I Fluid dynamics: drag coefficient and pressure at the stagnation point.

[happyparticle]

TL;DR

Pressure at the stagnation point of an incompressible fluid.

Hi,
In my textbook the author say that the drag coefficient is the drag force divided by the pressure at the stagnation point time the area perpendicular to the stream.
$c_d = \frac{2F_d}{\rho v^2 A}$

To get the pressure at the stagnation point I'm using Bernoulli for an incompressible fluid. If both ends are at the same level and knowing that the velocity at the boundary of an object (a sphere for example) is null. Bernoulli equation is now:

$\frac{u^2}{2} + \frac{P}{\rho} = \frac{P'}{\rho}$ Where P' is the pressure at the stagnation point.
If the pressure far from the object is 0. We get exactly the pressure at the stagnation point used in the drag coefficient.

If this above is correct why exactly the pressure far from the object is 0?

 

[Lnewqban]

Cd represents the percent of the kinetic energy of the airstream that is wasted in friction and turbulence.
At the stagnation point, the whole dynamic pressure becomes static pressure.