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  • FLUENT - Flow over an Airfoil- Step 7

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Step 7: Validate the Results

Report Force

Info
titleForce Conventions
Wiki Markup
FLUENT report forces in term of pressure force and viscous force. For instance, we are interested in the drag on the airfoil.Drag = Drag{~}pressure~ + Drag{~}viscous{~}Similarly, if  we are interested in the lift on the airfoilLift = Lift{~}pressure~ + Lift{~}viscous~
 !force convention.jpg!
Drag due to pressure:
{latex}
\large
$$
{(Drag)_{pressure}} = {\oint \-P \hat{n}.\hat{e_d}dS}
$$
{latex}
Drag due to viscous effect:
\\
{latex}
\large
$$
{(Drag)_{viscous}} = {\oint \tau_w \hat{t}.\hat{e_d}dS}
$$
{latex}
where

_e{_}{_}{~}d{~}_ is the unit vector parallel to the flow direction. 

_n_ is unit vector perpendicular to the surface of airfoil.

_t_ is unit vector parallel to the surface of airfoil.\\

Lift due to pressure:
{latex}
\large
$$
{(Lift)_{pressure}} = {\oint \-P \hat{n}.\hat{e_l}dS}
$$
{latex}
Lift due to viscous effect:
\\
{latex}
\large
$$
{(Drag)_{viscous}} = {\oint \tau_w \hat{t}.\hat{e_l}dS}
$$
{latex}
where

_e{_}{_}{~}l{~}_ is the unit vector perpendicular to the flow direction. 

_n_ is unit vector perpendicular to the surface of airfoil.

_t_ is unit vector parallel to the surface of airfoil.\\

Let's look at the forces on the airfoil. We will first investigate the Drag on the airfoil.

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!force convention.jpg! Here's the convention for drag due to pressure: {latex} \large $$ {(C_d)_{pressure}} = {\oint \-P \hat{n}.\hat{e_d}dS} $$ {latex} \\ {latex} \large $$ {(C_d)_{viscous}} = {\oint \tau_w \hat{t}.\hat{e_d}dS} $$ {latex} where e{~}d~ is the unit vector parallel to the flow direction.  n is unit vector perpendicular to the surface of airfoil. t is unit vector parallel to the surface of airfoil. \\
No Format
Force vector: (0.99980003 0.02094 0)
                                pressure        viscous          total       pressure        viscous          total
zone name                          force          force          force    coefficient    coefficient    coefficient
                                       n              n              n                                            
------------------------- -------------- -------------- -------------- -------------- -------------- --------------
airfoil                        3.8125084              0      3.8125084   0.0024897052              0   0.0024897052
------------------------- -------------- -------------- -------------- -------------- -------------- --------------
net                            3.8125084              0      3.8125084   0.0024897052              0   0.0024897052
 
Info
titleForce Convention
Wiki Markup

This is an inviscid model, we are expecting a drag coefficient of zero, and yet there is a small drag coefficient that present. We should investigate more into this.
Cd =  (Cd)pressure + (Cd)skin friction

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Info
title

In reality, (Cd)skin friction has biggest contribution to drag but ignored because of the inviscid model that we specify. (Cd)pressure should be zero, but not because of inaccuracies and numerical dissipation.

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