Difference between Cd_i and Cd_v

Hiroshi TakeuchiHiroshi Takeuchi asked 5 months ago

Does anyone explain to me the difference betwen Cd_i and Cd_v with detail ? I believe Cd_i ‘s ‘i’ is inviscid and Cd_v ‘s ‘v’ is viscous. In case of using ‘ from Cl – xflr5 method’ option within the analysis definition, Cd_v is based on the polar data of airfoil, I guess.  Is Cd_v is the viscous part of Cd number or the total ( inviscid plus viscous ) Cd number ? I’d like to confirm that if I should use Cd_v number only in case of calculate the local drag force on main wing or not.

Main Wing
y(m) Re Ai Cd_i Cd_v Cl CP.x(%) Trans.top Trans.bot Cm_i Cm_v Bending.mom Vd.x Vd.y Vd.z F.x F.y F.z
-16.950000 133257.777027 -1.559368 0.009003 0.015059 0.330708 0.392781 0.814491 0.990264 -0.044319 0.143337 0.000000 0.000000 -0.074104 -0.191920 -0.017404 0.112491 0.284553
André Deperroistechwinder Staff replied 5 months ago

Yes, CD_i is the inviscid part of the drag and CD_v is the viscous part.
Cd = Cd_i + Cd_v
I tried to explain things in the “Theoretical background” presentations which you will find linked on the xflr5 web page.
Hope this answers the question.

Hiroshi TakeuchiHiroshi Takeuchi replied 5 months ago

Thank you for your quick response. I understand the definition of Cd_i and Cd_v and will refer the presentation later.