80 votes
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Why does rainwater form moving waves on the ground? Is there a name for this effect?

These waves are called "roll waves." They are due to an instability in shallow shear flows. The analysis is much too complex for a short answer, but if you google "Roll Wave" you will find more ...
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58 votes
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Why are aerodynamic / streamlined shapes always stumpy at the front?

You are correct if your boat will only travel in a straight line. In real life the motion of the boat will often have a yaw angle, so that it is moving slightly "sideways" relative to the ...
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  • 9,842
38 votes

Turbulent spacetime from Einstein equation?

Gravity can, of course, become turbulent if it is coupled to a turbulent fluid. The interesting question is thus, as John Rennie points out, whether a vacuum solution can be "turbulent". As far as I'...
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34 votes

Why is an airfoil shaped like a teardrop?

The airfoil shape is optimized for two features a maximal lift coefficient $c_L$, to get a big lift force a minimal drag coefficient $c_D$, to get a small drag force Maximizing the lift coefficient $...
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30 votes

Why are aerodynamic / streamlined shapes always stumpy at the front?

Any speculation about what shape might be best is meaningless without specifying the flow conditions. For the keel on a boat, the main one is the Reynolds Number, a parameter that is proportional to ...
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22 votes

Turbulent spacetime from Einstein equation?

Thanks to holography, we now know that solutions to the Einstein equation in certain $d+1$ dimensional spaces are equivalent (dual) to solutions of the Navier-Stokes equation in $d$ dimensions. This ...
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18 votes

Can a theory of everything solve the oldest problem of turbulence?

I’d like to answer by expanding the analogy made by @Charlie. A theory of everything would be like knowing the rules of chess. We could understand all the rules, the pieces, and their moves and ...
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14 votes
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Can a Styrofoam box bounce back into a moving van?

Is this possible; due to some effect of turbulence? Sure. I'd consider that video plausible, at the very least. Can the "suction force" due to a moving vehicle propel an object, faster ...
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11 votes

What does the Reynolds Number of a flow represent physically?

Reynold's number is defined to be: $$ \text{Re} = \frac{ v D }{ \nu } $$ where $v$ is the characteristic velocity for the flow, $D$ is a characteristic size and $\nu$ is the kinematic viscosity. Now,...
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10 votes

Turbulent spacetime from Einstein equation?

Update Recently, there was a talk titled Turbulent gravity in asymptotically AdS spacetimes which may be of interest. In these papers, spacetimes which are anti-de Sitter asymptotically with ...
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9 votes

Concept behind Reynolds number

Dimensionless numbers in fluid dynamics are always a ratio of two quantities. The expression that you share is only a results of that expression. The Reynolds number is defined as the ratio between ...
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8 votes
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Why is the Reynolds number "the way it is?" Why is its order the way it is?

The Reynolds number, with $\rho$ the density, $u$ the velocity magnitude, $\mu$ the viscosity and $L$ some characteristic length scale (e.g. channel height or pipe diameter) is given by $$\text{Re}=\...
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  • 3,216
8 votes

Can a theory of everything solve the oldest problem of turbulence?

Can we have unified theory of universe without solving the problem of turbulence? Yes. Because the "unified" in the name refers to the basic interactions, from which in principle everything ...
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8 votes

Why are aerodynamic / streamlined shapes always stumpy at the front?

As mentioned above, indeed, this shape is more aerodynamic when parallel to the vector field (flow direction) in particular. You see this shape often on long distance Kayaks and Canoes that move in ...
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8 votes

Concept behind Reynolds number

One real-life application of the Reynolds number (no apostrophe as it is named after Osborne Reynolds) is in the design of small scale simulations of fluid dynamics scenarios, such as wind tunnel and ...
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7 votes

Is Navier-Stokes a turbulence model?

The Navier-Stokes Equations are not a 'turbulence model', they are more fundamental than that: they are the fundamental equations that govern all of fluid dynamics (assuming the continuum assumption ...
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  • 3,954
7 votes

Can a Styrofoam box bounce back into a moving van?

Think about why race cars follow each other very closely. The reason is that the first car creates a suction behind it. This is actually a huge problem with transport trucks because it is very bad ...
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6 votes
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Why are the backs of airplanes curved?

The reason why the aft ends of airplanes are streamlined is to preserve a smooth flow of air. Just as the fore ends of airplanes are streamlined to smoothly cleave the air, so too the aft ends are ...
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  • 1,553
6 votes

How do I calculate the Reynolds number in multiphase flows?

There are three ways you can proceed in: 1. Homogeneous Flow Model Herein, you would assume single averaged flow quantities and then solve the Navier-Stokes equations as if it were arising from the ...
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6 votes

What is enstrophy?

Here's a relevant quote from David C. Wilcox's Turbulence Modeling for CFD. This is discussing the interpretation of $\omega_t$, which here is the specific turbulent dissipation rate*. (also, $k_t$ is ...
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6 votes

Turbulent spacetime from Einstein equation?

The fluid-gravity correspondence that Thomas referred to in his answer is a very concrete set-up where we can import intuition from fluid dynamics to suggest how we might get turbulence in vacuum GR (...
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  • 2,555
6 votes

Pressure loss in an infinite pipe

What comes first? Pressure or flow? Without potential, a pressure differential, there is no flow. It's pressure that needs to be there first. So rather than ask what pressure drop you get, it's ...
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6 votes

Why does rainwater form moving waves on the ground? Is there a name for this effect?

I think that the explanation is that waves of different size run with different speeds. This makes the faster waves run up to the slower which make them stack up, or constructively interfere.
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6 votes

Why are aerodynamic / streamlined shapes always stumpy at the front?

One way of looking at the problem is to consider the pressures over the surface. Streamlines curling outwards tend to indicate high pressure pushing water away, streamlines curling inwards indicate ...
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6 votes
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Concept behind Reynolds number

I think your handout is written in quite a confusing way. The main point of the Reynolds number is similarity. You can show that flows around a geometrically identical shape, but with different size, ...
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5 votes
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Benefits of rear spoiler in cars

From Wikipedia: A spoiler is an automotive aerodynamic device whose intended design function is to 'spoil' unfavorable air movement across a body of a vehicle in motion, usually described as ...
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5 votes

How to estimate the Kolmogorov length scale

Here's an empirical way to roughly estimate ε for a natural stream. You need: an audio recorder a thermometer a buoyant object tape measure a stopwatch freeware audio software (like Audacity) 1. In ...
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5 votes

What is the physical application of Navier-Stokes existence and smoothness?

IANAFD but I'll stick my neck out and say this: resolving the Clay problem one way or another won't cause people doing CFD to lose any more sleep than they already do. First of all, Jean Leray ...
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5 votes
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What do the differences between spatial and temporal spectra imply about the structure of turbulence?

Thanks for posing a great question. I would love for Taylor's hypothesis to be uniformly valid! It would mean my dissertation would be finished immediately. Let me take on your cluster of questions ...
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5 votes
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Why don't surfboards fly out the back of a utility truck when driving on the highway?

A crude model of the UTE is used simulate the pressure field created by the car as it travels through atmospheric air. We must determine the pressure in truck bed because it applies force on the ...
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