Is circulation real?

Thanks. What is the speed of the airfoil(s)?

 

5 m/s. The length of the foil is 0,5 m, a NACA0010, angle of attack 10°. This is a very groce simulation, at 1400 elements only, but I don't think it matters for the purpose - I did try at higher resolution, it won't change much but the arrows get so small you would not see them anymore.

 

This would be a case of sailing in a river on a totally calm day, with no wind...

 

I have sailed low AR sweptback rigs where most short telltails in the field are spanwise up into the LEV, and long TE telltails are pulled up along the TE into the LEV, perhaps joined by the tip vortex. Seems the flow has no down wash, but high lift. Is there circulation?

There are actually several good arguments that VLM and other panel
methods are unable to capture the strength of the leading-edge singularity
and how its strength varies around the leading-edge and towards the tip for
curved planform wngs.

 

I have sailed low AR sweptback rigs where most short telltails in the field are spanwise up into the LEV, and long TE telltails are pulled up along the TE into the LEV, perhaps joined by the tip vortex. Seems the flow has no down wash, but high lift. Is there circulation?

 

low aspect ratio spept back rigs generate lift with large vortex forming just aft of the LE, which is what you are seeing with the tell tales, they are getting drawn into the vortex. The vortex lift can create a lot of drive, but is costly in terms of drag, they are also very stall resistant so it can allow you to point high into the wind without having the sail stall.

The lift is generating by warping or bending (accelerating) the far field flow far off the surface, which the tell tales can not detect. the vortex energies the flow to keep it attached, but that energy comes from the forward thrust of the boat. High lift but poor L/D, which means it has an advantage in light wind and low speeds, but not so much in high speed reaches. The flow conditions (and optimum foil shape) for vortex lift is very different than for potential flow lift, which is more efficient and usually the direction modern designs go to improve performance.

 

"usually the direction modern designs go" This is all experimental and unlikely to to pay off.

 

??? I have no idea what you are referring to. Perhaps I should clarify. A low aspect swept back sail, like a lateen or lug sail, is an obsolete design in terms of performance. They do have certain advantages, but performance is not one of them. If you want to improve the performace of this type of sail you have to redesign it and make a more modern design.

The best performance come from minimizing the tip vortex, which means creating a surface that best approximates an infinite wing length. This is done with a high aspect ratio elliptical platform, or tapered with a flat top to approximate an elliptical plan form. This why you see very tall narrow sails with a flat top or rounded top to the sail on all modern designs. This eliminates the peak, and improve sail efficiency. Also, better foil shape by blending the mast shape to minimize separation at the aft edge of the mast where the sail starts, and to use full batten to maintain a good foil shape across the surface. You can not maintain a good shape without lots of full length battens, which is also the way all modern sails are designed.

These are not experimental, but the way all modern sails are designed. And of course there are the boats that use rigid wings instead of sails, which is the ultimate in controlling the shape of the surface. Though I would not put these in the same class since I am not sure they would be practical for a recreational or cruising sailboat.

So, if you want to improve the performance and efficiency of a low aspect ratio swept back sail, you have to redesign it in the direction of the more modern sails; high aspect ratio, full battens, better mast shape, better cut of the sail pattern, etc.

 

What determines the orientation of the markers in the second video? Are they aligned with the velocity vector, or do they represent the rotation of the fluid at that point?

I would think the flow around the leading edge would be irrotational - moving in circles, but not changing orientation. But the markers circulating the bound vortex appear to have similar orientation to the markers in the starting vortex, which should have a viscous core with rigid body rotation.

 

They are aligned with the velocity vector. The flow should be 'almost' irrotational around the leading edge. This won't be perfectly the case due to viscous effects and also the poor resolution won't help. But the vorticity module should be clearly stronger at the trailing edge and in the starting vortex.

 

To summarize, I feel that the simulations would suggest that the circulation theory of lift is more than just a theory - that it actually explains how lift is created. When the wing starts from the rest, the flow is accelerating at the trailing edge more rapidly on the pressure (lower) side than on the upper (suction side), simply because under the foil the flow has "a straight road" to the trailing edge. This creates a shear layer between the upper & the lower surface, extending from the trailing edge. The shear layer rolls into the spiral "starting vortex", and the starting vortex creates the circulation in the opposite direction, around the foil, while itself being left behind.

There's nothing new in what's said above, but usually the circulation model is dismissed as "a mathematical model and theory, which just happens to yield results close to experiment". A hundred years ago, as explained by David Bloor in "the enigma of the airfoil", the theory was overlooked by the British Cambridge School, since it was based on an inviscid flow, where no vortices could form or decay, thus unrealistic. More recently, I think it has been overlooked because most often the flow over an airfoil is presented in a coordinate system moving with the foil, equivalent to the wind tunnel experiment, where the foil is stationary and the fluid is moving. This way, the true circulatory nature of the flow is masked by the free stream superimposed on it. When the fluid is stationary and the foil is moving, the circulatory flow pattern is revealed. Circulation is real.

 

Then there is the question of "what causes the circulation"? How does a vortex create the circulation about the wing?

There is an expression in US English: "Which came first, the chicken or the egg?". Did the circulation cause the flow field, or did the flow field cause the circulation?

What is clear to me is that lift/circulation requires viscosity, though the viscosisty can be extremely small.

 

Is anyone disagreeing with the statement "Circulation is real"?

 

We are back on the page 1 of the thread...

It proves that circulation of discussions is real.

 

Missed this whole subject thread,...must have been asleep . I shall have to take some time and review it, since I have returned to some of these subjects recently