zeroGradient BC

jaason · July 19, 2013, 09:59

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

alexeym · July 19, 2013, 10:02

Hi,

zeroGradient is actually not time derivative (dF/dt = 0) but spatial derivative (dF/dx = 0).

July 19, 2013, 09:59	zeroGradient BC	#1
jaason New Member Jason Pearl Join Date: Jul 2013 Location: Burlington Posts: 10 Rep Power: 12	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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July 19, 2013, 10:02		#2
alexeym Senior Member Alexey Matveichev Join Date: Aug 2011 Location: Nancy, France Posts: 1,930 Rep Power: 38	Hi, zeroGradient is actually not time derivative (dF/dt = 0) but spatial derivative (dF/dx = 0).