Evaluating tangential derivatives on patch
 

Hi, I'm implementing non reflecting boundary condition for compressible LES calculations. In particular I'm interested in LODI boundary conditions (see Ref: Poinsot and Lele, Boundary conditions for direct simulation of compressible viscous flows).
In the first step I was tring to implement a partially non reflecting outlet and I'm facing these two major problems:

1. How could I evaluate normal derivative of a tangential component of velocity on a boundary?
2. How could I evaluate directional derivative of a variable on a boundary (direction tangential to the boundary)?

I looked in the boundary condtions library, but I didn't find anything helpful for my case. Any help or suggestion would be really appreciated. Thanks in advance.