What causes low pressure on a wing?

[boneh3ad]

The tendency to follow a convex surface is reduced in the case of a low viscosity flame jet in air across an airfoil. Note how soon the jet detaches from the surface of the airfoil in the video below. In this case, the low viscosity of the jet entrains very little of the surrounding air, mostly pushing the air directly in front of the jet.

I'll admit I haven't given that problem much thought, but this seems unrelated to the classical Coanda effect to me at first glance. Yours is a phenomenon that's ultimately related to boundary-layer separation. A hot gas actually has a higher viscosity, so my first inclination would be that this might suggest a lower Reynolds number and reduced resistance to separation.

 

[rcgldr]

Note how soon the jet detaches from the surface of the airfoil in the video ...

I'll admit I haven't given that problem much thought, but this seems unrelated to the classical Coanda effect to me at first glance. Yours is a phenomenon that's ultimately related to boundary-layer separation. A hot gas actually has a higher viscosity, so my first inclination would be that this might suggest a lower Reynolds number and reduced resistance to separation.

I struck through most of my prior post. You're correct that the Reynolds number is low, it's a small wing, and the jet speed is not that fast. I don't know what gas is being used to produce the flame, could this affect viscosity?

 

[boneh3ad]

I struck through most of my prior post. You're correct that the Reynolds number is low, it's a small wing, and the jet speed is not that fast. I don't know what gas is being used to produce the flame, could this affect viscosity?

It most certainly would. It's hard to glean much from that video without more information, but there could be any number of important factors when you have a flow such as that: high temperature gradients across the width of the flame/jet (meaning high density and viscosity gradients), chemical reactions between the combustion byproducts (and the resulting species concentration gradients), potentially ablation from the surface of the object if it isn't protected, compressibility, size... it's potentially a mess, and every one of those factors into Reynolds number and/or other aspects of the behavior of the boundary layer.

 

[Change in pressure]

I think water and a spoon are perfectly reasonable examples of the Coanda effects.

A common misconception is that Coandă effect is demonstrated when a stream of tap water flows over the back of a spoon held lightly in the stream and the spoon is pulled into the stream . While the flow looks very similar to the air flow over the ping pong ball above (if one could see the air flow), the cause is not really the Coandă effect. Here, because it is a flow of water into air, there is little entrainment of the surrounding fluid (the air) into the jet (the stream of water). This particular demonstration is dominated by surface tension. (McLean in "Understanding Aerodynamics" states that the water deflection "actually demonstrates molecular attraction and surface tension.

Air jet follow curved surface even in inviscid flow,indeed air viscosity is so small that is irrelevant...

Listen at 17:50


Listen at 21:10

 

[boneh3ad]

A common misconception is that Coandă effect is demonstrated when a stream of tap water flows over the back of a spoon held lightly in the stream and the spoon is pulled into the stream . While the flow looks very similar to the air flow over the ping pong ball above (if one could see the air flow), the cause is not really the Coandă effect. Here, because it is a flow of water into air, there is little entrainment of the surrounding fluid (the air) into the jet (the stream of water). This particular demonstration is dominated by surface tension. (McLean in "Understanding Aerodynamics" states that the water deflection "actually demonstrates molecular attraction and surface tension.

Air jet follow curved surface even in inviscid flow,indeed air viscosity is so small that is irrelevant...

Listen at 17:50


Listen at 21:10

You can repost the same things over and over again, but clearly I simply disagree with Dr. McLean's characterization of that problem. I would counter that the problem has contributionsfrom both phenomena contributing to the behavior. Arguing about whether that problem is the Coandă effect is essentially arguing about whether a problem featuring a large contribution from surface tension is still covered by a phenomena described only qualitatively by Coandă. This is why Coandă is rarely mentioned in any formal course of fluid mechanics: the effect isn't a mathematically- or quantitatively-rigorous phenomenon. It's just a broad description of a behavior.

So basically, we are now arguing about something that actual fluid mechanicians don't even care about.

 

[berkeman]

Thread is closed for a bit for Moderation...

 

[berkeman]

It looks like we have done the best we can to help the OP. Thanks everybody for the replies.

Thread will remain closed now.