Fluid Mechanics - airfoil

[enjoyablesounds]

After seeing this video it seems you would want to keep your symmetrical foil as thin as possible? I ask this because it seems the amount of drag is caused by the de-attachment of flow ( in water visible by airpockets/ bubbles ) when increasing the width of foil..in other words what are "fat" foils good for? Also am I correct to say foils react similar through air as they do through water? Thanks, great videos!

[enjoyablesounds]

@enjoyablesounds awesome song too

[burninmunkeys]

@enjoyablesounds Air and water are both fluids, so yes they react the same, only difference is density and other such properties between the different molecules

[jsm666]

@enjoyablesounds The trouble I see with keeping it thin (for a subsonic or low speed airfoil) is that the stall behaviour isn't as benign over the leading edge of a thinner foil - the fatter ones are better at maintaining attached flow at higher angles of attack and lift coefficients. Depending on the application, this may be worth a few percent extra in zero-lift drag coefficient. Of course that's for a lifting foil - horizontal and vertical stabilisers are another matter.

[sundancekid122]

the streamlines are passing over the foil at high speeds in the video. What practical things can we use the low speeds of air around the edges with?

[dansmith57]

PrandtlGlawert is correct. Even though it is heavily debated the Bernoulli principle is a very good one - as the bottom is generally flatter and the top curved the air moves faster over the top creating a lower pressure. In this example lift might not necessarily be generated but illustrates the airflow well.

[bluemagictube]

Im pretty sure this is an example of lift . A larger surface area on top with a smaller one on the bottom creates lift.

[PrandtlGlawert]

Uhm i don´t think so. This airfoil seems to be symmetric at all the time, and it is positioned at zero angle of attack. These two things combinated mean no lift is generated (aprox). Besides, take a look at the wakes: They are quite similar one to another at the lower part of the foil and at the highest part. If lift was generated, you would be able to see that the boundary layer is separated at one part of the foil but not at the other, probably. (depending on the conditions, of course)

[dansmith57]

i think it might be a vacuum effect caused by the laminar flow?

anyone else...........

[dansmith57]

is it me or is the flow going in the opposite direction in the stall area?

[PrandtlGlawert]

Yep, you´re right. Several things can happen to the flow downstream the separation point when stall happens. One of those things is reversed flow. It depends on several key variables.

[mikek1962]

@dansmith57

1. you are correct that the air in the stalled regions are moving in the opposite direction.

2. If the body (wing, in this case) deflects the flow downwards, by Newton's Third Law, the flow then pushes upwards on the body. No deflection, no lift! A flat plate could have the same effect IF the flow on the upper surface would not stall. Stall ruins the flow deflection. Airfoil shapes are designed to keep the flow attached to the upper airfoil surface.

[muddwell]

so thats a wake right ? which is a low pressure zone which create drag ?

[saturne46]

Très intéressant décollement des couches limites. Is it possible to known the ReynoldsNumber ?