# Tagged Questions

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

566 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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### 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 ...
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### How much lift does an airplane get from its wings, vs the rest of the airframe?

Consider a big commercial airliner, like a 727, 747, or a 787. At cruising altitude, under standard conditions, how much of the lift of the aircraft comes from the wings, and how much from the rest ...
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### What is the return probability for Brownian motion in three dimensions?

I would like to know the probability of return to the initial point in three dimensional Brownian motion. Does someone know an expression for the diffusion constant? (Suggestions of books on this ...
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### Minimum viscosity of liquids

In a lecture by Purcell he mentions that he notices that there aren't any liquids with viscosities much less than that of water, even though they go up seemingly unbounded. In an endnote (endnote 1 in ...
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### Multiple source Pipe Network: Given a known outflow, can we deduce the inflows?

I have the following simple Y shape water pipe network: Given $S_{out}$ ( in $m^3/s$), can we compute $S_1$ and $S_2$? For the pipe, we know for each pipe their corresponding diameter $d$, length ...
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### Why do non-Newtonian fluids go hard when having a sudden force exerted on them?

You can dip your hands into a bowl of non-Newtonian fluid but if you are to punch it, it goes hard all of a sudden and is more like a solid than anything else. What is it about a non-Newtonian fluid ...
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### Wave with mass transport?

I don't think this question has been asked on this forum before (at least I didn't find it). In the case of a tsunami, an earthquake generates a wave which will travel with the sea/ocean as the ...
94 views

### Bifurcation of convection of fluid in container, when adding temperature

I once read a paper, in which: a fluid in a container was heated from below, after reaching temperature $T_1$, a circular motion (convection) was clearly distinguishable, in form of cylinder, after ...
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### What do bullet trails really look like?

In the movie The Matrix, during "bullet time" sequences, bullets are shown trailing evenly-spaced refracting blobs: ...
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### How does one derive the equation for the speed of sound?

In my acoustics books I see $$c^2 = \frac{\mathrm{d}P}{\mathrm{d}\rho}$$ where $c$ is the speed of sound, $P$ is the pressure and $\rho$ is the density. Where does this equation come from? In my ...
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### Why does your car lurch toward an oncoming truck as it passes you?

I notice that the larger the truck the greater the magnitude of the lurch. Can anyone give a physical explanation to this?
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### How to model water flow with and without friction against other objects?

I am interested in modeling water flow including boundary flow (with friction) against other objects in the path of the water flow. I'm not a physicist but I am interested to learn the relevant ...
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### What are some ways that humans could have influence over what sequence a star was in?

How would a society go about either preventing our sun in its primary sequence from going into a Red Giant a billion years from now? Or perhaps, accelerating the process of going from main sequence ...
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### Is this a correct addition to a pipe friction loss calculation?

I've been using this online calculator to work out the difference in pressure and flow between two ends of a pipe: Pipe Friction Calculation for Fluid Flow in a Pipe So far most of it makes sense, ...
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### When does a pot not spill?

What shape for the edges of a pot and what surface properties are best for a tea pot not to spill? Apart from the cross-section from the side, how does the radius of curvature as seen from above play ...
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### Could motives aid in the study of the Navier-Stokes equations?

Recently, mathematicians and theoretical physicists have been studying Quantum Field Theory (and renormalization in particular) by means of abstract geometrical objects called motives. Amongst these ...
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### Hydraulic System in a Simulation

A simulation project we are on, we have a machine with hydraulics that are powered (They don't just look like they move something, they are the only thing moving/turning/lifting something) - However, ...
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### Change in Vapour/Liquid change point, at very low pressure

In a previous question, I was given an answer: "A quick Google suggests that the triple point of Hydrogen is 13.8K and the triple point of Neon is 24.6K, so neither can exist as liquids at ...
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### some hints/introductions/textbooks for LBM(Lattice Boltzmann methods) fluid simulation?

I come from a computer science background and I want to learn details about LBM in fluid simulation. I have been searching around for a concise tutorial that tells me how LBM is used to solve a ...
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### Why does the sound pitch increase on every consecutive tick at the bottom of a filled cup of coffee?

Since I don't know the proper physical terms for this, I describe it in everyday English. The following has kept me wondering for quite some time and so far I haven't found a reasonable explanation. ...
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### Helium plasma in space and its properties

It is said, that, "Most of the Helium in the Universe, is in a plasma state". Plasma's are now talked of, as the forth state of matter, but this does not seem to be a majority opinion. Plasma's are ...
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### Flow rate of a syringe

Suppose a syringe (placed horizontally) contains a liquid with the density of water, composed of a barrel and a needle component. The barrel of the syringe has a cross-sectional area of $\alpha~m^2$, ...
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### Does Helium just naturally display BEC properties at <1K, or does it become a BEC?

I am researching low temperature, near absolute zero, and in particular Bose Einstein Condensate. There is a lot of research information, but it is confusing, and not explained. Technically a BEC is ...
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### Why do rain waves form and what is their connection to the texture of the surface they're on?

When it rains and water flows down an inclined street, ripples may form that are carried along with the current. Here's a picture with an example of what I'm talking about I'd like to know what the ...
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### Why is compressible flow near the choke point so efficient?

Imagine a steady state, one-dimensional, compressible flow in a horizontal pipe of constant cross sectional area. This flow can be isothermal, adiabatic (Fanno), or diabatic (Rayleigh). As an ...
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### Are there good home experiments to get a feel for the behavior of yield-stress liquids?

What common household liquids / foodstuffs show Herschel-Bulkley fluid behavior? I assume tomato soup to be one, since when sloshing it around lightly on a spoon the visible surface moves as one (as ...
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### What is “surface fluid adhesion energy”?

This is related to my previous question. Pardon me for asking so many questions recently. My physics knowledge is not that good, and some answers are hard to find. In the question in the link, I ...
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### Can a the volume flow through a positive displacement pump be reliably measured by tracking revolutions of the drive shaft? [closed]

The question refers to non-compressible fluids. I'm mostly thinking about monopumps (excentric screw pumps), but this also should apply to any type of gear pumps as well, possibly also to piston ...
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### wind vs air resistance

I'm wondering which offers more resistance: pulling an object at some speed through air, or holding the object steady against wind at the same speed. I think initially people would think same ...
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### Does it take significantly more fuel to fly a heavier airplane?

I was reading in the papers how some-airline-or-the-other increased their prices for extra luggage, citing increased fuel costs. Now I'm a bit skeptical. Using the (wrong) Bernoulli-effect ...
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### What is the pressure drop in a venturi with a compressible fluid?

I would like to know if there is an equation to predict the pressure drop in a venturi device using a compressible fluid as the working medium. In particular, I'd like to use this equation to predict ...
1k views

### Video or animation for continuity equation in water pipes

I am looking for an animation or even better a video with a real experiment, which shows that at constant flow rate the speed of water in a pipe is higher at narrower parts of a pipe. I googled ...
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### Local Mach number for an airfoil

I would like to know how to calculate the local Mach number on the upper surface of an airfoil given the ambient temperature, the local velocity on the airfoil surface, the freestream velocity, and ...
2k views

### What do the dimensions of circulation mean, and how is circulation related to action?

The dimensions of circulation $\int_C \vec{v}\cdot d\vec{r}$ seem strange, but if you include (even a constant) density $\rho$, then $\int_C \rho\vec{v}\cdot d\vec{r}$ has dimensions the same as ...
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### Boundary condition for water waves generation

I am trying to model water wave propagation in 2D with CFD. What would be a suitable boundary condition to generate the waves on the left boundary? Currently I vary the fraction of water above the ...
9k views

### Why do ice cubes come out easier from top trays?

This is my "hey, I've noticed that too!" question for the week. If you stack two plastic ice cube trays with water in them in a freezer, the resulting ice cubes in the top tray will usually come out ...
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### Why do steam bubbles increase in size as they rise?

In the following video (a customer's review of a glass kettle), we can observe water boiling: http://youtu.be/jByY5I7Xk7w?t=2m55s As the kettle starts to boil at around 2:55, we can see large steam ...
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### Why is the unit of hydraulic impedance/resistance kilogram per second and hypermeter?

I'm trying to figure out the unit of impedance in the hydraulic analogy of electronic networks. Assuming $$Z=\frac{P}{Q},$$ with $P$ as pressure difference, $Q$ as volumetric flow rate and $Z$ as ...
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Some lecture notes I was reading through claimed that a pressure pulse propagates through a liquid-filled tube (blood in a vein) with the speed $$c=\sqrt{\frac{A}{\rho}\frac{dP}{dA}},$$ where $A$ is ...
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### Convective and Diffusive terms in Navier Stokes Equations

My question has 2 parts: I just followed the derivation of Navier Stokes (for Control Volume CFD analysis) and was able to understand most parts. However, the book I use (by Versteeg) does not ...
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### Ping pong ball levitation control system question, modeling as a damper? [closed]

I am building a control system to levitate a ping pong ball at a given height in a clear plastic tube using an arduino and implementing PID control. I have to come up with a transfer function... I am ...
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### I want to recreate a phenomenon I've noticed from our water tank [closed]

I'm trying to recreate a phenomenon I have noticed from our water tank at home. Ok so we have a deep well pump that fills our water tank everyday. But something odd always happen everytime the water ...
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### How does smoke move in the air and how can I direct it so it will go to a place I want it to go?

Let's assume a close room with 1-2 people who only one of them smoking cigarette. What is the equation describe the smoking spreading? is it diffusion? what are the parameters is so? Is there a ...
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### Calculating spacecraft water leak flow rate

A large container of heated water is floating in space. How can I calculate leak rate if a small puncture is opened? I'm assuming the water is kept at 298K and in the liquid state inside the chamber. ...
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### Can a wave propagate in an elastic fluid in the absence of volume forces?

A motion (wave) $\mathbf{x}: \mathcal{B}_0 \times [t_0,t_1] \to \mathcal{E}:$ such that $q-o = \mathbf{x}(p,t)=(p-o)+\mathbf{a}_0 cos(\mathbf{k}_0\cdot(p-o) - \omega_0 t)$ can propagate in an elastic ...
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### Strict general mathematical definition of drag

Is there a formal definition of drag, say, as some surface integral of normal and shear forces? There seem to be a lot of formulas for specific cases, but is there a general one? I need to accurately ...
236 views

### Air velocity in a double-skin facade

I was wondering how the air flows in a double skin facade? Say I have a glass facade with a depth of 1m, a height of 200m and a width of 10m. It is a single vertical shaft all the way up with openings ...