Static Pressure at stagnation point query....

[CraigBEng]

Hi,

I have undertaken a project to relieve stagnation point pressure on a NACA 23015.

Experimentation is complete,

However the only readings that could be taken with the available apparatus was static pressure at the LE to find the point.

This may seem trivial to some but what is the static pressure at a stagnation point of an aerofoil, My assumption would be that it is atmospheric, is this so?

Any assistance would be appreciated.

Regards

Craig

 

[btrueblood]

Static pressure at the stagnation point is stagnation pressure, by definition.

Stagnation, or total pressure, is Pinf + 0.5*rho*V²

where Pinf. is freestream (essentially atmospheric or tunnel wall static) pressure, rho is freestream air density, and V is the freestream velocity.

 

[rb1957]

i was thinking the same, but wanted a lemming to jump first !

yes the stagnation pressure is total pressure, but assuming this is an open wind tunnel (and not a pressurised, supersonic tunnel), then would the total pressure be sealevel atmosphere ? ie tunnel total pressure = SL atmosphere = tunnel static + tunnel dynamic ?

 

[reidh]

rb157,

There is no guarantee that the total tunnel pressure would equal the SL atmospheric pressure. In fact it would be highly unlikely.

Reidh

 

[rb1957]

can i ask why ? if you open the door into the tunnel (when it's off) is there a pressure to overcome ?

 

[reidh]

rb1957,

No there isn't a pressure to overcome.

However,the pressure outside of the tunnel (ambient) doesn't have to be 29.53 in HG, which is SL atmoshperic pressure.

Reidh

 

[btrueblood]

Err. Rb, an "open" tunnel (I think, I may not have this definition straight) is one in which the test section is open to atmosphere. Thus, the static pressure is = atmospheric in the test section, and total pressure is the same as what you'd have for the test body in flight (also, the Reynold's no. is the same as for free flight, which is the impetus for open tunnels).

Total pressure is stagnation pressure, by definition, and it is always per the equation I described; the variants depend on how you are defining the static pressure (Pinf). In a closed tunnel (closed test section), you can have the static pressure fall well below atmospheric in the test section, in a pressurized tunnel it can be well above atmospheric. In general, pressurized sections are used to achieve supersonic flight regimes (high Mach, and high Reynold's numbers).

 

[rb1957]

for me an open tunnel intakes and exhausts the airflow to the surrounding atmosphere, so that the total pressure is the same as the surrounding atmosphere; which i guess is reidh's point (that it doesn't need to be SL). of course the static pressure in the operating tunnel is reduced (because of the dynamic pressure of the airflow) but i think the total pressure is the same as the surrounding atmosphere. and of course a pressurised tunnel is different (the total pressure inside the tunnel is higher than the surrounding stmosphere).

 

[CraigBEng]

Lol, confusing isnt it ......

 

[btrueblood]

rb, you said: "for me an open tunnel intakes and exhausts the airflow to the surrounding atmosphere, so that the total pressure is the same as the surrounding atmosphere..."

Umm, only in certain types of tunnels. The driver (big fan or air compressors) that pushes the air thru the tunnel changes the total pressure of the airstream. The exception is when the driver is a huge suction fan or vacuum tank downstream of the test section, and there are no friction losses from the tunnel inlet to the test section. There are tunnels that operate this way, but they have limited capabilites relative to ones that pump air to high pressure on the upstream side, and so aren't very common.

 

[rb1957]

i'm not a wind tunnel expert, but i'd've thought that the fan created the dynamic pressure (from the speed it imparts to the airflow), and that this reduced the static pressure in the tunnel when it's working, but the total pressure remained the same.

if the total pressure inside the running tunnel is higher than the ambient atmosphere, how do you calculate it ? if you only know the dynamic pressure, how do you complete bernoulli's equation, with two unknowns, static pressure and total pressure ?

 

[btrueblood]

Rb,

Re your first question, a fan always creates a total pressure increase, due to the work done by the fan on the airstream.

2nd question: a good aerodynamicist always measures both - using a pitot/static probe. You can sometimes assume you know one or the other, but unless you measure 'em you can't be certain. Once measured, you then subtract one from the other to derive the dynamic pressure, and thus can calculate airspeed.

 

[rb1957]

yeah, i was thinking something like that (after i'd posted my question) ... at first i was thinking that the work done by the fan manefested itself in the airflow, but it makes more sense that the work done by the fan increases the total pressure in the tunnel. sure you can measure them in a working tunnel, but there should be some rational way to anticipate the tunnel conditions ... is the static pressure the same as the ambient static pressure ? ... but that dosen't seem right either ...

ie is there a way to answer the OP, without saying "measure it" ... and using fan power, and propulsive efficiency to determine airflow speed seems to be a very round-around way of determining dynamic pressure; and in any case, what's the static pressure inside the tunnel ?