Simple explanation for static and dynamic pressure?

Question

From what I understand, static pressure is just the pressure of still air and acts perpendicular to the airflow and surface. An airplane will feel static pressure regardless of whether it is moving or not. If there was airflow moving through a pipe, then static pressure is the pressure exerted on the walls of the pipe.

On the other hand, dynamic pressure is the pressure that results from the motion of air, or its kinetic energy. It acts parallel to the flow of air. Would it be appropriate to say that dynamic pressure is the pressure exerted on the windward side of a flat plate in the pipe like this:

If this illustration is correct, then does that mean maximum dynamic pressure is felt by a surface when it is perpendicular to the flow?

Answer 1

Static pressure is a force per unit of area due to fluid molecular motion leading to some statistical collisions with its environment (a solid submerged or another fluid for instance). Pressure gives an information of the fluid state (in a thermodynamics sense) and is a « measure » of the force apply by molecules as a whole. Wikipedia could be helpful on that topic.

Dynamic pressure is a bit confusing term as it is the kinetic energy of a fluid parcel (a unit of volume). This parcel has different form of energy (internal, pressure, potential) and this one is energy due to its motion. But it is not a pressure at all and does not act on the solid. However, disturbance created by the body (the flat plate on the picture) will slow down the flow in front, increasing pressure near surfaces. But this is still that pressure difference between front and back which is « felt » by the plate.

In fact, I believe this is the reason why we call it dynamic pressure. It is an energy form (known as kinetic energy) which is easily changed into pressure (or inversely pressure changing to kinetic energy) during fluid dynamics process: drag and lift are created like this on a flow around a body, gas expanding inside jet engine nozzle for thrust generation, etc..

Answer 2

Static pressure is exactly as you describe it.

Dynamic pressure is not necessarily the pressure on your plate. It is given by the same formula as kinetic energy

1/2 * mass * velocity^2

except instead of "mass", we use "density".

The actual pressure from movement on your plate is given using a number called pressure coefficient, which is just the proportion of the dynamic pressure that your plate actually feels. So the actual pressure that your plate feels is given by:

Pressure = Pressure coefficient * Dynamic pressure + Static pressure

Your plate could have a pressure coefficient close to one, in which case you could be experiencing near dynamic pressure (+static of course). A very streamlined body might have a very low pressure coefficient, in which case the actual pressure experienced is lower.

And, taking another look at your static pressure example, if you close a valve and stop the flow, the pressure pushing outwards on the pipe will be equal to static pressure + dynamic pressure.

(Yes, the pressure coefficient is related to the drag coefficient)