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Lambda-2 criterion

Posted July 1, 2013 at 11:06 by sbaffini (NuTBox)
Updated December 21, 2016 at 09:10 by sbaffini

The lambda 2 criterion simply concerns the definition of the scalar lambda2 and how turbulent structures can be visualized by proper isosurfaces of lambda2 (like for the Q criterion). Hence, the real difference with the scalar Q is how you compute the scalar lambda2.

This is defined as the second (in magnitude) eigenvalue of the matrix:

$S_{ik} S_{kj} + \Omega_{ik} \Omega_{kj}$

where:

[LaTeX Error: Syntax error]...

Attached Files

lambda2.c (2.6 KB, 547 views)

sbaffini

Senior Member

Posted in LES

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3D Panel Method

Posted July 1, 2013 at 11:03 by sbaffini (NuTBox)

Some open/free 3D panel method tools:

APAME
http://www.3dpanelmethod.com/home.html

PANAIR (A502)
http://www.pdas.com/contents15.html (look for Panair - A502)

If you are looking for something more didactic, i suggest:

http://www.dept.aoe.vt.edu/~devenpor/aoe5104/

But it seems that the material is not available when the course is not active.

sbaffini

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The role of pressure for incompressible flows

Posted July 1, 2013 at 10:34 by sbaffini (NuTBox)
Updated December 21, 2016 at 09:11 by sbaffini

If we consider the continuity and momentum equations for a general compressible fluid:

$\frac{\partial \rho}{\partial t} + \frac{\partial}{\partial x_j}\left[ \rho u_j \right] = 0$

$\frac{\partial}{\partial t}\left( \rho u_i \right) + \frac{\partial}{\partial x_j} \left[ \rho u_i u_j + p \delta_{ij} - \tau_{ji} \right] = 0$

taking the time derivative of the first one and the divergence of the second one:
...

sbaffini

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Posted in Basic Fluid Mechanics and CFD

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Heat Exchanger Resistance

Posted June 28, 2013 at 07:31 by ryaron

The method I have given earlier is known as perturbation significance analysis.

Quote:

Originally Posted by ryaron

What are the ratios of the conductivities ?
i.e. k1/k2 ?
Here's a quick answer:
make two dimensionless parameters epsilon=k1/k2 and eta characteristic length tube/characteristic length plate, make sure they are << 1.

Denote them as epsilon and eta.
Now expand both expression you've written in powers of epsilon and eta.
The one with the higher order O(epsilon^n),

...

ryaron

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Workbench meshing - 1

Posted June 27, 2013 at 08:16 by kartrmswy

So I modeled (with some difficulty) a concentric heat exchanger pipe on ANSYS WB Design modeler, and found that it's not that difficult to start using. However, the main challenge encountered was in the mesh, wherein the automatically generated mesh proved to be too extensive (even with a coarse relevance center) and the total number of nodes was in the order of 20,00,000 - this was going to take very long even on the 8-core machine I was working on.

I need to change the mesh...

kartrmswy

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