The quantitative study of how fluids (gases and liquids) move.

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What causes acceleration of particles in the expansion section of a De Laval nozzle?

A De Laval nozzle has a compression section, where the propellant is compressed (and thereby accelerated) as it moves towards a narrow section (the throat). After the throat, the nozzle widens out ...
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0answers
154 views

Conformal symmetry of Navier-Stokes?

This question is in reference to the paper arXiv:0810.1545 Can someone help understand this scaling argument and the proof(?) that there is a conformal symmetry in Navier-Stoke's equation? (..am I ...
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0answers
196 views

Wall pressure of a fluid flow in a pipe of variable radius

Using all cylindrical coordinates, pipe with z-axis vertically upward and radius of $r = G(z)$, flow is incompressible, inviscid and steady, Using appropriate boundary conditions I want to find the ...
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2answers
417 views

Stripeless cleaning of windows

Cross post: http://chemistry.stackexchange.com/q/4377/22 Last week I was discussing with a friend how we thought the stripeless cleaning of windows is achieved when using a cleaner like Windex ...
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1answer
2k views

Drag on a spinning ball in fluid

I am a physics newbie (high school level) and I am wondering what happens when a spherical object is spinning on the spot in a bunch of gas (no gravity here, just an imaginary physics sandbox). Am I ...
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1answer
563 views

Pressure loss in a syringe

I'm currently working on a problem which is really giving me some issues. The problem concerns the force required to expel water from a syringe. We have a 20 ml syringe (which is $2\times10^{-5}$ ...
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2answers
158 views

Flux Over a Surface

I am teaching a multivariable calculus course and we are starting to go over surface integrals. I am a math professor with little knowledge of physics. At one point the book discusses fluid flow. ...
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1answer
122 views

Buoyancy fluxes in a stratififed fluid and units

I am calculating the buoyancy flux ($B$) for a stratified fluid as follows: $$ B=\frac{g\alpha S}{C_{pw}\rho_0} $$ where $g = 9.81$ $m/s$; $\alpha = 1.6 t\times10^{-5} + 9.6\times10^{-6} \times (20 ...
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3k views

What is the mystery of turbulence?

One of the great unsolved problems in physics is turbulence but I'm not too clear what the mystery is. Does it mean that the Navier-Stokes equations don't have any turbulent phenomena even if we solve ...
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2answers
362 views

Proof that flux through a surface is independent of the inner objects' arrangement

$$\Phi=\iint_{\partial V}\mathbf{g} \cdot d \mathbf{A}=-4 \pi G M$$ Essentially, why is $\Phi$ independent of the distribution of mass inside the surface $\partial V$, and the shape of surface ...
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2answers
5k views

Determine viscosity using falling sphere (Stokes Law, Ladenburg correction)

Introduction I am trying to determine the viscosity of a fluid. Therefore, I let a sphere of known mass m and radius r fall ...
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2answers
475 views

Whirlpools and Tornados

This may not be a great question. But whenever you drain water, a small whirlpool happens, obviously. This got me thinking... Can we model tornados with this effect, would it even be beneficial?
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1answer
193 views

Bernoulli's theorem: $\frac{p}{\rho}+\frac{1}{2}u^2+\phi$ is constant along a streamline

I am trying to understand the Bernoulli's theorem: $\frac{p}{\rho}+\frac{1}{2}u^2+\phi$ is a constant along a streamline I got that: $\frac{\partial u}{\partial t}$ + ($\nabla \times u)\times u$ ...
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1answer
2k views

how to determine if a vortex is laminar or turbulent

In a cylindrical chamber with a high diameter-to-height ratio; a fluid is tangentially injected. there is an axial exit to the cylinder. how do I determine if the vortex so formed is laminar or ...
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2answers
412 views

Pumping water from a closed column. Will the column get empty or just circualte?

As a picture is worth a thousand words, here is my problem: It is a closed water column with a bit of air in the top section. If I run the pump and make the water flow from IN to OUT, Will the ...
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1answer
306 views

What does Euler equation mean?

In order to prove the Bernoulli’s principle ($\frac{p}{\rho} + \frac{1}{2}u^2+\phi = constant$ ), I have to use the Euler equation: $\frac{Du}{Dt} = -\frac{1}{\rho}\nabla p + g$. I know how to prove ...
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0answers
126 views

Difference between a Fixed Point and a Limit Point in implementations of the Renormalization Group (RNG) in Large Eddy Simulation (LES) model

In the introduction of this paper, it is explained that and how the application of a dynamic subrid scale model for turbulence into a large eddy simulation (LES) model corresponds to doing one ...
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2answers
438 views

Membrane that allows liquid to pass only when forced under pressure

Is it possible to have a membrane that will not let a liquid through it at normal pressures due to gravity, but pass that liquid when substantially pressurised? For instance, a few inches of water ...
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1answer
280 views

Water, how do you make ripples

How do I make a ripple effect in a bowl of water. I have tried throwing small pebble sin but it just splashes and sinks. I have read that I need to make the water a thicker liquid ie add glycerine, ...
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1answer
432 views

Flow in parallel paths after pressure regulator

Consider the following: ...
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0answers
71 views

Water Stream from a Horizontal Surface

If water was projected from a flat surface where gravity was equal all over the surface. What would happen when the water fell in on itself? The water is in a continuous stream and is perfectly ...
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1answer
1k views

Wave propagation in an incompressible flow

Incompressible approximation of fluid flow is usually known to be lame in modeling the propagation of any disturbance in it, predicting a speed equal to infinity for the propagation of the ...
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1answer
89 views

Query about Bernoulli's principle

We know that the lower atmosphere has high pressure and as we go up, the pressure decreases, if it's so then why doesn't all gases fly up into the upper atmosphere from the lower following Bernoulli's ...
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1answer
107 views

Explanation for the next steps of lamb-chaplygin dipole

Can someone explain me the steps please (I mean steps 2 and 3)? I didn't understand it. 1) Any flow field represented by a stream function $\psi$(x,r), defined as: v = - $\frac{\partial ...
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3answers
833 views

Effective angle of attack of a wing

Is the angle of attack of a rectangular wing influenced by the dihedral ($\Gamma$) of the wing? If e.g. a wing exists with $\Gamma$ = 0$^{o}$ it has angle of attack $\alpha$. If the dihedral is set ...
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0answers
490 views

Can a divider “laminarize” turbulent flow and thus reduce friction?

Looking at the Moody chart I think to myself, the friction factor doesn't decrease much at all with Reynolds number after a certain point. I wonder if laminar flow is more efficient in a sense, and ...
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4answers
5k views

What's the surface area of a liquid? How does evaporation increase if the surface area of a liquid is increased?

Wikipedia says that a substance that has a larger surface area will evaporate faster, as there are more surface molecules that are able to escape. I think the rate of evaporation should decrease as ...
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2answers
3k views

Whistle Physics

I'm looking for a simple explanation of how a whistle operates. I know that forcing air over a sharp lip can set up a wave in a resonating cavity, but how? "Most whistles operate due to a feedback ...
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1answer
232 views

Measuring acceleration of a bus using water between two sheets of glass

I was riding a bus one day and noticed that the double windows had some water between them. As the bus accelerated, the water collected to the sides, first forming a trapezoid and then a right ...
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1answer
135 views

Calculating the dimensional wall-normal coordinate for a self-similar compressible boundary layer using Levy-Lees transformation

How can I convert my self-similar boundary layer solution that is a function of the nondimensional wall-normal coordinate $\eta$ to be a function of dimensional $y$? For instance, if I determine from ...
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3answers
264 views

Unclear how heat interacts with Navier Stokes

I am playing around with an Navier stokes solver and I'm having trouble introducing heat. Am I right in thinking this would be introduced in the ${\bf f}$ term of ${\partial{\bf u}\over\partial t} = ...
13
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4answers
708 views

Why does string not wick down?

I regularly drink tea at work and I often reuse the tea bags (yes I know I'm a cheapskate). Yesterday afternoon I used a tea bag once and kept it in the cup in case I wanted another cup before I ...
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2answers
394 views

continuity equation for a fluid with variable density

I am trying to derive the equation for buoyancy frequency in a stratified fluid and am struggling with some of the equations. I have a limited background in fluid dynamics so I basically just need ...
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61 views

Is sonoluminescence relevant to the behaviour of Navier-Stokes (or converse)?

More precisely, could Sonoluminescence be a singularity of Navier-Stokes(NS)? Is there some other connection that might be interesting, or is it completely irrelevant? Wiki page mentions NS, but says ...
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1answer
92 views

Oscillations of a fluid particle in a stably stratified fluid

Could anyone help me work through the following set of equations: I found these online at http://marian.fsik.cvut.cz/~bodnar/PragueSum_2012/Staquet_1-2.pdf and I'm trying to work through them. I ...
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1answer
857 views

How to calculate pressure field in potential flow

I haven't ever studied fluid dynamics before and may mix something here, so please, be patient :). Given flow potential of the form (homogeneus flow over a dipole): $$ \phi = u_\infty x ...
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1answer
268 views

When moving something by entrainment in a fluid stream, where does the energy come from?

The particular Something I had in mind here would be air bubbles that are pulled downwards against their buoyancy by a stream of water falling down a shaft. The work required to push those bubbles ...
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1answer
270 views

In a column of rising hot air, is the velocity higher at the top?

Since the movement of the air is induced by buoyancy, i. e. there's a constant force acting on the air, so I would expect the velocity to increase during ascent, much like an object falling down due ...
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1answer
172 views

Sideways motion between a vertical launch from a planet and landing [duplicate]

I saw a video some days ago (Hello Kitty in Space) of a schoolgirl successfully launching a balloon into space which later popped and landed ~47 km from launch site. If I vertically launch an object ...
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1answer
802 views

How fast does water fall in the middle of a very very thick waterfall?

Let me create a very artificial experimental set up. Take a bathtub the size of Delaware and suspend it a mile above the ground. Fill it with water (though I'm not sure to what depth - and it might ...
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2answers
1k views

Water vs Milkshake being sucked through a straw

Consider water in a glass being sucked through a straw. The water rises up in the straw because of a pressure gradient introduced by the sucking action. Now, change the liquid from water to something ...
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2answers
2k views

How to estimate the Kolmogorov length scale

My understanding of Kolmogorov scales doesn't really go beyond this poem: Big whirls have little whirls that feed on their velocity, and little whirls have lesser whirls and so on to viscosity. ...
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4answers
10k views

What causes ballpoint pens to write intermittently?

After a while, a ball point pen doesn't write very well anymore. It will write for a little distance, then leave a gap, then maybe write in little streaks, then maybe write properly again. It seems ...
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2answers
3k views

What is enstrophy?

In turbulence, the enstrophy of a flow in a domain $\mathcal{D} \subset \mathbb{R}^{D}$ $$ \mathcal{E} = \int_{\mathcal{D}} |\vec{\nabla} \times \, \vec{v}|^2 d^{D}x $$ appears sometimes, it's cool ...
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1answer
362 views

Satellite droplets in a breaking liquid jet

The famous example of a dripping faucet is an example of a Rayleigh-Plateau instability in which there is a certain jet radius below which perturbations on the surface will grow to break the jet into ...
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3answers
5k views

A fly in an accelerating car

A fly is flying around in a car, the fly never touches any surface in the car only fly’s around in the air inside the car. The car accelerates. does the fly slam in to the rear window. or does the fly ...
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2answers
94 views

How does the correlation length of weather emerge?

The question is pretty simple: If I know the weather where I stand, I can estimate the weather 5 meters or 1 km away away pretty well, but I'll have a hard time guessing what the weather is, say, 50 ...
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3answers
350 views

A simple logical problem regarding pressure and force [closed]

Suppose a 2 meter long pipe, with area of cross section is $1 m^2$. The pipe is vertically placed, with the bottom end closed. Inside the pipe, at the closed bottom end, there is $1 m^3$ of air, ...
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1answer
382 views

Compressible fluid flow through a branched pipe junctions

Backdrop: Designing a dust extraction system (LEV) with a branched junction. What principles allow for the calculation of volume flow rate and pressure at the inlets if the volume flow rate and ...
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1answer
534 views

Best way to move teaspoon to dissolve sugar in a cup of tea

I know that there is asked questions about something like cooling down cope of coffee..etc, but this one is little different. My friend was watching TV program of English eating etiquette, and they ...