zeroGradient BC

jaason · July 19, 2013, 10:59

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, 11:02

Hi,

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

July 19, 2013, 10:59	zeroGradient BC	#1
jaason New Member Jason Pearl Join Date: Jul 2013 Location: Burlington Posts: 10 Rep Power: 13	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

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
second order schemes	marine	OpenFOAM	67	April 11, 2022 19:19
multiphaseInterFoam: timestep error by simulating a co-extrusion nozzle	Quatschinsky	OpenFOAM Running, Solving & CFD	7	March 27, 2014 06:08
Temperature Issue in OpenFOAM-2.2.0	prasant	OpenFOAM	0	March 12, 2013 08:17
[swak4Foam] Air Conditioned room groovyBC	Sebaj	OpenFOAM Community Contributions	7	October 31, 2012 15:16
Pressure instability with rhoSimpleFoam	daniel_mills	OpenFOAM Running, Solving & CFD	44	February 17, 2011 18:08

July 19, 2013, 11:02		#2
alexeym Senior Member Alexey Matveichev Join Date: Aug 2011 Location: Nancy, France Posts: 1,938 Rep Power: 39	Hi, zeroGradient is actually not time derivative (dF/dt = 0) but spatial derivative (dF/dx = 0).