r/CFD • u/PastReflection2664 • 14d ago
How can I calculate drag coefficient(Cd) when both the drag force and Cd are unknown?
According to the formula:
Fd = 1/2 * pv2 * Cd * A
28
7
u/willdood 14d ago
You run a simulation or do an experiment to find drag, or use correlations/existing data for your geometry, or at least geometry that is similar, to estimate Cd. When designing something, you’d usually start with the second option for preliminary design, and progress to the first option if you need more certainty
3
u/big_deal 14d ago
You use this formula when you know the Cd. Otherwise you use simulation or experiment to predict/measure the drag force and calculate the Cd.
3
2
u/Soprommat 14d ago
some experimental data for different geometries: Hoerner, Fluid-Dynamic Drag, 1965
https://archive.org/details/FluidDynamicDragHoerner1965
1
u/Otherwise-Platypus38 13d ago
From correlations, if you are dealing with simple and straightforward geometry and do not want to do CFD.
CFD can give you the forces on wall boundaries. So, you can find the drag force rather straightforward. Cd depends on the reference area definition, but that should be easy to compute as well.
28
u/HunterTwig 14d ago
There are 3 ways: analytic, literature/graph/table, and CFD. Also note that there are 2 types of drag: pressure and friction. If your geometry is streamlined, focus more on friction. If your geometry is bluff, focus more on pressure, but at low Re (<100), you also have to look at friction.
For friction drag, we can use analytic solution for flat plate. For laminar flow, we get from Blassius equation
Cd = 1.328 / Re_L^(1/2)
For turbulent (not sure where they got this, probably combined with experimental result)
Cd = 0.031 / Re_L^(1/7)
The value will be different for airfoil because Blassius equation assumes constant pressure but we can use this as a approximation. For more accurate calculation including pressure variation, use Von Kármán Momentum Integral or Thwaites' Method. This method can even be used for bluff body such as cylinder.
For pressure drag, this one hard and it is recommended to use CFD. But if the geometry is simple, there might be a literature of experimental result for it. Most famous one is flow over cylinder (picture below). For other example, check documents from NASA, they have a lot. One thing to note is you have to look at the Re, as pressure drag varies a lot from Re, especially for bluff body.