What's the requirement of the derivative order of boundary conditions?
 

What's the requirement of the derivative order of boundary conditions?

If my governing equations contain 3 variables, vorticity,u and v, the derivatives order of which are:

vorticity: 2nd/1st
u: 1st
v: 1st

Can I use 2nd order boundary conditions like d^2(u)/d(x)^2=0 on the outflow boundary?

Thank you in advance.