- **OpenFOAM Meshing & Mesh Conversion**
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- - **Diffusivity [dynamicMeshDict]**
(*https://www.cfd-online.com/Forums/openfoam-meshing/97299-diffusivity-dynamicmeshdict.html*)

Diffusivity [dynamicMeshDict]Hi everybody.
I have a dynamic mesh and I want to keep the orthogonality between cells. I use a directional diffusivity, it seems like: diffusivity directional ( a b c ); in my dynamicMeshDict archive. a,b and c are numbers, can anybody tell me what mean these numbers? I found two versions. One of them says that this numbers are the value of the coefficient of diffusion in the 3 directions of the space. The other says that the first number determines the mean cell non-orthogonality, while the second number determines the mean cell skewness (and the third?). But in my experience I can not determine this numbers correctly. How can I determine the value of a,b and c? What other types of diffusivity I can use to keep the orthogonality between cells? Thanks a lot. |

Dear Darlopez,
I am also facing the same issue. I didn't able to get those numbers. Did you able to find out? If so, Please tell me the what are those numbers. Regards Prasanth. |

Dear Parasanth
I solve my problem using other type of diffusivity, concretly I use inverseDistance type, but inverse quadratic diffusivity works also ok. If you should use directional diffusivity, I think that the three numbers are the value of the coefficient of diffusion in the 3 directions of the space (I saw this running different cases). How to determine the value of this numbers is a mystery. I suggest you to use an inverse distance diffusivity. And you do not forget put the convenient patches type "slip". Regards Darío |

Dear lopez,
Thanks for your reply, I will use inverse distance and I will back to you about my experience regarding this. Regards Prasanth. |

Diffusivity directional (a b c)Hi,
I think it depends on what you have to do... for example, if you want to move a boundary patch in the y direction you can put "diffusivity directional (1 1e-6 1)": the b value sets the extension of the moving grid in the y direction. Value 1 for a e c implies that the cells in the x and z directions are "transported" without distortions of the grid. I think there is no way to set "a priori" these values since they depends on the relative dimensions of the problem. I use the moveDynamicMesh utility until the mesh moves in the way I want and then I pass to the solver run. This is only my experience...:confused: |

It seems to me that a,b,c define a vector.
I tested it with a 2D case (x-z-plane). -b seemed to make no difference. makes sense cause its not in the plane. -(0 0 1) gives the same result as (1e-7 0 1). makes also sense, cause the vectors are nearly the same. greetings Alex |

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