# Is circulation real?

Discussion in 'Hydrodynamics and Aerodynamics' started by Mikko Brummer, Jan 25, 2013.

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### patzefranpatzefran

Aerodynamiscists define the lift and drag relating to the apparent wind, lift is the component perpendicular to the apparent wind .
According to this definition the component of the aero force in the direction you want to go is not the lift but only a thrust force !

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### jehardimanSenior Member

GOD; I hate people that only reply in the negative without thinking about their statements. Lift is a single direction, but Drag is the vector ...NOT perpendicular to the flow..."Lift" is the direction you wish to go...Gravity/"drag" is the "thrust" you need to supply....This is so simple but so many people miss this...Like gliders they don't understand that this is simple...Energy is converted from one direction to another by the mass of the fluid moved... It is not that the sail or wing is pushed "forward"....But that it is not moving "downwind" as fast as it should...I should just give up trying to teach/inform up because so few are willing to learn...The willingness to learn is what makes a student...Not the attempt at reading.

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### patzefranpatzefran

You look more like a Guru than a physicist, you use your own definitions and visions at your pleasure !

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### HowlandwoodworksMember

There is no lift in aerodynamics, gravity is not a force, and the sun doesn't rise or set.
These terms only make it harder to understand the physics behind the concepts.
jehardiman,
Thanks for sharing your knowledge freely.

I still have much graphite to lay down and much to erase in Niña's lines.
As well as the coanda effect and it’s inevitable aftermath of circulation to define in the sea and sky.

NIÑA
Winner of the Fastnet Race 1928
W. Starling Burgess Design
Off the Isle of Wight
At The Needles

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### DogCavalrySoy Soylent Green: I can't believe it's not people

Lift is made by a suddenly perturbed foil, before the onset of circulation, described as the Wagner effect.
Lift is made by a planing surface at the interface between two fluids. It is particularly sad how folks try to rationize circulation into that system.
Lift is made by a foil moving faster than the speed of sound in a fluid.
Lift is made by the blades of a surface piercing propeller, where the individual blades enter and exit the water in paths too short for circulation to develop.

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### ErwanSenior Member

So lift is not the result of a difference of pressure which is the consequence of a difference of fluid velocity ?

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### DogCavalrySoy Soylent Green: I can't believe it's not people

Lift is a consequence of different pressure, but that is a consequence of changing direction in the fluid. Difference in fluid velocity comes from changing direction. Lift comes from momentum transfer from the lifting body to the fluid, where the fluid's vector leaving the lifting body is redirected normal to it's vector encountering the lifting body. The reaction force is lift.
Consider the bottom of a planing hull. The water encountering the hull is redirected downward, and slightly forward in the direction of travel. There is no circulation.
Not to say that circulation isn't real. Of course it's real, and when it is fully established, after about 7 chords of travel by the foil in a steady state system, the resultant forces on the foil are much higher lift and lower drag than if circulation were not established. The conceptual error is in thinking that circulation is the reason for lift. And even that isn't a problem if you never have to think outside the box. But a fully cavitating prop, a surface piercing prop, a planing hull bottom, a supersonic wing, all make plenty of lift without circulation.

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### patzefranpatzefran

Good summary . Hopefully, this could end the topic !

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### DogCavalrySoy Soylent Green: I can't believe it's not people

Thank you @patzefran! You are most gracious. When I first encountered circulation, back in '90, it was in CA Marchaj's Aerohydrodynamics of Sailing. It made my brain hurt. Marchaj was a brilliant fellow, but he didn't disassemble the 4 quadrants of circulation in a way that fostered a solid intuitive understanding, and he gave far too much ink to shedding the starting vortex, as if that inevitable state change could tell us anything.

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### Pablo SopelanaJunior Member

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### jehardimanSenior Member

I'd rather say that "The reaction force is the body force and the dot product of the body force vector in the direction you want is lift". Realistically; "lift" and "drag" are just components of the body force in arbitrary user defined directions. I'd also stress that only in the most simplistic forms is it only a change in the fluids momentum; it really is a change in the total energy of the fluid. This is why there are wake waves and hydraulic jump and soup getting hot in a blender...all these things involve changes/exchanges in energy through the fluid. As Pablo's article suggests, sometimes people take the terms too literally.

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### DogCavalrySoy Soylent Green: I can't believe it's not people

Most excellent! Like I said, if you never think outside the box, then oversimplification is tolerable. But if you want to do something genuinely interesting, you really need to be clear in your thinking. I've read papers from fellows trying to explain how circulation is present in a surface piercing prop. Spoiler: it isn't. But the earnest student took it to heart when his prof told him that there is no lift without circulation. There is lift with a SPP, ergo there is circulation. And so a paper explaining how circulation can follow a blade up into the air in a sparkling cloud of spray, and from that air, back down into the water again. Had he clarity of thought, no such waste of a research grant.

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### sandhammaren05Senior Member

Lift is the consequence of circulation. Only circulation provides the net pressure difference that generates lift. That's shown in every text on aerodynamics. All propellers generate lift via circulation, whether fully or partially submerged. There is no other way to get a large lift/drag, thrust/drag, ratio.

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### sandhammaren05Senior Member

As Prandtl explained so clearly in 1917, lift is generated on a wing by circulation when the trailing vortex is shed. Regarding surface piercing, the mechanism is the same as for full submersion. No one will try to argue that a fully submerged prop generates lift by any mechanism other than circulation. Now, any propeller where only some fraction of the the tip pierces the surface once per revolution is surface piercing. The circulation about the submerged part of the blade is not interrupted and ends on the surface if the blade fraction above the water would be dry (it is not, as photos of the helicoidal wake shed by props above the waterline on tunnel boats clearly show). But let us continue as if the blade section above the waterline would be totally dry. Raise the propshaft further toward the surface. The argument is not the same, the circulation grows over the blade to the tip upon full reentry. If the prop is only half of less submerged then you have the hard to formulate and unsolved boundary value problem of how the circulation restarts upon reentry. But restart it does. there is no other known mechanism for a high lift/drag ratio.

Summarizing, there is no high lift/drag ratio without circulation, there is no other mechanism.

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### jehardimanSenior Member

I take it you have not actually read the paper? Or Lanchesters 1907 work "Aerodynamics"? Or the parallel work of Betz and Munk? They all hang their hat on circulation/Joukowski theories and the Kutta condition; they are all heuristic arguments that are patently false for real fluids. And they know it.
It would be better to start at Lerb's 1954 “Moderately Loaded Propellers with a Finite Number of Blades and an Arbitrary Distribution of Circulation“. In that paper he defines the background of the theory and that it does not apply to lightly loaded or heavily loaded propellers like a surface piercer. You can get a copy from SNAME or it, and the insightful commentary, are in the 1954 Transactions.

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