CFD Online Discussion Forums

CFD Online Discussion Forums (
-   Main CFD Forum (
-   -   Please help a newbie find the drag on a 3d model (

David Amer March 5, 2002 20:05

Please help a newbie find the drag on a 3d model
I Wanted to implament a program that could compute the total drag on a given shape (a 3d model) and show the amount of drag being generated at anypoint on the model by shading the model. As im not to bothered about it being accurate to the point where heat and "wind" from the model itself changing the net wind acting on each poly get involved. i thought i could work out the drag on each poly by using one of the easy equasions.

By poly i mean a plane with a definate area, if you know what i mean.

What i thought was:

the force reaction from the "wind" on the poly is

0.5(p(V^2)S) = Force

Where p=Air density

V=Velosity of wind

s=Surcace area of poly

^=to the power of

Would i be correct in thinking that to find out the force against a poly that wasnt directly facing the wind i would just multiply the result from the last equation by cos(theta) where theta is the angle between the normal of the poly and the direction of the wind. I know there are many other factors invloved such as the deflection of wind form the rest of the model changing the direction of the wind on each poly, heat generated, etc..., etc. but is what i have worked out so far correct. if what i have worked out so far is good i could manage the shading bit my self. Any help would be greatly appresiated.


sylvain March 6, 2002 04:33

Re: Please help a newbie find the drag on a 3d mod
For each poly, the Pressure Drag Force (PDF) vector is equal to P.N where N is the normal to the so call poly with ||N|| = S and P is the pressure (scalar).

You also have a Skin Friction Drag Force (FDF) vector which can be compute if you know the deformation D (tensor) at the skin of your model by

the dimesionless drag coefficient Cd is defined by :

Cd = (PDF*i+FDF*i)/(0.5*rho*V^2*S)

if i is the unity vector align with the flow direction, rho is the air density and V is the flow velocity.

This drag coefficient is usefull if you want to compare the aerodynamic design of several cars.

From Hucho's book, this drag coefficient is equal to 1.17 for a circular plate, 0.47 for a sphere, 0.42 for half a sphere. This 3 shapes are view as a disc by the flow.


All times are GMT -4. The time now is 14:25.