Questions tagged [fluid-dynamics]

The quantitative study of how fluids (gases and liquids) move.

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257
votes
15answers
72k 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 ...
132
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12answers
12k views

Why does my tea periodically alternate its rotational speed after stirring? (Link to video below)

I noticed that after stirring, a bubble in the centre of my mug of tea changed the speed it was rotating at periodically. Speeding up, then slowing down, then speeding up again, etc. Almost like when ...
109
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8answers
47k views

Why does a helium filled balloon move forward in a car when the car is accelerating?

I noticed that when I had a helium filled, latex balloon inside of my car, it moved forward in the cabin as I accelerated forward. The faster I accelerated forward, the faster the balloon went from ...
105
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6answers
24k views

Why is oil a better lubricant than water?

How come mineral oil is a better lubricant than water, even though water has a lower viscosity? When two surfaces slide over each other with a gap filled with a fluid, the different layers of the ...
93
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6answers
96k 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 ...
81
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4answers
7k views

What causes the water in this fountain to reverse direction?

In a children's museum, I ran across this fountain. You can adjust the flow rate with a valve, visible at the bottom. At low flow rates, the sheet of water does more or less what you'd expect: it ...
73
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5answers
55k 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 ...
67
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3answers
6k views

Why is dry soil hydrophobic? Bad gardener paradox

When I forget to water my plants, and their soil becomes very dry, during the next watering I can see that the soil becomes hydrophobic. I can even see pockets of air between the repelled blob of ...
66
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3answers
10k views

Why does water falling slowly from a tap bend inwards?

This is something anyone could easily verify. When we open a tap slowly, water bends inwards (towards the axis) while maintaining its laminar flow. After a certain height below the opening, the flow ...
62
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4answers
11k views

If water is incompressible, how can sound propagate underwater?

Since sound travels as longitudinal waves, sound waves should only be able to propagate in a medium through compressions and rarefactions. However, water, as a liquid, is generally treated as an ...
61
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15answers
28k 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.
60
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12answers
28k 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 ...
56
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4answers
6k views

Why are aerodynamic / streamlined shapes always stumpy at the front?

I'm building an autonomous boat, to which I now add a keel below it with a weight at the bottom. I was wondering about the shape that weight should get. Most of the time aerodynamic shapes take some ...
50
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2answers
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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), arXiv:1008.1911)...
49
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5answers
16k views

How effective is speeding?

At a simple level, speeding in a car attempts to minimize the time required to travel a distance by utilizing the basic relationship: $$d=st$$ So for a given distance, time should be inversely ...
48
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3answers
14k 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 ...
46
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8answers
6k views

Explaining to a five year old - why don't bubbles run like water

My five-year old daughter was asking about astronauts the other day and why they float in space. After me showing her a few bits on the kids section on the NASA web site I started explaining about the ...
46
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5answers
7k views

Why does pitch increase when you blow harder into a whistle?

When you play recorder or whistle, the pitch depends on how hard you blow into the tube. E.g. when you blow a whistle, initially the pitch is slightly lower when there is less air flow. This seems ...
44
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2answers
5k 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 ...
44
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13answers
4k 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): ...
43
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4answers
4k views

What is the minimum pressure of a medium for which a sound wave can exist?

At what pressure will be particles in a medium be unable to form a sound wave when disturbed? How can this pressure be described mathematically? My guess is that this would correspond to the point at ...
43
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7answers
133k 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 ...
42
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4answers
22k views

Does the sea level increase if an iceberg melts?

It was claimed that if an iceberg melts in the ocean, the sea level won't change as the ice displaces as much water as there will be melted water. The other claim was that the sea level should rise ...
42
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5answers
8k 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 is ...
41
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4answers
3k 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 ...
40
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4answers
24k views

What is the mystery of turbulence?

One of the great unsolved problems in physics is turbulence but I'm not too clear what the mystery is. Does it mean that the Navier-Stokes equations don't have any turbulent phenomena even if we solve ...
39
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7answers
6k views

Supergun Launching of Satellites

I should say first that I don't believe this is a feasible launch method, otherwise NASA and other space agencies would be using it by now. It's based on this BBC news story Saddam Hussein's Supergun ...
39
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4answers
5k views

Why do you have to blow sideways on a flute?

If you were to play a flute, you would want to blow sideways across it, kind of like a bottle, like in Fig 1. Why would this be better than blowing straight in to the hole, like in Fig 2? It seems ...
38
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5answers
10k views

Why does a weather vane arrow point in the direction of the wind?

It seems that a weather vane will rotate in order to minimize energy and thus orient itself parallel to the wind. What I do not understand is why it is implied that the weather vane arrow should ...
38
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3answers
25k views

What is going on in front of and behind a fan?

Why is it that when you drop paper behind a fan, it drops, and is not blown/sucked into the fan, whereas if you drop paper in front of a fan, it is blown away?
36
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4answers
5k views

Why can't the Navier Stokes equations be derived from first principle physics?

At the 109th UCLA Faculty Research lecture, Seth Putterman gave a talk on Sonoluminescence. During the lecture he emphasized that "The Navier Stokes equations cannot be derived from first principles [...
35
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5answers
6k views

How were the Navier-Stokes equations found in the first place if we can't solve them?

I was reading up on the Clay Institute's Millenium prizes in mathematics. And I noticed the Navier-Stokes equations were described as minimally understood. As far as I was taught in physics a few ...
35
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5answers
5k views

Why do tidal waves appear very suddenly in some rivers?

As tide approaches in lower part of some rivers (e.g. Ganges), a several feet high tidal wave enters from the sea against the flow of the river (making a great noise), and the water level suddenly ...
35
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3answers
4k views

What is the shape of the upper boundary of water hitting a screen?

When I saw that thing I did not understand how that shape is formed? To be ideal, take a vertical smooth plane. Now aim at the wall with a thin water tube. Then the outer layer forms a "parabolic ...
35
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2answers
10k views

Pouring oil on choppy water to calm it , does it work and if so how?

Near where I live, local fishermen often bring cans of castor oil with them, to calm the water around their boats, if they feel bad weather is due. They claim this method of sea calming works, (...
35
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2answers
21k 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 ...
35
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4answers
7k views

Floating a ping pong ball in the air just using a pen!

Caution: Apparently this problem is harder than it seems! There is a well known phenomena, which I first learnt about when I was a 10 years old kid. You can levitate a ping pong(or whatever not-so-...
34
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3answers
68k views

Why does water pouring from a glass sometimes travel down the side of the glass?

If you have a glass of water, say, three quarters full and you pour it at an angle of say, $45^{\circ}$ with respect to the the table, the water comes out of the glass and goes directly down towards ...
32
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3answers
11k views

Emptying a bottle faster by swirl

A moment ago, I was emptying bottles filled with water (2 liters) that are on the terrace of my house. As I did so I remembered something I saw on TV a some time ago (I don't remember when or where or ...
32
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2answers
1k views

What forces are at work causing sand to migrate to the centre of a spinning bucket of water? [duplicate]

A bucket is filled with water and a handful of sand. The water is then spun. Why and what forces are in play which cause the sand particles to congregate in the centre of the bucket?
32
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3answers
3k views

What really is a fluid?

My question may be pretty basic, but I feel it is important to ask this as I've gone through several texts and none offer me the clarity I seek. The question is: What is a fluid? What is flow? If we ...
32
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6answers
3k views

Do gases have phonons?

A phonon is a quantized unit of sound; they are encountered when quantizing lattice vibrations in solids. Now, even an ideal gas supports sound waves, but in this case, interactions between atoms are ...
32
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3answers
2k views

Why do particles in the molten wax near a burning candle wick get pushed away?

Have a look at this clip I made of my candle burning: I can accept that the molten wax gets sucked into the wick (though not quite sure why: capilary action?). But why does it also seem to get ...
30
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3answers
7k views

Is this really a golden ratio spiral?

In this blog post, I found this picture: There are other similar photos: and Does the water really form golden ratio spiral in such cases? Or is the photo just a provocative example, without ...
30
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7answers
41k views

Why do tsunami waves begin with the water flowing away from shore?

A sign of a tsunami is that the water rushes away from the shore, then comes back to higher levels. It seems that waves should be both + and - polarized and that some tsunamis should go in the ...
29
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6answers
8k views

Are there any fluids that flow slower in a constricted region, unlike water?

It is well known that when water flows through a tube you can make it flow faster by making the tube narrow. Now consider what happens when a group of people are moving and the space becomes narrower....
29
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6answers
4k views

How can fast moving particles gain energy from slow moving ones?

Imagine a large diameter piston filled with water connected to a small funnel. When you press on the piston slowly but with considerable force the water will move very quickly from the funnel in form ...
29
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3answers
6k 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 ...
28
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6answers
3k views

Frequency difference when water splashes at different temperatures

When I pour hot water (near boiling) and cold water ($5 \unicode{x2103}$) from a height on a platform, there is a distinct difference in the sound that is generated. I feel that hot water splashing ...
28
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5answers
8k views

Can I make the water boil simply by spinning it in a glass?

Correct me if I'm wrong, I think the pressure in a fluid reduces when the speed increases(The airplane rises because the air above the airfoil moves faster than the air below it). Next, looking at ...

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