[datle]

I look at a compressible flow model for a gas through a long tube in the paper titled "Constructive Lyapunov Control Design for Turbocharged Diesel Engines" They model it as an lumped orifice model as in the picture. From the model, I see that regardless of p2>p1 or p1<p2, the flow direction doesn't change. This is not consistent with what I have with incompressible model where flow = sign(deltaP)*sqrt(deltaP)...
One thing I notice is that their equation (2.6) is well-known for compressible. However, as I understand two pressures used in the equation are different in concept. One is stagnation pressure while the other is static pressure. Hence, p1 and p2 used in the case p1<p2 of equation (2.6) cannot be used in the case p1>p2.
Could anybody explain to me why and correct me if I am wrong here?
Thanks