Tagged Questions

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

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Fluid Dynamics - Water Bottle Drink Mix: Air or No Air?

If you take an ordinary sized plastic water bottle full of water and pour a packet of powdered (or liquid) drink mix into it, Will shaking the bottle with the cap screwed on to dissolve the mix into ...
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2k views

Physics behind Water drops during falling from a tap

what is physics behind Water drops during falling from a tap. water drop animation A drop or droplet is a small column of liquid, bounded completely or almost completely by free surfaces. Why Water ...
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2answers
620 views

What would be an appropiate shape for a parachute?

I'm trying to design a parachute that minimizes the descent velocity, but I'm not sure what shape I should use. From what I've read, ellipse-shaped parachutes are too aerodynamic and minimize drag, ...
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1answer
812 views

Are coffee's properties different enough from water's to cause increased spillage while walking?

I recently found this article, which describes how... It just so happens that the human stride has almost exactly the right frequency to drive the natural oscillations of coffee, when the fluid is ...
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1answer
409 views

What is the particle residence time for given flow rates of gas mixture components?

For two different chemical substances there are two open valves with flow rates $$Q_1=a\frac{m^3}{h}\ \text{ and }\ \ Q_2=b\frac{m^3}{h},$$ leading into seperate cables. Next, the cables join, the ...
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253 views

How to compute the speed necessary for an airplane to fly?

I give some physics lessons to a friend. She asked me a question that I am unable to answer. Could you help me ? A plane has a weight of $2\times10^6$kg. The surface of the wing is $1200 \text{m}^2$. ...
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1answer
546 views

Change in appearance of liquid drop due to gravity

A liquid drop is spherical in shape due to surface tension. But why does it appear as a vertical line under the free-fall due to gravity? (E.g. During a rain - falling raindrop) Is there a specified ...
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1k views

Classical Limit of Schrodinger Equation

There is a well-known argument that if we write the wavefunction as $\psi = A \exp(iS/\hbar)$, where $A$ and $S$ are real, and substitute this into the Schrodinger equation and take the limit $h \to ...
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2answers
207 views

Why is there a wave on the water surface when I put something in the water?

We've started doing waves in our physics class, but we're doing things very quickly and the teacher doesn't explain anything. And I don't understand why waves work. I was thinking that when I fill a ...
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201 views

Reduction in velocity in pipe

I have two pipes divided from main pipe. If I add one more pipe, what would be the amount of velocity in each pipes. I know the velocity of main pipe, then diameter and radius of each pipe. Edit: 1) ...
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Why does the higher of two medical IV bags empties first?

I'm a nurse, so my physics background is about that of a lay person. Please explain on that level. Ordinarily an Intravenous (IV) fluid bag is one liter. An IV piggyback is usually 2500ml. usually the ...
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4answers
585 views

Shock speed in air/vacuum shock tube

Some of you are probably aware of What If, xkcd's blog about interesting physics problems. One episode, Glass Half Empty, concerns itself with what would happen if a glass of water is half water, ...
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Free fall of two spheres made of same materials, different masses, with air resistance

Two people falling with the same parachute will gain different speeds if their masses are different. The upward air drag will needed to be bigger for a heavier person, since gravitational force is ...
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3answers
735 views

Equations of fluid dynamics and differential geometry [closed]

Where can I look for equations of fluid motion written in terms of nifty things from differential geometry like exterior derivative, Hodge dual, musical isomorphism? Preferably both with and without ...
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4answers
602 views

Zigzag flow of water along a vertical glass window

I've observed this behavior many times. When it rains, the rainwater will form vertical channels along a glass window. The flow of water is mostly confined within these vertical channels and the ...
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2answers
559 views

Need help understanding dynamic and static contact angles

I've run into a conceptual road block. I'm coming to you guys because I think my adviser is getting annoyed with me. The concept involves a meniscus being pulled up a cylinder. I understand that ...
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3answers
7k views

What is the physical meaning of diffusion coefficient?

In Fick's first law, the diffusion coefficient is velocity, but I do not understand the two-dimensional concept of this velocity. Imagine that solutes are diffusing from one side of a tube to another ...
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4answers
431 views

Can ship/boat propellers be placed, with adequate protection, alongside the fuselage instead of at the back?

On small private aircraft the engine is placed at the front of the fuselage, or on the wings E.g. Stationair, Otter On combat aircraft the engine/s is placed at the end of the fuselage E.g. Raptor, ...
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124 views

How does a particle move in Water?

Is there any precise model (More precise than the random walk model) that describes a particle's movement in water ?
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274 views

Why is my ballpoint pen spatially temperamental?

I'm trying to write with a ball-point ink pen on a plastic[1] sandwich bag. When I try to write on some parts of the bag, no ink transfers to the bag: I can continue trying for quite a while in vain. ...
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1answer
94 views

how to stop water reaching up inside an under water structure (see video)

in this video at arround 0:47 they deliver the 'dry pots' to the under water lab, bellow the water and they seem to swim up into an opening where it is completly dry.. how is this dry, surly the water ...
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2k views

Why and how is sound produced when two objects hit each other?

When two objects collide and undergo a partially inelastic collision (so every one we experience in every-day life), they rebound to a certain degree, but kinetic energy is not conserved. Thus, the ...
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1answer
124 views

Flow of freezing liquid or a melting solid

If a liquid is freezing, is equation of continuity violated? As the liquid flows, some portion of it is getting frozen. The mass of the fluid thus keeps dropping. Similarly, when a molten fluid flows ...
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6k views

Flow of liquid among branches

If water is flowing through big pipe is branched into 4 branches of small pipe. Lets say the flow is around 4 m/sec. I have the following questions: What will be the flow rate in each of the ...
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3answers
3k views

Non-Newtonian Fluid Stop a Bullet?

I just saw a YouTube video about Non-Newtonian fluids where people could actually walk on the surface of the fluid but if they stood still, they'd sink. Cool stuff. Now, I'm wondering: Could a pool ...
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1answer
2k views

Why is pressure in a liquid the same in all directions?

I'd like answers both in the more intuitive side an on the more precise side. Thinking of water as "cubes" of water, for example, would allow pressure in the z axis to be independent of the y or x ...
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1answer
300 views

How would this rapid depressurization of water really behave?

An in-development game(Starbound) recently released a demo of their water physics. At about 39 seconds into the video, a volume of water in a container is released when the cap on the container is ...
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1answer
2k views

Why does water flow out of an upside-down bottle? (Rayleigh Taylor Instability)

I am currently reading the excellent book An Indispensable Truth: How Fusion Power Can Save the Planet by Francis F. Chen and I came across this explanation. The Rayleigh–Taylor Instability ...
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1answer
577 views

Glass - paper: Stevin's Law

In my understanding the well-known experiment of the glass full of water in equilibrium with a piece of paper, the atmospheric pressure acts on a small layer inside the glass (on the top) and under ...
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1answer
1k views

Paper plane between two fans - is this possible?

The setup: two fans facing each other, distance around 1m. Both are turned on. In between them, place a simple paper plane and according to this video, it will fly. ...
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2k views

Frequency of Vortex Shedding

Suppose we have a long cylinder with diameter $D$, then we move it parallel to its axis with a velocity $V$ in a medium such that the Reynolds number of the flow is in the range ...
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1answer
127 views

Which topics & books should I read to understand whirlpool, tornado, hurricane etc.?

I want to be expert to understand physics behind whirlpool, tornado, hurricane. Which topics are needed for this?
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1answer
176 views

Pressure loss after water goes up and back down 7 stories

I live on the first floor of a seven story apartment building. We are considering our hot water options and one of them is a solar-powered water heater in which water is heated in a specialised device ...
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2answers
241 views

What equations govern the formation of droplets on a surface?

When some smooth surface (like that of a steel or glass plate) is brought in contact with steam (over e.g. boiling milk) then water is usually seen to condense on that surface not uniformly but as ...
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938 views

How is the stickiness of a fluid measured?

How does one compare the "stickiness" of different substances on a certain surface? For example: Does glue A stick better to steel than glue B? Does water stick better to steel than oil?
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2answers
640 views

How is energy transferred from one incompressible fluid to another?

When you apply pressure to an incompressible fluid the pressure is transferred. If you stop applying the pressure there is no motion. Volume and density are integral parts of calculating how much ...
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1answer
237 views

What equations/constants were used to calculate the Kármán Line for Earth?

I am interested in how the original value of ~100 km was calculated for the Kármán line of earth. What equations and constants would need to be used to reproduce this value? Note: By constants, ...
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2answers
989 views

How can I calculate the flow rate necessary for a given cross sectional area, angle, and length

I'm making a toy for my kids and this problem came up. I have a channel on a slight angle (angle is between ground and length of channel) and I'm pouring water into it. I want to know how quickly I ...
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3answers
2k views

What is the principle behind centrifugation?

What is the principle behind centrifugation? I understand the idea that you spin something around the centripetal force will cause an apparent force on the spinning system. However I don't quite ...
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1answer
2k views

Is there an analytical solution for fluid flow in a square duct?

I couldn't find one but assumed it must exist. Tried to find it on the back of an envelope, but got to an ugly differential equation I can't solve. I'm assuming a square duct of infinite length, ...
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3answers
2k views

Does irrotational imply inviscid?

Let us consider a 2D irrotational flow, such that $\nabla\times\boldsymbol u =\boldsymbol 0$. Defining the stream function such that $\boldsymbol u =\nabla\times\psi \boldsymbol n$ where $\boldsymbol ...
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1answer
152 views

Does turbulence violate Galilean relativity?

Fluid flows become turbulent beyond a certain velocity. The velocity is almost always with respect to a fixed boundary. However, an observer in a frame of reference travelling with the fluid will also ...
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1answer
279 views

Can cannonballs go through water?

In the recent Spielberg/Jackson Tintin movie, there is a scene where Red Rackham and Captain Haddock's ships are fighting, and cannons are fired. The cannonball is shown at one point to go through a ...
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1answer
1k views

Continuity equation for compressible fluid

A question is given as Consider a fluid of density $ \rho(x, y, z, t) $ which moves with velocity $v(x, y, z, t) $ without sources or sink. Show that $ \nabla \cdot \vec J + \frac{\partial \rho ...
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3answers
1k views

Why airplanes fly: the final truth

The questions about the reasons the aircraft fly are frequent among scientist, since the high school until now, even if I work on the other side of fluid mysterious world (the Low Reynolds numbers ...
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1answer
217 views

Capillaries in series

The velocity of fluid of viscosity $\eta$ through a capillary of radius $r$ and length $l$ at a distance $x$ from the center of the capillary is given by; $v=\frac{P}{4l \eta }(r^2-x^2)$ (where $P$ is ...
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1answer
319 views

How to deal with conflicting “no-slip” Navier-Stokes boundary value constraints?

The no-slip boundary value constraint for Navier-Stokes solutions was explained in my fluid dynamics class as a requirement to match velocities at the interfaces. Now that my class is done, I've been ...
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0answers
97 views

Concentration of fluid pumped through a tube?

I have a bottle filled with a fluid A, and a tube of volume $V_0$ filled with a fluid B leading away from it. When I now start pumping fluid A out of the bottle, I am interested in the concentration ...
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3answers
207 views

What's the most efficient strategy to shake a bottle to create a global flow?

Consider a bottle of orange juice with pieces of the fruit gathered at the bottom of the bottle. You need to shake the bottle to evenly distribute the pieces in the bottle. The first part of the ...
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1answer
258 views

Darcy law yields extreme speed for gas flow throgh packed spheres?

The darcy law is as follows: $u=-\frac{k}{\mu}\nabla p$. Assume we have a gas, then $\mu$ is about $10^{-5}$. $k$ for packed spheres a few mm in diameter is of order $10^{-8}$ $m^2$. Say the ...