Quick Xfoil plot question
 

Hi All,

I'm wondering what the horizontal dotted line at Cp = -1 is in the plot below.

Thanks for any help.
Chris

Hi, if i'm not wrong the dotted line is the sonic Cp, which means the area above the dotted line will be the area where the local mach number is >= 1.

Btw since this is a sonic case, can we trust the XFOIL result?

Hi,

Thanks. And yeah, that does make sense. The first instance where that line crosses the Cp distribution occurs at about Mach 0.5, which seems surprisingly low (but not unlikely!). I'll check the edge velocities to confirm what's happening.

I know Xfoil's results are not valid for transonic flow. The boundary layer analysis is based on the ISES code, as described in "Viscous-Inviscid Analysis of Transonic and Low Reynolds Number Airfoils" (Drela & Giles, 1987) which seems to handle transonic flow. How then is the ISES code different to Xfoil?

To try and answer my own question, Xfoil uses a panel method to calculate the inviscid solution, but the ISES results, like Figure 13 (attached), were calculated using Euler nonlinear potential equations, I guess maybe in more of a mesh rather than just along the surface? Is there more to the story, and do I have my facts straight?