3
$\begingroup$

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. enter image description here

inviscid: $C_l=0.89$ and $C_d=2.91\times 10^{-2}$

viscous : $C_l=0.67$ and $C_d=1.9\times 10^{-2}$

mach=$0.78$, aoa=$2.5 $

$\endgroup$
  • $\begingroup$ How are you computing drag? $\endgroup$ – tpg2114 Mar 26 '16 at 19:05
  • $\begingroup$ It use AUSM method for inviscid flow . $\endgroup$ – meisam nemati Mar 27 '16 at 0:16
  • $\begingroup$ That doesn't answer my question... AUSM is used to compute the flow field. How are you computing the drag from the resulting flow? $\endgroup$ – tpg2114 Mar 27 '16 at 0:17
  • 1
    $\begingroup$ 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. $\endgroup$ – tpg2114 Mar 27 '16 at 2:00
  • 1
    $\begingroup$ 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. $\endgroup$ – tpg2114 Mar 27 '16 at 2:15
2
$\begingroup$

No. There must be something wrong with your code.

Inviscid, subsonic flow around a 2D object should not produce drag - I guess you look at supersonic results, but there, too, viscosity should only add drag, not reduce it.

If you provide more information, I can be more specific. Why don't you plot the pressure distribution for both cases? This will maybe help to find the reason for the wrong result.

| cite | improve this answer | |
$\endgroup$
  • 1
    $\begingroup$ Although I'm not disagreeing with you about a potential error in the code, is it possible that the viscous solution predicts more lift than the inviscid one? That would lead to a higher L/D, although it's not entirely clear how there is drag at all in the inviscid unless it is wave drag from transonic/supersonic results. In which case the viscous drag may be very small anyway, so if viscosity does increase L, that may explain the result. $\endgroup$ – tpg2114 Mar 25 '16 at 20:18
  • $\begingroup$ @tpg2114: I agree, but see no real-life mechanism where viscosity increases lift beyond a change in viscosity (i.e. comparing results at two different Reynolds numbers). Adding viscosity will reduce lift in all cases I can think of, at least when looking at ordinary airfoils. $\endgroup$ – Peter Kämpf Mar 25 '16 at 22:53
-1
$\begingroup$

There seems to be definitely something wrong in the code since the CFD result should give a lower L/D ratio and definitely a greater drag for subsonic flows. Also, inviscid flow can have an induced drag if I'm not wrong.

| cite | improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.