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2
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2answers
40 views

What are examples of Couette flow in nature?

What are examples of Couette flow in nature? All I could find are theoretical discussions. It seems to be the flow of viscous fluid between two moving plates. I am imagining slipping on an oily ...
0
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1answer
28 views

Derivation of the equation for flow of a viscous liquid on a rotating disk

In this paper, Emslie, Alfred G., Francis T. Bonner, and Leslie G. Peck. “Flow of a Viscous Liquid on a Rotating Disk.” Journal of Applied Physics 29, no. 5 (1958): 858. doi:10.1063/1.1723300. ...
0
votes
2answers
64 views

How does viscosity cause dissipation? [closed]

I am not a physicist so bear with me. I'm trying to understand the mechanism through which energy is dissipated in a fluid or solid. Often the explanation is that it happens due to viscosity or ...
1
vote
1answer
34 views

Why is oil more slippery than water? [duplicate]

It is very well known that the higher the viscosity the higher the resistance offered. But why do we slip more on oil as compared with water even if oil has higher viscosity than water?
2
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0answers
21 views

Viscous accretion disk, green formula

I have this problem. I am studying a the viscous accretion disk problem and in the assumption of constant viscosity the equation of diffusion of the superficial density becomes: $$ \frac{\partial}{\...
0
votes
1answer
39 views

Matching Bernoulli equation and Hagen Poiseuille law for viscous fluid motion

I thought that Bernoulli equation could be used only in the case of non viscous fluid. But doing exercises on 2500 Solved Problems In Fluid Mechanics and Hydraulics (Schaum's Solved Problems Series) ...
1
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1answer
49 views

Why does viscosity effect the normal component of stress in a Newtonian fluid?

The constitutive law for a Newtonian fluid is $$ \boldsymbol{\tau} = 2\mu \mathbf{D} + \lambda\left(\nabla\cdot\mathbf{v}\right)\mathbf{I}$$ where $\mu$ is the dynamic viscosity. Assuming we have a ...
1
vote
3answers
76 views

Pressure just before the hole in a draining tank

I get extremely confused in fluid dynamics problem where the pressure in some points is required. Consider the ideal fluid in the tank in the following picture. I would like to understand what is the ...
1
vote
1answer
40 views

Viscosity and mean free path

I have troubles understanding how to derive the formula for viscosity in terms of the mean free path $$\eta\sim \rho \lambda \bar v$$ where $\bar v$ is the average molecular velocity of the gas, $\...
1
vote
1answer
114 views

Why does the parallel component of the velocity gradient affect viscous force in a fluid?

The force density due to viscosity in an incompressible fluid is $-\mu \nabla^2 \mathbf{u}$ where $\mathbf{u}$ is the velocity and $\mu$ is the dynamic viscosity. Let's suppose some particular small ...
2
votes
0answers
26 views

What are the dynamics of the “break the surface tension with a drop of soap” experiment?

There's a common experiment exemplified in the first few seconds of this video: https://www.youtube.com/watch?v=WsksFbFZeeU You put some food coloring in the center of a plate of milk, then put a ...
0
votes
0answers
27 views

What is the relative velocity term in drag and lift force in the following case?

When a stationary particle is introduced in a fluid moving with horizontal velocity $V_fx$, what is the relative velocity terms in the drag force ($F_D$) and lift force ($F_L$) equations. Am I correct ...
0
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1answer
37 views

Difference in viscosity-concentration and reduced viscosity-concentration graphs

I prepared 3 different polymer soultions with varying concentration of 100, 50 and 25 ppm in order to measure their viscosity . When I plot concentration vs viscosity graph in the excel, I have a ...
4
votes
1answer
73 views

Why is turbulence caused?

In high Reynolds numbers we have turbulent flow. This is because the inertial forces are much greater than the viscous forces. I understand inertial forces to be actually the fictional forces due to ...
0
votes
1answer
53 views

Newton and Stokes regime in settling velocity

Can you tell me please the differences between Newtonian and Stokes regime in settling velocities of particles in moving fluids? I know that Newtonian applies to higher Reynolds numbers. Does that ...
1
vote
2answers
104 views

Rotational and irrotational flow

Can a rotational flow take place if there is no viscosity? If yes then will it always be rotational or will it become irrotational after some time?
3
votes
3answers
52 views

Why does a pot start rotating when coffee is stirred inside?

I usually make Turkish coffee as my morning coffee. I have a small somewhat rounded pot with handle on one side. I noticed that when I pour water in and start stirring, pot has a tendency to start ...
0
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0answers
32 views

Pendulum with Viscous and Frictional Damping

I am trying to model a pendulum with both viscous and frictional (Coulomb) damping. The problem is that the viscous damping only occurs in one direction because I am modeling a dashpot that only has ...
0
votes
3answers
42 views

Wind restistance as function of temperature

Is temperature-dependent wind resistance the reason there's a significant increase in fuel consumption in my Prius car when air temperature drops by 30 degrees K from 300 degrees K? I think I see a ...
0
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0answers
9 views

Is viscosity excluded by Lorentz symmetry in spatio-temporal discreteness?

"In the case of spatio-temporal discreteness, viscosity is excluded by Lorentz symmetry". This statement is from here. Can anyone explain why this is the case? I'm looking for an intuitive way to ...
1
vote
1answer
37 views

Physical reason why Prandtl number is order unity for gases?

Is there a physical reason behind the fact that for gases the thermal diffusivity is on the same order of magnitude as kinematic viscosity (and as such a Prandtl number of order unity) and if so what ...
0
votes
1answer
51 views

Working Out Shaft Torque in a Stirred Tank with a Cylindrical Cavern Formation (using a non Newtonian fluid)

Here is some background to the problem (in a stirred tank): "With yield stress non-Newtonian (viscoplastic) fluids, it is possible to generate an agitated volume around the impeller, defined as a ...
1
vote
1answer
83 views

Patterns in falling viscous fluids

It is a common observation that honey(or any other viscous fluid),tends to overlap/coil/wind up as it reaches the rigid surface. There is this little bend near the pile-up. Why is this so? How does a ...
0
votes
1answer
62 views

Ball touching the side of a tube — DOUBT

I have a small doubt in understanding a physics concept. If a steel ball is dropped in a thin tube of glycerol, it will eventually reach its maximum or terminal velocity. However, if the steel ball ...
1
vote
0answers
14 views

percolation permeability threshold in Helium before and after lambda point transition

There's an experiment demonstrating the properties of super-fluid Helium compared to normal fluid Helium, in which the Helium is put in a jar who's bottom is porous. The super-fluid can pass through ...
1
vote
1answer
36 views

Why do we need the kinematic viscosity?

Kinematic viscosity is the well known dynamic viscosity divided by density. What difference does it make to divide by density? What is so useful about this?
1
vote
1answer
46 views

Newton's second law of motion and viscosity [closed]

In a Coutte flow, the applied force on one of the plates can be expressed by $F = \eta A \frac{dv}{dz}$. F here can also be defined using Newton's second law of motion, $F = m \frac{dv}{dt}$. The ...
2
votes
4answers
74 views

What makes the magnitude of the force of friction velocity dependent in one scenario, but not the other?

When a solid objects makes contact with another solid object, I believe the magnitude of the force of friction between them does not depend on the relative speed of the two objects. When a solid and a ...
5
votes
1answer
42 views

How should I think of a liquid in terms of interatomic potential and molecular speed?

A rather simple question for liquids specialists I guess but I have hard time finding information about this. Here is my problem. I understand the ideal gas theory and the Maxwell's speed ...
0
votes
1answer
66 views

Terminal Velocity in Water

Another question concerning classical physics. On Earth, the combination of air resistance and gravity interact in a way that creates the phenomenon of terminal velocity. Do other fluids (for example, ...
1
vote
1answer
58 views

Free expansion of ideal gas, transient phase (3 questions)

We all know the classic scenario of free expansion. A contained gas expands into a vaccum and in the end we have $\Delta T = \Delta U = \Delta H = 0$ and $\Delta S = R \ln \frac{V_2}{V_1}$. This is ...
0
votes
1answer
23 views

If there was an object such as a bucket travelling through the atmosphere very fast could the air caught in it transition to a liquid?

The question came to me after thinking of the pressure waves in front of a spacecraft during re-entry. Would the pressure caused by the air being compressed cause the air to liquefy and if so at what ...
0
votes
1answer
85 views

Basis for Derivation of Stokes Friction Law for Spheres

When deriving Stokes law one uses the Navier Stokes equation with the assumptions: low Reynolds number stationary flow in compressible flow leading to this version of the N.S : $$\nabla p = \...
2
votes
1answer
86 views

Effect of paint on drag force [closed]

Aerospace engineering as well as automobile engineering gives a particular significance to the shape of a vehicle to enable proper and more effective transportation.What I want to know, is, about the ...
0
votes
1answer
268 views

Application of Stoke's Law

I was looking at Stoke's law and it says that you use it to calculate the drag force on a sphere passing through a fluid. Can i also use this equation to calculate the drag force on a car going ...
0
votes
2answers
83 views

Viscous forces with asymmetric gradient velocity in fluid mechanics

In fluid mechanics, the stress tensor writes $\sigma = -p 1 + \tau$ where the deviatoric part $\tau$ corresponds to shear. The viscous (volumic) forces are $\operatorname{div}\tau$. For a Newtonian ...
1
vote
1answer
56 views

Derive drag coefficient of plate

There is any analytical way to derive drag coefficient of flat plate aligned perpendicular to the flow? Wikipedia says it's between 1.98~2.05 but I want to get this value in calculation, not ...
0
votes
1answer
37 views

Ratio of force of resistance due to liquid to that due to gravity

What is ratio of force of resistance due to liquid to that due to gravity? For any object for example a ball falling in a liquid with constant velocity, what will be the ratio?
0
votes
1answer
29 views

Without knowing fluid density, can it be correctly determined by measuring both kinematic and dynamic viscosity directly?

Without knowing a fluid's density, can its density be correctly determined by measuring both kinematic and dynamic viscosity directly, and then dividing these values? Can one determine cStokes and ...
2
votes
1answer
107 views

Can dynamic viscosity be measured directly and without knowing fluid density?

Can dynamic viscosity be measured directly and without knowing fluid density? If so, how? My understanding is that only the kinematic viscosity can be measured directly (with the devices that I am ...
0
votes
1answer
55 views

Velocity of liquid molecules in turbulent flow

I was solving some questions when I came across this: what is the velocity of liquid molecules in contact with the walls of the tube? and the answer was given that it can have any velocity and then ...
0
votes
0answers
46 views

How to determine which one is dominant shear rate vs. flow rate

I have a question for the image below. Now we have a pump connecting to a votator for transferring some slurry. The pump yields a flow rate Q, and the votator yields a shear rate 𝛾. I wanna ...
3
votes
2answers
106 views

Dissipative forces and reversible processes

A book that I have contains the following lines: For a process to be reversible, the dissipative forces such as viscosity and friction should be absent. My question is why?
1
vote
1answer
53 views

Shear stress in cylindrical coordinates?

In cylindrical coordinates the momentum flux is given by (in the $r$ direction): $$ \Pi=-\eta \frac{\partial (r\omega)}{\partial r}$$ Where $\eta$ is the viscosity. Therefore one would expect that the ...
0
votes
0answers
23 views

Plateau–Rayleigh instability according to liquids $Ca$ number

For examining whether viscosity or surface tension dominates each other effects, we can't refer to $Re$ or $We$ numbers because they just tell us about one of them ( surface tension and viscosity ). ...
2
votes
1answer
32 views

The validity of constitutive diffusive fluxes

In transport phenomena the diffusive fluxes for mass, energy and momentum are the constitutive laws: $$\boldsymbol{j}_c=-D\boldsymbol{\nabla}c \quad \boldsymbol{j}_T=-k\boldsymbol{\nabla}T \quad \...
0
votes
1answer
48 views

Plateau–Rayleigh instability for liquids with low $Re$ number

I'm working on a project about Plateau–Rayleigh instability for liquids. But I've a question. we can examine the fluids with high $Re$ number that the influence of viscosity is negligible, but what ...
1
vote
1answer
72 views

Why does my coffee stop?

As I was mixing my coffee this morning, wishing that it would do it on its own, I started wondering: What makes my coffee stop swirling? I mean what exactly? Is it the walls of the cup or the coffee ...
9
votes
3answers
688 views

Derivation of viscosity using basic kinetic theory?

This question has been asked in part before in the question Kinetic theory derivation of viscosity of a gas although the given accepted answer does not give the required detail for the part of the ...
1
vote
2answers
76 views

Viscosity forces and Depth of water

I was wondering if the viscosity force depends on the depth of water. I mean it is much more harder to swim in the ocean at a depth of 1km than swimming at the surface due to the viscosity forces ?