Editing Drag coefficient (section)

== Cauchy momentum equation ==
{{see also|Cauchy momentum equation#Nondimensionalisation}}

In the non dimensional form of the Cauchy momentum equation, the skin drag coefficient or '''skin friction coefficient''' is referred to the transversal area (the area normal to the drag force, so the coefficient is locally defined as:

<math display=block>c_\mathrm d  = \dfrac \tau q= \dfrac{ 2 \tau}{ \rho u^2}</math>

where:
* <math>\tau</math> is the local [[shear stress]], which is by definition the stress component in the direction of the local [[flow velocity]];<ref>See [[lift force]] and [[vortex induced vibration]] for a possible force components transverse to the flow direction</ref>
* <math>q</math> is the local [[dynamic pressure]] of the fluid
* <math>\rho</math> is the local [[mass density]] of the fluid;<ref>Note that for the [[Earth's atmosphere]], the air density can be found using the [[barometric formula]]. Air is 1.293 kg/m<sup>3</sup> at {{convert|0|°C}} and 1 [[atmosphere (unit)|atmosphere]].</ref>
* <math>u</math> is the local [[flow speed]] of the fluid