Poiseuille just didn't come to play last week, but I'm sure his mother still loves him all the same. Poiseuille is a very important guy in the sprinkler business. Without his equations it would be next to impossible to get all of your sprinklers working properly. His law equates the flow of the system to pi * R^4 * |Delta P| divided by 8 * the viscosity * the length. In fact the only easy "solution" is to over engineer the system, like mine. See, most home use sprinklers work from 25 - perhaps 40 psi. My system (using the same previously mentioned sprinkler heads) runs at full city water pressure, about 120 psi. This gives me plenty of flow, enough to blow the tops of some of the sprinklers if I am not careful.
Enough about me, lets talk about Poiseuille and why he lost. I see two main points (not counting him being french). First, Poiseuille's law requires Newtonian fluids, which don't exist. Enough discussion has already been made in comments about the significance of this in the context here. Second, his law only works on laminar flow.
Laminar flow is when the flow if the liquid occurs in paralell streams; the layers being undisturbed by each other. Nice and predictable, even for many non-Newtonian fluids. This is typically the case for a Renoylds number below about 2100-2300. Above this turbulent flow occurs. Turbulent flow, as you may surmise from the name, is violent(read the footnote here to see why turbulence is good at propagating violence ). Some may argue that laminar flow is required for planes and helicopters to fly. To them I say "you just need more thrust."
6 comments:
Are Reynolds numbers related to Reynold's wrap?
No, Reynold's numbers are basically a measure of the importance of viscosity(stickyness) or a fluid compared to pressure effects. So on very small objects (like bug wings) the air acts as if it were much 'thicker' than you or I experience it, more like water.
As for whether laminar flow is required...it isn't required, it just(usually) makes things more efficient. Like running lanes on a track - each runner (air particle) stays in its own lane going forward. When the flow gets turbulent then the poeple (or particles) go in all sorts of directions and spend much more energy bouncing around - like the people in a stadium trying to beat the traffic after the game.
Props to Kit for a very well written answer but I'm still going to go with the Reynold's Wrap explanation.
Well, I guess you could say they are related. They do both have to do with the ability of a substance to stick to itself. So as long as you want to equate air and plastic that way, go ahead.
But I do recomend keeping them far enough separate in your mind to avoid trying to breathe Reynold's Wrap. That would be what we call "A bad thing".
Reynolds never suggested you breathe his wrap.
Reynolds numbers also don't seem to have to have something to do with air and wings. What is it with aeronautics engineers and wings anyways?
Besides, based on the earlier explanation, if I was really small, air would be more like water to me. I wouldn't want to be breathing water either...
Maybe we should just leave breathing out of this.
Reynold's number doesn't HAVE to have anything to do with wings, true. You can read more about it here:
http://en.wikipedia.org/wiki/Reynolds_number
But for wings, the characteristic length is the chord (or width) of the wing. So the size of a wing is a very important parameter in Reynold's number - when dealing with wings.
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