Why is static pressure called "static"?

[fog37]

Hello,
I am still perplexed on why the pressure term $p$ in Bernoulli and Euler equations is called "static", even if the fluid is flowing (moving). Are there historical or other specific reasons?

In hydrostastics, the pressure at a certain depth is called hydrostatic which makes sense since the fluid is indeed static...

Thank you!

 

[jedishrfu]

Using static usually implies that some quantity doesn't change. However, it looks like some authors prefer its use to distinguish it from dynamic pressure or total pressure.

https://en.wikipedia.org/wiki/Static_pressure

The concepts of total pressure and dynamic pressure arise from Bernoulli's equation and are significant in the study of all fluid flows. (These two pressures are not pressures in the usual sense - they cannot be measured using an aneroid, Bourdon tube or mercury column.) To avoid potential ambiguity when referring to pressure in fluid dynamics, many authors use the term static pressure to distinguish it from total pressure and dynamic pressure; the term static pressure is identical to the term pressure, and can be identified for every point in a fluid flow field.

 

[GlenLaRocca]

Even though the fluid is moving, you a measuring the pressure in such a way that the fluid is not moving towards your measurement device. Thus your measurement does not see (is not impacted by) the momentum of the moving fluid. Think of it a measuring the pressure at right angles to the direction of fluid motion.

 

[fog37]

So, in summary, the pressure is called "static" because it is measured (by an instrument) as the relative speed between the flow and the instrument is zero. The term is used to distinguish it from total and dynamic pressure but, fundamentally, there is only one pressure and the dynamic "pressure" is just a possible contribution to the static pressure.

For example, to measure static pressure, the instrument would have to freely flow in the stream...However, piezometers (which are vertical tubes perpendicular to the wall of a pipe), are fixed in place and measure the pressure in a direction perpendicular to the water flow speed. So I guess there are two ways to measure $p$ static:

a) with a manometer moving with the flow and at the same speed as the flow;
b) with a piezometer which measures the pressure in a direction perpendicular the the flow speed so as not to engage any forward momentum in the determination of the pressure;

If that is correct, what is now the difference between "static" pressure and thermodynamic pressure? I think they are the same concept...

Thanks!

 

[Chestermiller]

The valuable time you spend wondering about why the pressure is sometimes called static pressure decreases the time you have to get practice solving problems (which is a million times more important).

 

[GlenLaRocca]

So I guess there are two ways to measure pp static:

a) with a manometer moving with the flow and at the same speed as the flow;
b) with a piezometer which measures the pressure in a direction perpendicular the the flow speed so as not to engage any forward momentum in the determination of the pressure;

Yes

fundamentally, there is only one pressure

Thanks!

I wouldn't say that though. Think of putting one end of a thin glass tube into the flow and measuring the pressure at the other end of the tube. If you oriented the tube along the direction of flow, you would measure the total pressure. You would feel all the pressure of the fluid moving at you (dynamic pressure based on fluid speed and density) plus the static pressure (due to N atoms being confined to a volume at a given temperature). If the tube was perpendicular to the flow, you would not feel the fluid momentum and would only measure static pressure. Orientations in between would feel some of the fluid momentum and get an answer in between static and total pressure. If you oriented the tube so that the open end pointed upstream in the flow, the fluid momentum would suck on the end of the tube and you would measure a number less than static pressure.

The relative motion of the measurement could be because the measurement device is moving and the fluid is moving, like an aircraft flying on a windy day. You put a tube facing forward to measure total pressure, and mount some on the sides of the aircraft to measure static pressure.

In a moving fluid, the pressure, you feel, is dependent on the orientation (and relative movement) of the measurement device.

I'll let someone else answer the question about thermodynamic pressure, but it looks to me to be more than just static pressure as you are working on the system moving the fluid and maybe there may be just one thermodynamic pressure.

 

[fog37]

I learned from Cherstermiller and others that pressure (called total pressure) is one and is always isotropic.

When the fluid is brought to a stop, at a stagnation point, the "total" pressure is the stagnation pressure is it is also isotropic there. The fact that the pressure increases and is due to the term "dynamic pressure" but it is simply the momentum transfer at the point of impact.

If a hypothetical moving fluid parcel at speed $v$ is not brought to a stop, its (static pressure) can increase, in theory, by the contribution from the dynamic term. However, when the fluid parcel is not stopped, all the pressure is the static pressure measured either by a piezometer in a direction perpendicular to the flow or by a manometer moving at the same speed v together with the flow...