The tag has no usage guidance.

learn more… | top users | synonyms

1
vote
1answer
96 views
+50

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 ...
5
votes
2answers
379 views

Viscous Burgers equation physical meaning

The viscous Burgers' equation: $$ q_{t}+q\:q_{x}~=~\nu\:q_{xx}, \mbox{ where } \:\:\nu >0, $$ combines the nonlinear propagation of $q(x,t)$ and the diffusion. What is this equation for? (in ...
1
vote
3answers
71 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 ...
3
votes
2answers
100 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?
0
votes
1answer
33 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) ...
2
votes
1answer
529 views

Equilibrium between two water tanks

I've been reading about hydrostatic theory, however I couldn't find a similar case/exercise about my basic problem. I have two water tanks (same size) filled with different water heights, and ...
0
votes
2answers
240 views

Viscosity for steam in the Reynolds number

In calculating the Reynolds number for a flow of steam in a pipe, this is the general formula I am trying to use: $$Re=\frac{\rho d v}{\mu}$$ with density $\rho$, pipe diameter $d$, flow steam $v$ ...
1
vote
1answer
42 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
votes
0answers
15 views

Difference between turbulence and instability

One of the tasks of my current research is to study the instability of a viscoelastic fluid in a co-rotating cylindrical system. I have observed some patterns but cannot say if they are unstable or ...
1
vote
1answer
32 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, $\...
0
votes
1answer
226 views

How to calculate fluid(oil / hydrocarbon) loss under pressure

I'm trying to calculate the amount of fluid that would flow through an area dependant on the amount of pressure that there is. I'd also like to know the rate at which it would flow. Essentially I ...
0
votes
1answer
236 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 ...
1
vote
2answers
193 views

Viscosity calculation of a rarefied gas

I am studying the rotational-translational relaxation of a diatomic gas (like oxygen) using a GPU in order to accelerate the calculations; during the calculations I get the translational temperature, ...
2
votes
0answers
25 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 ...
-1
votes
0answers
25 views

Is the change in viscocity of water at 1C vs 99C discernable visually?

Waters viscosity varies from 1.7 at 0C to .3 at 100C. Is this visually confirmable?
0
votes
0answers
26 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 ...
1
vote
0answers
448 views

An equivalent for the Bernoulli equation for viscous liquids

I was wondering whether there is any equation for viscous liquids (probably derived from energy and mass conservation principles) relating the pressure, rate of volume flow, area of cross section and ...
0
votes
1answer
28 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
72 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
41 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
65 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?
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 ...
0
votes
0answers
709 views

How to calculate the ship resistance caused by water viscosity

If there is a ship going in the sea at 50km/h, and a length of 5m, width of 2m, how do I calculate the ship resistance caused by water viscosity? in other words I want to calculate the drag Force that ...
3
votes
3answers
44 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
votes
0answers
28 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
39 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
votes
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 ...
1
vote
1answer
80 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
53 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
34 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
45 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 ...
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 ...
2
votes
4answers
68 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 ...
0
votes
1answer
58 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
2answers
10k views

Determine viscosity using falling sphere (Stokes Law, Ladenburg correction)

Introduction I am trying to determine the viscosity of a fluid. Therefore, I let a sphere of known mass m and radius r fall (...
1
vote
1answer
57 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 ...
1
vote
3answers
116 views

Shaking water inside bowl causes waves but why does the water stabilize?

Suppose you shake water inside a container, then at first the waves goes up and down strongly but they gradually dissipate. What makes them dissipate?
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
74 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
77 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 ...
4
votes
1answer
236 views

Explicit form of the entropy production in hydrodynamics

I'm trying to understand how hydrodynamics arise from a precise, mathematical formulation of thermodynamics, learning mostly from Landau's "Hydrodynamics". So Landau starts from formulating the ...
7
votes
4answers
5k 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 ...
0
votes
2answers
79 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 ...
8
votes
3answers
2k views

How is viscosity described on the molecular level?

What is the 'molecular' origin of the viscosity? The molecular origin of elasticity is almost clear for me: at the very bottom the 'elasticity' comes from the attraction and repulsion between atoms ...
1
vote
1answer
53 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
36 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?
2
votes
1answer
100 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
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 ...