All Questions
1,856 questions with no upvoted or accepted answers
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How can we interpret a system in which the probability distribution obeys the forward and the backward Fokker-Planck equation simultaneously?
For a system involving randomness, there is no longer a unique derivative and hence no longer a unique definition of velocity. But for the forward (Ito) derivative, we can find a probability ...
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2
votes
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228
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Symmetry of Stress-Tensor is to "local" isotropy what Symmetry of stress-energy tensor is to general covariance?
I am trying to wrap my head around why the (3d) stress tensor is symmetric when rotational invariance is a symmetry of the system, and its relation to (what I understand to be) the reason for why the (...
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2
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177
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Analytical Solution of the Sod Problem with Different Gammas?
Is there a known analytical solution to the Sod Shock Tube problem where the two gases have different specific heat ratios $\gamma$? I have only been able to find solutions where the $\gamma$ values ...
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2
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72
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Are dipole interactions considered short range in 2d
Every paper that I read about dipole-dipole interactions always call them long-range interactions. Is dipole-dipole interactions can be considered short-range in 2D? As we know it behaves as $\...
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33
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Effective viscosity in bubbly liquid
Motivation: I have screwed some crepes big time this morning and it made me think...
Consider a bubbly liquid (such as a crepes batter if you whisk it too much). Is there an approximate measure of ...
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2
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1
answer
100
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Adjoint operators and the method of variation for the Orr-Sommerfield problem
My question relates to the transient Orr-Sommerfield Squire problem, described on page 20 of the thesis by Eaves.
Here I briefly describe the context of my question.
We consider an infinitesimal ...
- 201
2
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174
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Vertical spike like water drop
Slow motion guys created this Energy Spike ( at 1 min 53 sec ) that looks like a massive water drop or like a negative vortex in the Edinburgh Hydraulics laboratory.
Using momentum of radial inward ...
- 1,094
2
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1
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104
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Speed of sound in thermodynamically unstable phases
The general thermodynamic definition for the speed of sound is
$$c_S=\sqrt{\left(\frac{\partial P}{\partial \rho}\right)_{S}}$$
But what if the material is in a non-stable state, e.g. during tension ...
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2
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1
answer
205
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Relationship between bridge pylon and water flow
Recently, I was met with the following question:
When observing water flow around a bridge pylon, of the following statements, which one(s) are true:
a) The bridge pylon applies force to ...
- 21
2
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444
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Analytical solution to damped harmonic oscillator - Fokker-Planck equation
In the paper "Numerical solution of two dimensional Fokker-Planck equations" (available at: https://doi.org/10.1016/S0096-3003(97)10161-8), the authors quote an analytical solution to the damped ...
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2
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1
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Diver's air torus rising from 10m underwater (edited)
[Edit: earlier version was even more messy]
I just watched a video of a jellyfish caught in a diver's 'air ring' - a torus blown for the sake of watching it rise. The jellyfish gets drawn into the ...
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2
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66
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Is the force due to mass flow proportional to the square of the mass velocity?
I'm trying to calculate the force on a surface due to some flow of water against that surface- for simplicity, assume that the mass flow vector is orthogonal to the surface.
Using dimensional ...
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2
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3
answers
632
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Will mass flow rate be the same in these tank-pipe-ideal fluid systems?
The equation of continuity for pipes
$$\frac{\Delta m_{1}}{\Delta t} =\frac{\Delta m_{2}}{\Delta t}$$
states that mass flow rate inside a pipe doesn't depend on pipe diameter. I'm confused on ...
2
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calculate CFM of a ceiling fan
I'm trying to calculate, or even understand what data do i need to calculate the CFM of a ceiling fan. in many countries this is a given data in the product details but not where I'm from. I have the ...
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Are conservation of mass and conservation of energy *independent* constraints for physical systems?
I'm modeling fluid systems and want to use conservation of mass (aka 'continuity') and conservation of energy as constraints to help solve for certain system parameters. But it occurred to me that the ...
- 11.7k
2
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347
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The physics of the startup of a lava lamp
I noticed a strange phenomenon observing a lava lamp.
We all know the complex flow of bubbles when the lamp is "hot" and running, but I would like to have some explanation about the startup behavior ...
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Viscous Force On A Rotating Cylinder
In this question asked in Irodov, it is taken that the friction force acting on a unit area of a cylindrical surface of radius $r$ is given by $σ = ηr(∂ω/∂r)$.
A fluid with viscosity $η$ fills the ...
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54
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How do vortex filaments move?
In fluid dynamics (Navier-Stokes, incompressible case), a vortex filament, as I understand it, is a curve of points $x\in\mathbb R^3$ such that the velocity field $v\colon \mathbb R^3\times\mathbb R\...
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53
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Does a thickness of a bubble become constant over time?
Assuming no gravity, imagine you have a bubble floating in the air, does its thickness have a tendency to become constant?
State a bit differently, if we start with a bubble with variable thickness, ...
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Is there a physical circumstance where the flow of a system changes from a nodal source/sink to a saddle?
Phase portraits for sinks can often like eddies in water, so I would assume there is some fluid or aerodynamic system where these curves appear. But, the only difference between a saddle and a nodal ...
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2
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122
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Does tilting a liquid develop shear stress on its surfaces?
Please help me these confusions I have.
If I tilt a cuboidal container containing an ideal fluid (non viscous liquid), does a shear stress develop on the liquid due to some shearing force acting ...
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2
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1
answer
193
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Laminar Flow From Outdoor Faucet
I ran across a video recently that really caught my attention. It's been circling the internet over the past couple weeks, but I'm trying to figure out what's going on. Basically, it's a video of an ...
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What would happen if we test the Hagen-Poiseuille law with a superfluid?
When we have a laminar flow we can test the Hagen-Poiseuille law from the pressure difference created by the flow of the fluid. This process is created due to conservation of total energy but the ...
- 1,159
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74
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Proof that pressure rises in a fixed control volume
I am looking for a proof using Bernoulli's equation that that for $Q=(Area)(Velocity)$, $Q_{in}>Q_{out} \Rightarrow P_{inside}>P_{outside}$. I read that for a computer, if the case fans blow a ...
- 21
2
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158
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At what critical Reynolds number does vortex shedding begin?
In:
"Fluid Dynamics", Chapter 3 (Turbulence), Section 26,
Landau and Lifchitz analyze the problem of the stability of a steady flow past a body of finite size.
The fluid is assumed to be ...
- 457
2
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1
answer
283
views
How can a vortex be created?
I have seen this video. It is not completely clear how a difference of velocity can cause a vortex, can you explain it to me?
- 454
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155
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Computing the velocity of fluid along the length of a cylinder
I am modelling the flow of fluid through a cylinder of length L.The cylinder is divided into three segments(each segment of length L/3), there are four nodes(i=0,1,2,3). I'm trying to compute the ...
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275
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Forces acting upon body on impact with water
I am investigating the water-entry of a steel ball-bearing at different speeds. Ideally, I would like to model (to an extent) the forces experienced by the ball bearing as it transitions from air to ...
2
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63
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When can we say $x$ and $p$ are "independent variable", in order to find the Vlasov equation?
I have a question about "independent variable" in physics, and especially variable in Lagrangian or Density Function. I read several questions about it in this forum and although I have the feeling I ...
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1
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560
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Vapor pressure of sublimating ice in vacuum
I am trying to find a vapor pressure of sublimating ice as a function of thermal energy being absorbed by ice. Basically, I am starting with a sample of ice (t=200K) in a vacuum. From there, I am ...
2
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49
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Why do bubble rings stop rising near the surface, and then spontaneously dissipate?
When I've blown bubble rings in my pool I've noticed that every single time a ring rises to the surface it stops rising about 1-2 centimeters from the surface, stopping in place. Then, it ...
- 1,416
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121
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How to model pressure loss over time for air in a cylinder with a hole
Related to an engineering project I will be working on.
The situation is that essentially I will have a closed cylinder with a small hole on the side, which allows the air inside to be at equilibrium ...
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28
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Is it possible to have a vortex with more than one attachment point?
In my study of two vortexes, fire whirls and tornadoes, I have come to hear a bit about vortex attachment. As far as I can understand, a vortex is attached to its "power source" for lack of a better ...
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2
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62
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Thermodynamic formulation of pefect fluid with spin
The spin phenomena in an asymmetric spin-related Einstein-Cartan candidate solution is due to microscopic spin of small volumes of a globally "ordinary" perfect fluid.
The only description I have ...
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330
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How to determine the maximum size of a hole that can keep ink/water stay in an ink-cartridge for fountain pens?
What can be the maximum size of the hole? The physical forces to be taken into consideration are: external pressure, internal pressure, surface tension of the fluid (ink=water). What is the formula?
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229
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What are the perpendicular components of a tensor?
In a paper by Landau et. al. (Hydrodynamic theory of multiple formation of particles), the conservation of energy-momentum is used:
$$
P^{\nu}\equiv\partial_{\mu}T^{\mu\nu}=0,
$$
This is a vector ...
2
votes
0
answers
464
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How to prove that Navier-Stokes equations are Lorentz invariant?
I've been hearing recently that the Navier-Stokes (NS) equations are invariant under a Lorentz transformation, so I tried to prove it just changing terms of transformed velocity instead of the ...
2
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0
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284
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Can Normal Shock Waves be stationary in a flow?
When solving problems of converging diverging nozzles, a shock wave is formed in the flow. For the purpose of problem solving we consider it to be stationary in one point of the flow.
Is this ...
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Velocity of efflux by method of forces
I've been trying to derive the Torricelli's law by using Newton's first and second law but I'm not coming up with an exact answer. Here's the problem.
Consider a large cylindrical vessel with a very ...
user150098
2
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0
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103
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Why does water flow up the sides?
Ive noticed this on my faucet and wonder why water flows up the sides? Is it dark magic?
- 330
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395
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Total force generated in a pneumatic cylinder-piston with known gap size
Assuming that we have a pneumatic cylinder-piston with arbitrary but known surface area A, a known clarence c, and a known length L. What is the best model to describe the total steady-state force ...
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307
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Why is it said that vorticity gets produced at a wall?
I am currently taking a fluids mechanics class and during the lecture the Prof. explained the following: when dealing with a 2D incompressible newtonian flow without external forces (gravity), ...
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366
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Can static charge on a fan blade affect how air flows over it?
So there is this hypothesis that the reason why fan blades accumulate so much dust click here to see why
I was thinking what would happen if I did the opposite. Let's say if I attach the blades to a ...
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4k
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Derivation of the energy equation in the differential form
I'm trying to understand the derivation of the energy equation from fluid mechanics, that is presented in the book "Fluid Mechanics" (4th ed.) by Frank M. White (as you can see here, page 231).
I can ...
- 1,092
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516
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Settling Velocity of Particles in a Flowing Fluid
I'm trying to calculate the settling velocity of particles traveling through a horizontal section of pipe. I've not been able to find information about settling velocities in moving fluids, only still ...
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42
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In what sense can a "manned model" boat handle identically to a much larger boat?
A YouTube video(relevant part is 1:24 - 1:46) suggests that if you take an oil tanker, scale it down 25 times, and give it an engine with 0.4 horsepower, "they behave exactly like a [larger] ship".
...
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2
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2
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649
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Why does potential energy time-derivative depend only on the mass flow?
This is a homework exercise, but I don't want specifically to tell me how to resolve the exercise. I want you to help me what's wrong here, and why I can't do it.
Water is pumped from a lake to a ...
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3
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5k
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Will flow rate always increase if the pressure drop across piping system is decreased?
I have a test stand that is used for testing hydraulic pumps. We are adding a pump that we have never tested before and we are getting low flow readings. Will increasing piping to one size bigger than ...
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72
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Pressure and velocity change at window
When the pressure and velocity of atmosphere(out side of room) are Pa and Va, how do pressure and velocity of wind change at window?
Set the window size 'A'. Please explain this. Thank you.
- 21
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101
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Frequencies of stellar g-mode oscillations in astroseismology
It's a simple derivation that in a gravitating fluid, if the density decreases with height then a perturbed fluid element should oscillate at the Brunt-Vaisala Frequency,
$$N^2 = - \frac{g}{\rho_0} \...
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