static pressure vs. dynamic pressure

woleka

I am having trouble relating the two quantities of dynamics and static pressure in: 1/2p*v^2 + p = constant, bernoullis equation, to corresponding physical descriptions. In what way do the air molecules act to create both dynamic and static pressure, respectivley? As far as i know, dynamic pressure results from the collisions of air rushing over an airfoil colliding with its surface, as the velocity (and momentum) increase these collisions are more forceful and thus the pressure increase. But if this is the case, then what type of collision during this process defines the static pressure? My main concern here is with static pressure, how does it occur? and why does it decrease with increases in flow speed?

russ_watters

Static pressure is just how it sounds - it is static. It is the pressure you have if the fluid isn't moving (or if you are moving with the fluid). So it doesn't involve the air 'crashing' into the surface of the airfoil.

It decreases with an increase in speed because of conservation law. The best way to look at it is with a large pressure vessel with a small opening. The large pressure vessel has a certain static pressure and no velocity pressure. As the air moves through the opening, it accelerates, releasing the stored energy of static pressure to push and accelerate the air.

woleka

" So it doesn't involve the air 'crashing' into the surface of the airfoil. "
I thought that pressure by definition is the 'crashing' into the surface, otherwise how else could it impart a force?
Secondly, I understand the conservation law and how it works, im just wondering what makes it work the way it does, what exactly happens to the air molecules that causes the dynamic pressure increase to be equal to the static pressure decrease? I like your example with the pressure vessel that , it is very intuitive thanks.

russ_watters

You are talking about individual molecules - I'm talking about a fluid as a continuous medium. Either way, this force is measured in the same way and the momentum of the fluid causes the force to be larger for an equal number of molecular 'crashes'. Well, what happens as the air moves out of the pressure vessel and accelerates through a pipe? The static pressure at the end of the pipe must be zero (gage pressure) because the end of the pipe is open to the atmosphere. So you have a pressure gradient inside the pipe.

And what else is different when pressure is different? Density. The force of pressure that pushes the air molecules through the opening and into the pipe causes the molecules be pushed apart, lowering the density and thus the pressure.

woleka

Alright thanks. So i gather that if we considered an area of a wing section;
-if the static pressure > dynamic pressure , the particles collide more often (the fluid is denser) and with smaller force(the momentum is smaller)
-relative to the condition where:
-the static pressure < dynamic pressure (greater momentum, fewer collisions).

FredGarvin

This pretty much sums it up:

http://www.grc.nasa.gov/WWW/K-12/airplane/dynpress.html

nasir123

so if you say that as dynamic pressure increases, static pressure decreases in keeping with constant total pressure, shouldn't the altimeter and VSI now mis read.. i.e. show false climbs??

russ_watters

No, an atlimeter measures static pressure only, as it varies with altitude. When a plane accelerates, it isn't in a constant total pressure situation: static pressure stays the same but velocity pressure increases.

nasir123

AAh.. so total pressure is constant only for a given airspeed...
What you are normally lead into thinking is that since Total Pressure is constant, an increase in airspeed (dynamic P) results in a drop in Static P...

So, now with comparison to venturi effect... as a streamline of air is accelerated upto the throat of a venturi.. we say the pressure drops.. this is static pressure isn't it.. now as per what you say, this should not happen because only the velocity pressure is going to increase.. Im a little confused.. thanks...

Also.. wrt to aerofoils.. the region of low pressure above is due to the venturi effect isn't it?

Cyrus

Total pressure is not constant for a given airspeed. It is constant along a streamline.


The pressure at the throat is not static pressure, it has velocity. It is dynamic. The total pressure is constant, and the static pressure reduces substantially.

Yes, you can think of the upper surface of a wing in that respect.

nucleus

paulwh
First the pitot static system consists of an altimeter, vertical speed indicator and the airspeed indicator. All are connected to the static port, while only the airspeed indicator is connected to the pitot tube, hence it has two connections.

Second: Stagnation pressure.
If a symmetrically shaped object is placed in a moving airstream, like the leading edge of a wing, the airstream at the very nose of the object would stagnate and the relative flow velocity at this point would be zero. The airstream dynamic pressure will be converted into an increase in static pressure at the stagnation point. Static pressure at the stagnation point is equal to the airstream total pressure --- ambient static pressure plus dynamic pressure. In the surface of the object the airflow will divide and local velocity will increase from zero at stagnation point to some maximum value.

The measurement of free stream dynamic pressure is fundamental to indication of airspeed. Airspeed indicators are simply pressure gauges, which measure dynamic pressure, related to various airspeeds. The flow in a venture and flow over a wing the air keeps moving, but in an airspeed indicator the air is stopped and becomes stagnation pressure plus heat.
From the site below:
“At a stagnation point the fluid velocity is zero and all kinetic energy has been converted into pressure energy. Stagnation pressure is equal to the sum of the free-stream dynamic pressure and free-stream static pressure. [2] Stagnation pressure is sometimes referred to as pitot pressure because it is measured using a pitot tube.”

http://wapedia.mobi/en/Stagnation_pressure

On the top of page 29 of the next site is a cutaway of an airspeed indicator. It consists of a sealed case with a connection to the static port and a bellows connected to the pilot tube. Static ports sense true static pressure of the free airstream. This is a differential pressure indicator with static pressure on the outside of the bellows and dynamic and static pressures on the inside of the bellows. You are left with dynamic pressure q and through gears it moves a needle over a face marked in mph or knots.
http://books.google.ca/books?id=A0bu...age&q=&f=false

Cyrus

What does "paulwh" mean?

nucleus

What does "paulwh" mean?

Nothing! I don't know where that comes from.

Cyrus

Well you wrote it, so you must know what it means. Otherwise, why did you write it?

dr dodge

in the simplest terms static pressure is like DC voltage. dynamic pressure is like an AC signal. It can vary in amplitude and frequency, and like the AC/DC relationship, dynamic pressure can "ride on top" of a static pressure (AC with a DC offset). Many uses besides aerospace applications EX: By reading static and dymanic pressures in a pump output, you can tell volumns about its health and efficiency

dr

Cyrus

What an odd analogy.

kotreny

But wait, that can't possibly be right, can it? Unless I'm mistaken, the "greater momentum" associated with dynamic pressure is only parallel to the wall--not perpendicular. If you don't believe me, try using a straight piston to accelerate the fluid. Thus, it must have no effect on the pressure arising from molecular collisions with the wall. And besides, venturi meters show that the pressure from molecular collisions actually decreases, rather than remains the same (right?). So the static pressure must be the variable representing these collisions, not the total pressure.

What then, is the physical significance of the dynamic pressure, and by extension the total? Where is the force in the equation P=F/A for these pressures? The area? If the molecular collisions with the wall are already manifested in the static pressure, what could possibly be manifested in the dynamic pressure? (If the answer is "kinetic energy", please specify how that translates into a force acting over a certain area.)

Thanks for any attempt at helping me.