Originally Posted by beyond
Skier219 - You sound like you know your stuff, so I'm hesitant to disagree. Obviously, SA is the big dog. But it seems to me that whatever the physical fact of snow, we don't buggy on top of it. We ski in it. Even a big fatty at speed is partly embedded, and fluid attributes, such as bow shock and a wake, are visually evident in light pow. Moreover, lift calculations allow us to consider other variables, like velocity, snow density, and shovel/tail shape, that clearly affect ski performance. Otherwise, my Mantras would plane the same at all speeds and at all resorts, and manufacturers wouldn't worry about prow-shaped vs. low angle shovels, or smearing turns with pin-tails vs scallops.
Well, you know I try to sound smart -- that's half the battle
. Actually, I am an aerodynamics engineer at NASA and have been doing research in fluid dynamics for about 20 years. It's cool stuff to discuss, especially in the context of sports.
I wasn't going after the "on it" versus "in it" aspect of the sand or snow -- that wouldn't matter so much. I think most of the "lift" we feel is coming from riding the snow particles with the bottoms of the skis. And here, things like velocity, surface area, density, etc. do matter, just not in a fluid dynamic sense. There would have to be a true difference in the flow between the top and bottom of the ski to create a pressure difference and then result in aero/hydro-dynamic lift. And unfortunately snow does not obey the basic characteristics of a fluid that would allow it to have "pressure" in a traditional sense. Like I said, it's a collection of solid particles. In fact, one reason it can stand on a sloped surface and not "flow" is this non-fluid property. If you look at the mechanics of how avalanches start, it's usually some sort of solid behavior (shear, fracture, etc).
One basic definition of a fluid is that it can take the shape of a container, be displaced (say, stick your hand in a jar of water), and then resume the previous shape (take your hand out of the jar of water). Just by leaving permanent tracks in the snow, we sort of disprove that last part. Contrast this with a boat -- there's a wake but the water eventually settles back to it's undisturbed state.
Now, if you're in light powder where there really is a lot of air, I can definitely understand seeing flow-like features such as wakes. Even on hard snow, you are creating those features in the air around you. Powder just gets mixed/wrapped into the air flow and allows us to visualize it.
One of the few true "flow" aspects to skis that I know of occurs when you're skiing on packed snow and a film of water forms between the ski and the snow. There is actually a flow there -- it's known as a low Reynolds number "creeping" flow. Even at high speeds it behaves like a low speed flow because the thin gap between the ski and the snow creates intense frictional forces in the fluid, which outweigh the momentum forces significantly. It can actually create a suction, which is one of the reasons you used to see a groove on some skis -- to break the suction (not sure if it really worked -- I don't see it on skis anymore). Another example of creeping flow is when you get two plates to suck together in soapy dishwater. Same deal.
I do think there is probably some sort of analogy to be made here between snow skiing and flow, and like I said, things like surface area, velocity, and density would have to be important for skis. I would just stop short of drawing a direct analogy to a real flow with aero/hydro-dynamic forces. So I agree with the theme of this discussion, just not some of the details.
By the way, I have heard that the principle behind powder ski shaping is simply to distribute the load from ski to snow in a certain way -- to tailor the forces for improved handling/feel. By tapering, scalloping, or scarfing the tail, it transitions the force a little more smoothly than having an abrupt "step" change in the geometry. Thus the tails blends away some.