# Diffusivity [dynamicMeshDict]

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 February 14, 2012, 06:36 Diffusivity [dynamicMeshDict] #1 New Member   Darío López Join Date: Oct 2011 Location: Vigo, Spain Posts: 10 Rep Power: 8 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.

 April 3, 2012, 07:00 #2 New Member   prasanth Join Date: Jul 2010 Location: Chennai, India Posts: 17 Rep Power: 0 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.

 April 3, 2012, 12:56 #3 New Member   Darío López Join Date: Oct 2011 Location: Vigo, Spain Posts: 10 Rep Power: 8 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

 April 4, 2012, 08:20 #4 New Member   prasanth Join Date: Jul 2010 Location: Chennai, India Posts: 17 Rep Power: 0 Dear lopez, Thanks for your reply, I will use inverse distance and I will back to you about my experience regarding this. Regards Prasanth.

 September 6, 2012, 11:50 Diffusivity directional (a b c) #5 New Member   Enrico Boesso Join Date: Mar 2011 Location: Padova, Italy Posts: 16 Rep Power: 8 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...

 March 6, 2015, 08:03 #6 Member   Alexander Bartel Join Date: Feb 2015 Location: Germany Posts: 97 Rep Power: 4 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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