Help,I have some questions about UDF of mass diffusivity in fluent
 

Hi,all. I have some questions about UDF of mass diffusivity in fluent
By default, FLUENT uses the dilute approximation, under which the diffusion flux can be written as
http://edu.emuch.net/attachment/84/5...637610_209.jpg
that means the mass diffusion coeffcient Di,m is a isotropic value,but in some papers,the mass diffusion coeffcient is an anisotropic value, the mass diffusion coeffcient in radial direction is different from that in axial direction. How can i define this in UDF. Thank you!