88 votes
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Why doesn't oil produce sound when poured?

The noise is generated by turbulent flow. Turbulence in the flow generates turbulence in the air at the interface between the air and the liquid surface, and that turbulence in the air is what we hear ...
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34 votes

Why doesn't oil produce sound when poured?

Actually, two questions: The existence of the sound: The most common source of the flowing liquid sound is air bubbles popping. Surface waves (both over the open surface and inside the bubbles ...
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  • 6,113
30 votes
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Frequency difference when water splashes at different temperatures

This is a guess since I have never done the experiment, but the viscosity of water falls by a factor of 5 on heating from 5°C to 100°C. The viscosity is one of the two factors (the other being density)...
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28 votes
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Reason for 6π factor in Stokes' law

It is not determined experimentally, it is an analytical result. It is verified experimentally. As @Mick described it is possible to derive the velocity and pressure field of a flow around a sphere ...
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  • 3,216
24 votes
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What does air "feel" like to a flying mosquito in terms of viscosity?

What you need to compare when looking at bodies of different sizes and asking how the forces relate, is in general, the Reynolds Number as you included in your question. This is defined as: $$ Re = \...
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  • 16k
24 votes
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Can a water vortex (whirlpool) be used to efficiently store energy?

Yes, this would be horribly inefficient compared to pumped hydro or even a regular flywheel. With a rotating fluid, there's a lot of viscosity. This viscosity generates heat and slows the fluid down....
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  • 14.8k
20 votes
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Ball falling through viscous fluid experiment - strange results

You have pointed out the difference between rain drops (large radius) and mist drops (small radius) which fall much slower. When terminal velocity $v$ is reached the viscous drag on a sphere of ...
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  • 79.7k
14 votes
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What is a Physically Accurate Explanation for the Kutta Condition?

The Kutta condition is completely artificial. The potential equations are completely artificial. The potential equations are a mathematical construct we use because it's much simpler than the full ...
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  • 16k
14 votes

How is viscosity described on the molecular level?

Imagine two two trains side by side - one going faster than the other. Frictionless rails. Start shoveling coal from the slow train to the fast one, and from the fast to the slow one. Every shovel of ...
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  • 117k
13 votes
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Dive into a pool of mayonnaise. What happens next?

Mayonnaise is made of a mixture of vegetable oil, water and some egg yolk. It might be possible to swim in mayonnaise (see below) for a short amount of time, but given it's high viscosity (mayonnaise ...
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  • 25.6k
12 votes
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Why are non-Newtonian fluids called non-Newtonian when they follow Newton’s third law?

Newtonian fluids are named after Isaac Newton, who first used the differential equation to postulate the relation between the shear strain rate and shear stress for such fluids. For Newtonian fluids ...
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9 votes

How is viscosity described on the molecular level?

Yes there is. Let's focus on the kinematic viscosity ($\nu$), which is defined as the diffusion constant for momentum in the fluid. That is, it tells us how quickly a momentum disturbance would ...
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  • 13.1k
9 votes

Viscosity and surface tension

Both viscosity and surface tension are connected theoretically to inter-molecular forces, but they are still very different concepts. Viscosity force is a force that acts only when the fluid is ...
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9 votes

What happened here in this Taylor-Couette Flow experiment?

This is what's happening in the video. I've drawn just a single drop, and for convenience I've ignored the curvature of the plates (it's harder to draw curves!): It looks as if the (red in this ...
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9 votes

Frequency difference when water splashes at different temperatures

My immediate guess is that it is because of the difference in viscosity due to temperature. At 5 degrees celsius, water is around five times more viscous (dynamic viscosity ~ 1.5 mPa.s) than it is ...
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8 votes
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Kinematic Viscosity

In mechanics, "kinematics" means describing the motion mathematically, so, for example, if the acceleration is known I can integrate to find the velocity and position. "Dynamics" means analyzing ...
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8 votes
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Height of Water 'Splashing'

Basically the whole kinetic energy is transfered to pressure, and then this pressure will be transfered to kinetic energy again; this time only the direction is as defined by hydrostatic pressure; ...
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  • 2,316
8 votes
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What is the viscosity of Sun surface?

What is the viscosity of Sun surface? By "solar surface" I am going to assume you mean the photosphere. In that case, then the question is whether you are asking about dynamic ($\mu$) or kinematic ($...
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8 votes
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Deriving Stokes' law ($f_v=6\pi\eta Rv$) in a simple way

As already stated I am not familiar with a simpler way than the standard derivation. I don’t even think it is possible to derive it in a easier manner: For Stokes’ formula it is necessary to find ...
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  • 1,529
7 votes

Intuitive explanation of Poiseuille's law -- why $r^4$?

No-slip condition requires that the velocity is $0$ at the boundaries, thus you can see intuitively that the velocity profile has to behave in $a^2-r^2$ (at least as leading terms in its expansion), ...
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7 votes
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Is viscosity a function of density only?

For the most part, temperature is the dominant factor in viscosity, not density. Unless you are also considering multi-component fluids, in which case the components of the fluid are the biggest ...
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  • 16k
7 votes
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Is viscosity simply "slowing down time"?

Not necessarily, it depends on how different the viscosities are. @MonkeysUncle got it right. If the Reynolds number is < 2,000 the flow is laminar; if it's > 4,000 the flow is turbulent. Since ...
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7 votes
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Why is the damping force proportional to $v$ and not $v^2$?

At low velocity $v$ the flow of the fluid around the object is mostly laminar and the drag force a viscous response, which is proportional to $v$. But at higher velocity, flow becomes turbulent and ...
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  • 34.5k
7 votes

Can a water vortex (whirlpool) be used to efficiently store energy?

As mentioned by other writers, water viscosity will rapidly convert any kinetic energy into heat. However, efficient energy storage could be attainable with superfluids. Superfluids are related to ...
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7 votes

What does $ \mu \nabla^{2} \vec V$ mean in the Navier-Stokes equations?

Since you are asking for some intuition, forget about the positive coefficient $\mu>0$ and about the other terms in the Navier-Stokes apart from: $$ \partial_t \bf v = \mu \nabla^2v $$ All the ...
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  • 2,925
7 votes

Why are non-Newtonian fluids called non-Newtonian when they follow Newton’s third law?

The non-Newtonian denotion is not about Newton's 3rd law. It is just a referral to the, shall we say, "expected behaviour". And such "expected behaviour" is that when you push ...
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  • 47.2k
6 votes

Viscous Burgers equation physical meaning

1D Burger's equation is not meant to model a physical phenomenon. Rather, it is a simplification of homogeneous incompressible Navier-Stokes equations that preserves (some of) its mathematical ...
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6 votes

Height of Water 'Splashing'

Indeed, this seems to be very complicated problem. But let's throw away all this complexity and concentrate in the core of the phenomenon. So consider an ordinary stone with volume $V$ that falls ...
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  • 2,602
6 votes

What does air "feel" like to a flying mosquito in terms of viscosity?

Viscosity of air will be same for both fly and human. In the case of flies, from the point of view of the fly, it would seem to it that the viscous force is very high as it keeps the fly afloat. In ...
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  • 365
6 votes
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What happened here in this Taylor-Couette Flow experiment?

Two typical quantities which characterize these systems are the Reynolds and Schmidt number: $$\mathrm{Re}=\frac{vL}{\nu} \qquad \mathrm{Sc}=\frac{\nu}{\mathcal{D}}$$ where $v$ and $L$ are ...
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