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Is fast moving air always low pressure? I know this isn't true, but some of the explanations of shock diamonds I see use this analogy.

So how does the speeding up of air over an airfoil create low pressure, but fast moving air doesn't create low pressure by itself? What differs to make fast moving air low pressure? I've also seen that the curve of an airfoil will make low pressure. Why is that? (Picture of shock diamonds below because they look cool)

Wyatt
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    With a wing, it isn't the moving *air* that creates low pressure, but the moving *airfoil*. Air by itself at whatever speed (with no object around to affect its flow) will be constant pressure. – Ralph J Dec 18 '23 at 15:10

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Fluid flow behaves in different ways depending on whether it is subsonic or supersonic.

Subsonic fluid which is not constrained in a constant volume is incompressible, and it's total pressure remains constant. But pressure is a vector, because it is just force per unit area, and the orientation of the surface you are measuring the force against matters, So as you increase velocity, the component of the force per unit area on a surface that is perpendicular to the direction of flow (the dynamic pressure), increases, (the air molecules hit the surface with higher velocity), and the component of the force per unit area on a surface that is parallel to the direction of flow (the static pressure), decreases. The pressure measured by the pitot tube increases as you go faster (indeed that is how airspeed indicators work), and the pressure measured by the static ports decreases (they have to be mounted on a airframe surface that is parallel to the aircraft velocity).

This is the Bernoulli principle. This principle is often stated to say that the faster the flow, the lower the pressure. But when stated this way the pressure it is describing is just the static pressure, not the total pressure. The vector sum of the two components (the total pressure), remains the same. it must remain the same or the principles of conservation of energy and momentum would be violated.

Put a pressure sensor in a flow, and measure the pressure. The direction of the sensor will tell the story. If the sensor points directly into the flow the reading will be higher (it is just a pitot tube). They put these on the nose of the plane as far out in front as possible, to minimize the effect of the airframe itself on the flow hitting the tube. Turn it 90 degrees to the flow. It will now measure static pressure. Point it backwards, the measured pressure will decrease even lower.

Supersonic flow is compressible. The air molecules cannot get "out of the way" of the other molecules that are coming at them at higher than the velocity of the molecules within the fluid due to their heat energy, so they pile up and create a higher density, compressed fluid. Different rules apply.

So, in talking about aerodynamic force on a wing (or on any surface), i.e., Lift, we are talking about the force pushing on the surface, which by necessity, must be pushing perpendicular to the surface, (and to the flow), so, for subsonic velocities, it is the static pressure, which (by Bernoulli) decreases as the flow speed increases.

Charles Bretana
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  • A little late to the party, but I was going back through this post and re-reading answers and developed a question : On planes, the surrounding air isn't moving relative to the ground, but it is relative to the plane. So the air around a plane shouldn't be low pressure, right? Because there is no pressure field affecting the air around the plane (besides the plane itself) If that doesn't make sense I'll happily clarify. – Wyatt Jan 20 '24 at 04:02
  • @Wyatt, ALL physics must be understood as only being applicable, or "true", when examined from within a consistent frame of reference, (FOR). So, yes within the FOR of the ground, the air is not moving and so the pressures do not change. But relative to the aircraft frame the air is moving and the pressures do change. The most basic idea behind Einstein's revolution is that the laws of Physics have to work in ALL frames equivilently. Much of the complexity in doing Physics arises from having to carefully manage how we measure things in the coordinates of the FOR in use. – Charles Bretana Jan 20 '24 at 12:44
  • As an example "thought experiment", consider two scenarios. One plane in flight at a given velocity, and another identical plane, immovably mounted in a giant wind tunnel where the *air* is moving at that same velocity. Every aerodynamic effect should be the same. The "pressures" you might measure in these two scenarios should be the same, right? YES! Because they depend on what FOR you put the pressure guage in - (how fast is it moving, and its orientation relative to the flow – Charles Bretana Jan 20 '24 at 12:51
  • Oh okay, thanks. So Bernoulli basically states that low pressure air will pull higher pressure air from around it, and in doing so 'stretches' out the air making it low pressure, I think (also speeding it up). I could see that working in a wind tunnel, but how does that work in real flight? The wind tunnel is creating a high pressure at the front, but in real flight there is no such function, right? – Wyatt Jan 25 '24 at 22:49
  • To add to that : There's nothing affecting the air besides the plane itself. Now beside the plane, the air isn't getting affected by anything. In a wind tunnel the fan (or whatever wind making device), is making the air low pressure beside the wing, if that makes sense. – Wyatt Jan 26 '24 at 03:09
  • Well, no, Bernoulli principle is that Pressure is a *VECTOR, and unless constrained by walls or some other boundary, (like supersonic shocks) will remain constant* in magnitude. This is required by the conservation of momentum law. Only its components (as measured in different directions), can change. Bernoulli says that as the velocity of the flow increases, the component parallel to the flow, (dynamic or pitot tube pressure), will increase in magnitude, and the component normal (perpendicular) to the flow, (static pressure), will decrease. – Charles Bretana Jan 26 '24 at 17:03
  • To add, you can't apply any physics principle that applies in specific narrow contexts or scenarios, to scenarios in which it does not apply, or requires a more detailed deeper understanding (like "this is about vectors not scalars"), to scenarios where it does not apply without introducing unanticipated errors. – Charles Bretana Jan 26 '24 at 17:08
  • The reason why the principle does not apply to supersonic flow is that the molecules cannot "get out of the way" of the molecules banging into them due to the relative motion of the airfoil, and "bunch up" increasing the density and creating a shock wave. In this shock wave (just like in a closed container), the pressure increases. Since the Pressure is changing, the Ideal gas Law (PV=nRT) applies, and so the temperature goes up, the energy contained within the system is no longer constant, and Bernoulli no longer applies. – Charles Bretana Jan 26 '24 at 17:14
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so how does speeding up of air over an airfoil create low pressure, but fast moving air doesn't create low pressure by itself?

Last one first. Fast moving air is the result of air moving from high to low pressure.

Speeding up air over an airfoil creates low pressure by pulling air molecules away from the surface of the wing. This is because, to a certain extent, air molecules are attracted to one another. This is known as viscosity.

Air molecules also have mass, and moving them gives momentum, mv.

If one can visit a water fall with a fast flowing river, one can see the momentum of the water carries it past the edge, leaving a gap between the cliff face and the water fall. In the same way, the momentum of the accelerated air prevents the gap between the airstream and the wing from closing.

In your (excellent) picture, no doubt the exhaust stream will be pulling some of the ambient air along with it.

Robert DiGiovanni
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