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Archive 1 has been created with a link at above right. It is an exact copy of the talk page as it was before this edit. Archive 2, when needed in the future, should be a new subpage (same as creating an article) titled "Talk:Static pressure/Archive 2" and the link added to the template on this page's code. For further information on archiving see Wikipedia:How to archive a talk page. See also User:5Q5 for the used archiving procedure. Thank you. Crowsnest ( talk) 21:34, 20 May 2008 (UTC)
On 20 April Wiki-editor Djhnsn added links to the following two sites:
On 22 April Wiki-editor Giuliopp removed the two links, commenting that a “sneaky advertisement” was being removed.
These sites contain valuable and relevant reports produced by the US National Aeronautics and Space Administration. However, these web-sites are operated by a commercial organisation (SpaceAge Control, Inc.) and each of the two web-sites contains one page of promotional material supporting the commercial organisation.
The above actions raise the question of how much advertising is tolerable at a site linked to Wikipedia? Wiki advice given here says:
If the link is to a relevant and informative site that should otherwise be included, please consider mentioning it on the talk page and let neutral and independent Wikipedia editors decide whether to add it. This is in line with the conflict of interest guidelines.
In the spirit of this advice, I am mentioning this matter on this Talk page. I believe these two NASA reports are highly valuable and relevant to the concept of static pressure. The reports were written when William Gracey was head of NASA’s Langley Research Center. Gracey subsequently incorporated this material in his book, and that book is quoted and cited in Static pressure. The reports are therefore particularly relevant because they give all Wiki readers access to information that was previously only accessible to those few readers who have access to Gracey's book. I also believe that one page of corporate promotional material is well within the bounds of what can be tolerated. What do other Wiki-editors think? If there is no suitably-substantiated objection to restoring these two links within a week or so I will be inclined to restore them.
If there is a rule that might prevent these two valuable NASA reports from being accessible via Wikipedia I consider that Wikipedia:Ignore all rules is relevant – “If a rule prevents you from improving or maintaining Wikipedia, ignore it.” It is true that each of the links to the two NASA reports contains a page of corporate promotional material but, on balance, I believe these two links improved Wikipedia (until they were deleted.) Dolphin51 ( talk) 12:30, 22 April 2008 (UTC)
This article does not appear to at all address the common use of static pressure in HVAC systems. That seems strange, since I think it probably the most common usage. (E.g. a fan might produce 140 CFM of air blowing into a static pressure of 0 in h20, but only 110 CFM blowing into 0.5 in h20.)
I think that's part of what the first archived comment is about, as well... jhawkinson ( talk) 18:04, 14 March 2009 (UTC)
It seems from my perspective you guys are going about this from completely the wrong standpoint. The article keeps making the statement comparing static pressure to Bernoulli's equation. This is not the origination of static pressure in any sense of the meaning. Approaching it from a energy conservation standpoint, all energy and momentum concerns should keep in mind that fluids have macro and micro properties. While dynamics pressure is a macro energy, the pressure term in bernoulli's equation measures the microscopic energy associated with the fluid. This makes more sense viewing the equation as such: P*dV + 1/2*m*v^2=total energy. Now I'm also going to give a derivation from statistical thermodynamics. Static pressure is most easy to conceptualize in that it is the pressure you would measure the fluid having while moving in the same inertial reference frame as the local fluid. From a control volume analysis within the reference frame of the fluid, the pressure times the velocity can actually be found to be 1/3 of the kinetic energy of the gas, directly relating it the density and temperature of the gas. Keep in mind that static temperature is entirely a measure of the kinetic energy of the molecules as observed from the reference frame of the fluid. My point in all this is to say that the static pressure of a fluid is much more than the article seems to be making it out to be. It is a direct measure of the force the fluid applies to its surroundings (whether that is other fluid or a surface). This defines all aerodynamics through Mach=5. It is equivalent to the temperature of the fluid and kinetic energy associated with the microscopic affects of the fluid. So yes, Bernoulli's usage of Pstatic is entirely 100% factual given the incompressible assumption and it is much more than simply something measured from some reference point in the fluid, as the article states. If anyone wishes to debate this, let me know. Iron_Engineer ( talk) 19:54, 1 July 2009 (UTC)
This bears reference to the topic "Static Fluid Pressure". My question to the members is:
When we say Pitot-tube pressure we mean Dynamic Pressure and when we say Static pressure we are reffering to simple hole drilled on the surface of the conduit (say pipe) opening in to a tube open to atmosphere and long enough to hold the water thrown by STATIC Head!
Now if instead of the Water we assume Steam (560 deg.C, 160 bar(a)) as being the flowing media and the small hole be replaced with a "Tee" followed by a very long topsy turvy Piping closed with a valve at the other end (to visualise, assume simple By-pass connection of Main steam supply pipe)
My question arises here as follows :
1. Will the fluid, flowing into the branching pipe, have Static Pressure only i.e; to say Elevation Head plus the fluid pressure corresponding to its pressure at that temp?
2. Since the fluid is being stopped at the end of its travel (assuming branch run long enough than the Dynamic head), would the pressure and temp in the branch be termed as Stagnation Temp & Pressure? If so, the stagnation pressure and temp. should come in to picture in the branch line only after the Dynamic head length has passed. This again is perplexing as to how in a particular conduit one can have two pressures!!, though at a distance?
3. Also, if the steam is flowing in the main line (continuously consumed by Turbine), and the bypass line (branch) is closed at the other end. Wouldn't the Fast flowing steam in Main pipe try to SUCK-in the fluid (steam) in branch. (The principle on which Gas stove works viz; Fast flowing Gas sucks the air in through the holes provided in the gas supply tube)??
Kindly shed some light as to how to picturise the phenomenon.
regards, —Preceding unsigned comment added by Pipeyoga011 ( talk • contribs) 19:21, 6 September 2010
Thanks Dolphin51 for your reply. I shall try to go through the contents and onwards put such queries over SIENCE REFERENCE DESK ! Regards, pipeyoga —Preceding unsigned comment added by Pipeyoga011 ( talk • contribs) 18:12, 16 September 2010 (UTC)
While studying the forces on sails, I came across the term 'static pressure' without a proper explanation, so I looked it up here. I assumed that it is the pressure the fluid (in this case the air) would have if not in motion (so in this case atmospheric pressure). The article seems to want to say this too, but it doesn't. But is it is so simple, then why doesn't it simply say it? Or am I wrong? DirkvdM ( talk) 17:17, 8 August 2011 (UTC)
Thanks for your responses. I really want to understand this because it is essential to my present study.
It seems to me that all three alternatives, 'static pressure', 'free stream pressure' and simply 'pressure' are confusing. The fluid doesn't have to be static. 'Free stream pressure' covers that, but it is incomplete because it also existst (and is then different) when the stream is no longer free. And simply 'pressure' I would interpret as 'total pressure'. Since 'static pressure' is the most widely used and doesn't have an alternative meaning (*), I suppose that is the best choice (the lesser of three evils :) ).
(*) That is, assuming that for fluid statics it doesn't matter if the fluid is in motion, just that a flow, if any, is homogenous and can therefore be considered internally static, even if in a larger scope it isn't. So the pressure of a fluid that is static (relative to what?) is a non-issue.
This is how I now understand it:
Is that about right?
A few more issues:
1. A further explanation of what causes this dynamic pressure would be helpful. As I understand it, that is either, when the object moves through it, because it compresses the fluid, or, when it flows past an object, it has less space to move through, so it has to speed up. But the two effects are the same, right? But then I can't think of a good way to formulate it.
2. What if the flow is not homogenous? I suppose that is the case for the fluid some distance from the object, but still deflected by it. That would then be how the pressure propagates throuh the fluid, with the streamlines acting as deflecting objects upon each other (with the total force being spread over a larger area, so the dynamic pressure dropping and the static pressure rising). And what about colliding ocean currents? They will merge to some extent, complicating matters tremendously, I assume.
3. Is the fact that the total pressure is constant a case of conservation of energy?
4. Is the drop in static pressure the same as the underpressure created by a flowing fluid? That is a concept many people, including myself, will be more familiar with, so if correct it would be helpful addition to the article.
5. Can one say that static pressure works perpendicular to the object's surface (or rather the streamlines), and dynamic pressure tangential to it? I know pressure is not a vector (so should I say a force (potentially) caused by the pressure?), but looking at it this way would nicely explain how the transferral of static to dynamic pressure would create a lift-force on a sail or wing. And it would give us some nice terminology: normal pressure and tangential pressure. But of course we can't make up our own terminology, however good it may be. :)
DirkvdM (
talk)
09:21, 14 August 2011 (UTC)
Ah, that analogy between the components of mechanical energy and total pressure is really hepful. Thanks Dolphin! That conservation of energy is the reason for total pressure being constant should definitely go into the article. But also the analogy? How far can one take that? In the book I'm reading ('Sail Performance' by C.A. Marchaj), the author writes "... kinetic energy [...] is usually called the dynamic pressure ...", which I thought was such nonsense (energy is not pressure) that I sort of skipped that bit. Maybe I should have given it more thought.
That dynamic pressure can't be measured still bugs me. Well, of course it can, by using a pitot tube and substracting the static pressure from the total pressure. But if there is a pressure, it should be possible for it to work on something, and thus be measured. Giuliopp, you say the body doesn't sense it on its surface. Anywhere? Then the pressure doesn't work in any direction? (Whereas the static pressure is equally strong in all directions, I assume.) This sounds a bit like dark energy in cosmology, which can't be measured and is thought up to explain the accelerating expansion of the universe. Likewise, you know the total pressure has to remain constant, but still you see a drop in pressure. so there has to be some pressure you can't measure. 'Dark pressure'? :)
DirkvdM (
talk)
17:42, 14 August 2011 (UTC)
@DirkvdM: It is fairly simple to convince yourself that dynamic pressure is the kinetic energy of one unit of volume. Energy, including kinetic energy, is specified in joules, and volume is specified in cubic metres. So the kinetic energy of each unit of volume is specified in joules per cubic metre. One joule is equivalent to one newton.metre so the kinetic energy of each unit of volume can be specified in newton.metres per cubic metre or newtons per square metre. One newton per square metre is a measure of pressure (or stress) and is equivalent to one pascal which is the SI unit of pressure. You can see that dynamic pressure is the kinetic energy of each unit of volume of fluid; static pressure is the potential energy of each unit of volume, and total pressure is the mechanical energy of each unit of volume.
The reason I say dynamic pressure and total pressure can’t be measured is probably rather obscure. Kinetic energy, dynamic pressure, mechanical energy and total pressure are all dependent on the choice of reference frame. (I am sitting at my computer and I can take the view that my speed is zero so my kinetic energy is also zero. Alternatively I can choose an inertial reference frame fixed to the center of the Earth, take the view that my speed is 465 m.s-1 times the cosine of my latitude and calculate my kinetic energy with the usual formula. In doing so I haven’t measured my kinetic energy – I have selected an inertial reference frame, measured my speed and calculated my kinetic energy.) It is possible to measure a unique value for forces, masses and accelerations; but displacements and speeds (and velocities) are dependent on the choice of reference frame so it isn't possible to measure a unique value for any displacement or speed. See Galilean invariance. (Prior to Galileo, people tried to determine the absolute velocity of the Earth. Galileo showed that there is no such thing as an absolute velocity. If we knew the mass of a body or the density of a fluid, and we could measure the kinetic energy of that body, or the dynamic pressure of that fluid, we could determine its absolute velocity. Galileo convinced the scientific community it couldn't be done.)
For the same reasoning I say we can’t measure dynamic pressure directly. First we select a reference frame and measure the speed of the fluid relative to that reference frame. Then we use the half rho speed squared formula to calculate dynamic pressure. If we select a different reference frame we measure a different speed and end up with a different dynamic pressure.
Strictly speaking, a pitot tube doesn’t measure total pressure – it brings the fluid to a stop and measures the stagnation pressure. Stagnation pressure is the static pressure in a region where the fluid speed is zero - such as inside a pitot tube. It is equal to total pressure in one reference frame only.
We measure static pressure using a gauge, and we measure speed using a pitot-static system or any one of a number of devices. Once we have selected a reference frame we can determine the speed and calculate dynamic pressure and total pressure. The important conclusion for the purpose of this discussion is that if we plug a mercury column or aneroid or bourdon tube into a body of fluid we see the pressure (static pressure) of that fluid. We don’t see dynamic pressure or total pressure. In the special case of a reference frame attached to the body of fluid, the speed of the fluid is zero, its dynamic pressure is zero and total pressure is equal to static pressure. You can argue that in that situation total pressure can be measured using the mercury column, aneroid or bourdon tube, but that is a coincidence that only occurs with that one reference frame. Dolphin ( t) 23:32, 14 August 2011 (UTC)
If that satisfies you that's good, as long as you don't put it in the article :) You say that "if the formula comes out wiht the same unit it has to be the same thing. It has to be pressure". That conclusion is completely arbitrary and I'll leave this discussion giving you another example: work and torque have both in the same unit measure (Newtons times meters) but express two completely different physical quantities. One represents energy exchanged by a system, the other one expresses how strongly a force can make a body rotate around an axis (even when no energy is exchanged at all). How about that? Regards. -- Giuliopp ( talk) 13:20, 16 August 2011 (UTC)
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Hey dolphin51 and Giuliopp, judging from now-archived entries in this talk page, both of you at least were interested in this article. So I wanted to let you know I've now stuck my own toe into the water. The lead really needed a definition, so I've ventured one. Then, while I was there, I also did some stylistic twiddling to improve clarity and ease of use. I hope the heavens don't come crashing down on me.— PaulTanenbaum ( talk) 18:30, 3 May 2022 (UTC)
![]() | This article is rated Start-class on Wikipedia's
content assessment scale. It is of interest to the following WikiProjects: | |||||||||||||
|
|
Archive 1 has been created with a link at above right. It is an exact copy of the talk page as it was before this edit. Archive 2, when needed in the future, should be a new subpage (same as creating an article) titled "Talk:Static pressure/Archive 2" and the link added to the template on this page's code. For further information on archiving see Wikipedia:How to archive a talk page. See also User:5Q5 for the used archiving procedure. Thank you. Crowsnest ( talk) 21:34, 20 May 2008 (UTC)
On 20 April Wiki-editor Djhnsn added links to the following two sites:
On 22 April Wiki-editor Giuliopp removed the two links, commenting that a “sneaky advertisement” was being removed.
These sites contain valuable and relevant reports produced by the US National Aeronautics and Space Administration. However, these web-sites are operated by a commercial organisation (SpaceAge Control, Inc.) and each of the two web-sites contains one page of promotional material supporting the commercial organisation.
The above actions raise the question of how much advertising is tolerable at a site linked to Wikipedia? Wiki advice given here says:
If the link is to a relevant and informative site that should otherwise be included, please consider mentioning it on the talk page and let neutral and independent Wikipedia editors decide whether to add it. This is in line with the conflict of interest guidelines.
In the spirit of this advice, I am mentioning this matter on this Talk page. I believe these two NASA reports are highly valuable and relevant to the concept of static pressure. The reports were written when William Gracey was head of NASA’s Langley Research Center. Gracey subsequently incorporated this material in his book, and that book is quoted and cited in Static pressure. The reports are therefore particularly relevant because they give all Wiki readers access to information that was previously only accessible to those few readers who have access to Gracey's book. I also believe that one page of corporate promotional material is well within the bounds of what can be tolerated. What do other Wiki-editors think? If there is no suitably-substantiated objection to restoring these two links within a week or so I will be inclined to restore them.
If there is a rule that might prevent these two valuable NASA reports from being accessible via Wikipedia I consider that Wikipedia:Ignore all rules is relevant – “If a rule prevents you from improving or maintaining Wikipedia, ignore it.” It is true that each of the links to the two NASA reports contains a page of corporate promotional material but, on balance, I believe these two links improved Wikipedia (until they were deleted.) Dolphin51 ( talk) 12:30, 22 April 2008 (UTC)
This article does not appear to at all address the common use of static pressure in HVAC systems. That seems strange, since I think it probably the most common usage. (E.g. a fan might produce 140 CFM of air blowing into a static pressure of 0 in h20, but only 110 CFM blowing into 0.5 in h20.)
I think that's part of what the first archived comment is about, as well... jhawkinson ( talk) 18:04, 14 March 2009 (UTC)
It seems from my perspective you guys are going about this from completely the wrong standpoint. The article keeps making the statement comparing static pressure to Bernoulli's equation. This is not the origination of static pressure in any sense of the meaning. Approaching it from a energy conservation standpoint, all energy and momentum concerns should keep in mind that fluids have macro and micro properties. While dynamics pressure is a macro energy, the pressure term in bernoulli's equation measures the microscopic energy associated with the fluid. This makes more sense viewing the equation as such: P*dV + 1/2*m*v^2=total energy. Now I'm also going to give a derivation from statistical thermodynamics. Static pressure is most easy to conceptualize in that it is the pressure you would measure the fluid having while moving in the same inertial reference frame as the local fluid. From a control volume analysis within the reference frame of the fluid, the pressure times the velocity can actually be found to be 1/3 of the kinetic energy of the gas, directly relating it the density and temperature of the gas. Keep in mind that static temperature is entirely a measure of the kinetic energy of the molecules as observed from the reference frame of the fluid. My point in all this is to say that the static pressure of a fluid is much more than the article seems to be making it out to be. It is a direct measure of the force the fluid applies to its surroundings (whether that is other fluid or a surface). This defines all aerodynamics through Mach=5. It is equivalent to the temperature of the fluid and kinetic energy associated with the microscopic affects of the fluid. So yes, Bernoulli's usage of Pstatic is entirely 100% factual given the incompressible assumption and it is much more than simply something measured from some reference point in the fluid, as the article states. If anyone wishes to debate this, let me know. Iron_Engineer ( talk) 19:54, 1 July 2009 (UTC)
This bears reference to the topic "Static Fluid Pressure". My question to the members is:
When we say Pitot-tube pressure we mean Dynamic Pressure and when we say Static pressure we are reffering to simple hole drilled on the surface of the conduit (say pipe) opening in to a tube open to atmosphere and long enough to hold the water thrown by STATIC Head!
Now if instead of the Water we assume Steam (560 deg.C, 160 bar(a)) as being the flowing media and the small hole be replaced with a "Tee" followed by a very long topsy turvy Piping closed with a valve at the other end (to visualise, assume simple By-pass connection of Main steam supply pipe)
My question arises here as follows :
1. Will the fluid, flowing into the branching pipe, have Static Pressure only i.e; to say Elevation Head plus the fluid pressure corresponding to its pressure at that temp?
2. Since the fluid is being stopped at the end of its travel (assuming branch run long enough than the Dynamic head), would the pressure and temp in the branch be termed as Stagnation Temp & Pressure? If so, the stagnation pressure and temp. should come in to picture in the branch line only after the Dynamic head length has passed. This again is perplexing as to how in a particular conduit one can have two pressures!!, though at a distance?
3. Also, if the steam is flowing in the main line (continuously consumed by Turbine), and the bypass line (branch) is closed at the other end. Wouldn't the Fast flowing steam in Main pipe try to SUCK-in the fluid (steam) in branch. (The principle on which Gas stove works viz; Fast flowing Gas sucks the air in through the holes provided in the gas supply tube)??
Kindly shed some light as to how to picturise the phenomenon.
regards, —Preceding unsigned comment added by Pipeyoga011 ( talk • contribs) 19:21, 6 September 2010
Thanks Dolphin51 for your reply. I shall try to go through the contents and onwards put such queries over SIENCE REFERENCE DESK ! Regards, pipeyoga —Preceding unsigned comment added by Pipeyoga011 ( talk • contribs) 18:12, 16 September 2010 (UTC)
While studying the forces on sails, I came across the term 'static pressure' without a proper explanation, so I looked it up here. I assumed that it is the pressure the fluid (in this case the air) would have if not in motion (so in this case atmospheric pressure). The article seems to want to say this too, but it doesn't. But is it is so simple, then why doesn't it simply say it? Or am I wrong? DirkvdM ( talk) 17:17, 8 August 2011 (UTC)
Thanks for your responses. I really want to understand this because it is essential to my present study.
It seems to me that all three alternatives, 'static pressure', 'free stream pressure' and simply 'pressure' are confusing. The fluid doesn't have to be static. 'Free stream pressure' covers that, but it is incomplete because it also existst (and is then different) when the stream is no longer free. And simply 'pressure' I would interpret as 'total pressure'. Since 'static pressure' is the most widely used and doesn't have an alternative meaning (*), I suppose that is the best choice (the lesser of three evils :) ).
(*) That is, assuming that for fluid statics it doesn't matter if the fluid is in motion, just that a flow, if any, is homogenous and can therefore be considered internally static, even if in a larger scope it isn't. So the pressure of a fluid that is static (relative to what?) is a non-issue.
This is how I now understand it:
Is that about right?
A few more issues:
1. A further explanation of what causes this dynamic pressure would be helpful. As I understand it, that is either, when the object moves through it, because it compresses the fluid, or, when it flows past an object, it has less space to move through, so it has to speed up. But the two effects are the same, right? But then I can't think of a good way to formulate it.
2. What if the flow is not homogenous? I suppose that is the case for the fluid some distance from the object, but still deflected by it. That would then be how the pressure propagates throuh the fluid, with the streamlines acting as deflecting objects upon each other (with the total force being spread over a larger area, so the dynamic pressure dropping and the static pressure rising). And what about colliding ocean currents? They will merge to some extent, complicating matters tremendously, I assume.
3. Is the fact that the total pressure is constant a case of conservation of energy?
4. Is the drop in static pressure the same as the underpressure created by a flowing fluid? That is a concept many people, including myself, will be more familiar with, so if correct it would be helpful addition to the article.
5. Can one say that static pressure works perpendicular to the object's surface (or rather the streamlines), and dynamic pressure tangential to it? I know pressure is not a vector (so should I say a force (potentially) caused by the pressure?), but looking at it this way would nicely explain how the transferral of static to dynamic pressure would create a lift-force on a sail or wing. And it would give us some nice terminology: normal pressure and tangential pressure. But of course we can't make up our own terminology, however good it may be. :)
DirkvdM (
talk)
09:21, 14 August 2011 (UTC)
Ah, that analogy between the components of mechanical energy and total pressure is really hepful. Thanks Dolphin! That conservation of energy is the reason for total pressure being constant should definitely go into the article. But also the analogy? How far can one take that? In the book I'm reading ('Sail Performance' by C.A. Marchaj), the author writes "... kinetic energy [...] is usually called the dynamic pressure ...", which I thought was such nonsense (energy is not pressure) that I sort of skipped that bit. Maybe I should have given it more thought.
That dynamic pressure can't be measured still bugs me. Well, of course it can, by using a pitot tube and substracting the static pressure from the total pressure. But if there is a pressure, it should be possible for it to work on something, and thus be measured. Giuliopp, you say the body doesn't sense it on its surface. Anywhere? Then the pressure doesn't work in any direction? (Whereas the static pressure is equally strong in all directions, I assume.) This sounds a bit like dark energy in cosmology, which can't be measured and is thought up to explain the accelerating expansion of the universe. Likewise, you know the total pressure has to remain constant, but still you see a drop in pressure. so there has to be some pressure you can't measure. 'Dark pressure'? :)
DirkvdM (
talk)
17:42, 14 August 2011 (UTC)
@DirkvdM: It is fairly simple to convince yourself that dynamic pressure is the kinetic energy of one unit of volume. Energy, including kinetic energy, is specified in joules, and volume is specified in cubic metres. So the kinetic energy of each unit of volume is specified in joules per cubic metre. One joule is equivalent to one newton.metre so the kinetic energy of each unit of volume can be specified in newton.metres per cubic metre or newtons per square metre. One newton per square metre is a measure of pressure (or stress) and is equivalent to one pascal which is the SI unit of pressure. You can see that dynamic pressure is the kinetic energy of each unit of volume of fluid; static pressure is the potential energy of each unit of volume, and total pressure is the mechanical energy of each unit of volume.
The reason I say dynamic pressure and total pressure can’t be measured is probably rather obscure. Kinetic energy, dynamic pressure, mechanical energy and total pressure are all dependent on the choice of reference frame. (I am sitting at my computer and I can take the view that my speed is zero so my kinetic energy is also zero. Alternatively I can choose an inertial reference frame fixed to the center of the Earth, take the view that my speed is 465 m.s-1 times the cosine of my latitude and calculate my kinetic energy with the usual formula. In doing so I haven’t measured my kinetic energy – I have selected an inertial reference frame, measured my speed and calculated my kinetic energy.) It is possible to measure a unique value for forces, masses and accelerations; but displacements and speeds (and velocities) are dependent on the choice of reference frame so it isn't possible to measure a unique value for any displacement or speed. See Galilean invariance. (Prior to Galileo, people tried to determine the absolute velocity of the Earth. Galileo showed that there is no such thing as an absolute velocity. If we knew the mass of a body or the density of a fluid, and we could measure the kinetic energy of that body, or the dynamic pressure of that fluid, we could determine its absolute velocity. Galileo convinced the scientific community it couldn't be done.)
For the same reasoning I say we can’t measure dynamic pressure directly. First we select a reference frame and measure the speed of the fluid relative to that reference frame. Then we use the half rho speed squared formula to calculate dynamic pressure. If we select a different reference frame we measure a different speed and end up with a different dynamic pressure.
Strictly speaking, a pitot tube doesn’t measure total pressure – it brings the fluid to a stop and measures the stagnation pressure. Stagnation pressure is the static pressure in a region where the fluid speed is zero - such as inside a pitot tube. It is equal to total pressure in one reference frame only.
We measure static pressure using a gauge, and we measure speed using a pitot-static system or any one of a number of devices. Once we have selected a reference frame we can determine the speed and calculate dynamic pressure and total pressure. The important conclusion for the purpose of this discussion is that if we plug a mercury column or aneroid or bourdon tube into a body of fluid we see the pressure (static pressure) of that fluid. We don’t see dynamic pressure or total pressure. In the special case of a reference frame attached to the body of fluid, the speed of the fluid is zero, its dynamic pressure is zero and total pressure is equal to static pressure. You can argue that in that situation total pressure can be measured using the mercury column, aneroid or bourdon tube, but that is a coincidence that only occurs with that one reference frame. Dolphin ( t) 23:32, 14 August 2011 (UTC)
If that satisfies you that's good, as long as you don't put it in the article :) You say that "if the formula comes out wiht the same unit it has to be the same thing. It has to be pressure". That conclusion is completely arbitrary and I'll leave this discussion giving you another example: work and torque have both in the same unit measure (Newtons times meters) but express two completely different physical quantities. One represents energy exchanged by a system, the other one expresses how strongly a force can make a body rotate around an axis (even when no energy is exchanged at all). How about that? Regards. -- Giuliopp ( talk) 13:20, 16 August 2011 (UTC)
Hello fellow Wikipedians,
I have just modified one external link on Static pressure. Please take a moment to review my edit. If you have any questions, or need the bot to ignore the links, or the page altogether, please visit this simple FaQ for additional information. I made the following changes:
When you have finished reviewing my changes, please set the checked parameter below to true or failed to let others know (documentation at {{
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This message was posted before February 2018.
After February 2018, "External links modified" talk page sections are no longer generated or monitored by InternetArchiveBot. No special action is required regarding these talk page notices, other than
regular verification using the archive tool instructions below. Editors
have permission to delete these "External links modified" talk page sections if they want to de-clutter talk pages, but see the
RfC before doing mass systematic removals. This message is updated dynamically through the template {{
source check}}
(last update: 5 June 2024).
Cheers.— InternetArchiveBot ( Report bug) 23:19, 1 December 2016 (UTC)
Hey dolphin51 and Giuliopp, judging from now-archived entries in this talk page, both of you at least were interested in this article. So I wanted to let you know I've now stuck my own toe into the water. The lead really needed a definition, so I've ventured one. Then, while I was there, I also did some stylistic twiddling to improve clarity and ease of use. I hope the heavens don't come crashing down on me.— PaulTanenbaum ( talk) 18:30, 3 May 2022 (UTC)