All Questions
1,857 questions with no upvoted or accepted answers
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Problem of fluid velocity and pressure drop inside a horizontal pipe, and an explanation I found myself
I'm a college student majoring in chemical engineering and I was simulating a pipe design using a versatile plant designing and chemical engineering tool, Aspen Plus.
The thing I wanted to ask a ...
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3
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37
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A question in the book of Frisch&Kolmogorov at Chapter 4.5
in the book of Frisch’s Turbulence (http://users.uoa.gr/~pjioannou/mechgrad/Frisch_Turbulence.pdf), at chapter of 4.5 The spectrum of stationary random functions, as following picture shows, i have a ...
- 183
3
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141
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Are the force frequencies the same in the discrete and continuous case?
Let's say I have an ideal gas of particles and the strike zone of area $\sigma=\pi d^2$ of a molecule of the molecule is what is the sub system considered. The end of the face of the cylinder is the ...
- 1,349
3
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1
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Does the distribution of velocity matter in the fluid approximation?
So for a perfect fluid in General relativity, my impression is the average velocity only matters and the distribution of velocities does not in the fluid approximation (otherwise I would have some ...
- 1,349
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What are hydrodynamic modes?
What are hydrodynamic and non-hydrodynamic modes in a fluid? Apparently, they are connected with the analytic structure of the correlation function of conserved quantities. I need a pedagogic ...
3
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89
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Burning a match in a one-exit tunnel (or Heavy Rain trial of the butterfly scene)
If you light a match in a narrow labyrinth tunnel where there is only one way the air is coming from, will the flame of a match bend in that direction?
In other words, I want to know if the trick one ...
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88
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Why is floating debris above a vortex stationary with respect to the container?
I noticed the phenomenon that a spinning vortex with an initially corotating surface (as identifiable from the movement of floating debris or bubbles in it) appears to quickly stop spinning when ...
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80
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Is it possible to view the GR metric as a fluid in Newtonian physics?
I'm just wondering, are there any existing formulations of General Relativity where instead of thinking about objects following geodesics in spacetime, they are instead objects sitting inside a "...
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51
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How does a plate stick to a tablemat?
So at my dinner table, I witness this weird phenomenon almost everday.
A wet plate if placed on the table mat sticks to it(works better if the plate is made up of steel).
I account surface tension and ...
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3
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1
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594
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Is the velocity in parallel pipes the same as a single pipe according to Bernoulli's equation?
This system pushes water through a nozzle. I can use Modified Bernoulli's equation to calculate the nozzle velocity as such:
$$V_3 = \sqrt{2g\left(\frac{P_1}{\rho g}+ z_1-\Delta h_L\right)}$$
where ($\...
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Shower power -- I can get a more effective rinse by removing a flow restrictor. Have I increased water pressure? What principles apply here?
I watch a plumber on social media; he explains how to do simple fixes. Sometimes his customers want more "pressure" coming from the shower head; when they feel like it's just a "trickle&...
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1
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Pressure term in Bernoulli's principle
I have encountered two different explanations of why the pressure term appears in the equation for Bernoulli's principle. The first one is:
(1) "The fluid has to speed up as it enters the ...
3
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0
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153
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Water flowing across an inclined plate
There is a blue plate between the two black plates and the water flows between the plates in the direction of the red arrow. Is the flow velocity greater at the top of the blue plate or at the bottom? ...
- 2,165
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Is it valid to add energy densities of *interacting* perfect fluids?
In several papers on interacting perfect fluids in cosmology, the authors assume that we still can add the energy densities and pressures of the individual fluids, as if there wasn't any interaction:
\...
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Can turbulence be predicted from entropy?
Frozen water has lower entropy than liquid water.
This is true for most solid/liquid transitions.
Clearly a solid can't flow with turbulence ( although glass does flow over many years as can be seen ...
user291777
3
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Geometrical acoustics derivation
In acoustics for non-uniform media (speed of sound is $c(\mathbf{r}, t)$, dependent on space and time), I want to solve
$\frac{\partial^2\phi}{\partial t^2}-\nabla^2(c^2\phi)=0$, where $\phi$ is the ...
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3
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407
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Is there an equation depicting a compressible fluid flowing into a cone, increasing in pressure and velocity as the cone narrows?
I'm trying to write an equation which shows a compressible fluid entering a cone with radius R and length L, where the pressure (density) and velocity of the compressible fluid increases as the exit ...
- 287
3
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365
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Normal Viscous Stress (Newtonian Incompressible Flow)
I have been struggling to understand why normal viscous stresses are often neglected in elementary fluid mechanics analysis (see examples below). Usually, the justification is something in the lines ...
3
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79
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Implications of thermodynamic inconsistency in CFD calculations
During my PhD work I had to use tabulated values of thermodynamic properties of gases in some Computational Fluid Dynamics (CFD in short) simulations.
CFD simulations consist in the numerical solution ...
- 67
3
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90
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Dominant component of drag in a streamlined body (sounding rocket) at 0.8 Mach
We are currently working on CFD solutions on ANSYS Fluent for the drag on our sounding rocket. We are modeling its drag profile through out its velocity range. We currently hit a top speed of approx ...
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145
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In fluid mechanics, why are the forces on the fluid the same as the forces on the control volume?
From the momentum equation of fluid mechanics:
$${\frac{\partial}{\partial t} \int_{V_f(t)} \rho \vec{v} \text{d}\text{V}} = \int_{\sum_f(t)} \vec{\bar{\bar{\tau}}} \cdot \vec{n} \text{d}\sigma +
\...
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3
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256
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Why does a spinning disc turn before it fades?
I have recently gotten into disc golf, and an interesting phenomenon happens when you throw a disc hard. If you throw a disc right hand back hand, so that the disc spins clockwise, at high velocities ...
3
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Will the beaker be acted upon by the force in the direction of the green arrow?
The black circle is the top view of the beaker. In the gravity field, the beaker is vertical down along the beaker axis. The beaker is still. The water in the beaker rotates at a uniform angular speed ...
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3
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109
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If a hollow sphere connected to a rope and half-filled with water forms a physical pendulum, what does the water surface do?
Suppose I make a pendulum consisting of a long string connected to a hollow sphere, then fill the sphere half way with water and set the pendulum in motion by giving all the water and the sphere some ...
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109
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Why is hydrodynamics a diffeomorphism invariant theory?
The fact that general relativity is a diffeomorphism invariant theory means that there is no preferred co-ordinate system in GR. How is it possible to understand this in the context of relativistic ...
- 91
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149
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Characteristics of the Navier-Stokes equations as a set of PDE's
I am not entirely sure if I should ask this question here or not, but here goes: can anyone suggest any reference (book, article, etc.) about the Navier-Stokes equations from a mathematical point view?...
3
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1
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4k
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Can you explain the perfume bottle with the Bernoulli effect?
Someone explained the perfume bottle with the Bernoulli effect:
squeezing the bulb over the fluid creates a low pressure area due to the higher speed of the air, which subsequently draws the fluid up. ...
- 2,165
3
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75
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Dimension reducing of heat equation 2D problem
I have two metal objects of different thickness and conductivity:
1) thicker, but poorly conductive. It's properties: $k_1, H$
2) thin and much better conductive. It's properties: $k_2, h$
...
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54
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Boltzmann transport equation for granular gases
I am researching about granular gases and their collisions, and have come across this Boltzmann transport equation:
$$\frac { \partial f ( v ) } { \partial t } = \iint d u _ { 1 } d u _ { 2 } f \left( ...
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219
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Fluid mechanics, symplectic structure, the Hamiltonian, and vorticity
Consider an inviscid irrotational fluid in two dimensions. There are some explicit connections with symplectic geometry that I do not understand. I am not well versed in the later topic, so please ...
- 1,706
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535
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Swimming pool skimmer floating weir forces
How would I calculate the forces required to open the weir gate depending on the depth of the water? The pump is an impeller type, I want full flow to the pump. I assume there are also forces from ...
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Is there any use of a quadratic equation of state in FLRW cosmology?
Consider standard FLRW cosmology. Usually, the relation between energy density $\rho$ and pressure $p$ of a cosmological fluid component is linear:
\begin{equation}\tag{1}
p = w \, \rho,
\end{...
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205
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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, ...
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60
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Minimal drop size
I am trying to do something that requires very high precision, so I got stuck in a question:
What is the lower bound of a size of a water droplet (and what is the equation, I want to use different ...
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256
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Derive mechanical pressure from strain energy density
Similarly to thermodynamic pressure, when we have:
$$P_{therm} = (\frac{\partial U}{\partial V})_{S}$$
Can we define the mechanical pressure for a fluid as:
$$P_{mech} = \frac{\partial \psi}{\...
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60
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Good resource to "deduce" bathtub vorticies from navier stokes?
I am a mathematician that is interested in how physicists justify the creation and persistence of vortices (such as bathtub vortex or tornados) from Navier Stokes.
The explicit solutions I have come ...
3
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1
answer
109
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Coupling Navier-Stokes and stochastic models for particle tracking in micro-scale free convection?
I have been using a commercially available software to simulate laminar free convection in a specific small domain (let's use channel w/ heated lower wall as an example). The scale is approx 50-100 ...
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188
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Surface fluctuations in the combustion of ethanol in a woosh bottle
A whoosh bottle demonstration typically involves igniting ethanol vapor in a large bottle with a narrow opening. The rapidly expanding gas creates the whooshing noise as it vacates the container. This ...
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Water vapour behaviour during hypersonic expansion
I have a question about the behaviour of air with a small mass fraction of water vapour at extremely low pressures and temperatures.
I am currently running experiments in a large vacuum facility that ...
- 131
3
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1
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672
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Confused about applications vs misconceptions about Bernoulli principle
I've been reading on wikipedia that the Bernoulli principle is often incorrectly applied to the example of blowing on pieces of paper because
A third problem is that it is false to make a ...
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126
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Two phase mixing by virtue of high Reynolds number alone
Consider a long horizontal tube into which I inject two immiscible fluids. Say, water and benzene. If one increases the pumping velocity i.e. flowrate / Reynolds number would one get intimate mixing ...
- 131
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141
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Power in a wind turbine
An wind turbine starts turning by a wind speed of $v_b$ and it stops turning (rated wind speed, to prevent overloading) by a wind speed of $v_g$. By wind speeds between $v_b<v<v_g$, the power ...
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144
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Time scale turbulence flow -distance between two particles
I want to estimate the time interval, $\tau$, during which the original distance $L$ between two fluid particles become much larger. I assume that the motion takes place in the inertial interval.
I ...
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How are boundary consitions implemented correctly in time dependent hydrodynamics?
I posted this question more than one year ago and got an answer recently. This answer looks good to me, but indicates that something is wrong in my original approach to the problem.
Can someone tell ...
- 4,415
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84
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Physical Meaning of Over- / Underdetermined Acoustic Eigenvalue Problem
I am performing an eigenmode study on a system of acoustic ducts. The system consists of two large cylindrical volumes connected by several smaller cylindrical volumes (modeling a combustion chamber ...
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89
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Why would oscillation of a gas column inhibit rather than promote mixing?
Many years ago I helped to support an experiment conducted in Japan which investigated the effects of high frequency oscillation ventilation (HFOV) on the mixing and distribution of gas into the lungs....
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101
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A question about droplets formation
This picture shows a fuel ejector of a diesel engine that propels ships.
When the fuel line pressure exceeds 380 bars, the tension of the spring is overcomed and fuel is ejected into the combustion ...
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Backing pressure for a turbo-molecular pump
It is known that turbo-molecular pumps are to be operated at already low levels of pressure. So the turbo is always backed by another pump (usually a scroll, rotary pump) that ensures this. One should ...
- 25.3k
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How does air going through silica gel change its humidity
Assume that the properties(temperature,humidity,velocity and so on) of the incoming air flow stay constant. The air flow goes through a silica gel plate. Humidity of the air will decrease, but how to ...
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Origin of spin and direction in the magnus effect
If you solve the Bernoulli equation:
$$p=p_0-\rho_0{v^2 \over 2}$$
using a complex flow potential for a flow around a cylinder:
$$W(z)=v_0 z + {v_0 R^2 \over z} - {\Gamma \over 2 \pi } \ln(z)$$
you ...
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