Expert: Paul Soderman - 7/9/2011

Question
Thanks again. This is Vidhyasagar disturbing you for the last time,

Please excuse me.

Your reply is in literature and I have received it from a few other experts.

But none accepts the deficiencies I have pointed out in it and they do not point out the deficiencies in my alternative opinion.
So, I approached you.

I respectfully feel that a scientific new thinking must be refuted by laws, logic and data, not subjectively.

Kindly evaluate the deficiencies in your reply and my ideas in the three questions.

Thanks.
Vidhyasagar

Answer
OK Vidhyasagar - this is getting a little repetitive, but perhaps I can use some better words to describe things.  This is your as yet unpublished question.  My answer follows.

QUESTION:
Thanks again, this time much gratefully, for going through and answering so many questions. It shows you are a very kind person and a very good teacher. Thanks a lot. I restrict to your replies for three questions for which only I approached initially.

1. Question 11:

= The cambered airfoil is suspended, so stationary, in atmosphere; billions of atmosphere's molecules will be immediately surrounding it and colliding on the upper and lower surfaces applying pressure, so a force of around 10,000 N at sea level. Now when air stream of sufficient density,velocity and cross sectional area flows across it at zero angle of attack it is lifted.

Q: Which body lifts it?

There are four possible answers:
 (a) The free stream (because it will apply a collision force, by NSSL, on the leading slopes. Is this the lifting force?)

 (b) The upper stream (by its lateral/static pressure)

 (c) The lower stream (by its lateral pressure)

 (d) The atmosphere by its pressure.

Neither your reply nor literature have named the body. The only answer one gets or has to infer is,' Lower stream by its (net) lateral pressure'.

My thinking:

Before accepting this answer let US consider one essential question,' What happened to the atmosphere's billions of molecules?' Are all of them still present and applying pressure or none of them is present; so its pressure is nil or some of are present (if so how much); so still applying some pressure.

The lower stream will surely remove/deflect away molecules by collision using its kinetic energy. It can because its kinetic energy is greater. So, all of them will not be present.
There is no experimental or other proofs that show that none of them is present.
So, one will not be wrong in considering some of them will be present. On this, one logical answer:

The upper and lower stream will remove some atmosphere molecules by collision work. So, atmosphere's pressures/forces on the upper and lower surfaces will decrease. The upper stream removes more molecules because
   (i) it flows faster (cause to be found)
   (ii) its mass is greater (due to the cambering)
So, the atmosphere's force on the upper surface decreases more. So, a net atmosphere's force by the lower atmosphere results. This force lifts.

So, the correct answer: Lower atmosphere lifts. By its (contact, mechanical)collision force using its kinetic energy.

Please tell what is wrong here.

2. For the question 3, 4 and 5:
' Which body does the work of speeding up the free stream over the leading slopes? Why is work greater for the upper to make it flow faster than the lower stream?'
 (Sorry, I didn't frame the question correctly.)
'Which body does the work of creating pressure gradients?'

 Your answer is (most probably) the propulsion system transferring its energy to the airfoil by which the airfoil does either work.

My thinking:

 There is no propulsion system here. The airfoil doesn't have any kinetic energy to do the work. So your answer seems to be wrong.

A propulsion system is present/needed in the lift of an airfoil moving in atmosphere; it is to move the airfoil continuously because if there no motion of airfoil there is no lift. Considering the kinetic energy of moving airfoil as that of the stationary airfoil is surely wrong.

Explaining the formation of opposite pressure gradients by the stream:

The free stream is speeded up (by some mechanism)at the leading edge and reaches maximum velocity at the point x ahead of the crest. So , from the leading edge to the point x, more and more atmosphere molecules will be deflected off. So, from the leading the pressure goes on decreasing to the minimum at the point x.

At the point x speeding up stops. Thereafter the stream has to travel with its kinetic energy without gaining energy. It does the work of removing air molecules. Two consequences:
(a) its kinetic energy is lost; so it gradually slows down.
(b) it removes lesser and lesser number of air molecules downstream. So, more and more of air molecules remain. So, pressure increases as ome goes downstream from point x.

So, answer: The pressure gradients are in the atmosphere, formed by the stream with its kinetic energy.

What is wrong here?

So, two things remaining to be explained are: How does the free stream become faster and how is the upper stream faster.

If you permit me I will give an explanation. It will be different from any other. So, please evaluate.

Grateful Vidhyasagar.


ANSWER
1.  By definition, the free stream is the flow field far removed and unaffected by the wing.  This cannot provide lift yet.  As the free stream reaches the wing it divides into upper surface and lower surface flows.  Those flow fields generate pressures along the wing surface that are more or less than the atmospheric pressure. Although pressures are not vectors, the resulting forces are vectors and are normal to the surface everywhere. These forces must then be resolved in the lift direction and summed over the entire wing to get lift.  At the stagnation point, the strong ram pressure causes a strong force in the aft direction.  Along the upper surface, the pressures are lower than atmospheric causing weak forces.  Forces on the lower surface are stronger causing an upward differential force. By summing all the forces everywhere we get lift.  Therefore, it does not make sense to ask which flow or which pressure causes lift.  All the surface pressures must be accounted for to find the final resultant.  This is best done with CFD (computational fluid mechanics).  

These forces do not require a 'collision' of the flow with the body.  To introduce the idea of molecular kinetic energy only confuses things.  Aerodynamicists consider airstreams to be fluids, continuum that have state properties such as pressures and velocities that respond to well established laws of physics.  Although pressures are the result of molecular motion, the random nature of that process is so difficult to quantify in detail that fluid properties work much better.

2.  A wing in flight has kinetic energy provided by the propulsion system equal to 1/2 mv^2.  The energy eventually heats up the air and aircraft.  A wing fixed in a wind tunnel, which is what I think you are thinking of, has no kinetic energy.  But the airstream has kinetic energy delivered by the wind tunnel fans.  In both cases the energy is equivalent if the velocities are the same.  In both cases the aerodynamic forces are same which is why we use wind tunnels.

3.  I have explained, as has Weltner et al, how airfoil upper surface pressures and velocities are related to surface curvature, Coanda effect, and centrifugal forces. Alternatively, you could imagine the flow to be entering a venturi between the surface and free field and accelerating.  As the flow accelerates the unit volumes of flow expand and pressures reduce.  I prefer Weltner's explanation as it leads to pressures that then drive velocities.  Some prefer to think of velocity fields that drive pressures via Bernoulli's law.  In any case, basic laws of physics work well for fluid flows.  There is no need to worry about molecules deflecting off each other in such a way as to create airfoil lift. I see no basis to justify that idea. The only way gas molecules lose or gain kinetic energy is if temperature changes.
Paul