# 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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 $v$ ...
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 ...