# Stagnation or Total conditions in a pipe with moving fluid?

In Flow Science, they provide an example of boundary conditions and how to specify them for CFD simulations. The following is stated:

Pressure Boundary Example

For example, consider the problem of flow in a section of pipe. On the one hand, if the upstream end of the computational region coincides with the physical entrance to the pipe then a stagnation condition should be used to represent the external ambient conditions as a large reservoir of stationary fluid. On the other hand, if the upstream boundary of the computing region is inside the pipe, and many diameters away from the entrance, then the static pressure condition would be a more reasonable approximation to flow conditions at that location.

which I don't really understand, because inside the pipe, the fluid is already in motion, how come we shall use the static pressure?

I think the answer is in the definition of "Static Pressure", which is given, according to the British Standards Institutions (https://en.wikipedia.org/wiki/BSI_Group), as

The pressure at a point on a body moving with the fluid

So the term "static" it is not to be intended as absence of motion. Also, every point in a steadily flowing fluid, regardless of the fluid speed in that point, has its own static pressure .

• so what's the difference between saying "a volume at total pressure" and "a volume at static pressure" ? – user2536125 Nov 4 '15 at 12:44
• $P_{total} = P_{stagnation}$ if the gravity head of the fluid at a particular point in a fluid flow is zero. And $P_{stagnation} = P_{static}+P_{dynamic}$ – Rhei Nov 4 '15 at 13:50

Because Static Pressure is just another way of saying pressure, but specific to fluid flow/dynamics. It is the pressure associated with something moving with the fluid.

Dynamic pressure is the kinetic energy per unit volume of a fluid particle. (dynamic pressure usually only for incompressible flows - For compressible flows, try "impact pressure").

Stagnation pressure (or pitot pressure) is the static pressure at a stagnation point in a fluid flow... aka it is the same idea as the static pressure BUT instead of there being cohesive/constant/"not changing"/"static" fluid motion ... At a stagnation point the fluid velocity is zero and all kinetic energy has been converted into pressure energy

**static pressure + dynamic pressure = stagnation pressure**