# Courant number calculation and min. grid size

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 September 3, 2018, 06:22 Courant number calculation and min. grid size #1 New Member   Soheil Join Date: Mar 2014 Location: Helsinki Posts: 21 Rep Power: 11 Hi, For calculating the Courant number (Co = U*timeStep/meshSize), minimun edge length is needed. My 3D Mesh is generated in Ansys meshing and it's all Tetrahedron (V=a^3/6*(2)^0.5). Is it OK to make an approximation of the smallest grid length by the volume of the cell, which is volume of a regular tetrahedron? Fluent generates the "minimum volume (m3)". Thanks for any advice,

 September 4, 2018, 14:11 #2 Senior Member   Micael Join Date: Mar 2009 Location: Canada Posts: 156 Rep Power: 17 Create a custom field functions so you can visualize courant number field. It will give you a better picture to judge if your courant number is good. this text command might work or need a bit adaption: Code: `define/custom-field-functions/define "courant-nb-gas" "velocity_magnitude*delta_time/cell_volume^(1/3)"` soheil_r7 likes this.

 September 5, 2018, 13:50 #3 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,142 Rep Power: 61 For tetrahedrons it is not right/wrong to use the cell volume (I would do it and throw in a factor of safety). The faces are not oriented in any particular direction with the flow. Even if you knew the minimum edge length, you wouldn't know it's oriented with respect to x,y,z coordinates and you still don't know the u,v,w velocity components. So in general you should run a few test cases to confirm the courant number that you get from your mesh. The best you can do is guess. There already is a courant number field in Fluent. You don't need to create a custom field function. But this doesn't help you predict anything without meshing and simulating a case first. soheil_r7 likes this.

 Tags courant number, fluent