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Questions tagged [turbulence]

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Characteristic scales in Thin Shear Layers Equations (Aerodynamics)

I am struggling to understand what characteristic scales mean in the context of thin shear layer equations. I am referring mostly to the following paragraph from the book "Turbulent Shear Layers ...
Bogdan's user avatar
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What are the forces acting on a rectangular polymer film submerged in a turbulent flow (stirred tank)?

I am new to research in fluid mechanics and for one of my experiments, I'd like your help. I need to write a force balance on a polymer film in a cylindrical tank at a fixed position. The film moves ...
flowcat's user avatar
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Dimples on a golf ball, dimpling on the Mythbusters car, and the terminology used common to both

This post is strategic. I have an answer to the Mythbusters dimpled car question which was asked elsewhere on SE, but since I have a 1 reputation score at the moment and cannot post its answer, ...
The DOuGTrainer's user avatar
2 votes
1 answer
68 views

Why are Reynolds Stresses time-varying?

The time-averaging operator definition gives us $$\overline{u_iu_j}=\lim_{T\rightarrow\infty}\frac{1}{T}\int_{0}^{T}u_i(x,t)u_j(x,t)dt.$$ From my understanding, this should mean that the Reynolds ...
Ats's user avatar
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Turbulent flow, fluctuating part of velocity

Let's say we have a fully developed turbulent flow on x axis (simple shear flow). Time-averaged components of velocity on y and z axis are 0. If a lump of particles is moving upwards, then the ...
Sok's user avatar
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1 vote
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Why is the turbulent kinetic energy $k=\overline{v_1'^2}+\overline{v_2'^2}+\overline{v_3'^2}$ even in 2D cases?

I'm writing an essay which partly includes writing about the Reynolds stress model where the dissipation term $\varepsilon_{ij}$ is assumed to be isotropic (since dissipation mostly occurs at small ...
Erik Sahlin's user avatar
1 vote
1 answer
97 views

Help with friction factors for non-Newtonian fluids

I've been researching about friction factors for non-Newtonian fluids and I've found quite inconsistent results. The power-law model correlations I've tested do not agree (within some interval) as to ...
Carlos Gouveia's user avatar
2 votes
2 answers
185 views

Imposing/ensuring a constant mass flow rate in turbulent flow simulation

I'm trying to simulate using Finite Elements (non commercial software) the Tube Bundle benchmark problem: http://cfd.mace.manchester.ac.uk/ercoftac/doku.php?id=cases:case078&s[]=tube&s[]=...
Terry's user avatar
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1 vote
1 answer
437 views

What is 'wavelength' with reference to turbuence?

When reading about turbulent flows and turbulence spectrum I frequently come across the terms 'wavelength' 'wavenumber'. I understand that the energy is transferred from larger eddies to smaller ones ...
GRANZER's user avatar
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1 answer
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Does turbulent river flow classify as uniform?

I found this meme in a facebook group on engineering concepts: I selected A (steady and uniform). In selecting A I assumed the width of the river to remain constant along its path and no parameters ...
Jules Manson's user avatar
3 votes
1 answer
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Ideal funnel design for powders

I'm designing a funnel/bottle for a powder. My goal is to have all of the bottle's powder contents come out when inverted. However, the powder has a tendency to stick to itself (like flour or brown ...
Davbog's user avatar
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Distribution of turbulent energy dissipation in a stirred tank

Is detailed experimental information on the distribution of energy dissipation in a stirred tank(45o Pitch blade) available? I require this variation in all directions in a standard stirred tank ...
Saad's user avatar
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5 votes
1 answer
570 views

Von Karman mixing length $l=k \frac{du/dy}{d^2u/dy^2}$

In a fully developed turbulent flow of an in-compressible fluid inside a pipe of radius $R$, velocity at the center is $U_m$. If we define $U^*=\sqrt{\tau_0/\rho}$, where $\tau_0$ is the wall shear ...
Ghartal's user avatar
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3 votes
2 answers
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Turbulence model for low reynolds problem

I'm simulating the flow in a capillary with a sudden expansion. The small capillary has a diamater of 250 µm. The big capillar has a diamater of 1000 µm. My flow rate ist $ 1\frac{ml}{min} $. Using ...
Gesetzt's user avatar
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267 views

Why Taylor's microscale gives us information about the smallest scales of the flow?

So I understand the principles of two-point time correlations (covariance) and how you can just normalise that to get the autocorrelation. I also understand the integral time scale which is quite ...
thephysicsguy's user avatar
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Direction of rotation of turbulent eddies in channel flow

Suppose we have a isotropic homogeneous turbulent channel flow. We know that this flow is associated with many swirling eddies of different length and time scales. I need to know the direction of ...
Mohammad M Faghih's user avatar
2 votes
0 answers
263 views

Obtaining the correct flat plate turbulent boundary layer thickness in a CFD simulation

I used a commercial CFD software to simulate incompressible turbulent flow of air over a flat plate. I setup a 2D domain 1.5m long in the x direction and .2m in the y direction. The boundary ...
Paul's user avatar
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5 votes
1 answer
560 views

How to measure turbulence parameters experimentally?

When running CFD simulations with turbulence on, I often need to specify inlet parameters such as the turbulent intensity, turbulent viscosity ratio and/or the turbulent length scale. If these ...
Paul's user avatar
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4 votes
0 answers
479 views

How should I set the turbulent prandtl number in a turbulent compressible flow simulation?

I'm working on simulating compressible fluid flow over a flat plate with turbulence. The energy equation for compressible flow with turbulence requires modeling a turbulent heat flux of the form $$...
Paul's user avatar
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6 votes
1 answer
3k views

Deriving the turbulent kinetic energy equation

I'm trying to understand the derivation of the turbulent kinetic energy equation, as described in this link: Evaluation of RANS turbulence models for flow problems with significant impact of boundary ...
Paul's user avatar
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10 votes
2 answers
380 views

Best way to get rid of turbulent flows?

I am making a testing facility for pumps. This facility has to be able to handle 500 m3/h with at little turbulence as possible. The basin itself is 5 meters long, 2,5 meters wide and 2 meters tall. ...
Arjo's user avatar
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4 votes
2 answers
684 views

How safe is this assumption for turbulence modelling in porous media?

I am studying the performance of volumetric solar receivers (specifically ceramic foams) versus the change of many parameters: solar flux, surface temperature, porosity, pore diameter, flow pressure ...
Algo's user avatar
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7 votes
4 answers
15k views

Ways to reduce turbulence in internal flows

Often it's desirable to reduce the turbulence level in an internal flow without changing the Reynolds number or fluid. This could be measured by the turbulence intensity, though other measures might ...
Ben Trettel's user avatar
5 votes
1 answer
329 views

Wall functions and separation in turbine blade simulation

I am simulating a turbine and I have some question about standard wall function in FLUENT. Standard wall function requires $y^+$ value between 30 to 60 on the blade. but the following quote states: ...
user19061's user avatar
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8 votes
1 answer
375 views

How do I select an appropriate turbulence model?

Turbulent flow involves extremely small scale fluctuations which are usually too small to resolve by using the Navier-Stokes equations directly. To overcome this, we model each transient variable $\...
Paul's user avatar
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5 votes
1 answer
834 views

What is a non-equilibrium boundary layer?

I studied a manual of Ansys CFX about turbulence models and I came across the term "non equilibrium boundary layer". What does this term mean?
user19061's user avatar
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7 votes
2 answers
8k views

Is inviscid flow necessarily turbulent?

The Reynolds Number is defined as $Re = \frac{\rho v L}{\mu}$, where $\rho$ is the density, $v$ is the fluid velocity, $L$ is the characteristic length, and $\mu$ is the dynamic viscosity. By ...
Paul's user avatar
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6 votes
2 answers
438 views

Why does the standard $k-\epsilon$ model use zero equation models?

According to Hoffman, the standard $k-\epsilon$ turbulence model incorporates a two-layer approach (inner region and outer region formulations to reperesent mixing length) when it comes to regions ...
Algo's user avatar
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