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

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What is the different between Poisson Nernst–Planck and Poisson Navier–Stokes in the Electrolyte?

I am now studying about electroosmosis flow phenomena which is governing by the Navier-Stokes equation and Poisson equation. By combine these two equation we can describe the electrolyte flow velocity ...
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17 views

Breakup of liquid jet hitting another liquid surface: Droplet size?

If a jet of liquid #1 impacts the horizontal surface of liquid #2, liquid #1 will break up into dispersed droplets below the surface of liquid #2. (The liquids are immiscible.) Is there a method for ...
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45 views

Surface energy of water [closed]

If $10^3$ small drops of water each of radius $10^{-7}\,\mathrm{m}$ combined to form one single large drop then what should be the energy ? If the surface tension is equal to $0.07\,\mathrm{N/m}$ . I ...
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32 views

The validity of constitutive diffusive fluxes

In transport phenomena the diffusive fluxes for mass, energy and momentum are the constitutive laws: $$\boldsymbol{j}_c=-D\boldsymbol{\nabla}c \quad \boldsymbol{j}_T=-k\boldsymbol{\nabla}T \quad \...
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46 views

Plateau–Rayleigh instability for liquids with low $Re$ number

I'm working on a project about Plateau–Rayleigh instability for liquids. But I've a question. we can examine the fluids with high $Re$ number that the influence of viscosity is negligible, but what ...
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33 views

How fast does water have to be moving for it to gain a lot of heat? [duplicate]

A random question popped into my head a few minutes ago: objects, when moving fast enough, start to get resistance from the air. Since there is friction between the air and the object [water], there ...
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2answers
374 views

Why does the pressure of fluid increase when the diameter of the pipe increases?

Assuming mass flow rate doesn't change and it's a closed system. Why does going from a smaller pipe to a large pipe cause an increase in pressure? What is the kinetic energy converted to as the ...
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90 views

Barometer barometric pressue

the barometric pressure and height on earth are 10^5 pa and 760 mm respectively . Then what is barometric height if it is taken at moom . I think at moon the gravity will decrease then pressure ...
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74 views

Rate of flow of water fluid mechanics

If we have a cylindrical tank of area $A$ which has a small outlet of area $a$ at the bottom of the surface . And the container starts filling with a constant rate $k [m^3/sec]$ . Now the max. level ...
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36 views

Aircraft lift theory vs energy conservation [closed]

I spent years trying to find out that when an aircraft flies or anything works it consumes energy more than it delivers back, means no perpetual motion exists. My query is that when an aircraft is at ...
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45 views

Pressure difference along horizontal in accelerated fluids

In accelerated fluids, fluid in a container can orient itself in a direction due to acceleration. In that case, pressure at different heights (at the surface) is same (atm). Then at the same height, ...
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244 views

Torricelli's speed of efflux law

Torricelli's law states that if we make a hole in a container of fluid open to atmosphere, then the speed of the fluid spouting out is: $$\sqrt{2gh}$$ Does this velocity correspond to the horizontal ...
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72 views

Why does my coffee stop?

As I was mixing my coffee this morning, wishing that it would do it on its own, I started wondering: What makes my coffee stop swirling? I mean what exactly? Is it the walls of the cup or the coffee ...
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53 views

How can pressure and velocity fluctuations in acoustic plane waves be in phase and still hold to the B.C. that velocity must be zero at a solid wall?

In linear plane-wave acoustics (no mean flow, small perturbations, etc.), it is often derived that the phase of a traveling pressure fluctuation wave and a fluctuation velocity wave are the same. For ...
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60 views

What is an intensive property of a fluid?

Let's assume we are considering a fluid which occupies the bounded domain $\Omega_t\subseteq\mathbb R^d$ at time $t\ge 0$. Let $c\in\Omega_0$ be a particle of the fluid and $$x_c:[0,\infty)\to\mathbb ...
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35 views

Does *advection* describe the change of density of massless infinitesimal tiny *thingies* injected into a fluid?

I'm considering an incompressible Newtonian fluid with uniform density and try to figure out what's meant by the term advection. Let $\Omega_0\subseteq\mathbb R^d$ be an (infinitesimal small) bounded ...
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52 views

why is pressure more convenient than force while dealing with fluids? [duplicate]

If two equal and opposite forces act on a surface, can I say the pressure is zero because the net force is zero? How can pressure be non directional when force as a direction? How can the pressure of ...
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18 views

Effect of heating a fluid

When a liquid or a gas is heated from below, the fluid terms to organise itself into cylindrical rolls. Can anybody help me in reasoning why so?
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567 views

Rayleigh-Plateau instability and optic [closed]

Let a liquid jet fall onto a surface. If the contact point is illuminated by a lase beam, rings of light around the jet can be observed . (pic.1) and we know it that the reason of this phenomenon is ...
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24 views

Pressure Change in Supersonic Wind Tunnel [closed]

As far as I know, the total pressure changes across a shock wave. How is this possible if you are in the atmosphere and atmospheric pressure isn't changing? Is energy lost due to entropy in ...
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32 views

Pressure Variations due to Flow in an Inviscid Fluid

Let's say we have an irrotational, inviscid fluid with steady flow. Then the Bernoulli function (ignoring gravity) $$H=\frac{p}{\rho}+\frac{u^2}{2}$$ is constant everywhere. My notes then make a jump ...
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63 views

How do fluids behave in space?

In outer space under absence of any external force (like gravity) can a free liquid develop pressure due to cohesive forces? Will the liquid be in spherical shape and will pressure increase towards ...
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55 views

Rate of applied force versus material failure?

Question: Does the rate at which a force is applied to an object determine the maximum force reached that mechanically fails the object? If so, what are the concepts that need to be understood to ...
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35 views

Period of small oscillations of liquid in a bottle

I have seen that water in a bottle of water when perturbed invariably starts oscillation back and forth, with the shape of the water surface remaining intact. I was wondering if there was any way to ...
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51 views

Fluid flow around a cylinder

For an irrotational, incompressible fluid we can solve Laplace's equation for the velocity potential in a fluid in order to obtain the velocity field. This can be done for flow around a cylinder to ...
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88 views

How to calculate the speed of a jet of liquid coming out of a nozzle? [closed]

I'm working on a project and i need to calculate speed of a jet falling from a nozzle. But my question is that, is it correct to use $$v=\sqrt{2gh}$$ Or should the speed of the jet be independent ...
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78 views

The Optics of the Vortex (in Water): why there is a bright ring also in reversed flow?

This question is closely related to this one; The optics of vortex (in water): why there is a bright ring? In this case "physics girl" gives somehow an plausible explanation, and it's only debatable ...
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103 views

How does a pop pop boat work?

I have been reading about the pop pop boat on wikipedea. What I don't understand is after the hot water is ejected the vacuume gets created, so when the same amount of water will be sucked in the boat ...
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38 views

Fluid Dynamics - How does air behave out a window?

I often smoke out a window, and I know that if you're driving a car, the passing air will affect the pressure inside and "suck" out the smoke for instance. But what if you're in a window pane? I have ...
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40 views

Do liquids creep from cold to hot like gases?

Consider the U-tube water experiment below with the left red block at 10C and the right red block at 90C. I think the right level will become about 3% higher than the left level simply because hotter ...
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185 views

Rotation of fluid filled cylinders [closed]

Given two cylinders of same mass , one completely made out of solid metal and the other a hollow one which has been filled with a viscous fluid say oil. Both have the same mass.Now if they are placed ...
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17 views

How to make glass surfaces with different hydrophobicity level?

I have a number of small glass beads (d= 2.0 mm) and I need to use a method to make the glass surface hydrophobic. How can I create glass beads with different wettability levels (i.e. various contact ...
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Self similarity for slowpokes

I am struggling with the concept of self similarity. If I google search, I either find examples that don't explain all too much or detailed explanations that go waaaaay over my head. I've found a ...
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148 views

Fluid filled harmonic oscillator

A vessel (preferably circular) filled with water is accelerating unidirectionally such that the level of water is higher on one end than the other. What I want to know is that if the vessel is ...
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36 views

Momentum equation in collision of a particle and a droplet

When a solid particle which moves with a constant velocity has a head-on collision with a water droplet, at the exact moment of collision, a sudden decrease happens in particle velocity, and then ...
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104 views

Explanation of the liquid rope coil effect

I was recently introduced to the liquid rope coil effect by this video: https://www.youtube.com/watch?v=zz5lGkDdk78 I noticed while the video mentioned the equations governing the motion, it didn't ...
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302 views

Pressure exerted by a liquid on walls of a container (quantitative)

I want to calculate the total pressure exerted by a liquid of density ρ on the walls of a container, say a cylinder, for convenience. $$P = hρg$$ Therefore integrating from $0$ to $h\; ,$ we get : ...
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68 views

Application of Bernouli's Theorem

We have to calculate the velocity of efflux in this problem. My issue is, when I apply Bernoulli about the open end of the fluid and a point just outside the orifice, I seem to get a different answer ...
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58 views

Stokes' law and settling particle speed

What are the forces that are exerted when a particle is dragged by air flow in a tube and settles within some distance? Does Stokes' law calculate the net force, taking into consideration of all these ...
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27 views

Lift provided to an aeroplane [duplicate]

We say that aeroplanes find lift due to an effect explained by Bernoulli's theorem. But isn't newton's third law of motion applicable along with Bernoulli's theorem? otherwise, an aeroplane wouldn't ...
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How to calculate the time it takes water to travel 100' [closed]

100 foot of pipe(lets assume it is straight) Diameter of pipe 2" Flow rate at point of entry into pipe and exit 50gpm Temperature of water 100F 50 psi Or stated another way if I was able to ...
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30 views

Is a creeping flow with $u_r \sim \ln r$ physically possible?

I was wondering if it is possible to have a 2D cylindrical flow where the radial velocity scales with $ln (r)$. I understand that a flow with $u_r \sim 1/r$ corresponds to a line source or sink. Also $...
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933 views

Why doesnt flow of liquid from a orifice depend on atmospheric pressure?

While calculating the velocity of efflux, ie, the velocity of a liquid when it flows from a orifice which is made in a beaker filled with liquid. Since at the orifice, atmospheric pressure acts along ...
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2answers
141 views

How does a rising bubble take a dome-like shape?

Since I have swam on the swim team for most of my life, I am very familiar with bubbles. I know a raindrop falling through the sky gets its shape because it is the most aerodynamic shape, but how come ...
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142 views

General Definition of Steady State

According to many sources (including Wikipedia, Stephani&Kluge, D.J. Acheson) a steady state ist: In systems theory, a system in a steady state has numerous properties that are unchanging in ...
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232 views

Pressure and state variable: Difference between compressible and incompressible

My question: If I am simulating compressible fluids, then pressure is a "state-variable" - if incompressible, then not. Could anyone explain the difference :)? Thank you very much.
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kinetic energy of flowing fluid

When a quantity of fluid (let's say equal to volumetric flow rate Litre/s) collides during a second with an object, do all the molecules of this quantity collides with it? I am asking this question ...
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254 views

Why sphere minimizes surface area for a given volume?

I was studying surface tension recently. Rain drops or bubbles of any kind which form are always of a spherical shape. This is because the liquid tries to minimize the surface area as the molecules ...
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Need to know height of wave generated by spherical object falling into water [duplicate]

Working with an architect at present.I need to know the height of wave/ripple that would be created by a spherical object weighing approximately 10 tonnes and measuring 42 metres in diameter falling ...
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Why do particles in the molten wax near a burning candle wick get pushed away?

Have a look at this clip I made of my candle burning: I can accept that the molten wax gets sucked into the wick (though not quite sure why: capilary action?). But why does it also seem to get ...