Static Pressure

1. Nov 15, 2014

umair20

In many books & articles that I have read; static pressure has been defined as pressure of a fluid at rest. In some articles, it is said to be the pressure which is applied against frictional forces in a fluid. But I want to know how a fluid develops static pressure & what is basic force in this pressure ? ...just like dynamic pressure is due to velocity & hydro-static due to height!

2. Nov 15, 2014

Staff: Mentor

It is typically due to the fluid being constricted in a container.

3. Nov 15, 2014

Staff: Mentor

Terms like static pressure and dynamic pressure give me a real pain, particularly since they imply that there are different kinds of pressure that can exist. Pressure is pressure, period, and, at least for an incompressible fluid, corresponds to the isotropic part of the stress tensor.

Chet

4. Nov 15, 2014

Staff: Mentor

Those terms just describe how the pressure is created. In Bernoulli's principle scenarios, you need a way to keep them separated. Otherwise, people often mistakenly think that "pressure" drops in a moving fluid.

5. Nov 16, 2014

Staff: Mentor

Can you please elaborate. I still don't see why you have to keep them separated. To me, it would only confuse things. I have seen many posts on PF in which the OP thinks that two "kinds of pressure" can exist simultaneously, whatever that means.

Chet

6. Nov 16, 2014

Staff: Mentor

Consider the question: "Why does pressure drop in a moving fluid? Doesn't that violate conservation of energy?" We get questions like that a lot. It is a wrong question because pressure doesn't drop in a moving fluid. The three forms (or causes, if you prefer) of pressure are always equal to each other. The question arises from an explanation of Bernoulli's principle that labels static pressure as "pressure" but doesn't adequatly describe velocity pressure or head pressure, leading people to believe they aren't pressure.

A related reason why I prefer treating them separately is that if I am measuring it with a pressure gauge and it is different from another orientation of a similar probe, it is tough to say the two "pressures" are measuring the same thing.

7. Nov 16, 2014

Staff: Mentor

I don't think I'm going to be able to figure this out. After all these years of experience with fluid mechanics, I still fail to see what the issue is, and why the pressure has to be thought of in terms of separate contributions. I'm sure this is not the way I learned the Bernoulli equation. I'll try to pay more attention to other posts in this area and see if I can get a better understanding of what the issue is.

Chet

8. Nov 16, 2014

It's really just nomenclature. Static pressure is the only "real pressure" in terms of figuring out the force on a surface. Dynamic pressure is so called because it has units of pressure but that's about it. You can't feel it. It's really just kinetic energy per unit volume. In the context of energy, static pressure is akin to spring potential energy per unit volume. The two combined (or three if you care about Gravity) are total (or stagnation) pressure, which is the total energy of the system and is the conserved quantity in a conservative system.

But like I said, static pressure is the only "real" pressure that you can feel.

9. Nov 16, 2014

Staff: Mentor

This is what I simply call "pressure." But why add the adjective "static?" I contend that this is just a source of confusion to students. Thoughts?

Chet

10. Nov 16, 2014

Well I think it serves a couple of purposes. First, it helps differentiate the terms in Bernoulli's equation, which is obviously an important equation even if a lot of people don't seem to understand its limitations.

More importantly, there are a number of parameters that scale with dynamic pressure (such as drag), so having a name for it is useful. It has units of pressure and a definite relationship to the static pressure, so calling it a pressure seems reasonable to me.

Also, total/stagnation pressure is, in a sense, an actual pressure in that it physically represents the static pressure at a reference condition (that being isentropically slowing a given flow down to zero velocity).

So I think calling them all pressure is fine. I think the shortcoming is in how this is taught in some sources. For example, saying "dynamic pressure is pressure that arises from the velocity" doesn't really make any sense and seems to indicate a lack of understanding of what it really is.

I suppose that having originally learned these things as a mechanical engineer as an undergrad and then seeing it from the aerodynamics perspective as a graduate student, the nomenclature makes more sense from an aerodynamics perspective than it does from a piping or chemical process perspective.

11. Nov 16, 2014

Staff: Mentor

I really don't understand. How can you explain Bernoulli's principle without the adjectives? For example:

I put a pressure probe into an air duct and measure 1" w.g. of pressure. What is the speed of the flow?

Here's a hint: no matter what you answer, I'll tell you that you are wrong.

12. Nov 16, 2014

Staff: Mentor

What's w.g.? Water gage pressure?

13. Nov 16, 2014

Staff: Mentor

Water Gauge. It's what a manometer measures directly (well -- any unit of distance will do) and is used in my industry as a convenient unit for airflow since the numbers are low but not too low.
http://en.wikipedia.org/wiki/Pressure_measurement#Units.

Let's make it multiple choice:

A. 4000 feet/sec
B. 0
C. 100 feet/sec
D. Not enough information provided

[edit2] I don't want to make this too hard because it is just an example for discussion of my point. So if you aren't comfortable with the English Units math, here's a chart that may or may not be helpful: http://www.airmonitor.com/pdfs/industrial_public/brochures/BRO_Conversion_Chart.pdf

Last edited: Nov 16, 2014
14. Nov 16, 2014

Staff: Mentor

What direction is the gage facing? Of course, if it's facing the flow, its very presence changes the pressure at the location of the gage. In this case, it is measuring the stagnation pressure. So you need the pressure from a gage that doesn't disturb the flow (say upstream, or by rotating your gage perpendicular to the flow) and you need the stagnation pressure. This combination will tell you the velocity of the undisturbed stream. I'm sure I'm not telling you anything that you don't already know. OK, so I had to use the adjective stagnation; but that is just what specifies how this particular kind of gage is applied, or what happens when a flow velocity is forced to zero. I don't know. To me stagnation pressure has more physical significance than "static pressure" or "dynamic pressure." Maybe I'm just biased by my training.

Chet

15. Nov 16, 2014

Staff: Mentor

So you get my point. Adding one word to the problem statement changes it completely and you recognize the value of adding such a word (also, from my link you can see it is a convention in my industry in the US). I'm a bit confused by the last sentence though: since stagnation pressure is dynamic pressure plus static pressure, I can't see how it could be said that stagnation pressure has more significance than static or dynamic. Either all of them should have significance or none should.

Also, the "pressure probe" could be a pito-static probe. So it could get you dynamic pressure on its own, without another probe.

16. Nov 16, 2014

Staff: Mentor

Russ,

Until today, I didn't even know what the terms "static pressure" and "dynamic pressure" referred to. Stagnation pressure, I did know. I thought that when the term "static pressure" was used in Physics Forums, it referred to hydrostatic pressure. No wonder I was confused. Anyway, I was unfamiliar with the terminology that was being used.

Chet

17. Nov 16, 2014

Staff: Mentor

This will be slightly argumentative, but I disagree. I think total pressure is the only "real pressure". Why? Because a fluid imparts pressure against something else by hitting it -- whether the motion is random or organized. Without motion, there is no pressure.

Anyway, I recognize that in a manometer or other pressure gauge the tubes are filled with a non-moving (in bulk) fluid so you measure only static pressure even if it is created by a moving fluid stagnating at the opening of the pito-tube. But that's not the only scenario where you might "feel" it: if someone hits you with a fire hose, you will feel the dynamic pressure directly as the water achieves stagnation against your skin.

18. Nov 16, 2014

Staff: Mentor

Fair enough -- I'm still confused though. What would you call the terms in an equation that finds stagnation pressure? In the previous post you described how to find them without giving them names, which is fine, but in an equation, the terms kinda need names.

19. Nov 16, 2014

Staff: Mentor

Do you mean the following equation:
$$p_{stag}=p+\frac{1}{2}\rho v^2$$

Chet

20. Nov 16, 2014

Staff: Mentor

Yes -- or better yet:
$$p_{???}=\frac{1}{2}\rho v^2$$

I know you would just call the "p" term in your equation "pressure", but what would you call the last term (if anything)? Or in my equation? In this case, I'm really not trying to be argumentative. The reading you get from a pressure gauge is "pressure" and I would think that in order to insert that reading into an equation you would need to have a convenient name for it. Or, for convenience, if the pressure reading needs to be displayed on a control panel, it would be cumbersome to have a sentence describing it instead just a two-word name.