Expert: Paul Soderman - 11/26/2009

Question
why do the dynamic pressure is created and how  

Answer
Mohamed
We know from the conservation of energy in a flow that the total energy is composed of the potential energy, the kinetic energy, and the flow energy.

E = PE + KE + FE

The fluid weight is common to each term and when it is divided into the energy terms we get terms that relate to pressures, which is referred to as Bernoulli's equation:

E/w = Z + V^2/2g + p/gamma

The first term is called the elevation head, the second is the velocity head and the third is the pressure head.  gamma is the specific weight:  w/volume.  From this we can write:

Pt = Ps + rho V^2/2  (neglecting elevation head, assuming horizontal flow)

where Pt is the total or ram pressure, ps is the static pressure, and rho V^2/2 is the dynamic pressure (rho being density).  Hence, the dynamic pressure comes from kinetic energy and is not really a pressure in the usual sense, but it has units of pressure and can be found by taking the difference between the total and static pressure in a flow.  This is what a pitot-static tube would measure.
Paul