I am trying to access UDS value on faces. The function F_UDSI(c,t,i) proposed by Fluent returns always 0 (as if the value were not kept in memory). Has anybody some experience in using this function?

Thanks in advance.

Are the face variables available ONLY at exterior boundaries?

yes face variables are only available on domain boundaries. For interior cells face values are given by (c0 + c1)/2 where c0 and c1 are the cell values on either side of the face.

thanks. This method is certainly not very accurate for unstructured mesh.

I have tried to evaluate the UDSI variable on the face using the gradient C_UDSI_G. But, some face values get out of the range defined by the surrounding cells.

Apparently, Fluent does not provide Reconstruction Gradient for UDSI variable. Do you know whether there is a simple method to calculate them ?

there is no easy way to compute the graident but it is possible. have a look at section 5.2.3 on Cell Grid Definitions in the UDF manual. lots of info on getting gradients and face values on unstructured grids. best of luck!