Talk:Impact pressure

Uncategorised comments
This topic should redirect to the dynamic pressure page. It is the very same subject. —Preceding unsigned comment added by 69.221.157.5 (talk) 00:26, 13 April 2009 (UTC)

From what I can tell, this is slightly different from dynamic pressure in its application to supersonic flows. Note its definition as the difference between pitot measured pressure and static pressure. In supersonic flow, the flow will cross a shock before entering the pitot tube, thus altering it from the true dynamic pressure. I have never used the term before myself, but it appears this is what's meant, making it a slightly different phenomena then dynamic pressure. It seems better fitted though to slap a redirect onto it to a new section in dynamic pressure covering this topic. Iron_Engineer (talk) 18:49, 1 July 2009 (UTC)


 * Impact pressure is not dynamic pressure, regardless of the flow being subsonic or supersonic. There is very close agreement as far as airspeed measurement is concerned, until about Mach 0.3 when the assumption of an irrotational, incompressible, etc, flow becomes inadequate. For supersonic flow there is an additional problem of entropy increase across the strong bow shock wave in front of the pitot probe, and hence a loss in total pressure - for this see the Rayleigh supersonic pitot formula. See US Navy Hydrographic Office Publication 216. Washington: US Government Printing Office, 1963, p. 93; or any 'mature' handbook on compressible flow & airspeed measurement (J. D. Anderson's Fundamentals of Aerodynamics comes to mind), or NACA Technical Report 837, Standard Nomenclature for Airspeeds with Tables and Charts for use in Calculation of Airspeed. (Langley, 1946.)


 * The importance in distinguishing between impact pressure and dynamic pressure cannot be overemphasised. (Weirpwoer (talk) 04:06, 31 October 2009 (UTC))


 * The majority of authors, and certainly NACA Technical Report 837 mentioned above by Weirpwoer, define dynamic pressure as $$\tfrac{1}{2} \rho v^2$$. This is the difference between stagnation pressure and static pressure only in incompressible flows, and is usually acceptable in compressible flows up to approximately 0.3 Mach.  Impact pressure is the difference between stagnation pressure and static pressure in compressible flows, including compressible flows above 0.3 Mach.  Clearly, impact pressure and $$\tfrac{1}{2} \rho v^2$$ are not the same.  For this reason, Impact pressure and Dynamic pressure should not be merged.


 * A minority of authors in the field of compressible flows, most of them British, do not use the expression impact pressure. Instead, they use the expression dynamic pressure but they do not define it as $$\tfrac{1}{2} \rho v^2$$ because that would restrict its meaning to incompressible flows.  Instead, they define dynamic pressure as the difference between stagnation pressure and static pressure; thereby matching the definition of impact pressure.


 * On 21 October 2009 I edited Dynamic pressure to add information about this use of the expression dynamic pressure. My edits can be seen in the diff.


 * If the article Dynamic pressure was dedicated solely to its minority use with compressible flows, then it could be merged with Impact pressure. However, seeing the article primarily covers incompressible flows, the two cannot be merged. Dolphin51 (talk) 01:14, 17 November 2009 (UTC)

Comment on equivalent airspeed
The remark about the ADC deriving equivalent airspeed from impact pressure is slightly misleading. Equivalent airspeed is defined by convention and relies upon a restricted solution to Euler's equation where density is considered to be constant - as such it is based upon dynamic pressure and not impact pressure. Whilst on board electronics may provide for a an appropriate numerical conversion, the comment tends to lend unnecessary confusion. (Weirpwoer (talk) 01:27, 25 November 2009 (UTC))


 * I agree that equivalent airspeed is a concept that only has significance in incompressible flows. Air Data Computers are fully aware that air is compressible so any discussion about them should talk about calibrated airspeed, true airspeed and/or Mach number.  Dolphin51 (talk) 03:32, 25 November 2009 (UTC)

Even though EAS is defined in terms of incompressible flow, the significance of EAS is not limited to incompressibe flows. ADCs in some very high speed aircraft do display EAS. In isentropic compressible flow EAS can be expressed as a function of qc and Ps:

EAS = sqrt(7Ps/rho_0((qc/Ps+1)^(2/7)-1))

Having said that, this article is about qc. The only airspeed metric which is a function of qc alone is CAS. It would therefore make sense to delete all references to ADCs computing Mach number, EAS and TAS since these are all functions of other variables in addition to qc.

The expression for ratio of stagnation pressure to static pressure as a function of Mach number is also not relevant to this article since it is not an expression for qc.

I would like to use the term Pt for stagnation pressure rather than P0, since P0 is used eleswhere in Wikipedia for standard sea level pressure. Aslo Ps for static pressue to be consistant with the article on dynamic pressure and the airspeed articles.

In the isentropic flow section, the expression 1/2 gama P M^2 is just an expression for q, so I think this should be made explicit. —Preceding unsigned comment added by Rivet gun (talk • contribs) 15:42, 17 January 2010 (UTC)

Compressible or incompressible?
Can someone clear up the potential discrepancy in Ref.'s 3 and 4, specifically,

"Some authors in the field of compressible flows use the term dynamic pressure or compressible dynamic pressure instead of impact pressure.[3][4]"

yet Ref. 4 states, 4. "the dynamic pressure is equal to half rho vee squared only in incompressible flow.". This contradicts the initial statement of "compressible flows". Perhaps it should just read "some authors in the field of fluid dynamics use the term..." — Preceding unsigned comment added by 86.1.71.202 (talk) 19:19, 25 September 2011 (UTC)

Everyone agrees that in incompressible flows the difference between total pressure and static pressure is half rho vee squared.


 * $$P_t - P_s = \tfrac{1}{2} \rho V^2$$

Also, everyone agrees that in compressible flows the difference between total and static pressures is greater than half rho vee squared:


 * $$ P_t - P_s = \tfrac{1}{2} \rho V^2 (1 + \tfrac{M^2}{4} + \tfrac{M^4}{40} + ... )$$

Unfortunately a divergence appears when people begin to define terms to assign to these concepts. There are authors who use the term impact pressure and authors who don’t use that term.

Firstly, the authors who use the term impact pressure define dynamic pressure to be half rho vee squared. They say that in incompressible flows the difference between total and static is dynamic pressure. They also say that in compressible flows the difference between total and static pressures is called impact pressure and is greater than dynamic pressure. These authors say:
 * $$ \tfrac{1}{2} \rho V^2 (1 + \tfrac{M^2}{4} + \tfrac{M^4}{40} + ... )$$

is called impact pressure.

Secondly, the authors who don’t use the term impact pressure define dynamic pressure to be the difference between total and static pressures. They say that in compressible flows the difference between total and static pressures is greater than half rho vee squared. Houghton is one of these authors and he says "the dynamic pressure is equal to half rho vee squared only in incompressible flow." These authors say:
 * $$\tfrac{1}{2} \rho V^2 (1 + \tfrac{M^2}{4} + \tfrac{M^4}{40} + ... )$$

is called dynamic pressure (or compressible dynamic pressure.)

Neither of these two categories of authors is wrong. It is like some authors using the spelling labor, honor and color; while others use labour, honour and colour. Different, but both neither is incorrect. Dolphin ( t ) 03:48, 26 September 2011 (UTC)

Supercritical fluids
Could we please get a confirmation of whether these also apply to supercritical fluids? For such fluids the ratio of specific heats can be huge: e.g. larger than 8 (see slide 23 of "Fundamentals of Supercritical CO2" by Jason C. Wilkes, 2014).

—DIV (137.111.13.4 (talk) 07:52, 11 November 2020 (UTC))