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Questions tagged [viscosity]

The viscosity of a fluid is a measure of its resistance to flow, or be deformed, stirred, and changed shape.

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In turbulent flow, where is the friction of the pipe exactly? Is it between the laminar layer and the pipe walls or the turbulent layer and laminar?

In turbulent flow as we get closer to the pipe walls we encounter an overlap layer and then a layer of laminar flow. Also at the walls of a pipe, in turbulent flow, the laminar shear stress dominates. ...
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Bernoulli principle (classic form) in irrotational viscous flow (a paradox?)

We derive the Euler equation for inviscid flow. Then, for irrotational flow, we use the Euler equation to get Bernoulli equation (classic form) and show it holds in the irrotational region (still ...
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How to conceptually explain diagonal elements of viscous stress tensor in a fluid?

How can the diagonal components of viscosity tensor be explained conceptually? In other words, how can viscosity affect the normal stress (pressure) on the cube element of the fluid? $$\sigma_{ii} = 2\...
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Recovering the classical Navier-Stokes equation from Landau's relativistic tensor [closed]

I'm trying to get to the classical compressible Navier-Stokes equation from the relativistic tensors in the Landau/Eckart frame. I understand these have their own problems about entropy. In several ...
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Is it necessary that a particle attains terminal velocity?

In a viscous medium, consider a ball travelling under the influence of gravity and viscous force. Is it always necessary that during its time in air, the ball attains terminal velocity
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Green Kubo Relation for viscosity

I am calculating viscosity of WCA fluid using the Green Kubo relation. I am also following the paper of Zhang et al. for the Time decomposition method https://doi.org/10.1021/acs.jctc.5b00351 where, ...
Lifelong Learner's user avatar
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Difficulties understanding fluid flow claims in the context of animal physiology/cardiovascular networks

A topic that has repeatedly given me confusion is the notion of fluid flow through animal vasculature. I find that many of the 'physics 101' basic notions of fluid dynamics are not well-suited to deal ...
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Does air create a high pressure zone once the low pressure zone is filled in?

So say you have a sphere of low pressure air (let's say a 0.5 atm sphere in a 1 atm environment). Air would rush to fill in the low pressure, therefore it would accelerate to the center of it. Wouldn'...
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Direction of shear-stress on fluid element

This image is regarding flow of viscous fluid between two parallel plates. I don't understand how they determine direction of shear stress above and below the element. As the flow is towards right ...
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Low Reynolds number flow and Potential flow

In an experiment conducted in my lab, we observed a low Reynolds number flow between two parallel plates with narrow gap between them resemble Potential Flow which is quite counter-intuitive as low ...
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Can frictional force be un-aligned with flow direction?

There is a well known solution to Navier-Stokes equations for atmospheric boundary layer according to Ekman (see for instance PalArya's book "Introduction to Micrometeorology" or Holton's &...
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Where did the idea for the approach to solving this boundary layer problem come from?

I found the derivation of Blasius-Equations here: A free stream velocity hits a flat plate and the goal is to derive boundary layer behavior. Everything in this tutorial is clear - but the most ...
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Name of Equation $Q = \Delta P / R$

A very fundamental equation in understanding fluid flow is $Q = \Delta P / R$. When the flow is through a cylindrical pipe of constant radius, $R=8\eta L/\pi r^4$ can be substituted to give Poiseuille'...
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Soap film retraction dynamics

Let’s say you have a planar soap film, which thickness increases linearly. If you burst it, the film will retrace and the retracing speed will decrease. How exactly does the velocity of bursting ...
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Can angular momentum be transported between scales in turbulent flows?

Consider a turbulent rotating flow. You are interested in its average features so you use the Reynolds-averaged Navier-Stokes equation. Now, conservation of angular momentum implies the viscous ...
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Energy of a compressible viscous fluid

I'm trying to derive an expression for the total energy of a compressible viscous fluid. I know that the energy per unit volume of a compressible inviscid fluid is given by the sum of its kinetic ...
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In Stoke's law how terminal velocity depending directly proportional to the velocity of object?

In common examples and understanding, like bullet firing, higher the velocity the distance traveled will be higher in a given time, but in the case of terminal velocity concept it is said that the ...
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When we calculate the total force of an object falling through a fluid we calculate it by the formula weight - upthrust

Why when we calculate net force on an object falling through a a fluid we generally do not include viscous force?
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If a damped mass-spring oscillator is equivalent to a resonant bandpass filter, then in audio signal terms, what is the input signal for both?

Background I am working on some personal audio processing and synthesis experiments in the sample domain. I posted here about how a resonant bandpass filter with a given $Q$ and frequency $f_0$ is ...
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Why is Volumetric stress change in pressure not final pressure?

While reading about volumetric stress, I found that volumetric stress on a body is equal to restoring force per unit area if force is normal to the surface and is proportional to the area. At ...
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Solving the decay $Q$ of transverse waves in a viscoelastic string based on this Kelvin-Voigt equation of motion?

Goal I am trying to solve a wave equation of motion for the transverse vibrations of a viscoelastic string fixed at each end to get the $Q$ (decay rate) of each harmonic (partial). The goal is to ...
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Modeling a viscoelastic string with a collection of damped spring oscillators? (To replace finite difference model.) How to find $Q$ per harmonic?

Background I have simulated a vibrating viscoelastic string fixed at each end under tension using finite difference modeling. Most simply this can be done using Kelvin-Voigt style mass-spring dampers ...
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Shear stress for non-linear vs linear velocity profiles

In this question, I would like to consider 3 scenarios: 1) Fluid of uniform viscosity between 2 parallel flat plates with linear velocity profile, 2) 2 different fluids with different viscosities ...
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Critical velocity of a fluid

Critical velocity($v$) of a fluid is given by $v = \frac{Rr}{\rho\eta}$ where, $R \rightarrow$ Reynolds number $r \rightarrow$ Radius of tube $\rho \rightarrow$ Density of fluid $\eta \rightarrow$ ...
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Reynolds number = inertial force/force of viscosity

In class 11 physics ncert Reynolds number Re= ρA(v^2)/ (ηAv/d) Where inertial force = ρA(v^2) And viscous force is = ηAv/d In the book d is described as dimensions of the pipe But they don't mention ...
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How is shear stress exerted onto a fluid?

I am currently studying fluid mechanics and one of the most important concepts we come across is that of shear stress. I understand that it is a force that is acting on the fluid in a direction that ...
Tan Yong Boon's user avatar
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What is exactly particle inertia in a fluid-particle mixture?

In a mixture of a cylindrical particle and the carrier fluid, what exactly particle inertia refer to? (let's neglect gravitation force and Brownian motion). When the size of particles are small (...
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Falling through a thixotropic ocean -- what would happen?

Drop a heavy incompressible object into water and it would splash and then presumably reach a certain terminal velocity where acceleration is nill. If you dropped a heavy incompressible object into a ...
Robert Law's user avatar
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The viscosity of Natural gas

At low pressures, gas viscosity increases as temperature increases, which is mainly due to increase in the intermolecular collision that is caused by an increase in molecular friction. However, at ...
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Why we use thin films of lubricants if viscous forces are inversely proportional to separation?

When layers of fluid move relative to each other, viscosity creates frictional force. wiki: magnitude of the force, F, acting on the top plate is found to be proportional to the speed u and the area A ...
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How does a fluid element have vorticity if the stress tensor is symmetric

Considering incompressible, Newtonian fluid as an example. The stress tensor is dependent on the symmetric part of the strain rate tensor, which is also symmetric. What I don't understand is, with a ...
Ishan Tandon's user avatar
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What can cause viscosity in a BEC gas?

I know that Bose-Einstein condensate is not always a superfluid. So what can cause viscosity in a BEC gas?
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Fluid dynamics of a balloon

This is a question which might not seem relevant to physics, but I was always curious if the shape of the balloon were to be not the shape that we all know of, when we release the tip will air flow ...
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Experiment about a sphere in a tube in presence of viscous resistance

Recently, I carried out an experiment at home with little equipment, but I can't get to the bottom of it. Consider a fluid-dynamic resistance force of intensity $F_L$ ($\vec F_L = - \alpha \vec V$) ...
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How to mathematically derive the pressure contribution to drag force over a with viscous contribution determined?

I've looked in a lot of books on how it would be possible to derive the drag force over a sphere using PDEs. I've also learned in my fluid dynamics class how to derive the drag force of a rotating ...
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How to mathematically derive the pressure contribution to drag force over a sphere after the viscous stress is determined? [duplicate]

I've looked in a lot of books on how it would be possible to derive the drag force over a sphere using PDEs. I've also learned in my fluid dynamics class how to derive the drag force of a rotating ...
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Sphere submerged in a thin layer of gel (fluid with yield stress) that is spread on a vertical wall

Imagine a uniform thin layer of a viscous gel sticking on a vertical wall. A small sphere is submerged in this gel. The gel has a yield stress (Bingham model). How can I calculate the yield stress ...
Movey Sasar's user avatar
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Force exerted by water flowing in a pipe bent into a circular arc

my school teacher told me that when water flows in a pipe, it exerts a force equal to $ρAv^2$ where $ρ$ is density, $A$ area and $v$ velocity. he further said that when the liquid enters the pipe, the ...
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Is the wing drag of induced drag + 2d drag at 0lift still accurate for cambered airfoils?

I have some work at uni that requires some aerofoil analysis to create a wing, but from the polar plots its clear that the cambered aeros have lower total 2d viscous drag (from xfoil) at the cl its ...
George kirby's user avatar
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Can an ideal fluid undergoes a turbulent flow?

I have read before that the existence of viscosity is the main reason for the appearance of vortex. However, by definition, an ideal fluid has $\eta=0$. So, can an ideal fluid undergo a turbulent ...
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Unexpected result: flux using Poiseuille greater than using Bernoulli

I have a simple, easy to solve, Poiseuille type of problem, to which I added a second question, in which viscosity is zero, so I can solve it using Bernoulli. I am arriving a greater flux using ...
everything.is.fine's user avatar
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How can the panel method be used to find drag and lift if it is for invicid flow?

If simple CFD softwear like XFoil and OpenVSP use the panel method to find an estimate of the drags and lifts/pressure distributions. How do they find drag and lift if the panel method works based on ...
George kirby's user avatar
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Terminal velocity of two raindrops

Question: "Consider a small (less massive) raindrop, and a large (more massive) raindrop (consider both to be spheres), falling with air resistance (drag). Which will have a larger terminal ...
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Can direction of frictional force be opposite to velocity?

The frictional force in incompressible Navier-Stokes equation is given by $$\vec F = \nu \nabla^2\vec v$$ Lets assume simple 1D-flow along $x$, where $v$ depends on $z$: $$v_x = v(z)$$ Then the ...
MichaelW's user avatar
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Viscous fluid under constant force?

For a one-dimensional fluid with viscosity $\eta$ subject to a homogenous acceleration $a$ in periodic boundary conditions, in my understanding the momentum equation is $$\rho\left(\frac{\partial u}{\...
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How to calculate damping coefficient of accumaltor?

I'm currently exploring the fascinating world of hydraulic accumulators and aiming to delve into understanding their damping characteristics. I'm curious if anyone can shed light on how to calculate ...
Ron Levi Yahid's user avatar
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How can someone visualize frictional losses in pipes?

Suppose there is constant diameter pipe where a liquid flows through it, and if pressure of a liquid is due to external force or its weight when it is constrained or semi constrained (according to the ...
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Viscosity of fluids near the surface

is the viscosity of a fluid maximum at the layers adjacent to the surface? I think this because the molecules of liquid now experience a greater friction force because the adhesive forces also resist ...
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How does one calculate the viscous term in the integral form of navier stokes?

I learned about the integral form of the navier stokes equation and I am trying to find an explanation for this viscous term at the end but I've searched everywhere and I can't find anything. The ...
Mitsos YT's user avatar
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In beer, why do bubbles travel faster upwards in the center?

This is a very odd question, but I just so happened to be observing a pint of beer and realized that the bubbles floating up seemed to travel faster towards the center of the glass than towards the ...
Corey Mitchell's user avatar

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