UDF for momentum source term
 

How can I write UDF for the source term (formulation given below) in the x-momentum equation of the solid phase in a Eularian multiphase flow in Fluent?

-e^150(ε-0.6)*∂ε/∂x + b[∂/∂x (∂u/∂x) + ∂/∂y {1/2(∂u/∂y+∂v/∂x)} + ∂/∂z {1/2(∂u/∂z+∂w/∂x)}]

In the above expression ε is the solid volume fraction, b is a constant.

The above term contains ∂ε/∂x which is unavailable as gradient vector macro in UDF manual. How to calculate this term in UDF? Also, I need to know about how to calculate terms like ∂/∂x(∂u/∂x) etc.

Please help.