Follow-up Question to: The Concept of Pressure in Bernoulli’

[rdgn]

I have been looking all over the net for answers for the unintuitive relationship between pressure and velocity in Bernoulli's equation and this thread (https://www.physicsforums.com/threads/the-concept-of-pressure-in-bernoullis-principle.585231/) answered most of my questions.

I have one last question lingering in my mind though. What exactly is the difference between static and dynamic pressure?

The examples here: http://www.engineeringtoolbox.com/dynamic-pressure-d_1037.html
Hint that there is nothing magical about dynamic pressure. Same units, same effect.

Except that (from what I think) in contrast to static pressure that pushes in all directions, dynamic pressure only pushes in the direction of its velocity.
It's as if the pressure that was supposed to be pushing in all directions, some of that pressure is being converted into pressure that acts in only one direction by virtue of the shape of the tube?
Is this correct? Or am I wrong?

Thanks

 

[sophiecentaur]

dynamic pressure only pushes in the direction of its velocity.

Why would you say that? Have you read that explicit statement anywhere? Think Newton's Third Law.

 

[rdgn]

No, it's just something I've thought of since if dynamic pressure also behaves the same way as static pressure, then in the diagram of my first post, all three water levels should be equal right?

I'm not really sure about this to be honest, and you're hinting that it's probably wrong.
The way I came up with this explanation is, by Bernoulli's equation, the total pressure in any cross-section is equal throughout a pipe.
If the total pressure throughout the pipe is the same, then it doesn't make sense that the water levels due to pressure would be unequal.

For example:
P_S1, P_V1 = Static, velocity pressure for wider part
P_S2, P_V2 = Static, velocity pressure for wider part
P_S1 + P_V1 = P_S2 + P_V2

P_S1 > P_S2
P_V2 > P_V1

Their total pressures are the same but the amount of water pushed upwards is different.
The amount of water pushed in '1' is greater than '2'. The only quantity that's greater in 1 is P_S1, so I assume it's the quantity responsible for pushing the column of water upwards.

What would be a better way of thinking about static vs velocity pressure?

 

[Chestermiller]

Dynamic pressure is the same as the "kinetic energy per unit volume" of a fluid. It is not real pressure...yet. It is equal to the increase in pressure that could potentially be achieved if the fluid motion were stopped. Since it is a measure of kinetic energy, it does not have direction.

 

[rdgn]

I see. That makes more sense, thanks!

I guess that's the reason it doesn't contribute to making the column of water above it go up right?