What is coanda effect?

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Why are fluids clingy? Coanda Effect? 



Lift on an airfoil is a by-product of the attached flow over the airfoil. (this is explained in the article on lift linked below). But how do the air molecules know that they have to climb up the curvature and follow the contour of the airfoil. Is there a rule in nature which says that being clingy is socially acceptable behavior?
Yes, in fact, there is, and it's called the Coanda effect.
Image result for streamline flow airfoil
fluids- where an attachment is not a disorder

What is the Coanda effect?

It is the tendency of a jet of fluid (gas/liquid) to curve towards a surface. In other words, it's the attachment complex of a fluid. 

Why are fluids clingy? Let's delve into physics behind the Coanda effect.

Setting: a stream of water flows over a convex object (works on concave objects as well)

Cast: 
Fluid: anything that has molecules not too closely packed to be considered a solid.

Bernouilli equation: energy equation that says if pressure increases (pressure energy), velocity decreases (kinetic energy) and vice versa. So as to maintain constant energy.

Image result for pressure in a container
Pressure on the walls of a container
Hydrostatic pressure: The pressure that comes out of particles in the fluid bombarding against one another. The pressure at a point in a non-moving fluid is directly proportional to the height of the fluid above it. It can be thought of as the pressure exerted by the fluid on the walls that restrain it.
 Continuity equation- the equation that says an area is inversely proportional to velocity. (If you wanna remember this: picture a fat man run.
Image result for boundary layer
Viscosity prevails only in the boundary layer
Viscosity: the friction that exists between the layers of the liquid. When the fluid flows over boundary the boundary acts like it’s deadbeat husband, holding it back, not giving it the permission to go as fast as it can. So the layer above it has significant velocity decrease(for subsonic flow). The layer over that is a little faster, and so on, until it’s out of the boundary layer, and the fluid layer moves as fast as it can. Viscosity tells you all that you need to know about a dead weight, the further you move away from them, the better your life can be.

Entrainment: The movement of one fluid by another. (can exist between two layers of the same fluid at different velocities). You can visualize this concept as how an audience sways to a musician’s tunes. standing awkwardly not knowing what to do one minute, to rhythmically shaking what your mother gave you the next. It’s not magic, it’s just emergent behaviour….but yeah it’s pretty close to magic.

Movie:  Tiny little fluid particles are on their way over the curved surface. So at time t=0 they listen to inertia and they don’t curve downwards around the curve, they move on down the same line (they don’t get far but wait for it). This moving layer entrains the layers right below it (past the curve). This entrainment causes the layers to leave that region leading to a reduced pressure there. Why reduced pressure? where there is a lack of fluid particles (owing to the fluid layer eloping with it’s second cousin), there is a lack of pressure being exerted on the walls of the surface. (Like when you’re finally done with high school ,but you feel a pang of loss for some reason). At this point in time, there’s a higher pressure on top and lower pressure below, this pressure difference creates a force which acts downwards. Force gives you displacement so this facilitates the movement of the fluid to move from high P to low P curving downwards.

Crazed fan’s plot interpretation on the pretentious blog: All of this happens at t=0.0000000000001, and so you don’t really see them trying to move down the same line. They get curved by the fluid military (force arising from pressure difference), leaving the law of inertia in a blubbering mess. Valiant attempt, my little-wet friend.

Click over here, go on, I dare you. How do wings generate lift?

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