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-   -   Initial prism layer height (http://www.cfd-online.com/Forums/engrid/132157-initial-prism-layer-height.html)

taxalian March 26, 2014 15:54

Initial prism layer height
 
Hi,

I have been using engrid now for quite a while and i always used default prism generation settings to obtain the viscous mesh. But at the moment i would like to control the initial prism layer height manually i.e user defined. I tried to modify the boundary layer parameters but to no success at all.

I am using Engrid V1.4 and the parameters related to prismatic boundary layer generation are seems to be default fixed values because if you try to tweak them engrid complains about that and mostly it gives some internal error and then you have to restart engrid:

relative height of first cell = 0.001 (default)
absolute height of first cell = 1.0 (default)
ratio last layer/farfield = 0.8 (default)

So now here the question is engrid uses kind of top - down prism generation approach. First of all the single prismatic layer is generated around the viscous wall. Then later on this single layer is divided into further prismatic layers. What if i want to have a specific boundary layer thickness say at trailing edge of the wing or at certain distance downstream over the flat plate or cylinder etc. How can i achieve this i tried to tweak these parameters and also some additional parameters but no improvement.

Thanks and regards.

ogloth March 27, 2014 04:32

Hello,

you cannot specify the total height of the boundary layer mesh. This used to be possible, but proved to nit be required. The farfield ratio leads to a decent transition to the far field mesh. Once you have an isotropic mesh (appr. ratio of 0.3-0.8) you don't need to use prisms.

An important parameter, however, is the weighting between relative and absolute height of the first cell. Relative size means the height is a fraction of the edge length on the surface and absolute is absolute. So, you can set this parameter to 0 or 1 -- anything in between didn't prove to be particularly useful ...

The next release will possibly have the option to prescribe the initial height in the same manner that the surface resolution is prescribed at the moment.

'hope this helps!

Regards,
Oliver

taxalian March 27, 2014 12:33

Quote:

Originally Posted by ogloth (Post 482366)
Hello,

you cannot specify the total height of the boundary layer mesh. This used to be possible, but proved to nit be required. The farfield ratio leads to a decent transition to the far field mesh. Once you have an isotropic mesh (appr. ratio of 0.3-0.8) you don't need to use prisms.

An important parameter, however, is the weighting between relative and absolute height of the first cell. Relative size means the height is a fraction of the edge length on the surface and absolute is absolute. So, you can set this parameter to 0 or 1 -- anything in between didn't prove to be particularly useful ...

The next release will possibly have the option to prescribe the initial height in the same manner that the surface resolution is prescribed at the moment.

'hope this helps!

Regards,
Oliver

Hi,
Thanks for your reply, when is the next engrid release coming.
Regards.

ogloth April 4, 2014 09:48

Hi -- sorry, but it took a bit longish to reply...

The next release will possibly be ready at some point in the summer.

We are currently trying to use TetGen as an alternative to Netgen which would enable a more "1-click" approach to the meshing process. I'd expect that to start being usable in the next week or two. So, if you are happy to play with an unstable version, you might have some of the features a lot sooner. I'll announce it on our homepage (via Twitter), as soon as it can be tested.

Regards,
Oliver

taxalian April 4, 2014 12:33

Hi Oliver,
That sounds great, just let me know about the release of unstable version so that i can test that one.
Regards,
Taxalian.
Quote:

Originally Posted by ogloth (Post 483888)
Hi -- sorry, but it took a bit longish to reply...

The next release will possibly be ready at some point in the summer.

We are currently trying to use TetGen as an alternative to Netgen which would enable a more "1-click" approach to the meshing process. I'd expect that to start being usable in the next week or two. So, if you are happy to play with an unstable version, you might have some of the features a lot sooner. I'll announce it on our homepage (via Twitter), as soon as it can be tested.

Regards,
Oliver



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