Is circulation real?

For testing panel methods for non-zero thickness bodies which allow specified non-zero normal velocity, I've generated "exact" solutions by distributing singularities of known strength inside the body and and calculating a-priori the corresponding normal velocities on the surfaces which are then used a input boundary conditions. Lift can be included using vortices with the appropriate "wake" extending to downstream infinity. Flow field singularities can be included using singularties of known strength placed on the surface though that complicates calculation of the boundary conditions. I assume a similar proceedure could be used for zero-thickness methods.

 

Mikko, is someone arguring that "circulation" as defined by daiquiri in Post #4 does not exist for wings and other bodies generating aerodynamic lift?

 

I haven't seen anything like these de-singularised methods applied to
zero-thickness lifting plates, although they have been used to great effect
on many ship-related problems. The main difficulty I heard reported was that
it was almost impossible to fully automate the process of selecting the
location of the singularities inside the bodies of interest, and that some
human input was usually required. Does that square with your experience?

Offhand, can you remember who first used de-singularised methods? I
thought Josheph Keller might have tried very early. Beck and others at
Michigan (who I believe you worked with) were advocates but quite a bit later.

 

Not a de-singularized panel method. Rather a method to develop an "exact" potential flow solution to use in testing panel methods.

Simple example is a dipole aligned with a uniform flow. Away from the dipole the "field" is a potential flow. Enclose the dipole in a volume, calculate the component of velocity normal to the surface and specify that as the normal boundary velocity condition. Then use the shape of the boundary and normal velocity on the boundary as input to a panel method which works with finite thickness volumes and non-zero normal velocity boundary conditions. Solve using the panel method and compare the results with the exact solution obtained directly using the uniform flow and doublet.

 

Thanks for the clarification. I have seen similar methods used for calibrating "vortex-in-cell" methods and Havelock source approximations.

 

Oops.. maybe I better fly in an ultralight helicopter, then. I was thinking of drifting with the balloon, but then you go with the true wind, not the apparent wind. On the other hand, I am flying at the apparent wind velocity in the true wind field, so I should have some wind under my wings until close to the sailboat?

About the apparent wind: There is no such thing in reality, it's a man invented name for the vector substraction true wind velocity and the boat velocity. Even if we talk on board about the apparent wind, we actually mean the local flow. The wind vane at the top of the mast does not show the apparent wind angle, and the rotor doesn't give you the apparent wind speed, but the local flow, affected by the sails & rig & & hull & motions of the boat.

The true wind shown by the instruments is a calculated value based on the apparent wind, and as we know it is difficult to calculate true wind - based on the measured apparent wind, the apparent wind which does not exist ;-).

Of the 3 quantities true wind, apparent wind and boatspeed, true wind and boatspeed certainly exist as physical realities. But apparent wind?

I was thinking maybe flying at the apparent wind speed would eliminate the remainder of the flow field, leaving me with the circulation only?

Well yes, of course. First the truck pushes you away and then sucks you towards it. Same thing when a bus approaching from behind you is overtaking on the other lane, and you have the window open, you feel the breeze blowing in the window.

 

Not sure if the rings on the water in Arvel's experiment are really circulation - or something left over from a tip vortex forming when the plate is suddenly lifted from the bottom of the tub? Definitely a nice simulation object I should run when I find the time.

 

David, looks like you do dispute the definition of circulation as given in my post, which is the one used in aerodynamics:

Frankly, I have never heard that way of using the term "circulation" in aerodynamics, at least not until now. I would have used terms like "downwash", "induced velocity", "wing-tip vortex" etc. to describe those lift-induced fluid disturbances. But you are a native english speaker, so who am I to argue?

 

Of course apparent wind is real. All velocity must be measured in some reference frame. Apparent wind is measured in a reference frame that is moving with the observer. Even what you called "true wind" is relative; it is measured in a frame of reference that is hurtling thru space and rotating about an axis.

 

OK, you got me there, in the frame of reference. So for the sailor, the thing that is real is the apparent wind, while for the landlubber true wind is real and he has no perception about apparent wind.

I still think that most of us confuse apparent wind for the local flow on board - apparent wind is something abstract that you cannot feel, or even measure. So is circulation, though, the topic of this thread.

So for me apparent wind is not "quite as real" - local flow on your face when on board is real, the motion of the boat through the water is real, and so is true wind on your face when you are standing on the peer.

I don't think there's much point in measuring true wind in anything but the global frame of reference. The atmosphere is a closed system within which the winds & pressure systems are formed - putting wind into a planetary system or universal frame of reference makes little sense to me.

 

But the apparent wind is as real for a moving boat as the true wind is for a tree standing on the shore. It is the apparent wind that the boat "sees" as the upstream inflow (Vinf), just as a car feels an airflow (and hence the aerodynamic drag) due to it's motion through a still air. Actually, from the point of view of a moving object, the apparent wind is the only wind that counts. A true wind is something that doesn't exists in the absolute sense - until we have decided a point of reference which we can consider fixed. Choose a point of reference which moves at some speed (relative to another, even more fixed point ), and you get a different wind which you can call true. What is true for you is relative for me, and vice-versa. It's a kind of theory of aerodynamic relativity. That's the thing which makes sailboats move - the relativity of motions of air and water (as you know).
Sure, the problem is conventionally resolved by assuming fixed ground as the reference point for establishing the true wind. But, think of it - strictly speaking, that is just a convention. We always need some zero point for our measurements.

 

Absolute no dispute at all. The definition you gave is the standard and accepted one.

this is what is frequently referred to as "circulation" was an attempt to help those who mis-use the term circulation understand what I was referring to when I said earlier in the same sentence the portion of the flow field that is due to lift generation. I didn't feel it was necessary to explain the proper use of the term circulation since you had already done so in post #4. I'll edit the post to eliminate that phrase.

"downwash", "induced velocity", "wing-tip vortex" etc are not the only lift induced portions of the flow field. For a "2D" airfoil they do not exist (in the standard use of the terms in aerodynamics) and for high aspect ratio wings they might be minimal.

 

Roger that.

 

Nice one, thanks! This is the absolute truth about circulation.

IMHO, neither the sailboat nor the tree knows what's real and what's not . For the resolution of forces, the relative motion counts, yes. But to me, the apparent wind is a construct, invented to make calculation of the forces easier and to enable wind tunnel tests… would be awkvard to move the model around in the windtunnel, to measure forces on it.

I don't know much about frames of references, but the choice of one does make a difference. If you are only interested in forces, it makes no difference whether the boat or the water is moving, the relative speed difference between the object & fluid matters.

- With the boat static, and the water moving (like in a water tunnel) your frame of reference is that of the sailor's: The sailor sitting in the boat has no knowledge of whether it's the boat moving or the water moving (if you exclude pitching etc.). This would be a little like a case with the boat anchored in a current.

- But if you are an observer standing on the shore (frame of reference of an outside observer), you will see that the water is inert and the boat is moving. If there are some debris floating in the water, they will perhaps move a little up & down when the boat passes but remain in place after the boat has passed. To me, this is the "real frame of reference", since in the first case the debris would be flushed back with the water streaming around the boat.

If you are interested in what really happens to the water particles around the boat when it is sailing through the water, you have to choose the second frame of reference. So, I think the frame of reference of the outside observer standing on the shore is "more real".

(Not very clear here, sorry, this is about boat and water, not wind, copy-pasted it from an earlier discussion with a software developer - I hope you can get the idea behind my reasoning).

 

Let's do the experiment again, now with the boat sailing at 5 kn against a current of 5 kn, so essentially staying put. The apparent wind and the true wind are now the same, so I can use the balloon for the experiment. I float in my balloon in front of the sail boat, at about 1/3 of its rig height, where the mean aerodynamic chord of the sails is, to eliminate all 3D-effects. What do I feel on my cheek when passing in front of the sail boat?