Before revision: The surface is defined as The slip velocity boundary condition leads to The downwash speed is approximated as For a freestream velocity impacting a surface of area , which is inclined at an angle relative to the freestream, the change in normal momentum is and the mass flux incident on the surface is , with being the freestream air density.
After revision: Hypersonic flowIn hypersonic flow, the pressure coefficient can be accurately calculated for a vehicle using Newton's corpuscular theory of fluid motion, which is inaccurate for low-speed flow and relies on three assumptions:The flow can be modeled as a stream of particles in rectilinear motion Upon impact with a surface, all normal momentum is lost All tangential momentum is conserved, and flow follows the bodyThe surface is defined as The slip velocity boundary condition leads to The downwash speed is approximated as For a freestream velocity impacting a surface of area , which is inclined at an angle relative to the freestream, the change in normal momentum is and the mass flux incident on the surface is , with being the freestream air density.