All Questions
1,856 questions with no upvoted or accepted answers
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What is the fluid approximation for BCS action?
For a system with free fermion gas at finite density (i.e., free Dirac action), we have an ideal fluid description with $\rho$, $p$, $n$. What is the fluid description for BCS action?
Suppose we ...
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2
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522
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Creating the proper suction
I'm a bit rusty on my fluid dynamics. I'm experimenting with creating my own leaf vacuum.
How would I determine how much suction a fan creates?
Bernoulli's equation would apply correct?
I would ...
- 131
2
votes
1
answer
97
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Can Total Derivative of Quantity Give Partial Derivative?
While reading a paper on oscillating water column by David Dorrell and others it was given for the water height:
$$\sum F = \frac{d}{dt}P$$
$$\frac{d}{dt}P = \frac{d}{dt} (\rho_s\eta_1A_1\frac{d\eta_1}...
2
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2
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103
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Submerged Landau jet
I am reading Landau & Lifshitz's Fluid Mechanics. On page 81, section 23, it reads
Determine the flow in a jet emerging from the end of a narrow tube into an infinite space filled with the fluid -...
- 613
2
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1
answer
55
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Fluid covers the floor of a container
I know this formal definition of a fluid:
1.) It is a continuum.
2.) It cannot absorb shear stress.
But how can you explain with that, that a fluid covers under the influence of gravity the whole ...
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2
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1
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137
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Using heat to lift a liquid – What's a good way of turning heat energy to gravitational potential energy?
Edit: A bit of context... Living in a country with extreme heat and a lot of hills, many farmers need irrigation pump systems (pump up water from a pond up to a reservoir). I have for example seen ...
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2
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1
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745
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Why we have a leak in the pipe if the fluid pressure is lower than the atmosphere?
I've read in many books that if our flow pressure is lower than the atmospheric pressure then we'll have a leak in our pipe, but I already know that the flow is always from the high pressure point to ...
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2
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1
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Drag force acting on a disk in a 2D system
I have a 2-dimensional system with behavior governed by Langevin dynamics in which disks (circles) move through a fluid. In the Langevin equation, there is a velocity-dependent term that accounts for ...
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2
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1
answer
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What is the drag coefficient of an open wedge?
To check my two dimensional CFD calculation I am looking for reference data on the drag coefficient of an open wedge. The geometry is shown below, together with the flow direction.
I have found ...
2
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2
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353
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What is the definition of a Collisional Fluid?
I am unsure whether this means the particles in the fluid must physically collide or does 'collisional' also apply to particles interacting in general, eg via gravity? I cannot find a good definition ...
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2
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Vortex shedding
So I was reading about vortex shedding, and got to know that it causes the building to resonate. Is there any way to use this resonance to generate electricity (maybe use quartz?)
I am trying to make ...
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1
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0
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11
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Why does a fluid's velocity affect the fluid's incompressiblity?
First of all, I'm a complete beginner of fluid dynamics, and I'm taking a course of it because it is compulsory. Please be kind.
I'm told that a fluid is incompressible if the volume of any fluid ...
- 195
1
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1
answer
17
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From material derivatives to partial derivatives in the wave equation
Consider the Cauchy momentum equation:
$$\rho \frac{d^2 \mathbf{u}}{d t^2} = \nabla \cdot \boldsymbol{\sigma} + \rho \mathbf{f}$$
where $\rho(\mathbf{x},t)$ is the density, $\mathbf{u}(\mathbf{x},t)$ ...
- 225
1
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25
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Questions on surface tension
I've learned that surface tension occurs due to the difference between the molecules of surface and the bulk molecules and the asymmetry between them. The particles next to the walls of the container ...
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1
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1
answer
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Why do we include both local and temporal acceleration in fluid mechanics but only consider temporal acceleration in solid-body mechanics?
I am a beginner in physics, and I was studying fluid mechanics, specifically Newton's second law, when I was surprised to find that the expression for acceleration was composed of both local and ...
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92
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Why can I hear when my shower gets hot?
I noticed recently that when I turn on my shower, I can hear the moment that the water heats up. The hissing sound gets slightly louder and its pitch changes. The pressure doesn't noticeably change ...
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Is the Mach cone the model of a shockwave?
I am in the process of writing a mathematics paper which aims to model the interaction of a supersonic (or transonic for that matter) object with a flat plane. I need to determine the shape that a ...
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1
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45
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Fluid dynamics in Newtonian regime from action
I struggle to understand how one can get the Euler, continuity and Poisson equation describing a fluid in the weak gravity regime from the Einstein-Hilbert plus fluid matter action. Say I start from
$$...
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1
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1
answer
32
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Why can't choked flow accelerate?
Why can't flow accelerate in the choked condition?
I think the best way to explain my question is through an example, so here it is:
Imagine you have $2$ boxes connected with a valve that is closed. ...
- 355
1
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0
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35
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Frank White's Fluid Mechanics pressure work derivation
So I have been trying to get my head around the derivation given by Frank White in his Fluid Mechanics book on pressure work (page 164,. He states that for mass going into the control volume, $$d\dot{...
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0
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37
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Explosions in high viscosity liquids
I was watching The Score (2001) in which De Niro manages to make a hole in a cobalt and titanium infused steel safe. he fills it with water. inserts an explosive and boom! the safe is open.
It made me ...
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1
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22
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Displacement thickness derivation in the Von Kármán momentum integral
The Von Kármán momentum integral is
$${\displaystyle {\frac {\tau _{w}}{\rho U^{2}}}={\frac {1}{U^{2}}}{\frac {\partial }{\partial t}}(U\delta _{1})+{\frac {\partial \delta _{2}}{\partial x}}+{\frac {...
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35
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Evaporation of water due to vacuum aspiration?
In my research, I have encountered the following problem:
I have a vacuum pump that I would like to use aspirate water from below an elastic surface. The water resides under the elastomere and a tube, ...
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1
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0
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113
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Finding the shape of the surface of water within a spinning bucket with a leakage at the bottom of the wall
Finding the shape of the surface of water within a spinning bucket with a leakage at the bottom of the wall
Let think in the classic example of a bucket half full with water that start spinning at a ...
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1
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0
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18
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What factors and forces affect the velocity of volcanic emissions in the atmosphere?
Niche question for a niche subfield of Physics. While exploring the Sulfur Dioxide ($SO_2$) data from the Geostationary Environment Monitoring Spectrometer (GEMS), I noticed that during the moment ...
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1
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56
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Smoke simulations and how to render them
I recently began a real time smoke simulation system, and I need some help with the rendering. I didnt know if to ask here or on the game development stack exchange, but I decided here as it is ...
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0
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52
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Books about analytical fluid dynamics
I am looking for literature about analytical fluid dynamics and by analytical I mean analytical mechanics (Lagrangian, Hamiltonian,...). On the general reference list I couldn't find that topic. Also ...
1
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0
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36
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Why can blowing compressed air through a vertical glass tube suck up small objects from the tube?
I saw a video on a famous website about a vertical glass tube. When a compressed air nozzle is used to blow the upper end of the glass tube, small objects at the bottom of the tube will be sucked up ...
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1
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68
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Fluid dynamics (Applications of Bernoulli's equation)
Let's do a thought experiment. Suppose I have an infinite tube of diameter $2d$ placed horizontally, and I put a piston of diameter $d$ in it and filled it with an ideal liquid (i.e., coefficient of ...
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0
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86
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What will the pressure of accelerated air be relative to the pressure that accelerated it?
Imagine you have a high pressure reservoir filled with air. This reservoir is $1 \,\text{psi}$ above ambient atmospheric pressure. There is also a valve attached to the reservoir.
Now, open this valve....
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1
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0
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108
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Potential function of a flow around a stagnation point
Following the Steve Brunton lecture about the potential flow. It is possible de find the potential function by solving the Laplace's equation. In his first example (the same as the following picture) ...
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1
answer
107
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Mathematical explanations for the difference between blowing & sucking
It is obvious that there is a big difference between blowing & sucking fluid into/from relatively large volume using a small opening. Blowed fluid keeps a coherent stream, on the other hand, ...
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1
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0
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94
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How the heck do liquids work?
Liquids are denser than gases with similar molecular weight, which seems to indicate a strongly attractive intermolecular force; on the other hand they have a negative Joule-Thomson coefficient which ...
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Why is there no Betz type limit for fans?
Betz's law says that only 16/27 of the power in a wind stream can be converted into electrical power by a wind turbine. However, it appears that 100% of the power supplied to a fan motor can be ...
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0
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39
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Power Number of Impellers in non-Newtonian fluids
I have been working on a project involving mixing a non-Newtonian fluid.
To begin with, the fluid exhibits a shear-thinning behaviour that follows the power law:
$$\tau = K \dot{\gamma}^n$$
As for the ...
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48
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Non-linear Eigenvalue problem with an ODE to solve for the Linear Stability Theory of a Boundary Layer
I am working on the Linear Stability Theory (LST) for analysing the stability of a boundary layer in fluid mechanics. I have conducted CFD simulations and extracted the base flow data. To form the LST ...
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56
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Why does the energy density of a conformal field theory scale as $T^4$ in $3+1$ dimensions?
I'm trying to understand hydrodynamics of relativistic CFTs. A paper I'm referring to is this article published in PRL by Itzhak Fouxon and Yaron Oz in 2008.
The paper states that hydrodynamics ...
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0
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50
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Does the isotropy definition of a perfect fluid imply no heat conduction?
Weinberg defines a perfect fluid (Chapter 2, Section 10) as one where each fluid element appears isotropic in a reference frame moving with that element.
From the definition of the stress-energy ...
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0
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37
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Wave interference and fluid dynamics question
I'm curious to find an answer to this question:
In the college canteen I would often slide my Styrofoam cup of milky coffee across the melamine table top. (unlike paper or plastic, Styrofoam is a ...
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0
answers
47
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Add pressure gradient to Falkner-Skan (FS) boundary layer (BL) equations and solve numerically for arbitrary $P(x)$
I have read quite a few tutorials / watched several clips on the derivation of the Falkner-Skan boundary layer equations using similarity and then solution using RK solvers such as ...
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0
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Bubble behaviour during water / vapour coexistence in the most idealised scenario without boundaries
Consider the following statement: during a 1st order phase transition, the temperature of the system stays constant and any extra heat goes into turning a larger portion of the system into the new ...
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In turbulent flow, where is the friction of the pipe exactly? Is it between the laminar layer and the pipe walls or the turbulent layer and laminar?
In turbulent flow as we get closer to the pipe walls we encounter an overlap layer and then a layer of laminar flow.
Also at the walls of a pipe, in turbulent flow, the laminar shear stress dominates. ...
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57
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How to conceptually explain diagonal elements of viscous stress tensor in a fluid?
How can the diagonal components of viscosity tensor be explained conceptually? In other words, how can viscosity affect the normal stress (pressure) on the cube element of the fluid?
$$\sigma_{ii} = 2\...
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Output of ventilator and air friction
Imagine you have a ventilator with straight rectangular ventilator blades which are rotated at some small angle from a perpendicular line to the rotating axis. The ventilator has 100% efficiency (all ...
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36
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Is this assumption about a vortex's angular velocity reasonable?
I am deriving a velocity flow function $\psi(r,t)$ that could be derived by (1) establishing the relation between two vortex area functions, $a(t)$ and $A(r)$, using the disk method of integration, ...
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1
answer
34
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Relationship between density and temperature - How to know the precise temperature at which a bell inside a Galileo Thermometer will sink?
A Galileo Thermometer consists of bells placed inside a tube that's filled with liquid. As the temperature increases, the fluid density decreases. This leads to a decrease in the buoyant force, and ...
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22
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What is the vortex pattern when the flow is passing an axis-symmetric vortex flowmeter?
It is wellknown that in 2D case, the vortices will be formed at the back of the bluff body and detached periodically from either side of the body in a pattern called Kármán vortex street. But what ...
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137
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How fast does a gas expand in vacuum?
A gas is initially confined to a small portion of an empty container.
At time $t=0$ gas is allowed to expand to homogeneously occupy the container after some time $t$.
What is the time $t$?
This ...
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65
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Finding formulas for molar internal energy and molar enthalpy of mixing fluids
There is a Y-shaped pipe connection. One fluid flows through one pipe with molar flow $\nu_1$, molar internal energy $U_1$, molar enthalpy $H_1$; through the second pipe - another fluid with molar ...
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22
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Measurement of pressure using pitot tube
We know that pitot tubes are used in measuring velocity of a fluid or dynamic pressure. Now,in stagnation point which is basically the entrance of the pitot tube,the fluid particles are brought to ...
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