The quantitative study of how fluids (gases and liquids) move.
68
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3answers
5k views
What really allows airplanes to fly?
What aerodynamic effects actually contribute to producing the lift on an airplane?
I know there's a common belief that lift comes from the Bernoulli effect, where air moving over the wings is at ...
38
votes
2answers
576 views
Analog Hawking radiation
I am confused by most discussions of analog
Hawking radiation in fluids (see, for example,
the recent experimental result of Weinfurtner et
al. Phys. Rev. Lett. 106, 021302 (2011), ...
36
votes
11answers
4k views
How long a straw could Superman use?
To suck water through a straw, you create a partial vacuum in your lungs. Water rises through the straw until the pressure in the straw at the water level equals atmospheric pressure. This ...
32
votes
10answers
4k views
Why does dust stick to rotating fan propeller?
Why does dust stick to rotating fan propeller?
Intuitively, most people (including I) think of the dust will not stick to rotating fan propellers.
EDIT 1:
Thank you for the great explanations. I am ...
32
votes
7answers
2k views
How can two seas not mix?
How can two seas not mix? I think this is commonly known and the explanation everyone gives is "because they have different densities".
What I get is that they eventually will mix, but this process ...
29
votes
8answers
9k views
Why does the atmosphere rotate along with the earth?
I was reading somewhere about a really cheap way of travelling: using balloons to get ourselves away from the surface of the earth. The idea held that because the earth rotates, we should be able to ...
26
votes
10answers
2k views
Mechanics around a rail tank wagon
Some time ago I came across a problem which might be of interest to the physics.se, I think. The problem sounds like a homework problem, but I think it is not trivial (i am still thinking about it):
...
23
votes
3answers
1k views
When water climbs up a piece of paper, where is the energy coming from?
Take a glass of water and piece of toilet paper. If you keep the paper vertical, and touch the surface of the water with the tip of the paper, you can see the water being absorbed and climbing up the ...
23
votes
2answers
2k views
Before a once-warm lake starts to freeze, must its temperature be 4°C throughout at some point?
This is a problem I just started puzzling over, and I felt this would be a good forum to check my reasoning. So here are the relevant observations followed by my question:
Water achieves its maximum ...
22
votes
3answers
1k views
What is the fallacy in this infinite motion machine?
I realize this isn't possible, but I can't see why not, especially if you change the model a little bit so that the balls simply travel through a tube of water on the way up, rather than exactly ...
22
votes
3answers
841 views
Wind generators - why so few blades?
Why commercial wind generators usually have just 2-3 blades?
Having more blades would allow to increase power OR decrease diameter.
Decreased diameter would also reduce stress due to different wind ...
19
votes
3answers
368 views
How much information about the scale of a waterfall can be obtained from its sound?
Is it possible to constrain the height, volume flow, or distance of a waterfall from the quantitative analysis of a high-quality recording of its sound?
As an aside, the simulated sounds of fluid ...
19
votes
1answer
660 views
What nonlinear deformations will a fast rotating planet exhibit?
It is common knowledge among the educated that the Earth is not exactly spherical, and some of this comes from tidal forces and inhomogeneities but some of it comes from the rotation of the planet ...
17
votes
3answers
793 views
Have we figured out how to analyze turbulent fluids?
I was surprised to read that we don't know how to analyze turbulent fluids. On page 3-9 of The Feynman Lectures on Physics (Volume One), Feynman writes:
Finally, there is a physical problem that ...
16
votes
3answers
1k views
Why does the sound pitch increase on every consecutive tick at the bottom of a filled cup of coffee?
Since I don't know the proper physical terms for this, I describe it in everyday English. The following has kept me wondering for quite some time and so far I haven't found a reasonable explanation.
...
16
votes
4answers
363 views
Why does the fundamental mode of a recorder disappear when you blow harder?
I have a simple recorder, like this:
When I cover all the holes and blow gently, it blows at about 550 Hz, but when I blow more forcefully, it jumps an octave and blows 1100 Hz.
What's the ...
16
votes
2answers
239 views
Stripeless cleaning of windows
Cross post: http://chemistry.stackexchange.com/q/4377/22
Last week I was discussing with a friend how we thought the stripeless cleaning of windows is achieved when using a cleaner like Windex ...
15
votes
1answer
625 views
How can the Moon have such a strong effect on the ocean?
The gravitational acceleration on Earth is approximately $ 10 \mathrm{m}/\mathrm{s}^2 $. Compared to this, the tidal effect of the Moon's gravity gives a local variation in the acceleration of ...
15
votes
1answer
252 views
Flames with no gravity?
I was watching "Solaris" (Tarkovsky) today, and noticed this: in some moment the space station changed orbit and the people inside experienced zero-gravity. At that moment, a candlestick passed ...
15
votes
1answer
369 views
Minimum viscosity of liquids
In a lecture by Purcell he mentions that he notices that there aren't any liquids with viscosities much less than that of water, even though they go up seemingly unbounded. In an endnote (endnote 1 in ...
14
votes
2answers
272 views
How deep is the Great Red Spot?
The Great Red Spot (GRS) is a very persistent storm system that's easily visible through a telescope on the surface of Jupiter. But what is the three-dimensional structure of the GRS, and how deep ...
13
votes
4answers
678 views
Why is exhaling more forceful than inhaling?
By blowing at pencil, a piece of paper, or another object up to fifty centimeters away, I can cause it to move away from me significantly. But I can't move an object toward myself by inhaling sharply ...
13
votes
2answers
451 views
What causes ballpoint pens to write intermittently?
After a while, a ball point pen doesn't write very well anymore. It will write for a little distance, then leave a gap, then maybe write in little streaks, then maybe write properly again. It seems ...
12
votes
1answer
3k views
Did Felix Baumgartner produce a sonic boom during his jump?
I really got to thinking about this. The speed of sound is measured at 761.2 MPH at sea level. But how does this number change as air density decreases? The lack of air density is what allowed his ...
12
votes
1answer
263 views
Why is there a breakdown in Kolmogorov scaling in turbulence?
Why is there a breakdown of Kolmogorov scaling in turbulence? What causes intermittency?
12
votes
2answers
121 views
Cascade in relativistic turbulence
The Kolmogorov theory of turbulence indicates an energy cascade in turbulence. Is there a corresponding version of relativistic fluid?
11
votes
7answers
1k views
Why there's a whirl when you drain the bathtub?
At first I thought it's because of Coriolis, but then someone told me that at the bathtub scale that's not the predominant force in this phenomenon.
11
votes
5answers
444 views
Is flying really easier on smaller scales?
In the book Playing with Planets, the author makes the following argument, pertinent to flying robots of the future:
As it is, an important law of physics says that smaller organisms fly much more ...
11
votes
2answers
660 views
Why doesn't water come out of tap/faucet at high pressure when I turn it on?
(tap=faucet)
When I turn a tap on full and then put my thumb over the spout covering, say, 90% of it, then the water spurts out. If I turn it on to, say 10%, then the water dribbles out.
What's the ...
11
votes
1answer
326 views
Why does my natural whistle have a maximum volume
When I whistle, I find that I can vary the volume by pushing more or less air through my mouth at once. However, when I increase volume past a point, I start to hear a blend of rushing air and a ...
11
votes
2answers
433 views
How to model/simulate pressures and flows in a network of pipes
I'm having a hard time finding information on how to model/simulate this. I attached a couple files, both of which show an example tank & pump network. It's just nonsense that I made up for this ...
10
votes
5answers
558 views
Why cant one see tidal effects in a glass of water?
Why cant one see the tidal effect in a glass of water like in an ocean?
10
votes
4answers
954 views
How does a steady wind flow generate sound?
When a wind blow through sharp edge, say, edge of a paper, you can see the vibration of the paper and hear the sound.
For this type of oscillation, it should be a damped oscillation with external ...
10
votes
2answers
259 views
Can vorticity be destroyed?
I have a professor that is fond of saying that vorticity cannot be destroyed. I see how this is true for inviscid flows, but is this also true for viscous flow? The vorticity equation is shown below ...
10
votes
3answers
503 views
Which direction will Coriolis forces deflect a bubble?
If I throw a ball straight up, it deflects slightly to the west due to Coriolis forces. If instead I watch a bubble float up in water, is the bubble deflected west, east, or neither?
I think the ...
10
votes
4answers
731 views
Vortex in liquid collects particles in center
At xmas, I had a cup of tea with some debris at the bottom from the leaves. With less than an inch of tea left, I'd shake the cup to get a little vortex going, then stop shaking and watch it spin. ...
9
votes
8answers
2k views
Why do ice cubes come out easier from top trays?
This is my "hey, I've noticed that too!" question for the week. If you stack two plastic ice cube trays with water in them in a freezer, the resulting ice cubes in the top tray will usually come out ...
9
votes
2answers
2k views
Are there any liquids with zero surface tension?
Having read the Wikipedia page on superfluids I'm still not sure if stuff like liquid helium at the lambda point actually have surface tension or not. Is superfluidity the same thing? And are there ...
9
votes
2answers
311 views
Whistle Physics
I'm looking for a simple explanation of how a whistle operates. I know that forcing air over a sharp lip can set up a wave in a resonating cavity, but how? "Most whistles operate due to a feedback ...
9
votes
3answers
544 views
Could a fish in a sealed ball, move the ball?
If you had a glass ball filled with water, completely sealed and containing a fish, could the fish move the ball?
9
votes
1answer
331 views
Energy from man-made tornadoes
Peter Thiel just paid $300,000 to Canadian inventor Louis Michaud who is working to construct useful "man-made tornadoes" or "atmospheric vortex engines" which could be components of future power ...
9
votes
1answer
74 views
How can one build a multi-scale physics model of fluid flow phenomena?
I am working on a problem in Computational Fluid Dynamics, modeling multi-phase fluid flow through porous media. Though there are continuum equations to describe macroscopic flow (darcy's law, ...
9
votes
3answers
150 views
What does the quantification of causes and effect look like, for clouds in offshore wind turbine wakes?
At Horns Rev windfarm off the coast of Denmark, sometimes in winter, clouds appears in the wake of the turbines. I've only seen photos of the phenomenon when the wind direction is exactly aligned with ...
8
votes
3answers
313 views
Why $\log \rho$ in the continuity equation?
I just saw the continuity equation, in a manuscript, written as
$$\frac{\partial \log \rho }{\partial t} + \vec v \cdot \nabla \log \rho= - \nabla \cdot \vec v.$$
Now, just calculating the derivatives ...
8
votes
3answers
348 views
Occurrence of turbulences in Fluid Dynamics from the equations of motion?
How can it be shown that turbulences occur in Fluid Dynamics?
I think poeple imply that they develope because of the $\text{rot}$ terms in the equations of motion, i.e. the Navier-Stokes equations, ...
8
votes
1answer
2k views
Why exactly does a boomerang return back to the thrower?
I was always intrigued by the phenomena that govern the returning back of a boomerang to the thrower.
Even if it is dependent on various factors such as velocity, aerodynamics, wind resistance and ...
8
votes
4answers
1k views
Water pressure in free fall
The increasing water pressure as you go deeper is generally explained in terms of the weight of the water column above the observation point pressing down. The question, then, is what would happen if ...
8
votes
2answers
428 views
Does a wing in a potential flow have lift?
I have a hard time understanding whether or not a wing placed in a potential flow, assuming there is no viscosity and no friction with the wing, will produce a lift. I've seen several contradictory ...
8
votes
1answer
485 views
Friction term in Navier-Stokes equation
The friction term in Navier-Stokes equation assumes that the viscosity coefficients are the same for the longitudinal and transverse directions. This doesn't seem intuitive, because the former is ...
8
votes
3answers
239 views
What is the microscopic picture for warm air rising?
The usual explanation for warm fluids rising past cooler ones is that the warmer fluid has a lower density. I'm trying to understand what this looks like at a molecular scale. The density seems to ...
