BCs with FAVOR method in partially filled cells

Lupocci · January 21, 2013, 17:50

Dear all,
I am interested in completely understand how boundary conditions work with the porous FAVOR approach. I read all the manual of version 9.2 and also the Hirt and Sicilian paper on the FAVOR method ( http://www.flow3d.com/pdfs/tp/mariti...acles-3-85.pdf ) . This paper at pag 13 says: "a simple device is required. the difference espressions for all fluxes are formulated in terms of velocity derivatives. Then, all the derivatives at interfaces are set to zero. In this way all boundaries become free-slip boundaries. (When viscous shear stresses are wanted they can be adds a separate force contributions.)"
So I this the way BCs are currently implemented in flow3D? How can you transform the fluxes in velocity derivatives?And how do you prescribe the shear stress? In which direction? Is the direction of the wall known or not? It does not seems to me that a reconstruction is done in orderto compute the direction of the the wall. What if it is moving and it is compressible? you need to reconstruct the interface as in VOF right? All this is not explained in the paper or in the manual.
Thanks
A.

Lupocci · February 21, 2013, 11:44

Guys any idea? In particolar I dont understand the boundary conditions of the Favor. What I understand is that they set all derivatives of velocity to zero at the boundary, so I guess they just copy the velocity on the nodes right inside the wall. But it seems too simple. Should not we add also the pressure force of the wall in the direction normal to the wall itself? Otherwise how can the flow curves like in here http://www.flow3d.com/cfd-101/cfd-10...R-no-loss.html ? thanks

JBurnham · February 21, 2013, 14:45

The Hirt & Sicilian paper is a description of the FAVOR method only, not a full treatment of how slip conditions and shear stresses are calculated on solid surfaces. The default settings for slip (FRCOF, OFRCOF) are such that the default setting is non-slip. The velocity and shear stress in the cell that contains the solid surface are computed either using the logarithmic law (y+ & u+ method) when turbulence physics are active, or an analytical solution for laminar flow when only viscous flow is modeled, or not at all when flow is inviscid. The direction of the wall and it's normal vector are known. If the object is moving, the orientation and normal of the wall are recomputed in every time step. See the Tech Notes on the FLOW-3D User Site regarding moving objects for more detail on how pressure and shear are computed when the object and fluid are moving relative to each other.

Lupocci · February 21, 2013, 15:20

Ok thanks,
but lets stay on the case of a fixed (not moving) object. How are boundary conditions prescribed? For example reflective boundary conditions.

January 21, 2013, 17:50	BCs with FAVOR method in partially filled cells	#1
Lupocci New Member Lupo Ci Join Date: Aug 2010 Posts: 16 Rep Power: 15	Dear all, I am interested in completely understand how boundary conditions work with the porous FAVOR approach. I read all the manual of version 9.2 and also the Hirt and Sicilian paper on the FAVOR method ( http://www.flow3d.com/pdfs/tp/mariti...acles-3-85.pdf ) . This paper at pag 13 says: "a simple device is required. the difference espressions for all fluxes are formulated in terms of velocity derivatives. Then, all the derivatives at interfaces are set to zero. In this way all boundaries become free-slip boundaries. (When viscous shear stresses are wanted they can be adds a separate force contributions.)" So I this the way BCs are currently implemented in flow3D? How can you transform the fluxes in velocity derivatives?And how do you prescribe the shear stress? In which direction? Is the direction of the wall known or not? It does not seems to me that a reconstruction is done in orderto compute the direction of the the wall. What if it is moving and it is compressible? you need to reconstruct the interface as in VOF right? All this is not explained in the paper or in the manual. Thanks A.

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February 21, 2013, 11:44		#2
Lupocci New Member Lupo Ci Join Date: Aug 2010 Posts: 16 Rep Power: 15	Guys any idea? In particolar I dont understand the boundary conditions of the Favor. What I understand is that they set all derivatives of velocity to zero at the boundary, so I guess they just copy the velocity on the nodes right inside the wall. But it seems too simple. Should not we add also the pressure force of the wall in the direction normal to the wall itself? Otherwise how can the flow curves like in here http://www.flow3d.com/cfd-101/cfd-10...R-no-loss.html ? thanks

February 21, 2013, 14:45		#3
JBurnham Senior Member Jeff Burnham Join Date: Apr 2010 Posts: 204 Rep Power: 17	The Hirt & Sicilian paper is a description of the FAVOR method only, not a full treatment of how slip conditions and shear stresses are calculated on solid surfaces. The default settings for slip (FRCOF, OFRCOF) are such that the default setting is non-slip. The velocity and shear stress in the cell that contains the solid surface are computed either using the logarithmic law (y+ & u+ method) when turbulence physics are active, or an analytical solution for laminar flow when only viscous flow is modeled, or not at all when flow is inviscid. The direction of the wall and it's normal vector are known. If the object is moving, the orientation and normal of the wall are recomputed in every time step. See the Tech Notes on the FLOW-3D User Site regarding moving objects for more detail on how pressure and shear are computed when the object and fluid are moving relative to each other.

February 21, 2013, 15:20		#4
Lupocci New Member Lupo Ci Join Date: Aug 2010 Posts: 16 Rep Power: 15	Ok thanks, but lets stay on the case of a fixed (not moving) object. How are boundary conditions prescribed? For example reflective boundary conditions.