All Questions
Tagged with thermodynamics fluid-dynamics
61 questions
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
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
2
votes
1
answer
700
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 ...
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
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
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 ...
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
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
4
votes
2
answers
1k
views
Describing pressure in incompressible fluids
The canonical process of determining the pressure, velocity, and density of a fluid under the influence (or not) of external forces is through simultaneously solving conservation of mass, conservation ...
- 3,235
4
votes
4
answers
13k
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Why do vapour cones form around jet fighters?
Apparently this phenomenon has nothing to do with jets breaking the sound barrier and has something to do with the Prandtl-Glauert singularity as described on Wikipedia. But, the Wikipedia article isn'...
user29305
3
votes
1
answer
446
views
Derivation of general equation of heat transfer & entropy
In Landau & Lifshtiz Volume 6 on fluid mechanics we derive the general equation of heat transfer by starting with the expression
$$
\partial_t \left( \frac{1}{2} \rho v^2 + \rho \varepsilon \right)...
- 161
3
votes
2
answers
2k
views
How to determine effect of obstruction on pressure?
I am struggling to figure out what theory explains a situation I observed. A steam boiler had a pressure control which would turn off the boiler if it sensed 2 psi. The boiler would frequently switch ...
- 91
33
votes
3
answers
99k
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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
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
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
4
votes
2
answers
1k
views
Significance of Stokes Hypothesis
When we derive the Navier-Stokes Equation, we come across a a common assumption made by Stokes that makes the two quantities namely Mechanical pressure and Thermodynamic pressure equal to each other.
...
- 349
4
votes
2
answers
310
views
Sonic Boom in Aircraft and Spacecraft
I would like to know why aircraft, and spacecraft produce a double sonic boom on breaking the sound barrier.
A while ago, I thought I got it, as there’s a start and finish point to every vehicle.
...
- 41
4
votes
3
answers
4k
views
Why does a bubble take a spherical shape?
I suspect this has something to do with thermodynamics and the isoperimetric inequality and I'm interested in a mathematical derivation of this result.
user29305
2
votes
2
answers
188
views
Closure conditions in the form of equation of state
While reading the book "Riemann solvers and numerical methods for fluid dynamics" By E. Toro, the very first paragraph is:
"In this chapter, we present the governing equations for the ...
- 113
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
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
6
votes
4
answers
17k
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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
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
4
votes
3
answers
2k
views
Sound in stirred hot fluids
If a hot beverage in a cup gets stirred, the sound of the spoon changes. You can easily hear this if you repeatedly cling the spoon to the cup ground after stirring. The cling sound will raise in tune ...
- 485
3
votes
1
answer
487
views
Derivation of vorticity equation (incompressible flow)
In the following derivation of the vorticity equation, I do not understand how $\nabla \cdot v=0$ implies $\frac{1}{\rho^2}\nabla \rho \times \nabla p=0$.
We start with the Euler equation
$$\frac{\...
- 113
2
votes
1
answer
434
views
Joule heating for a fluid
Say you have a conductive liquid with a changing magnetic field going right through it, causing an electric current. How exactly does the electric current travel and how could you calculate the ...
- 21
1
vote
2
answers
1k
views
Assumption of local thermodynamic equilibrium in fluid dynamics
Moving fluids are generally in a state of non-equilibrium. However, in fluid dynamics, people generally assume a state of local thermodynamic equilibrium and argue that in such a condition, ...
- 21
1
vote
1
answer
126
views
Isentropic fluid: cross product of gradients is zero, why?
In the vorticity equation we have the baroclinic term of the form: $$\frac{ {\nabla}\rho}{\rho}\times\frac{ {\nabla}{P} }{\rho}.$$
Why does it go to zero for isentropic flow?
I understand that, if the ...
- 157
1
vote
4
answers
3k
views
How does the dissolution of salt affect the solution density?
Suppose you have a container of water as a solvent and you a certain amount of salt as a solute sitting at the bottom of the container that has yet to start dissolving. Supposing temperature and ...
- 289
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 ...
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
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
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
4
votes
3
answers
623
views
Why friction increase subsonic speed through pipe?
I know equations of Fanno flow but can anyone explain physically how friction increases subsonic velocity and decreases supersonic velocity through pipe ? is that for sake of boundary layer ...
- 43
4
votes
3
answers
571
views
Flame shape and size (length) depending on gravity [duplicate]
How would the shape and size of a flame, e.g. from a simple candle depend on gravity? Suppose all the relevant information is known, including candle dimensions and chemical composition, atmosphere ...
- 139
4
votes
4
answers
3k
views
Shock speed in air/vacuum shock tube
Some of you are probably aware of What If, xkcd's blog about interesting physics problems.
One episode, Glass Half Empty, concerns itself with what would happen if a glass of water is half water, ...
- 438
4
votes
2
answers
23k
views
Hot Air Balloon and Buoyancy
This is a conceptual question in a solution I am trying to understand.
Problem statement:
I have a balloon with a volume of V $m^3$. The outside air temp is $K$ kelvin
and mass to lift is $m$ kg.
I ...
- 439
4
votes
1
answer
8k
views
Why do steam bubbles increase in size as they rise?
In the following video (a customer's review of a glass kettle), we can observe water boiling: http://youtu.be/jByY5I7Xk7w?t=2m55s
As the kettle starts to boil at around 2:55, we can see large steam ...
- 34.9k
4
votes
3
answers
6k
views
What cools a drink?
When you stick ice in a drink, AFAICT (the last physics I took was in high school) two things cool the drink:The ice, being cooler than the drink, gets heat transferred to it from the drink (Newton's ...
- 421
3
votes
0
answers
205
views
Onsager reciprocal relations and Darcy's permeability tensor for anisotropic porous media
The Wikipedia entry on soil tensor permeability states that
The tensor is symmetric by the Onsager reciprocal relations.
Bear & Cheng (2010, p122) state that:
The hydraulic conductivity tensor, ...
- 1,455
3
votes
3
answers
2k
views
Gas pressure and centrifugal force
I think about a rotating torus (simplified tire) filled with ideal gas. Mass of gas is $m$ and molar mass is $M$. Pressure in non rotating torus is $p_0$. Temperature is constant $T$. Inner radius of ...
- 3,074
3
votes
1
answer
157
views
Why do flows gain pressure with decreasing velocity?
I know it isn't always the case, but in many conservation equations velocity and pressure of a flow are inversly related, or sometimes velocity and enthalpy. My question is, "What about slowing ...
- 2,908
3
votes
0
answers
344
views
Why (and how) do foods stick to a pan?
We all (sooner or later) have noticed that foods relatively high in protein (especially those low in fat) are very prone to sticking to a pan, or in general to any non-specially-coated metal surface. ...
- 1,813
3
votes
1
answer
135
views
Is there a molecular diffusion component to wind?
I'm thinking of vapor pressure from terrestrial water: A highly evaporating area in the ocean creates a volume of high relative humidity, the vapor tends to move to less humid air and by molecular ...
- 137
3
votes
1
answer
462
views
Why internal energy in Lagrangian is treated as a potential energy?
To obtain Euler fluid equations of motion one can do variational principle on the following Lagrangian density where $\rho_0$ is reference density, $\Phi$ is displacement vector field and $u$ is ...
- 409
3
votes
2
answers
492
views
How do we find force to derive $pV=Nmv^2$?
Since momentum is not changing in time t , but instantaneously in very small time at wall B.
Why do we write $F=\frac{2mv}{t}$? Where $t$ is (twice the distance AB)/ velocity.
Why don't we take the ...
- 75
2
votes
2
answers
290
views
Air velocity flowing out of a pierced can [duplicate]
How can I calculate the velocity of air that would come out of a pierced can of compressed air if I assume that the pressure inside the can is 7 atm and the size of the hole is that of a regular nail?
- 153
2
votes
1
answer
2k
views
Is there a difference between solid-to-fluid convection and fluid-to-fluid convection?
I found an online calculator relating to convective heat transfer, but the calculator's description says it's for heat transfer between a solid surface and a moving fluid. However, the definitions I'...
- 183
1
vote
1
answer
1k
views
Convective heat transfer coefficients - how do you calculate them?
How do you calculate (or otherwise determine) $h_\text{inner}$ and $h_\text{outer}$ (convective heat transfer coefficients) for a heat exchanger?
For reference, the standard equation is:
$$
\text{UA}~...
- 13