# Slip boundary condition what is inside

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 April 26, 2007, 09:35 Hello! I would like to unde #1 New Member   Normunds Jekabsons Join Date: Mar 2009 Location: Riga, Latvia Posts: 10 Rep Power: 9 Hello! I would like to understand a few points about how boundary conditions for vectorial fields are implemented in OpenFoam (OF). I will be grateful for any help since "reverse engineering" from source code is quite time consuming and may lead to terrible mistakes. For background, my recent problem is related to boundary condition - introduction of shear stress on free surface of incompressible closed buoyant N-S flow. The shear stress comes from separate boundary element based EM solver (very developed skin effect)+Marangoni forces. I already learned and tested implementation of boundary condition for scalar field (concentration segregation boundary condition on crystallization front was needed for diffusion problem which utilize velocity from mentioned above N-S problem) thus questions are only about vectorial field related differences. The questions are 1. How conditions like "slip" condition is actually implemented in OF? My point is - manuals says OF solves separate linear equation system - one per each vectorial component. However, boundary condition in coordinate representation is system of three equations which for arbitrary orientation of free surface has appearance (mix) of all three components, Vx,Vy, Vz. Similar conditions usually can be written by something like that a_ij (V_j),n + b_ijV_j =C_i (1) where a_ij and b_ij (i,j=1,2,3=x,y,z) are some kind of transformation matrices and (V_j),n indicate a derivative in normal direction of j-th component of vector V, finally C_j are constant quantities (may be 0). Studies of source (+ OF discussion board) revealed that in order to set in OF a boundary condition one has to specify source terms and diagonal terms both for face gradients and face values. Diagonal terms will be mix of a_ii and b_ii from (1). How about a_ij and b_ij in (1) with i!=j? Are these terms in OF belongs to the source term and the values comes from previous time step? How exact is boundary condition than? 2. From source code I got an impression that "symmetryPlane" and "slip" conditions are almost the same. What kind of symmetry it exactly is - (local) translation symmetry or (local) mirroring symmetry? I believe it must be mirroring. Lets have symmetry plane given by equation z=0. Then global mirroring symmetry will be something similar to this V_x(x,y,z)=V_x(x,y,-z); V_y(x,y,z)=V_y(x,y,-z); V_z(x,y,z)=-V_z(x,y,-z) (2) while local I believe is (V_x),z (x,y,0)=0; (V_y),z (x,y,0)=0; V_z (x,y,0)=0 (3). The Eq(3) ensures that V_n=V_z=0 on boundary while (V_z),z my have finite value. Is Eq(3) implemented as symmetryPlane condition in OF or it's something different? From code I got something like (V_i),n-n_i n_j (V_j),n = 0 (4) and grad_n V = - (n_i n_j V_j )/d (5) where n_i is i-th component of normal vector and d is something like distance from cell center till face. It approx. gives (3) but I am confused by code in basicSymmetryFvPatchField.c functions snGrad() and evaluate() where (5) exist in form which looks like ((I-2n_i n_j)V_j-V_i )/d with following division by 2. Why too complex expression for (5)? Am I correct that (4) and (5) is really that what is build in OpenFoam? It seems that the matrix formation is in the file transformFvPatchField.C , however I gave up on finding what really is "this->patch.weights()" and what kind of field is is "*this" in valueBoundaryCoeffs(), so answer on question 1 is still very important for me... Thanks a lot for any help, comments, corrections! best regards /Normunds

 June 4, 2007, 06:38 Dear OpenFOAM users I am lo #2 Senior Member   Join Date: Mar 2009 Posts: 248 Rep Power: 10 Dear OpenFOAM users I am looking for help on specifying free-surface interface for a 3d geometry. The problem setup is following: I have a shake-flask geometry. I am using interFoam to simulate the mixing of two fluids. Right now I am stuck with how to specify the height of the free surface. In CFX it could be done easily using the step function (ref. tutorial 7 of CFX). I am attaching the for clarity. Please help me. Thanks in advance. Jaswi

 June 4, 2007, 06:45 Hello Normunds I am extreme #3 Senior Member   Join Date: Mar 2009 Posts: 248 Rep Power: 10 Hello Normunds I am extremely sorry for that i posted my doubt at the wrong place. I will be careful next time. Thanks Kind regards Jaswi

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