The quantitative study of how fluids (gases and liquids) move.
68
votes
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 ...
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 ...
19
votes
1answer
661 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
797 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 ...
10
votes
4answers
734 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. ...
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.
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 ...
3
votes
2answers
349 views
Bernoulli's equation and reference frames
So I was thinking about this while driving home the other day.
I've never been quite clear on why when you drive with the windows down air rushes into your car. I thought this might be explained by ...
3
votes
1answer
322 views
Is there something like the Poynting vector for hydraulic circuits?
The Poynting vector is a representation of the energy flux in electromagnetics, showing the amount and direction of power flow at different points in space. In electric circuits, the energy is not ...
7
votes
1answer
728 views
Why do galaxies and water going down a plug hole spin?
We all experience things spinning, whether it's water down a drain, the earth on its axis, planets round the sun, or stars in a galaxy - even electrons round an atom.
But why is spin so common in ...
7
votes
1answer
469 views
Resolving water sink problem
There is a common myth that water flowing out from a sink should rotate in direction governed by on which hemisphere we are; this is shown false in many household experiments, but how to show it ...
3
votes
1answer
294 views
Change in appearance of liquid drop due to gravity
A liquid drop is spherical in shape due to surface tension. But why does it appear as a vertical line under the free-fall due to gravity? (E.g. During a rain - falling raindrop) Is there a specified ...
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 ...
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.
...
8
votes
3answers
349 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, ...
7
votes
3answers
2k views
How does a hole's size affect the distance that water will squirt
I took a bucket, drilled 2 different sized holes on the side near the bottom and filled it with water. The stream of water the proceeded from the larger hole traveled further than the stream from the ...
3
votes
3answers
714 views
Define Pressure at A point. Why is it a Scalar?
I have a final exam tomorrow for fluid mechanics and I was just looking over the practice exam questions. They do not provide solutions. But pretty much I have to define pressure at a point and also ...
4
votes
3answers
1k views
How is the Joukowsky Transform used to calculate the Flow of an Airfoil?
As I read in The Road to Reality by Roger Penrose, the Joukowsky transform $$w(z) = \frac12\left( z + \frac1z \right)$$
after Nikolai Zhukovsky (transcribed in several versions from Никола́й Его́рович ...
3
votes
2answers
301 views
Why are continuum fluid mechanics accurate when constituents are discrete objects of finite size?
Suppose we view fluids classically, i.e., as a collection of molecules (with some finite size) interacting via e&m and gravitational forces. Presumably we model fluids as continuous objects that ...
13
votes
4answers
680 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 ...
7
votes
3answers
455 views
Boundary layer theory in fluids learning resources
I'm trying to understand boundary layer theory in fluids. All I've found are dimensional arguments, order of magnitude arguments, etc... What I'm looking for is more mathematically sound arguments. ...
6
votes
2answers
148 views
Why water in the sink follow a curved path?
When you fill the sink with water and then allow the water to be drained, the water forms a vortex.. And then it starts to follow a curved path downwards by effects of gravity..
Why this phenomena ...
2
votes
1answer
60 views
Oscillations of a fluid particle in a stably stratified fluid
Could anyone help me work through the following set of equations:
I found these online at
http://marian.fsik.cvut.cz/~bodnar/PragueSum_2012/Staquet_1-2.pdf
and I'm trying to work through them. I ...
2
votes
1answer
540 views
Why, when one opens 1 car window, does that noise occur?
When you're driving and you open 1 car window, say the front one, there comes a horrible noise, but when you open another window just the slightest bit, this noise goes away (I'm sure most people know ...
0
votes
1answer
189 views
Are there good home experiments to get a feel for the behavior of yield-stress liquids?
What common household liquids / foodstuffs show Herschel-Bulkley fluid behavior?
I assume tomato soup to be one, since when sloshing it around lightly on a spoon the visible surface moves as one (as ...
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):
...
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 ...
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), ...
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
842 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 ...
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 ...
6
votes
2answers
136 views
Where can I check a solution to 3D Navier Stokes?
A few years ago I developed a solution to the Navier-Stokes equations and as of yet have not been able to locate a similar version of the solution. I would like to know if anyone has seen a solution ...
4
votes
1answer
345 views
Hit a bottle of beer on the top with another causes the first to spit all the gas, why?
So, on the other day me and my colleges were discussing the following phenomena:
Pick two open bottles of beer. With the bottom of the first, hit the second on the bottleneck, in the following way: ...
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 ...
5
votes
1answer
318 views
Why is pressure in a liquid the same in all directions?
I'd like answers both in the more intuitive side an on the more precise side.
Thinking of water as "cubes" of water, for example, would allow pressure in the z axis to be independent of the y or x ...
5
votes
1answer
436 views
Viscosity/Entropy ratio and unitary evolution in quantum gravity
Calculations of the shear viscosity of the quark-gluon plasma (QGP), observed in heavy ions collisions, are performed via the AdS/CFT correspondence [ref1]. One finds that the shear viscosity of the ...
3
votes
4answers
350 views
Why does smoke go out the window of the car - and what if there's wind blowing instead of the car moving?
When driving a car while smoking with the window open (safety and legal issues aside), I've noticed that the smoke tends to go outside the window.
Why does the smoke go outside?
If the car is ...
3
votes
1answer
450 views
Is there an analytical solution for fluid flow in a square duct?
I couldn't find one but assumed it must exist. Tried to find it on the back of an envelope, but got to an ugly differential equation I can't solve.
I'm assuming a square duct of infinite length, ...
2
votes
3answers
784 views
What is the velocity area method for estimating the flow of water?
Can anyone explain to me what the Velocity Area method for measuring river or water flow is?
My guess is that the product of the cross sectional area and the velocity of water flowing in a pipe is ...
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
2answers
430 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 ...
7
votes
1answer
742 views
water flow in a sink
When one turns on the tap in the kitchen, a circle is observable in the water flowing in the sink. The circle is the boundary between laminar and turbulent flow of the water (maybe this is the wrong ...
6
votes
1answer
143 views
How does liquid in a ball affect its rotational acceleration down a ramp?
Suppose we have a shell with mass $M$ and radius $R$. If we let that roll without slipping down a ramp of angle theta to the horizontal, we can easily find the acceleration of the shell the instant ...
5
votes
5answers
833 views
Is it possible to accelerate air to supersonic speeds? What would it look like?
The speed of sound is the rate that disturbances in air propagate through it.
Is it possible to have a wind that itself is moving at supersonic speeds relative to stationary winds around it?
Or ...
5
votes
6answers
427 views
Why do liquids separate in space?
I've seen videos of people in space (on ISS) who squeeze a bottle or something and liquid comes out, it then separates into smaller balls.
Why is this surely it should stay pretty much together ...
4
votes
2answers
1k views
Shape of Fan Blades
How is the shape of the blades of an air fan determined? Trial and error, or is there a theory behind it? What are they trying to maximize, volume of air dislocated per rotation?
3
votes
2answers
629 views
How much lift does an airplane get from its wings, vs the rest of the airframe?
Consider a big commercial airliner, like a 727, 747, or a 787.
At cruising altitude, under standard conditions, how much of the lift of the aircraft comes from the wings, and how much from the rest ...
3
votes
1answer
1k views
How far will water squirt out from a hole in a can?
Say you have a bottle of water filled up to a height H. A small hole is drilled in its side at a height d, so that water squirts out. The squirting water travels in an arc as it falls, covering some ...
2
votes
1answer
92 views
Showing symmetry of the stress tensor by applying divergence theorem to $\int\int_{\delta V(t)} \vec{x}\times \vec{t} dS$
I'm currently working through the symmetry of the stress tensor, in relation to viscous flow. I am looking at this by examining the conservation of angular momentum equation for a material volume ...
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?
