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Old   July 19, 2013, 09:59
Question zeroGradient BC
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Jason Pearl
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Hi all,
after reviewing a number of forums it seems to me that the zeroGradient BC is often used as a filler conditions, when often the boundary condition should be extrapolated from the internal flow (supersonic outlets) or some other BC would better describe the actual physics.
I feel silly asking this but I take zeroGradient to mean d/dt of the field is zero at the boundary. So for velocity the acceleration would be zero, or in the case of T zero thermal conductive heat transfer etc.

Why is zeroGradient used in many cases were the gradient of the field is not actual zero?
Is it because there is not a readily available substitute?

It seem zeroGradient is one of the more stable BC (Ive been using rhoCentralFoam for microNozzle simulations that include some of the plume)

I looked at the source code and had trouble decyphering how the actual code works I just started teaching myself C++

and help is greatly appreciated
cheers
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Old   July 19, 2013, 10:02
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Alexey Matveichev
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Hi,

zeroGradient is actually not time derivative (dF/dt = 0) but spatial derivative (dF/dx = 0).
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boundary condition, physical model, zerogradient

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