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

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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
190 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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737 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
1k 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
1k 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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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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132 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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248 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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877 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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963 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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181 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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244 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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88 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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194 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
214 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 ...
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474 views

Wind Turbine Impact

OK, I'm not sure this is not a silly question, but here goes. I've often wondered what the impact is of wind turbines on the weather. My question is, since wind turbines transform wind energy into ...
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1answer
397 views

Capillary engine is impossible?

I read some articles saying that it is not obvious (or maybe impossible) to generate energy using capillary action. On the other hand in my understanding trees uses capillary action to extract water ...
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1answer
115 views

How do Kolmogorov scales work in shear thinning fluis?

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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1answer
304 views

How is external pressure transmitted when a fluid is not enclosed?

Pascal's principle states that pressure applied to an enclosed fluid is transmitted undiminished to every part of the fluid. How is the pressure transmitted in the case where the fluid is not ...
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1answer
766 views

How much water must flow trough canal to maintain a constant water deep?

In order to maintain a constant water deep in canal, how much water must flow trought the pipe ? As shown on picture, canal have a rectangular shape. I don't know if canal length have an influence. ...
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2answers
537 views

Vortex street and Reynolds number

Its been stated in Wikipedia regarding Reynolds number(Re) that "laminar flow occurs when Re<2300 and turbulent flow occurs when Re>4000. In another wiki file related to Kármán vortex street it has ...
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1answer
102 views

Critical bathymetric profile to maximize surge and minimize breaking?

Reading about storm surge, I found it fascinating that the gradual slope of the Gulf Coast of Florida resulted in a much higher storm surge but much lower energy release in breaking waves. Is there ...
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0answers
69 views

The sound of coffee [duplicate]

Possible Duplicate: Why does the sound pitch increase on every consecutive tick at the bottom of a filled cup of coffee? A colleague suggested this experiment this morning : Get a cup of ...
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1answer
269 views

Can we make an environment where the density of air equals the density of water?

If that is possible then will it be possible for a human to breathe in that environment.
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427 views

How does Earth's interior dynamo work?

I'm interested in getting a basic physical understanding of how Earth's magnetic field is generated. I understand that it's a "dynamo" type of effect, driven by convection currents in the molten outer ...
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316 views

What is the microscopic picture for warm air rising?

The usual explanation for warm fluids rising past cooler ones is that the warmer fluid has a lower density. I'm trying to understand what this looks like at a molecular scale. The density seems to ...
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42 views

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 have a box ...
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1answer
297 views

Is it possible to calcualte the yield-stress of a fluid by measuring the smalles bubble that rises through it?

Consider a yield-stress liquid in gravity. I assume that the buyouncy of a small gas-bubble will not be enough to overcome the yield stress (so the liquid doesn't behave liquid), thus leaving the ...
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4answers
11k views

Why can't helicopters reach mount everest?

Is there a reason why people can't just take the helicopter to mount Everest? Why is it that helicopters can't reach that high?
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114 views

How to integrate twice of this viscous term?

I am reading a paper, and I do not understand why the author said the following term when integrated twice will become, $\int\limits_\Omega {{\rm{d}}\Omega {{\bf{\psi }}^{\bf{u}}}\cdot\nabla ...
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1answer
206 views

Confused about the physical meaning of velocity variation

I am reading a paper and saw the author wrote something like, Because of no-slip wall assumption, so velocity vector $\vec{v}$ is $0$ on the wall, and also the variation of this velocity on the wall ...
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5k views

Why does water pouring from a glass sometimes travel down the side of the glass?

If you have a glass of water, say, three quarters full and you pour it at an angle of say, $45^{\circ}$ with respect to the the table, the water comes out of the glass and goes directly down towards ...
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1answer
1k views

Why, when one opens 1 car window, does that noise occur?

When you're driving and you open 1 car window, say the front one, there comes a horrible noise, but when you open another window just the slightest bit, this noise goes away (I'm sure most people know ...
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459 views

What is the roaring in a roaring fire?

I was just starting a barbecue fire by blowing on the smouldering coals when I realised I had no idea what the sound was actually caused by. I can make the sound by blowing at almost any flame I can ...
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329 views

Why do sea levels not rise uniformly around the world?

While reading the article Hawaii’s Beaches Are in Retreat, and Its Way of Life May Follow on The New York Times's website, I was surprised to find out that sea levels do not rise around the world at a ...
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1answer
392 views

Turbulence parameterization from gravity - fluid dynamics correspondence

I`m looking for a nice introductary reference that explains how the turbulence coefficient or any kind of turbulence parameterization (in view of applications to atmospheric turbulence for example) ...
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1answer
915 views

Hit a bottle of beer on the top with another causes the first to spit all the gas, why?

So, on the other day me and my colleges were discussing the following phenomena: Pick two open bottles of beer. With the bottom of the first, hit the second on the bottleneck, in the following way: ...
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0answers
113 views

How do I simulate a constant velocity flow in porous media

I am modelling gas combustion in porous media. Most contemporary models assume that the pressure drop from the porous media is small enough to disregard, but I want to include that in my ...
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1answer
487 views

Sound waves through a glass filled with liquid

I was pondering about the following 'experiment', and was curious about the formulae behind it. Imagine having a glass filled with a liquid. On one side there is a sensitive receiver that records ...
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3answers
24k views

Relation between water flow and pressure

Is there any equation that states the relation between pressure and water flow. I.e. Let's say that in 1 hour with 8mca (water collum meters) pressure I obtain 50m3. What if (giving the same ...
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9k views

'Applications' of surface tension [closed]

What are some common applications, uses, exploitations of the properties of surface tension? Here is what I mean. A water strider can walk on water, that is a consequence of surface tension. ...
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4k views

Before a once-warm lake starts to freeze, must its temperature be 4°C throughout at some point?

This is a problem I just started puzzling over, and I felt this would be a good forum to check my reasoning. So here are the relevant observations followed by my question: Water achieves its maximum ...
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1answer
267 views

How much more effective is it to stir in both directions?

I have been told that industrial mixing machines (say, for cake batter) switch directions periodically, first stirring in one direction, then the other, because this mixes the material more ...
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4answers
969 views

Why is exhaling more forceful than inhaling?

By blowing at pencil, a piece of paper, or another object up to fifty centimeters away, I can cause it to move away from me significantly. But I can't move an object toward myself by inhaling sharply ...
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3answers
184 views

Analogies between physics and packet-switched networks

Suppose two entities in a communication networks (sender and receiver) are connected with a tube so that water can be pumped from the sender to the receiver. If we also assume that between sender and ...
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1answer
1k views

What is a fluid flux?

I need a concise definition of a fluid flux and an accompanying example. I've never taken a single physics course before, but I'm required to understand this concept so I can do the calculations for a ...
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1answer
11k views

What exactly is the difference between advection and convection?

After reading Wikipedia articles on advection and convection, I still cannot determine whether there is a consensus on a difference between these two terms. Sometimes, the term convection seems to ...
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1answer
286 views

Why are turbines built differently when a change of entropy or temperature is involved?

We are talking about turbines for extraction of energy from a fluid. Consider a typical wind turbine, or a francis turbin for water - we have one hub with on set of vanes that are driven by the ...
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2answers
379 views

Forces on an airfoil

I'm building an airplane (Super Baby Great Lakes) and I'm wondering something about airfoils. In particular (this plane is fabric covered), I'm wondering about the lifting forces on the main wings. ...
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2answers
307 views

Why does the higher pressure of air underneath an aeroplane wing keep it flying?

With aeroplane flight, the wings are shaped so that the air that goes over the top of the wing has to travel faster than the air that goes below the wing. This means that the air below the wing has ...