Implementation of a relaxation boundary condition

fs82 · December 19, 2011, 06:04

Hello Foamers,

I would like to implement a new boundary condition for open boundaries for my LES. I found a well written paper of Palma and Matano (1998) "On the implementation of passive open boundary conditions for a general circulation model: The barotropic mode". They suggest various types of passive open boundary treatments, e.g. a radiation condition (named advectiveBC in Openfoam) or the one I am interested in: relaxation methods. The basic equation looks as follows:

$\Phi = \alpha\,\Phi_0 + ( 1 - \alpha\,\tilde{\Phi})$

$\alpha$ , the relaxation parameter varies from 0 at the inner edge to 1 at the boundary, $\Phi_0$ the reference state and $\tilde{\Phi}$ the unrelaxed value. The bc treatment requires for my understanding write access to the internal velocity field. My question is now: where is the best starting point? Would it be better to implement it as a boundary condition or as a wall function? May be it is also an opportunity to add it as a additional volume force to the navier stokes equation? Thanks for your suggestions in advance.

kind regards,
Fabian

ngj · December 19, 2011, 06:54

Hi Fabian

I have been doing exactly what you are requesting for free surface water waves. The entire framework is readily available, however, you might need to modify some of the classes to eliminate the need of the void ratio.

I have released the framework, and it is described here:

http://openfoamwiki.net/index.php/Contrib/waves2Foam

Best of luck,

Niels

fs82 · December 19, 2011, 08:04

Alright sounds beautiful. I will check it. Thanks a lot.

Fabian

December 19, 2011, 06:04	Implementation of a relaxation boundary condition	#1
fs82 Senior Member Dr. Fabian Schlegel Join Date: Apr 2009 Location: Dresden, Germany Posts: 222 Rep Power: 18	Hello Foamers, I would like to implement a new boundary condition for open boundaries for my LES. I found a well written paper of Palma and Matano (1998) "On the implementation of passive open boundary conditions for a general circulation model: The barotropic mode". They suggest various types of passive open boundary treatments, e.g. a radiation condition (named advectiveBC in Openfoam) or the one I am interested in: relaxation methods. The basic equation looks as follows: $\Phi = \alpha\,\Phi_0 + ( 1 - \alpha\,\tilde{\Phi})$ $\alpha$ , the relaxation parameter varies from 0 at the inner edge to 1 at the boundary, $\Phi_0$ the reference state and $\tilde{\Phi}$ the unrelaxed value. The bc treatment requires for my understanding write access to the internal velocity field. My question is now: where is the best starting point? Would it be better to implement it as a boundary condition or as a wall function? May be it is also an opportunity to add it as a additional volume force to the navier stokes equation? Thanks for your suggestions in advance. kind regards, Fabian

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December 19, 2011, 06:54		#2
ngj Senior Member Niels Gjoel Jacobsen Join Date: Mar 2009 Location: Copenhagen, Denmark Posts: 1,903 Rep Power: 37	Hi Fabian I have been doing exactly what you are requesting for free surface water waves. The entire framework is readily available, however, you might need to modify some of the classes to eliminate the need of the void ratio. I have released the framework, and it is described here: http://openfoamwiki.net/index.php/Contrib/waves2Foam Best of luck, Niels

December 19, 2011, 08:04		#3
fs82 Senior Member Dr. Fabian Schlegel Join Date: Apr 2009 Location: Dresden, Germany Posts: 222 Rep Power: 18	Alright sounds beautiful. I will check it. Thanks a lot. Fabian