why is it that the minimum pressure peak over the top surface moves towards the front of the aerofoil with angle of attack increase?
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2 Answers
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The minimum pressure point -- the suction peak -- is the location of the highest velocity around the airfoil.
The flow must accelerate to get around a turn, so this ends up correlating with the curvature of the airfoil.
At zero alpha of a symmetrical airfoil, there is a streamline that hits the leading edge exactly -- this streamline comes to a stop and is called the stagnation streamline. Every streamline above the stagnation streamline goes over the airfoil. Every streamline below, goes under.
The streamline immediately above the stagnation streamline can be thought of as the streamline 'nearest' the airfoil all the way around. It does the tightest turning. When the airfoil is at zero degrees alpha, its path is somewhat mild as it accelerates up and around the airfoil.
When the alpha increases, the stagnation point (where the stagnation streamline meets the airfoil) moves to the bottom surface (below the leading edge). The streamline just above now must flow up, around the leading edge, and then down the top surface of the airfoil. It has to make a very sharp turn all the way around the leading edge. This requires a lot of acceleration and leads to very high velocity -- a very strong suction peak.
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$\begingroup$ Hi Rob, how does the flow accelerate over the leading edge itself please? $\endgroup$ Apr 21 at 22:06
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$\begingroup$ @MintyPythonn Somewhat sloppy description (exact description is with a set of differential equations) is that the air can't go through the wing, so it is deflected up. It then tries to continue straight by inertia as the wing curves away from it, but then there is less of it near the wing, so low pressure region forms and the pressure gradient accelerates the air towards the wing and aft. The flow has to obey the Bernoulli's principle that lower pressure means higher velocity, but note that that is an equivalence, not cause and effect and the pressure gradient is closer to being the cause. $\endgroup$ Apr 26 at 5:29
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Whomever drew those "pressure distribution" vectors doesn't know what he/she is doing.
Two points: 1. Sometimes we "normalize" pressure values by subtracting the absolute value, so we can represent lower than free stream pressure as suction, (which it is not). To accurately portray pressure vectors, you need to display their absolute values. They should all point towards the wing surface.
2. Secondly, pressure is the result of aerodynamic force of air molecules impacting and rebounding off the wing. The force exerted on a wing from the result of the impact of those fluid particles on a surface is caused directly by the change in momemtum of the particle when it changes direction.
F=ma, or.
F=m(dV/dt) or.
F=d(mV)/dt i.e.,
F=dP/dt (P is mV momentum)
Understanding this makes it much more intuitive, (qualitatively), why the pressure distribution diagram looks like it does, and why it changes as AOA changes. Molecules impacting a surface are no different in this regard than a baseball impacting a bat. The steeper the impact angle, and/or the higher the velocity, the higher the force will be.
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$\begingroup$ Hi, sorry I still don’t get why the point of minimum pressure moves towards the leading edge with aoa? I just found them online $\endgroup$ Apr 21 at 13:43
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$\begingroup$ @Minty Python, If the higher pressure occurs at the higher impact angles and velocities, then the higher the AOA, the higher impact angles (and the higher velocities at impact), occur in front of the aircraft, not at the rear. $\endgroup$ Apr 21 at 14:43