Fluid-solid coupling n.P=n.sigma?
 

I am treating two-dimensional fluid-solid coupling. Dynamically to couple the fluid and the solid, the fluid pressure is usually used to apply the traction force acting on the solid through the normal vector.

It is not trivial to define the normal vector at the interface. Is there any easy way?

Re: Fluid-solid coupling n.P=n.sigma?
 

in 2D you can calculate normal as (-sin(th),cos(th)) where the angle theta is usually calculated using the atan2 function.Check any book or article on panel methods.