Drag coefficient#
$$C_D \stackrel{\text{def}}{=} \frac{F}{\rho U^{2} A} \sim \frac{\text{drag force}}{\text{dynamic pressure force}}$$
Description#
Measures drag force normalized by dynamic pressure and reference area. It compares the aerodynamic or hydrodynamic resistance of bodies across speeds and scales.
Quantities#
| Name | Symbol | SI units | Dimension |
|---|---|---|---|
| force | \(F\) | \(\mathrm{N}\) | \(\text L\,\text M\,\text T^{-2}\) |
| mass density | \(\rho\) | \(\mathrm{kg}\,\mathrm{m}^{-3}\) | \(\text L^{-3}\,\text M\) |
| velocity | \(U\) | \(\mathrm{m}\,\mathrm{s}^{-1}\) | \(\text L\,\text T^{-1}\) |
| reference area | \(A\) | \(\mathrm{m}^{2}\) | \(\text L^{2}\) |
Regimes#
Body drag
| Range | Regime | Description |
|---|---|---|
| 0 – 0.1 | low drag | Drag is small relative to the dynamic pressure force on the reference area, typical of streamlined bodies at favorable conditions. |
| 0.1 – 1 | moderate drag | Drag is a significant fraction of the dynamic pressure force and depends strongly on shape and Reynolds number. |
| 1 – ∞ | high drag | Pressure drag, separation, or form effects are large relative to the reference dynamic pressure force. |