All Questions
1,856 questions with no upvoted or accepted answers
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Microscopic description of drag
It is well known that a fluid opposes the motion of a moving object and we can represent this resistance in motion with a force, which we call it drag.
Different fluids exert different drag forces ...
- 1,688
2
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72
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Dynamical system model representation for the geometric motion of soap bubbles
I have been trying to describe the dynamical system model for soap bubbles proliferating, coming into existence and moving and pushing around other bubbles (i.e. water gushing into a sink, full of ...
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2
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0
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33
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Can water waves be weird shapes?
Can stable, non breaking water waves be shapes other than the sine wave or trochoid? Like trapeziums, triangles, asymmetrical waves, etc?
If so, how would these waves behave? Would the traditional ...
2
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answers
35
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Exciting standing waves in piston/valve ended pipe
Suppose a pipe is connecting a high pressure with a low pressure region. Within this pipe there is a piston/valve that can be opened or closed and a large pressure drop.
As the valve is opened, air is ...
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2
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38
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Will two rotating disks in a fluid be 'attracted' to each other?
Imagine two rotating disks in a fluid like this drawing here:
The rotating disks will create a rotating flow by friction in the fluid. The flow between the disks will be higher velocity than the ...
- 2,180
2
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37
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Buoyancy versus viscosity
A common problem in casting is removing the air bubbles that might might be in the mold material, like plaster or resin. This is typically done by degassing--putting the mold material under vacuum to ...
- 4,504
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57
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Numerical methods for modelling non-linear pressure wave propagation in fluids/solids
So far, I only have experience solving the Laplace/(inhomogeneous) Helmholtz/Maxwell equation with boundary conditions using the boundary element method (BEM). So far I have used open source tools for ...
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67
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What is causing the cups to lift up?
I came across this video on Reddit, and as you can see the guy is spinning one cup by putting it into another, and then blowing on it, and the cups lift up. Some are saying its Magnus effect, that it ...
2
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1
answer
124
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Boltzmann Transport Equation existence and smoothness - Is it proved?
Currently, Navier-Stokes Equation, its solution's existence and smoothness is not well established, making the problem as one of famous Millennium Prize Problems. On the other hand, I noticed that ...
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81
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Microscopic mechanical energy conservation equation derivation
The transport phenomena book by Bird states that the mechanical energy equation can be derived by taking the dot product of the momentum conservation equation with the velocity vector v.
This is the ...
2
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1
answer
251
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How high can water rise up your arm hanging in the water from a moving boat? (Bernoulli's principle)
I have the following problem where you have to apply Bernoulli's principle, which asks:
How high can water rise up one's arm hanging in the river from a boat moving at $u_1 = 1 \text{ ms}^{-1}$?
(That ...
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2
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105
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Solving for fluid velocity field given positions as a function of time
I have a distribution function that describes the number of particles as a function of time and their mass, $n(M,t)$, which I can calculate analytically for all $M,t$. I want to know the evolution of ...
- 14.3k
2
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1
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173
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Why cant there be a sonic velocity inside the convergent part of a nozzle? (ie upstream of the throat)
A convergent-divergent nozzle is typically used for accelerating or decelerating airflow to or from supersonic speeds. The typical configuration for such a nozzle is the de Laval nozzle that is ...
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2
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120
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Aerodynamic Problem in regards to a ping pong ball and an open hole tube
I have a ping pong levitation system with a ping pong ball in a tube being pushed by a DC motor fan (shown below) and I'd like to define a relation of the ball's height with the air velocity or ...
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2
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108
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Dynamic Small Airways Collapse - Understanding the physics of the pressure drop from alveolus to mouth in human lungs
EDIT:
I think I have thought of a much better phrasing of my question using the middle diagram below.
a) If intrapleural pressure is >0 does airway collapse always occur?
b) In a hypothetical ...
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2
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answers
54
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Ripples through a uniform ball of water
Let's say the earth was a giant blob of water, with a still surface. Now, suppose I drop a drop of water at the north pole. What happens next? I don't know how gravity is going to influence this. ...
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How do kitesurfers / kiteboarders move?
Explanations I've managed to find on the web either focus on how to practice the sport or how a kite flies. A full force vector decomposition that makes clear how downwind, upwind, and crosswind ...
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1
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2k
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How can we create suction, without the use of a vacuum?
How is it possible to create suction without the use of a vacuum?
For example: As I understand, positive displacement pumps transport fluid by creating a vacuum at their inlet to draw in a distinct ...
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2
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answers
35
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Oil pooling along grates in heating pan
Recently I went to cook something on my cast iron pan. I turned the burner on, drizzled in some oil (I think it was olive oil) and started chopping some vegetables. When I turned back a few moments ...
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2
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93
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Blob of falling water
If a "blob" of about 100-200ml of water is dropped from higher than about a first floor (using English nomenclature: ground, first floor, second, etc), it'll stay together as a blob until it'...
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Name of the Rayleigh–Taylor demonstrator toy
There is a common physics "toy" consisting of two liquids sealed into the space between two sheets of plexiglass. When turned upside down, it demonstrates the Rayleigh–Taylor instability.
I ...
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If the air-jet garden hose swings back and forth, does the inhaling garden hose swing back and forth?
If the air-jet garden hose swings back and forth, does the inhaling garden hose swing back and forth? I guess it will too, but I haven't experimented. I don't know what the real situation is. who ...
- 2,165
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"Pointlike" explosions: the transition to non-explosions as initial energy is decreased (or volume increased). How does their behaviour change?
If a large enough amount of energy is dumped in a small enough volume of Earth's lower atmosphere, events follow a standard pattern:
The air within the volume is fully ionized and heated to extremely ...
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2
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57
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Compressible Navier-Stokes Equations and Millennium Problems
The 4th Clay Institute Millennium Problem is about the existence and smoothness of the Navier-Stokes Equations. The definition of the problem is limited to the incompressible fluid equation. What ...
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32
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Do stationary objects intoduce drag on closely passing moving objects
Would it require more energy to drive down a narrow road lined with columns than a similar open road?
Would the columns intoduce drag on the passing vehicle, or effect its aerodynamics in any way?
2
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1
answer
155
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How does the structure factor reflect the characteristics of particle distribution?
The structure factor is defined as follows:
$$S(\mathbf k)=\frac{1}{N}\sum_i\sum_je^{-\mathbf k\cdot\mathbf r_{ij}\sqrt{-1}}$$
It is related to the radial distribution function by Fourier transform:
$$...
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2
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1
answer
97
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How is thermodynamic pressure a measure of total internal energy (translational, vibrational and rotational)?
I read that mechanical and thermodynamic pressures are different and the distinction is explained saying that:
"The mechanical pressure is a measure of the translational energy of the molecules. ...
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2
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answers
40
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Why do coordinates depend on time in an eulerian point of view?
In all sources I read so far about fluid dynamics, we adopt an eulerian point of view, where we focus on a fixed position in space $(x, y, z)$ and we study the variation of the velocity vector at this ...
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27
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How does roughening a surface help with Reynolds number matching?
I have a research paper describing a wind tunnel test conducted on a small scale model of a mast.
It is said:
Considering the similarity of the Reynolds number, the surface of the cylinder part ...
- 921
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2
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847
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Necessary and Sufficient Conditions for an Equilibrium to be Stable
In the 4th section The condition that convection be absent of the book Fluid Mechanics by Landau and Lifshitz, they give the following statement:
For the (mechanical) equilibrium to be stable, it is ...
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2
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1
answer
102
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Newton's Laws of fluid motion
Can someone explain the similarity between friction and viscous force?
This is what I have understood:
Friction and viscous force come into play in presence of relative motion. They are dissipative ...
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2
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1
answer
44
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Mokka pot coffee homogeneity
I've noticed some people pouring coffee from a mokka pot alternates between two cups instead of directly filling one after the other. The reason argued is related to the beverage homogeneity, as the ...
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2
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76
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Prove momentum is conserved
I have a question which I do not fully understand about conservation of momentum of the shallow-water equation in conservative form.
We are given a velocity $u(x,t)$ and the height $h(x,t) = d + l(x,t)...
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43
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What is the intuition behind a high Froude number causing instability in uniform flow?
Uniform flow in an inclined plane becomes unstable for high froude numbers.
I can follow the spectral analysis, but I am curious why I would expect high froude numbers to cause instability. What is ...
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1
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91
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Does capillary action work in moving bodies?
If I were to have a capillary tube on a moving body such as an ocean buoy, would it still be able to draw the liquid upwards? Or would the turbulence slow down or stop capillary action?
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Does: capillary_action * surface * distance = (force*surface) / area*distance == capillary_energy?
I'm trying to get the hang of the conservation of energy. Looking for that aha moment when it makes more sense that energy is conserved than not. When reading it seems to be a consensus that energy is ...
2
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0
answers
31
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Defining the electric field with a potential in axisymmetric, stationary fluids on Kerr black hole spacetime
So studying the RMHD (relativistic magnetohydrodynamics) equations of a fluid (/plasma) and specifically Maxwell's equation reads as
$$ \nabla \times E = - \dfrac{1}{c} \dfrac{\partial B}{\partial t} ...
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2
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76
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Water flow through a ventilated orifice
Before I begin, please excuse the rushed and poor schematic made in haste.
I am not very knowledgeable in fluid dynamics and was curious when I observed the scenario below.
A water droplet of mass M ...
2
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0
answers
29
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How environmental factors affect aerosol transmission?
I am totally an amateur in this field, but I need this for my own multi-disciplinary project.
I know this might be a basic question, and I actually did do some researches. I just want to make sure ...
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255
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Speed of sound and diffusion in relativistic hydrodynamics
In the following the second part of the question Relativistic hydrodynamics is presented:
Consider the hydrodynamic energy-momentum tensor in 3 + 1 dimensions in
at space with
the Minkowski metric $\...
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answers
258
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Stokes's law of sound attenuation and mechanisms of sound absorption
According to this website and this website, there are two mechanisms by which sound is absorbed in air: effects of viscosity and molecular relaxation.
According to Wikipedia, Stokes's law of sound ...
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2
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92
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What is the physics behind why incorrect cuff size for measuring blood pressure manually or automatically lead to inaccurate results?
The biophysics behind measuring systolic and diastolic blood pressure manually with a cuff and stethoscope is that
Determine the approximate pressure it takes for the cuff to occlude the brachial ...
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Why does the reduction of "sloshing" in tankers also reduce brake distance?
After watching this video, I was confused at why reducing the peak slosh force made braking easier, after all it's the same mass inside the truck, plus, shouldn't the overall force over the whole ...
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1
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199
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How does air flow within a melodica when two key are pressed?
I'm trying to understand the air flow within a melodica.
A melodica is a wind instrument that has a piano-like keyboard. Pressing on a key opens an airway so that air entering the melodica's air ...
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2
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266
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Why does poured honey move periodically when reaching the surface it's poured upon?
Here's the picture, known to most of us:
When honey is poured out of a container, as in the picture (look here for the flow in motion), we see a kind of periodic flow of the honey when it comes to a ...
2
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answers
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Simple analytical model for fluid flow in "Mushroom cloud"
In potential flow theory there are simple analytical models (formulas) for velocity-field of elementary features (like source, sink, dipole, vortex etc.)
Is it possible to write simple analytical ...
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188
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Sound wave in relativistic perfect fluid
I was trying to solve the following problem from the problem book on relativty:
Problem 5.22. Show that the velocity of sound $v_{s}$ in a relativistic perfect fluid is given by
$$v_{s}^{2}=\partial p ...
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1
answer
169
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Can I use Pascal's principle to destroy any airtight container?
I am looking at the following textbook problem (please note that I am not asking for a solution to the problem, only using it as a motivating example):
A host pours the remnants of several bottles of ...
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37
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Lubrication Theory Question
I am consideringflow between a rigid cylinder parallel to a rigid horizontal stationary surface. Assuming that we can apply the lubrication limit (so that the cylinder is very close to the surface) I ...
2
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2
answers
289
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When compressing liquid, how much energy is converted to increased temperature vs increased pressure?
If you compress liquid in an infinitely stiff and infinity insulated cylinder such that the cylinder does not expand and no heat can transpire, how much of the energy will converted to increase in ...
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