technophobe

I'm specifically talking about shocks occuring on an airfoil although I guess the theory would apply generally. I suppose I'm really asking about the reversibility of the Euler equations. I understand that expansion shocks are non-physical but I'm reading that they are a possible solution of the Euler equations. Essentially the Euler equations don't 'know' that the expansion shocks are unphysical and so in some applications an entropy boundary condition is required (which basically rules out expansion...