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# Amazing flow visualization - Page 3

Quote:
 Originally Posted by YoungOldTimer What's the units of pressure in that plot? Torr? mbar? Why don't you look at the pressure in the air (not just at the surface of the ski). Follow some parcels of air along their streamlines and see if there are any for which the pressure stays sufficiently low for a long enough time to produce condensation in conducive conditions. My guess is that because of the very small size of the vortices that you see from skis compared to those from aircraft, the max pressure drop will be much less and will last for a much shorter period of time before it equilibrates. Therefore, even with nucleation centers available, and favorible RH, while a vortex might form, vortices from skis won't be large and strong enough to be self-visualizing. To support this argument, how many self visualizing vortices do you see from small UAVs compared to full size vehicles at similar speeds? Cheers, YOT

That is pressure coefficient, which is non-dimensional. It is defined as:

Cp = ( p - p_inf) / ( 0.5*rho_inf*U_inf^2)

(where p is pressure, rho is density, U is velocity, and inf denotes a freestream value). Cp gives the ratio of the change in static pressure to the dynamic pressure, and is handy for aerodynamics. Cp=0 is freestream pressure, Cp<0 is low pressure, and Cp>0 is high pressure.

It's hard to say if the vortices off skis are ever visible, but my guess is no since we haven't really seen any (especially now that this image has been partially debunked). I'd have to pull in some other analysis to get condensation worked out, but the Cp in the vortex core is -2 or so, which is inline with values seen on similar types of vortices on aircraft. I have seen vortices show up on small scale wind tunnel models, so it's not totally out of the question. It really takes the right combination of ambient conditions to be possible in almost any scenario.

Craig
One problem here with the concept of a stationary object creating a vortex. With speed and wind creating the compression and release necessary to generate visible vapor, a fixed object would have to have a very "friendly" shape and now your calculations would be based at thirty odd MPH versus the estimated 80's into a 30 MPH wind that could be expected and as I originally assumed for this to be possible.
Quote:
 Originally Posted by skier219 Two of them lineup very closely, with a third close too, but the 4th doesn't line up at all. I am using the blurred crescent rock (upper left) as a reference mark where possible. Also note that the near-field snow surface doesn't line up when the supposed vortex does. It could be that the views and angles (and sun) are changing, so they might not always need to line up to prove it's a sunlit ridge in the background. Of course, the background (with rocks, mountain face, etc -- and also containing the hypothetical ridgeline) is very much out of focus, while the supposed vortex is sharp. This could still be light playing games with the lens, however. I guess I am 50/50 at this point!

I adjusted for the angle of the ground by rotating a few of the pictures. Looks pretty exact to me.

It's obviously something 'stationary' such as a hillside in the sun.
Cool overlays. Looks like the mystery is solved? Hope this isn't construed as a highjack, but is semi related:

I roughed out a (low res) Karma as a simple extrusion and put a texture map as the top sheet. I need camber and tip/tail curve info to model it correctly and see if the texture map still works.....or if you send me your flexed and unflexed skis, I can see if I can check that out. (Shift zooms in-Ctrl zooms out, arrows or curser rotates).

As an initial step I rotated, laterally and longitudinally a series of copies and plopped them on a slope model. Hopefully, you stay on the snow better. I can add a vortex to your Karmas if you wish. FWIW, I just received an enhancement that should allow me to lay a 3d path on the contours and a slide the ski (or whatever) while flying around the slope. We'll see.
Quote:
 Originally Posted by jonnythan I adjusted for the angle of the ground by rotating a few of the pictures. Looks pretty exact to me. It's obviously something 'stationary' such as a hillside in the sun.

Yep, I think that settles it. Thanks for the good graphics work man!
Quote:
 Originally Posted by Alpinord Cool overlays. Looks like the mystery is solved? Hope this isn't construed as a highjack, but is semi related: I roughed out a (low res) Karma as a simple extrusion and put a texture map as the top sheet. I need camber and tip/tail curve info to model it correctly and see if the texture map still works.....or if you send me your flexed and unflexed skis, I can see if I can check that out. (Shift zooms in-Ctrl zooms out, arrows or curser rotates). As an initial step I rotated, laterally and longitudinally a series of copies and plopped them on a slope model. Hopefully, you stay on the snow better. I can add a vortex to your Karmas if you wish. FWIW, I just received an enhancement that should allow me to lay a 3d path on the contours and a slide the ski (or whatever) while flying around the slope. We'll see.
Nah, no hijack, I think we beat the original topic into the ground at this point!

That is slick. I am headed off to ski tomorrow, but can try sending you my definition of the Karma next week. PM me if you want to discuss further. I can send the triangular surface mesh if there is a way for you to work with that. The file would contain x,y,z coordinates for all the node points, then a connectivity list giving the 4 node numbers for every triangle. I may also be able to export a "plot3d" surface, which would be a structured Cartesian surface. Could also just send the equations for the ski curves if you can work with those.

Craig
Quote:
 Originally Posted by Yuki One problem here with the concept of a stationary object creating a vortex. With speed and wind creating the compression and release necessary to generate visible vapor, a fixed object would have to have a very "friendly" shape and now your calculations would be based at thirty odd MPH versus the estimated 80's into a 30 MPH wind that could be expected and as I originally assumed for this to be possible.
That might be true. The only photos I have seen of visible vortices from wind were those on the deck of an aircraft carrier (where the "wind" was from the ship moving at 30+ knots). Vortices would actually form on the edge of the deck, originating from various antennas, protuberances, etc., and then wiggle around occasionally becoming visible. In fact they looked like the sun on a ridgeline in the mountains.
A 3d mesh or surface (NURBS?) would be best, or just the ski dimensions. Not sure about importing 'plot3d' format, but IGES, 3DS, SAT & DWG formats are good. It's not critical, but interesting to see how it goes.

Thanks,
Terry
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