Help required to understand dynamic pressures

[aero_g]

Hi all,

I am trying to understand the concept of dynamic and static pressures and would appreciate your help.

I've got a nozzle flow setup, such that the flow is exhausting into atmosphere. Am I right in assuming that the dynamic pressure at exit (relative to atmosphere) is equivalent to 1/2*rho*v² and the gauge static pressure = 0?

Thanks for your time in advance!

 

[tiny-tim]

Welcome to PF!

Hi aero_g! Welcome to PF!

(have a rho: ρ )

I've got a nozzle flow setup, such that the flow is exhausting into atmosphere. Am I right in assuming that the dynamic pressure at exit (relative to atmosphere) is equivalent to 1/2*rho*v² and the gauge static pressure = 0?

Yes, the gauge pressure = 0.

But I think saying "dynamic pressure at exit (relative to atmosphere)" is wrong …

I don't think there's any such thing as "gauge" dynamic pressure, so dynamic pressure is always defined as 1/2 ρv², and P_atm is irrelevant.

(from the PF Library on pressure …)

Static pressure, P, in a fluid (a liquid or gas or plasma), is measured across a surface which moves with the flow. It is the same in all directions at any point (unless viscosity is significant at that point). It is usually simply called "pressure".

Total pressure in a fluid is measured across a stationary surface.

Dynamic pressure in a fluid is total pressure minus static pressure. It is the macroscopic kinetic energy density, 1/2 ρv²

…

Absolute pressure is another name for pressure, sometimes used to distinguish it from gauge pressure.

Gauge pressure is pressure minus atmospheric pressure. For example, the devices usually used for measuring tyre pressure measure gauge pressure.

 

[aero_g]

Hi aero_g! Welcome to PF!

(have a rho: ρ )


Yes, the gauge pressure = 0.

But I think saying "dynamic pressure at exit (relative to atmosphere)" is wrong …

I don't think there's any such thing as "gauge" dynamic pressure, so dynamic pressure is always defined as 1/2 ρv², and P_atm is irrelevant.

(from the PF Library on pressure …)

Hi there!

Thanks for your reply. You are right, "dynamic pressure at exit (relative to atmosphere)" is wrong …

Following that, the total pressure at exit is then = dynamic pressure (since static pressure = 0). Am I right in saying that?

 

[tiny-tim]

Following that, the total pressure at exit is then = dynamic pressure (since static pressure = 0). Am I right in saying that?

Yup!

 

[aero_g]

Yup!

Thank you very much!