All Questions
Tagged with thermodynamics fluid-dynamics
609 questions
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
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
34
votes
3
answers
7k
views
How does a river freeze when the water keeps moving?
It's cold outside right now, and the biggest river in the country has frozen over. We're talking about a minimum of 500m in width, and I've no idea how deep (but some pretty big ships can sail there).
...
- 3,461
33
votes
3
answers
99k
views
What exactly is the difference between advection and convection?
After reading Wikipedia articles on advection and convection, I still cannot determine whether there is a consensus on a difference between these two terms.
Sometimes, the term convection seems to ...
- 507
32
votes
3
answers
4k
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 ...
- 3,728
30
votes
13
answers
8k
views
Why do turbine engines work?
I know roughly how a turbine engine (let's say a gas turbine producing no jet thrust) is supposed to work:
The compressor forces fresh air into a combustion chamber, where it reacts with fuel to ...
28
votes
5
answers
10k
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 ...
- 1,113
27
votes
3
answers
5k
views
What is cold wind?
As per my understanding, temperature is the movement of particles in an environment. A highly energetic environment where particles possess high energy has a high temperature,
and low energy means low ...
20
votes
6
answers
9k
views
How can I understand a Vortex Tube and its efficiency?
A Vortex Tube takes a pressurized input stream, most typically of a gas, and creates two output streams with a temperature differential. Apparently, it has been described as a Maxwell's Demon.
Both ...
- 2,142
19
votes
2
answers
3k
views
Why would the lack of air in a mine in an asteroid prevent you from flooding it?
In Isaac Asimov's I, Robot novel, two scientists stationed in an asteroid where mining activities are performed say "We could flood the mines, if this weren't an airless asteroid" when ...
- 199
18
votes
5
answers
24k
views
What is the difference between mechanical and thermodynamic pressure?
To start with I know thermodynamics deals with processes at equilibrium. Hence the thermodynamic pressure should most likely be the pressure of a fluid at equilibrium.
I'm not sure if a fluid flow (...
- 478
17
votes
5
answers
33k
views
What equation of state is needed for liquid states?
I'm familiar with the ideal gas law $$PV=nRT$$ but I don't think it applies to liquids like water. If I'm wrong, please correct me! If I'm right, then what equation of state applies to liquids such ...
- 703
16
votes
1
answer
8k
views
Does fluid speed affect liquid cooling?
If you have a liquid-based cooling system, like the loop in the picture below, does the fluid speed actually matter?
I can see how cycling the liquid too slowly would be bad, since the fluid would be ...
- 483
14
votes
3
answers
3k
views
Would a perfectly clean and perfectly smooth champagne glass have no bubbles?
My understanding is that nucleation sites for bubbles in a champagne glass are either due to defects in the glass or due to fibers in the glass (see this article for details on that statement). Does ...
- 1,294
14
votes
3
answers
6k
views
How to freeze the Niagara waterfalls?
Here is a picture of the usual vigorous Niagara Falls (in the winter).
Here is the picture of Niagara Falls frozen in 1933 (in the very cold winter).
Here is the picture of Niagara Falls frozen in ...
- 7,928
13
votes
2
answers
942
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 ...
- 181k
12
votes
3
answers
882
views
Why does a chimney sometimes create a "draft"?
Sometimes, while lighting a fire in a wood stove in a basement, the smoke does not exit through the chimney like it normally does. Rather, a large amount of the smoke seems to get "pushed back&...
- 2,740
12
votes
2
answers
853
views
Relaxation of the Boltzmann transport equation
My professor in kinetic gas theory said that when considering the Boltzmann Transport Equation (BCE)
$$
\partial_tf + \frac{\vec{p}}{m}\cdot\nabla_{\vec{q}}f + \vec{F}\cdot\nabla_{\vec{p}}f = (\...
- 324
12
votes
3
answers
782
views
Self Stirring Boiling Liquid
A couple of weeks ago, a friend and I were brewing a new batch of beer. We boil our wort in a turkey fryer. Pretty standard setup for beginners. Anyway, our wort was getting pretty close to boiling ...
- 121
11
votes
1
answer
2k
views
Why is compressible flow near the choke point so efficient?
Imagine a steady state, one-dimensional, compressible flow in a horizontal pipe of constant cross sectional area. This flow can be isothermal, adiabatic (Fanno), or diabatic (Rayleigh). As an example,...
- 698
10
votes
3
answers
509
views
Why do distinct temperature layers form in bodies of water?
This may not be the correct place to ask a question about limnology (which I just found out is the science of lakes) but it seems like the place most likely to produce a good answer.
In my diving ...
- 223
10
votes
2
answers
784
views
What makes pasta go into a vertical position while boiling?
I began boiling pasta on a power burner on our stove. After about ten minutes, I looked into the pot to see the pasta encircling the outer edge of the pot and standing vertically. Below is a picture....
- 111
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
8
votes
2
answers
8k
views
Difference between pressure and temperature
If I am given the average kinetic energy of the molecules of a gas or a liquid, how can I tell if the fluid will burn me/crush me/both if I immerse my hand in it?
Equivalently, what is the difference ...
- 181
8
votes
1
answer
677
views
Strange behaviour of water drops on a heated pan
I did an experiment in my home and I was surprised by the results. I heated a pan without anything in it. After some time it became very hot and I took some water (2 drops) and put it into the pan. It ...
- 1,152
8
votes
2
answers
876
views
Hydrodynamics, non-equilibrium thermodynamics and equations of states
Why is it that in non-equilibrium hydrodynamic modelling, e.g. when the solution is time dependent, people use equations of state that are derived from equilibrium considerations (i.e. the ideal gas ...
- 762
8
votes
1
answer
4k
views
Perfect fluids in cosmology?
In cosmology, it is often assumed that the equation of state of a cosmological fluid is of the form $p=w\rho$. Why is this? Is it the equation of a perfect fluid?
Why does $w=0$ for matter $1/3$ for ...
- 2,313
8
votes
1
answer
1k
views
Explaining the phenomenon relating to my cup of tea
I'm sitting in front of my computer and just made myself of cup of tea ("Brasilianische Limette") and see the water on the top evaporating into the room. But what caught my eye was a small rotating ...
- 183
8
votes
1
answer
3k
views
Is a venturi pump feasible at very low pressure?
I need assistance (or experience feedback) in estimating if a Venturi pump would work at low pressure.
A boiler under vacuum (10% atmosphere) will produce steam (at a given speed depending on the ...
- 1,604
8
votes
1
answer
774
views
What's the efficency of a steam jet pump?
Jet pumps or venturi pumps are often stated as having a "terribly low" efficiency, steam jet pumps specifically are usually describes as "only justifiable when there's an abundant steam supply anyway" ...
- 2,142
8
votes
0
answers
304
views
Why there is no chemical potential on this thermodynamic relation?
I was following the book by Landau and Lifshitz, Fluid Mechanics (2nd edition) and got stuck trying to understand one of their arguments. On the chapter about Relativistic Fluid Dynamics it is stated ...
- 542
7
votes
3
answers
12k
views
Is there an equation to calculate the average speed of liquid molecules?
I seem to remember from first year physics that we can calculate the RMS speed of a stationary, ideal gas with $v=\sqrt{\frac{3RT}{M}}$. Does a similar equation exist for liquids?
- 123
7
votes
3
answers
12k
views
Why do tea bags sometimes float and sometimes sink?
Over the past few weeks, when making tea, I've noticed that after filling the mug with water, the teabag that has been placed inside sometimes floats to the top, and sometimes sinks/stays at the ...
- 245
7
votes
3
answers
2k
views
Do objects sink in ice?
The title explains it all. Do objects suspended in ice sink over time? You may remember the story of World War 2 planes that were found 260 ft under the ice which would be anachronistic with seasonal ...
- 303
7
votes
1
answer
660
views
How to derive a physical model of temperature above the earth surface?
At a point on the surface of the Earth, if I go straight "up" a distance $z$ (radially outward from the Earth), I'd observe a temperature profile $T(z)$. The conventional wisdom from the meteorology ...
- 565
7
votes
1
answer
3k
views
Intuitive explanation for subsonic Fanno flow
In most situations in physics, the effect of kinetic friction is to reduce the macroscopic kinetic energy of a system and convert it into heat, thereby increasing its temperature.
but in the case of ...
- 1,187
7
votes
2
answers
615
views
Liquid column "recoils" in a sealed cylinder when hit by a piston -- is it possible?
Consider a cylinder filled partially with a liquid (e.g. water). The cylinder is sealed, and is at held at room temperature (e.g 298K). At equilibrium (or when no external disturbance is imparted to ...
- 211
7
votes
2
answers
774
views
Rayleigh-Taylor Instability dependence on acceleration direction
I'm trying to bolster my understanding of the Rayleigh-Taylor instability, and I've gotten stuck on the point of which fluid (more or less dense) is being accelerated into the other. Cases of uniform ...
- 14.3k
6
votes
3
answers
11k
views
How much heat from a fire actually warms your home?
A fire in a hearth disperses heat to, I guess, three places:
the bricks of the chimney
out the hearth (where the person tending the fire is standing)
out the chimney, above the house
How would you ...
- 587
6
votes
4
answers
17k
views
Why does blowing on hot coffee cool it down?
And will it cool off faster if you blow across the top of the cup or directly into the coffee?
Does it have to do with the fact that when you blow across the top of the cup the velocity of the air ...
- 69
6
votes
3
answers
25k
views
Why does pressure decrease as velocity increases?
Bernoulli's principle states that as velocity increase pressure decreases. But higher the velocity, greater is the temperature and pressure must be high. Can you explain the situation in both the ...
- 613
6
votes
1
answer
3k
views
Entropy is constant. How to express this equation in terms of pressure and density?
In hydrodynamics of an ideal, non-compressive flow we use 5 variables: pressure $p$, density $\rho$ and velocity field $\mathbf{v}$. So we need 5 equations. Landau's "Hydrodynamics" states that the ...
- 991
6
votes
1
answer
1k
views
Explicit form of the entropy production in hydrodynamics
I'm trying to understand how hydrodynamics arise from a precise, mathematical formulation of thermodynamics, learning mostly from Landau's "Hydrodynamics".
So Landau starts from formulating the ...
- 991
6
votes
2
answers
487
views
Proper interpretation of $\rho_\textrm{rad} \, a^4$ in cosmology
Radiation fluid is usually represented by the equation of state $p = \frac{1}{3} \, \rho$, where $p$ is the pressure and $\rho$ is the energy density. The local conservation of energy states that
\...
- 7,677
6
votes
1
answer
690
views
Irrotational fluid
Often, when threating some problem of fluid dynamics I have read that people make the approximation of irrotational fluid, i.e. the velocity field is assumed irrotational:
$$
\nabla \times \vec{v}=0
$$...
- 943
6
votes
2
answers
2k
views
Considerations and questions for a DIY heat pipe; how to choose a working fluid? [closed]
I plan to build a DIY heat pipe! For more info on what a heat pipe is, see Wikipedia.
I would like to use it to cool a desktop graphics card I have. I am having trouble deciding how to choose the ...
- 827
6
votes
1
answer
197
views
Do vortex tubes work with a reversed end plug?
Would a vortex tube still work if instead of a cone plugged into the 'hot' end you had a smaller hole on the 'cold' end? As I understand it, the point of the cone on the hot end is to only allow the ...
- 161
6
votes
0
answers
151
views
How to obtain hydrodynamics from many-body quantum mechanics?
Background: It is well known that the Schrodinger equation is equivalent to the Euler equation (with a "quantum potential" term) plus the probability conservation equation (which is formally ...
- 5,244
6
votes
0
answers
300
views
Why is the Fokker-Planck equation only valid for the forward and backward velocities but not for the flux velocity?
I noticed that the Fokker-Planck equation is often only written for the forward velocity $\vec b$ and the backward velocity $\vec b^*$:
\begin{align}
\partial_t \rho + \nabla (\vec b \rho) &= D \...
- 10.3k
5
votes
3
answers
3k
views
Unclear how heat interacts with Navier Stokes
I am playing around with an Navier stokes solver and I'm having trouble introducing heat.
Am I right in thinking this would be introduced in the ${\bf f}$ term of ${\partial{\bf u}\over\partial t} = -...
- 5,188