Calculation of total drag

[Carvinos]

hello!
i want to calculate th total drag (surface and pressure drags) for my prototype car, a project of shell eco marathon. Actually our prototype has the shape of an airfoil, its quite stream line.the Cd we use is 0.04 which is used for streamline shapes. ihave calculated the pressure drag using the following formula:
Df=0.5*v^2*rho*Cd*A
A = pi/4*d^2 where d is the diameter of the cross section of the airfoil shaped body from the front end. now tell me please how do i calculate the surface drag because for streamline shapes, surface drag becomes more significant.
reply soon please

 

[robinfisichel]

hello!
i want to calculate th total drag (surface and pressure drags) for my prototype car, a project of shell eco marathon. Actually our prototype has the shape of an airfoil, its quite stream line.the Cd we use is 0.04 which is used for streamline shapes. ihave calculated the pressure drag using the following formula:
Df=0.5*v^2*rho*Cd*A
A = pi/4*d^2 where d is the diameter of the cross section of the airfoil shaped body from the front end. now tell me please how do i calculate the surface drag because for streamline shapes, surface drag becomes more significant.
reply soon please

I think the drag you have calculatated includes the skin friction drag you are talking about, typically the force you have calculated includes a shear and pressure component, the shear being from the skin friction.

You need to have Cf (skin friction coef) to determine the value of shear drag force and also the surface area of the vehicle (not easy to obtain for either).

To calculate the shear drag by hand is very lengthy, if involves calculating the local shear force inside the BL at points on the body and then produce the average from these. There is also a lot of these equations as they are usually empircal formulas from experimental data such as;

Cd_laminar = 0.664/sqrt(Re.x)
Cd_turbulent = 0.0576/(Re.x)^0.2

so you also need to know where the BL is turbulent and laminar.

If you have Cf i think the shear drag is just Fsd = Cf*S*0.5*rho*v^2 (S being the surface area)

 

[darkside00]

If you want a good coefficient, put in a scale model in a wind tunnel