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# CFD - Cell Size h

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I wonder, if reducing or increasing the cell size h in a cartesian grid would cause any changes in the numerical behaviour of my fluid simulation? (Keeping in mind, that the number of cells doesn''t change.) Cheers, Oliver

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ad far as i know, the simulation would be more accurate, since you sample the equations more often, but your final output will be smaler, or youll have to add cells.

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But, the sample rate is still the same, because I assume, that the number of cells don't change. However, you are right, that my output gets tighter if I change h=1.0 to h=0.5, for example.

To be honest, I actually don't think, that the numerical behaviour will be affected, when changing the cell size h.

[edited by - pilarsor on August 7, 2003 3:55:20 AM]

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No, it would.

think you have a 1D function graph, and you want to compute its slope at a given point, and you dont how to analiticaly do that.
then you will discretize the function, sample it every h, and then just use:
slope = (y2-y1)/(x2-x1) to compute the slope, if your h is small, the derivative would be more precise.

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There is a limited numerical range in a floating point number. If you shift your cell size, then you shift the point where your numbers go denormal, or go to infinite, compared to your cell size.

Here''s a good brain teaser:

What is the exact result of adding two 32-bit IEEE floating point numbers, where one has the value 1e10, and the other has the value 1e-10 ?

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