February 11, 2015, 12:04
Question Trying to validate simulation of flow over a tractor-trailer
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Kartik Arora
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Okay so i took up this project for finding drag force over a truck.
I have liitle to no knowledge of cfd.Ihave made the geometry(attached) and have checked out some tutorials on youtube.My mentor said that you need to validate the problem.
Can you help me understand what is validation and how do i do it?
links would also be appreciated.
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Old   February 11, 2015, 12:11
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Cees Haringa
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Hello Kartik,
Validation is checking if your CFD solution is feasible.
Qualitatively, you can see if the vector field looks good (the flow around the vehicle looks like you would expect it to). But, of course a quantitative validation is much, much more desirable. This means looking up experimental data, and see if your calculations are compatible with experiments.

I am by no means knowledgable about the literature related to trucks, but what I can imagine is that there is data available on the drag coefficient (which you can compute with FLUENT as well), perhaps on the flow velocity at some points around the geometry, and about pressure fields in the wake or so. If you can find those you should be able to compare them with your work, hence validate it.
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Old   February 11, 2015, 12:48
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Thanks for the quick reply.
The geometry i have made is completely random. Won't it be a problem to match exp. data with my model because of difference in geometry?
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Old   February 11, 2015, 14:01
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If you need an exact match, yes. But in any way CFD will never give you the exact answer, since the model is full of approximations and assumptions.

So to an order of magnitude you may validate your results.
For example, engineering toolbox gives, for a tractor trailed truck, a value of 0.96 (and a truck in general 0.8-1.0) for Cd. So, if you find a value of 0.5 or 2 your model is probably not very accurate. But 0.9 would be fine.
Also, see if the results depend on your mesh! Try different mesh densities, and see if the values of cd agree (they should converge to a certain value with increasing mesh density)
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Old   February 11, 2015, 15:07
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Kartik Arora
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Thank you so very much.you have been really helpful to me.
Surprisingly CFD sounds interesting now
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