Fluid Flow Deflection Bending Ahead of Airfoil?

[mishima]

TL;DR Summary

A student was curious how the flow "knows" to start deflecting downwards before contact with the airfoil, as seen in certain simulations and smoke trail wind tunnel videos.

Hi, I am a high school teacher in need of some help explaining fluid flow. I don't have much experience with fluids at a college level.

We were reviewing lift (Bernoulli's velocity/pressure relation, deflection of air downwards, etc) and I showed them:
this online simulation.

One student asked how the flow ahead of the wing "knew" to start bending even before contact with the wing was made. They were talking about the black dots starting to curve on the left side of the sim, before the wing reached that timeslice. I made a comment about how water in a sink does the same, curving before it reaches the hole. However, I realized I really don't know the physical reason for this. Any insights? How is information downstream (a wing is coming) communicated upstream (flow starts bending)? How fast does this information travel?

 

[russ_watters]

How is information downstream (a wing is coming) communicated upstream (flow starts bending)? How fast does this information travel?

It's a pressure wave, traveling at the speed of sound (that's recursive). This becomes more evident when you apply/compare it to supersonic shock waves:

 

[berkeman]

TL;DR Summary: A student was curious how the flow "knows" to start deflecting downwards before contact with the airfoil, as seen in certain simulations and smoke trail wind tunnel videos.

One student asked how the flow ahead of the wing "knew" to start bending even before contact with the wing was made. They were talking about the black dots starting to curve on the left side of the sim, before the wing reached that timeslice. I made a comment about how water in a sink does the same, curving before it reaches the hole. However, I realized I really don't know the physical reason for this.

This PF Insights article may help you with some of your airplane wing lift questions:

https://www.physicsforums.com/insights/airplane-wing-work-primer-lift/

 

[A.T.]

How is information downstream (a wing is coming) communicated upstream (flow starts bending)? How fast does this information travel?

The air accelerates based on the local pressure gradient, and the information about pressure changes travels at the speed of sound. So as long as the wing is slower than sound, the air ahead of it will be affected.

 

[mishima]

Ah...that makes sense! So, in a way, this student might find supersonic flows easier to understand.

@berkeman, thanks that looks consistent with what I have been teaching. I do admit however I am confused on cause and effect, which the PF article somewhat alludes to in the last paragraph.

In my understanding the Coanda effect is what is actually turning the airflow downwards to create the reaction force of lift upwards. Because of boundary layer effects, near the surface of the wing the flow velocity is near zero but increases as you get further from the surface. This difference in speeds is like a car with one side of its wheels travelling more slowly than the other side, it turns towards the slow side.

I don't understand how Bernoulli's pressure/velocity relation 'causes' the Coanda effect, if at all. It seems more like just an observation one can make about the overall pattern of flow.

 

[russ_watters]

I don't understand how Bernoulli's pressure/velocity relation 'causes' the Coanda effect, if at all. It seems more like just an observation one can make about the overall pattern of flow.

I wouldn't say it does. The wiki article explains it well: the key is entrainment. Basically, a jet of air "pulls" in air around it, lowering the pressure, "drawing" the jet towards the surface. I put these words in quotes because a lot of people don't like viewing pressure as negative gauge, as opposed to less positive absolute.

Bernoulli is always hanging around in the background though.

 

[boneh3ad]

Ah...that makes sense! So, in a way, this student might find supersonic flows easier to understand.

@berkeman, thanks that looks consistent with what I have been teaching. I do admit however I am confused on cause and effect, which the PF article somewhat alludes to in the last paragraph.

In my understanding the Coanda effect is what is actually turning the airflow downwards to create the reaction force of lift upwards. Because of boundary layer effects, near the surface of the wing the flow velocity is near zero but increases as you get further from the surface. This difference in speeds is like a car with one side of its wheels travelling more slowly than the other side, it turns towards the slow side.

I don't understand how Bernoulli's pressure/velocity relation 'causes' the Coanda effect, if at all. It seems more like just an observation one can make about the overall pattern of flow.

Newton also found supersonic flows easier. He just didn't know his theories of fluid flow only apply to very high Mach numbers. Search "Newtonian impact theory."

I'd ignore everything you've heard about the Coanda effect and lift. It's wrong. The Coanda effect has nothing to do with lift, as it applies to fluid jets interacting with surfaces, not flow fields originating from a common source.

Fluid "sticks to" the surface because nature abhors a vacuum. Under some conditions, the boundary layer can detach from the surface, but then a bubble of reversed, attached flow fills in what would otherwise be a void. On a wing, this separation is what leads to stall.

 

[Swamp Thing]

I made a comment about how water in a sink does the same, curving before it reaches the hole.

Your students will enjoy this video from Steve Mould. Another situation where we want to ask, "How do the molecules know what's going to happen?"