# Is it possible that viscous flow has a higher lift to drag ratio than inviscid flow?

I have a code for airflow around an airfoil. When I compare viscous and non-viscous flow at the same Mach number for a cruise, viscous flow has a higher lift to drag ratio than non-viscous flow. Is it this possible?.condition is transonic. here is contour of pressure for non-viscous. for viscous like this but The difference is that l/d for viscous has 5 unit more.

## mach=$0.78$, aoa=$2.5$

• How are you computing drag? – tpg2114 Mar 26 '16 at 19:05
• It use AUSM method for inviscid flow . – meisam nemati Mar 27 '16 at 0:16
• That doesn't answer my question... AUSM is used to compute the flow field. How are you computing the drag from the resulting flow? – tpg2114 Mar 27 '16 at 0:17
• Without knowing what factors are included in calculating the drag, I can only guess (and this is why I won't post it as an answer). But lift decreased, this is expected. Drag went down also -- most likely, adding viscosity smoothed out the small shock that would form so it is no longer there. Viscous drag increased, but wave drag (the only kind you can get in inviscid flow) decreased by a lot because viscosity killed the only wave that shows up. Or there's a bug somewhere. Or you're computing drag incorrectly. – tpg2114 Mar 27 '16 at 2:00
• Also, it's possible however you are computing drag does not include viscous effects and so the drag for the viscous case is lower than it should be. Some advice for working in CFD -- always know what equations and methods are used to produce the numbers you see; you can't physically justify anything without knowing how you are computing it and what is included or not included. – tpg2114 Mar 27 '16 at 2:15