Questions tagged [viscosity]

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Pressure dependence of viscosity

Does viscosity depend on pressure? If depends, is there any equation by which I can calculate viscosity at a specific pressure?
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1answer
54 views

How to estimate the difference in viscosity between 2 liquids without lab instruments?

Having 2 liquids I wonder which of the two is more viscous. I'm not looking at precise values, but something like roughly "twice as viscous". Without lab instruments, only using kitchen ...
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32 views

Stress and Strain lag for viscous materials

As stated here, In purely viscous materials, strain lags stress by a 90 degree phase. How can we derive this statement? Is it experimental? If not, which is its proof and what is the physical cause ...
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Why do fluids not accelerate?

A fluid flowing in a horizontal pipe must be flowing at a constant velocity because of the conservation of mass. However, considering how there would be a pressure and hence force acting behind the ...
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1answer
43 views

Ellis's viscosity law: Demostration

Show that the velocity profile of a fluid that obeys Ellis's viscosity law and that flows through a tube of radius R and length L is given by expression: $$ v(i)-v(i-1)+\Delta r\left(\phi_{0}+\phi_{1}\...
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Why does poured honey move periodically when reaching the surface it's poured upon?

Here's the picture, known to most of us: When honey is poured out of a container, as in the picture (look here for the flow in motion), we see a kind of periodic flow of the honey when it comes to a ...
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How does gravity, viscosity, IMF help a liquid spread on a surface?

If I have a bottle full of water and I turn it upside down at a small height above a flat horizontal surface while it’s cap is open, the water spreads on the surface. If I used a metal solid in place ...
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1answer
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Viscosity equations for rockets

I'm doing an experiment about the effect of viscosity on water rockets, and was wondering if there are any equations that include viscosity that are significant to rockets/water rockets.
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1answer
43 views

Motion of ball (air viscosity concerned)

Suppose a ball of mass $m$ is thrown vertically upwards from the ground. I understand that the speed-time graph would be somewhat like a distorted parabola. But what about the velocity- time graph (...
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Is there any relation between turbulent energy dissipation and pressure loss?

It is known that pressure loss depends on viscosity of fluid. However I learned today that energy dissipation rate in turbulent flow is independent of viscosity. It sounds like there is a ...
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Finding the force acting on a fluid: compressible Navier-Stokes equation

The compressible Navier-Stokes equation is given by $$ \rho[\frac{\partial\textbf{v}}{\partial t}+(\textbf{v}\cdot\nabla)\textbf{v}] = -\nabla p +\eta\Delta\textbf{v}+[\zeta+\frac{\eta}{3}]\nabla(\...
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Is it possible to solve for pressure loss in turbulent flow along a pipe purely analytically?

I understand it's possible to find pressure loss using formulas such as the Darcy Weisbach equation works with turbulence - but to my knowledge the friction factor is derived using empirical means. Is ...
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Why do we need to do work to maintain motion of a fluid?

On page 355 of principles of physics by Resnick walker and Halliday, the author writes: Work must also be done on the system (at the input end) to push the entering fluid into the tube and by the ...
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Is there a contradiction between the continuity equation and Poiseuilles Law?

The continuity equation states that flow rate should be conserved in different areas of a pipe: $$Q = v_1 A_1 = v_2 A_2 = v\pi r^2$$ We can see from this equation that velocity and pipe radius are ...
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1answer
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Why does fluid the backward curl as it gushes out from the nozzel in the forward direction?

Which phenomena is responsible for this kind of backward curled flow of the fluid (air)? Kindly if anyone knows about it,please explain.
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Mixing Liquids of Certain Viscosity

I need a very small amount of 2 cSt liquid for an experiment I am doing, but I only have 5 cSt oil and water which has viscosity of 1 cSt. Is it possible to mix the two up in the right proportion and ...
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1answer
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Is there a drag in a moving fluid due to pressure (reaction) from the walls of the tube?

Consider a tube of linearly varying cross section... Now consider a section of the tube...There are pressure on the ends of this section due to the fluid... Now consider the walls, there is a ...
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Temperature dependence of viscosity in liquids

I have been going through the literature on viscosity of liquids and their temperature dependence. The most commonly used forms of temperature dependencies are Arrhenius (eq. 1) and Vogel-Fulcher-...
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32 views

Derivation of Newton's experimental law (of restitution) by using wave equations

Newton's law of restitution states that when two particles collide heads on, the speed of approach is directly proportional to the speed of separation. The ratio is called the coefficient of ...
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1answer
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What is the function for the magnitude of air resistance/drag?

In John Taylor's Classical Mechanics, he gives a basic treatment on air resistance/drag. In order to do this, he models air resistance as $\mathbf{f}=-f(v)\mathbf{\hat{v}}$, and then Taylor expands $f(...
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1answer
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Is $b$ in the drag force formula $F=-bv$ constant for a certain medium and object?

I heard that $F=-bv$, where $F$ is the drag force, $b$ is the damping coefficient, and $v$ is the velocity of an object, can be used to calculate the drag force exerted on an object moving at a ...
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1answer
68 views

Why friction losses depend on velocity squared?

This is the reason that suction pipes of centrifugal pumps are most of the times much wider than the discharge pipes and that restriction of blood flow in arteries is always an alarming event. Does ...
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What dynamic viscosity to use in Hagen–Poiseuille for water/air rig

I have a rig that is fairly similar to a water column manometer to measure the pressure of an airflow. Before the air reaches the $U$-shaped part of the rig, the one with the water, there is a certain ...
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1answer
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Viscosity Question

If the fluid has zero viscosity, does that mean it is at rest? Would it be possible that the fluid is both at rest and has zero viscosity? Please explain.
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1answer
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What is the difference between viscosity and shear viscosity?

Are the terms shear viscosity and viscosity for fluids interchangeable, or do they mean different things?
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Does the shape of the pipe (the length not the cross section area) affect resistive force?

Does the shape of the pipe (the length not the cross section area) affect resistive force acting on water? Does keeping the pipe straight or keeping it in curves folded over each other effect the ...
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Is it correct to derive the fluid viscosity from momentum conservation law?

Some books assumes molecules flowing in the x direction for a stationary newtonian fluid, it is defined a microscopic control volume and they says in the top cover (of the box) at level $y_0$ ...
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501 views

Question About Speed of a Stone Underwater [closed]

Why does the speed of a stone dropped into a lake increase as it moves to the bottom of the lake?
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Would a solid object submerged in a super fluid conserve its momemtum as it travels?

Reading about Helium at low temperatures has me scratching my head in confusion. Liquid Helium-4 at around 2K exhibits properties of a super fluid. Viscosity of a super fluid is equal to zero, which ...
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2answers
54 views

Is theoretically possible to have the laminar flow above $Re=10^7$?

The onset of turbulent flow ranges greatly, for pipe flow I came across the information that the onset of turbulent flow occurs at approximately $Re=10^4$, while for boundary layers on the airplane's ...
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1answer
96 views

Difference between Viscous stress tensor and Shear stress tensor?

I am trying to learn about fluid dynamics in the context of incompressible, Newtonian fluids, Laminar flows. Unfortunately, different authors use different notations and every time I found my self ...
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1answer
35 views

Reynolds number of a submerged jet

Considering a jet of gas A into quiescent gas B how would the Reynolds number be defined? The characteristic lenght is the diameter of the jet. The velocity is also obvious. But what about the ...
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1answer
31 views

Visco-elasticity and dissipation of energy

I have a doubt with the integral of equation 1 shown in the picture and what's the meaning of Im in the integral. I don't have a good knowledge about visco-elastic theory so a simple explanation would ...
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Would the weight of an object increase if the pressure of the surrounding is increased? [closed]

I'm thinking something like if I take a box and place a weighing machine and an object inside it...then if i increase the pressure inside that box would i observe any change the weight of the object? ...
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How do I measure the viscosity for different percentages of water contaminated oil?

I am a high school student and would like to investigate how the percentage of water affects the lubrication of oil (water contamination in engine oils can negatively on the performance of the engine ...
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Effect of bulk viscosity on the equation of state

The equations of state may be obtained by looking at the energy-momentum tensor $T^{\mu\nu}$. A well known example is the ultra-relativistic limit (massless particles), which gives a pressure of $p=\...
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Obtaining pressure force in the Poiseuille's equation derivation

While deriving the poiseuille's equation for the flow of a liquid in a cylindrical pipe, one considers the force-balancing equation of viscosity and pressure forces across a pipe of radius $R$ and ...
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1answer
45 views

Is Stokes' law, for drag force in fluids, accurate?

In high school, I was taught that Stokes' law is dependent on assumption that drag force is proportional to velocity, viscosity and radius of the sphere (and the powers/exponents are evaluated using ...
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24 views

viscoelastic term in the linear elasticity equation with thermal expansion

I am treating a linear elasticity problem with thermal expansion. The constitutive equation reads: $$\nabla\cdot\sigma=C\nabla_S u-C\alpha\Delta T, \tag{1}$$ where $\sigma$ is the stress, $C$ is the ...
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1answer
28 views

Sign convention for Maxwell's model of viscoelastic material

Maxwell's equation for viscoelastic body is $\dot{\varepsilon}(t)=\frac{\dot{\sigma}}{E}(t)+\frac{\sigma}{\eta}(t)$, where $\varepsilon=\frac{dl}{l}$ is strain and $\sigma$ is stress. I am a little ...
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1answer
403 views

Deriving Stokes' law ($f_v=6\pi\eta Rv$) in a simple way

Is it possible to derive Stokes' law (Viscous force on a spherical body moving in a fluid $f_v=6\pi\eta Rv$) without using the "$\nabla$" operator (at least not in that form) or other theorems/laws ...
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1answer
80 views

Linear or quadratic damping term in Euler's equation?

In the Euler equation with frictional damping $$ \begin{cases} \rho_t + (\rho u)_x = 0 \\ (\rho u)_t + (\rho u^2 + P(\rho))_x = \color{red}{-\alpha \rho u} \end{cases} $$ sometimes I see a different ...
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82 views

What is the minimum speed that the car must travel at to avoid hearing the noise? [closed]

An aluminum sphere has been tied to the rear bumper of a 2005 Toyota corolla by a nylon string. When the car moves the sphere bounces on the road and makes a loud noise. What is the minimum speed that ...
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1answer
26 views

Equation for “tangential stresses across boundary”

I am having trouble deciphering this sentence from Batchelor's An Introduction to Fluid Dynamics. It is at the end of the second paragraph of section 4.2 on Steady Unidirectional Flows. Such a ...
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How viscous is the sun core?

How the heat getting transfer from centre to outside the core? Is convection dominant here or radiation heat transfer mode? Is viscosity a relevant parameter in describing the macroscopic motion of ...
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1answer
72 views

How to choose the characteristic length in Reynolds number formula?

If we have a car of length $L$ and a simple cuboid of the same length $L$, and assuming both objects are moving at the same velocity, does it mean the Reynolds number will be the same in both the ...
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1answer
30 views

Reason for fluid rotating with the container

When a container filled with liquid is rotated with constant angular velocity the surface of liquid becomes paraboloid in shape, but when we say fluid is ideal we consider fluid to be non viscous, if ...
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1answer
35 views

Dynamic or Kinematic viscosity

Utilising the formula $\eta = \frac { \left( 2\left( { p }_{ s }-{ p }_{ l } \right) g{ r }^{ 2 } \right) }{ 9v } $ Where $p_s$ is density of a sphere with radius $r$ traveling at velocity $v$ ...
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Why does going close to absolute zero take out friction? [duplicate]

I read in a physics book that when you get close to absolute zero, a liquid stops having friction. Why is that (if it’s even true, that is)?
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Fluid dynamics between with 3 infinite planes

I've been studying the dynamic of a fluid trapped between two infinite plates at speeds $v_1$ and $v_2$. So far, I understand that between those two plates, the velocity of the particles in the fluid ...

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