All Questions
871 questions
291
votes
17
answers
82k
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 ...
- 77.3k
103
votes
7
answers
113k
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 ...
- 7,463
11
votes
4
answers
11k
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 ...
- 1,158
8
votes
1
answer
4k
views
Non-linear waves and shock formation
In the cases of non-linear acoustics, why is shock formation unlikely when the dispersion is strong when compared to the non-linearity of the wave?
- 247
122
votes
9
answers
64k
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 ...
- 1,189
5
votes
1
answer
3k
views
Can a hovering helicopter travel half the globe in 12 hours? [duplicate]
Suppose we have a helicopter that is able to stay stationary in flight for extended periods of time. If such a helicopter stayed at point A in the sky for 12 hours straight, would it reach the other ...
- 161
30
votes
4
answers
7k
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 ...
- 21.3k
18
votes
4
answers
50k
views
Explanation that air drag is proportional to speed or square speed?
A falling object with no initial velocity with mass $m$ is influenced by a gravitational force $g$ and the drag (air resistance) which is proportional to the object's speed. By Newton´s laws this can ...
- 439
22
votes
3
answers
15k
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 (...
- 322
20
votes
2
answers
3k
views
Could Navier-Stokes equation be derived directly from Boltzmann equation?
I know how to derive Navier-Stokes equations from Boltzmann equation in case where bulk and viscosity coefficients are set to zero. I need only multiply it on momentum and to integrate it over ...
- 8,971
12
votes
3
answers
18k
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 ...
- 297
38
votes
7
answers
4k
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 ...
- 105k
22
votes
4
answers
3k
views
How to show that the Coriolis effect is irrelevant for the whirl/vortex in the sink/bathtub?
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 ...
user68
17
votes
2
answers
14k
views
Height of Water 'Splashing'
Suppose from a height $H$, I throw a ball of mass $M$ and radius $R$ with initial velocity $u$ into a pool of depth $x$ having a liquid with density $\rho$ and coefficient of viscosity $\eta$.
Upto ...
- 293
8
votes
2
answers
868
views
Assumptions of thermodynamic & chemical equilibrium in fluid-dynamics
I am reading about the Euler equations of fluid dynamics from
Leveque's Numerical Methods for Conservation Laws. After introducing the mass, momentum and energy equations, some thermodynamic concepts ...
- 501
19
votes
9
answers
7k
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. ...
- 291
12
votes
3
answers
3k
views
Patterns in laminar flow of tap water
This is a simple experiment that anyone can do at home. Open your tap so that the water maintains a laminar flow, and the cross section of flow is considerably thin. Place your finger 3-4 cm below the ...
- 781
5
votes
2
answers
2k
views
Navier-Stokes Derivation
Someone knows a physical derivation of the Navier-Stokes equation? Mainly the stress tensor. A lot of authors simply "jumps" the stress tensor and it's the more important of physical motion and ...
- 155
18
votes
1
answer
18k
views
Book recommendations for fluid dynamics self-study
I have my bachelors degree in engineering science: computer science, option electrotechnic. I have a good understanding of basic mechanics and dynamics, calculus (differential equations).
The books I'...
8
votes
1
answer
2k
views
Is there a relativistic version of Navier-Stokes equations?
Just as the title says, is there a relativistic version of Navier-Stokes equations?
In electromagnetic hydrodynamics it would be very useful to have relativistic version of Navier-Stokes equations, ...
- 935
3
votes
6
answers
2k
views
Mathematical Reasoning for Fluid Pressure as a Scalar
This question from a while ago and answers/comments to this question from earlier today both make heavy mention of the fact that fluid pressure is a scalar. Although this information was surprising ...
- 2,993
15
votes
5
answers
37k
views
A fly in an accelerating car
A fly is flying around in a car, the fly never touches any surface in the car only fly’s around in the air inside the car. The car accelerates. does the fly slam in to the rear window. or does the fly ...
- 151
22
votes
5
answers
10k
views
Would a fast interstellar spaceship benefit from an aerodynamic shape?
Some (generous) assumptions:
We have a spaceship that can reach a reasonable fraction of light speed.
The ship is able to withstand the high energies of matter impacting at that speed.
Given the ...
- 513
7
votes
2
answers
8k
views
Platform diving: How deep does one go into the water?
I am a platform diver. I am trying to figure out how deep in the water I go depending on which platform I dive from.
The platforms are 1m, 3m, 5m, 7.5m and 10m above the water surface.
By ignoring air ...
- 599
5
votes
3
answers
1k
views
How do shock waves form in space?
Today my aerodynamics professor mentioned that the equations we were learning for atmospheric shock waves can also be applied by astrophysicists to study black hole-related shock waves in space.
How ...
- 161
63
votes
15
answers
31k
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.
- 2,729
44
votes
5
answers
11k
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 ...
32
votes
2
answers
2k
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?
- 321
22
votes
9
answers
25k
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.
- 363
20
votes
8
answers
10k
views
How much effect does the Bernoulli effect have on lift?
I understand that the Bernoulli effect is a flawed explanation for the cause of lift, and does not cause much at all, but how much?
Is there any experimental data on the force caused by the ...
- 1,580
17
votes
1
answer
16k
views
Fluid in a rotating cylinder
I have been wondering why a fluid in a rotating container has a parabola shape? Is it possible to prove this mathematically?
- 1,823
8
votes
2
answers
15k
views
How was Reynolds number derived?
I'm studying fluid dynamics and recently the formula $Re=\frac{\rho vd}{\eta}$ was presented to me. I'm curious to know how Reynolds came up with this relation between these different variables.
Did $...
- 548
7
votes
2
answers
77k
views
Calculation of pressure from flow rate of water
Anybody kindly help me to find how to calculate pressure in bar from flow rate. I have a pipe and from that I am transferring water at a constant flow rate of 5ml/min. At this flow rate, with a 0.5 cm ...
- 81
52
votes
3
answers
17k
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 ...
- 5,121
46
votes
2
answers
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 ...
- 2,462
22
votes
1
answer
1k
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 ...
- 948
10
votes
2
answers
10k
views
Why does a balloon spiral in the air instead of moving in a straight line?
When an air-filled balloon is released without its opening tied up, it moves in a circular path rather than a straight line. Why is that?
6
votes
3
answers
46k
views
How does the radius of a pipe affect the rate of flow of fluid?
Poiseuille's law states that the rate of flow of water is proportionate to $r^4$ where $r$ is the radius of the pipe. I don't see why.
Intuitively I would expect rate of flow of fluid to vary with $r^...
- 527
44
votes
4
answers
5k
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 ...
- 555
44
votes
3
answers
44k
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?
- 1,281
39
votes
3
answers
84k
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 ...
- 4,637
27
votes
2
answers
4k
views
Why does a nuclear explosion have directionality?
It just occurred to me that almost all images I've seen of the (in)famous mushroom cloud show a vertical column rising perpendicular to the ground and a horizontal planar ring parallel to the ground.
...
- 379
22
votes
1
answer
5k
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 ...
- 381
19
votes
3
answers
13k
views
Why and how is sound produced when two objects hit each other?
When two objects collide and undergo a partially inelastic collision (so every one we experience in every-day life), they rebound to a certain degree, but kinetic energy is not conserved. Thus, the ...
- 1,813
16
votes
3
answers
8k
views
How is viscosity described on the molecular level?
What is the 'molecular' origin of the viscosity? The molecular origin of elasticity is almost clear for me: at the very bottom the 'elasticity' comes from the attraction and repulsion between atoms ...
- 173
14
votes
2
answers
5k
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 ...
- 4,637
10
votes
7
answers
26k
views
Capillary tube of insufficient length
I was wondering if we have a very thin glass tube placed in a tub of liquid and the portion of the tube outside the liquid is lesser than the height to which the liquid can rise because of capillarity,...
- 1,823
9
votes
1
answer
12k
views
Stress-energy tensor for a perfect fluid in general relativity
Wikipedia reports this expression for the stress-energy tensor of a perfect fluid in general relativity
$$T^{\mu \nu} = \left(\rho + \frac{p}{c^2} \right) U^{\mu} U^\nu + p g^{\mu \nu}, $$
where $\...
4
votes
2
answers
2k
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 ...
- 563
2
votes
1
answer
702
views
Knudsen Number and pressure
When computing the Knudsen number to know if the continuum hypothesis can be applied as $\frac{k_B T}{p \sqrt{2} \pi d^2 L}$, do we use the static or total pressure of the free stream? My object is ...
