Questions tagged [turbulence]

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Why is turbulence dissipation $\epsilon$ defined this way? [migrated]

When deriving the turbulence kinetic energy equation $k$ by taking the trace of Reynolds stress transport PDE, this term appears : $$ 2\nu \overline{\frac{\partial u'_i}{\partial x_k}\frac{\partial u'...
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0answers
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Units of the Von Karman Energy spectrum?

The three dimensional Von Karman spectrum is defined as $$\phi_{ww}(k_x, k_y, k_z) = \frac{E(k)}{4\pi k^2 } (1-k_z^2 /k^2)$$ where $$\begin{align} E(k) &= \frac{55\Gamma(5/6)\bar{u^2}}{9\sqrt{\pi}\...
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1answer
70 views

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 ...
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1answer
125 views

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 ...
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0answers
13 views

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 ...
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1answer
285 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 ...
3
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2answers
79 views

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 ...
2
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0answers
248 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 ...
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1answer
81 views

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 ...
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229 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 ...
4
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1answer
380 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 ...
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376 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 $$...
6
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1answer
2k 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 ...
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2answers
204 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. ...
4
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2answers
577 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 ...
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4answers
8k 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 ...
3
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1answer
209 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: ...
7
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1answer
258 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 $\...
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1answer
580 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?
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3answers
7k 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 ...
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2answers
329 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 ...