Thank you for your response,

I try expression without UDF , but when i click on MHD module this error is being shown :

Error: wta(2nd) to string_eq

Error Object: __expr__

And when i choose the wall in MHD boundary condition box this error is being shown :

Error: ASSV: invalid argument [2]: improper list

Error Object: #f

when i run the fluent , this message is being shown:

===============Message from the Cortex Process================================

Compute processes interrupted. Processing can be resumed.