How to compute this simple but weird derivative?

Hello,

I am working on numerical solution of incompressible fluid dynamics problem. I need to compute the Jacobian of a viscous flux. I have to deduct some derivatives like this

$\displaystyle \frac{\partial (\frac{\partial U}{\partial X})}{\partial U}=?$

U, X are independent to each other.

Some people say the result is absolutely not zero, but I still could not find an analytical answer. I wonder this will turn out into a derivative operator rather than an expression of fixed value. If an analytical expression is not possible, how to handle this in a finite difference method?

Thank you and have a good day!

Bearcat

sorry, I need to add more info

In my case, U is the unknown velocity in N-S equation, x is the coordinate. they are independent to each other. What I am looking for is an analytical expression for my coding. I am sure it's solvable in some way as I am not the first to encounter this problem.