iMechanica - FEM - Comments

Why FEM is not used in CFD

Edwin Vollebregt — Sat, 22 Nov 2008 03:46:33 -0500

I just came across this nice discussion, even though it was posted a long time ago.

In my opinion you are quite critical with respect to the "complex geometry" advantage of FEM. FEM is very good at handling complex geometries because it can handle triangles. FDM is very awkward with triangles. FDM can handle complex geometries by using generalized curvilinear/boundary fitted coordinates, by variable grid sizes and by local grid refinement, but none of these are easy to do.

Then why is FEM not used in CFD? Simply because directional derivatives are difficult on an unstructured grid. This concerns transport phenomena: advection processes. Advection is discretised much easier on a structured (i.e. locally orthogonal) grid, particularly when you want high accuracy and be able to capture shocks as well (e.g. using flux limiters). Probably FEM-people have found appropriate ways to deal with advection too, but not as powerful and simple as it is done in FDM.

FVM methods are somewhere in between and combine some of the advantages and disadvantages of both. You can use rectangular volumes in large parts of the domain and polygonal volumes near boundaries or internal interfaces. At the expense of complex administration and complex approximation schemes, you may get flexibility *and* accuracy.

thank you very much Mr.

sameer2008 — Fri, 17 Oct 2008 01:43:09 -0400

thank you very much Mr. Temesgen

You should include the BC's

Temesgen Markos — Wed, 15 Oct 2008 12:29:15 -0400

Hi Sameer,

With out the BC's, the stiffness matrices are simply singular.
This isn't a numerical issue. Even when you solve a DE analytically your
solution is not uniquely defined until you supply appropriate boundary
conditions. So include the BC before you go ahead.

I am Temesgen and I approve this message!

thank you very much. i

sameer2008 — Wed, 15 Oct 2008 11:27:27 -0400

thank you very much. i will do the things that u told.

but if i dont include the boundary condition and simpliy find the stiffness matrix for each local tetrahedron and to get the global stiffness matrix, i just add the local matrices in there corresponding positions with respect to the global matrix, even after that it will show any problem?

for assembling the local matrices i just did these things

kk=zeros(total_nod,total_nod); global stiffnes matrix

kk(t(iel,:),t(iel,:))=kk(t(iel,:),t(iel,:))+k; where k is local stiffness matrix for t(iel,:) element

i am using matlab software

once again thank you ( if u have any suggestion plz suggest )

Re: Problem with 3D Fem sparse matrix ( plz help me)

Alejandro A. Ortiz — Wed, 15 Oct 2008 02:34:14 -0400

What I would do in your case is to try with a simple example done by hand. Thus, you know what numbers the stiffness matrix should have. There are some reasons to get a singular matrix. One of them might be that the global stiffness matrix is not variationally correct meaning that the essential boundary conditions are not properly included in your stiffness matrix. After you form the global stiffness matrix you need to modify it along with the force vector to include the essential boundary conditions. Otherwise the matrix will be singular. If this is not your problem then a wrong assembly procedure might be the cause.

Best wishes in your project,

Alejandro A. Ortiz

Positions for PhD Students in Mechanics: University of Trento

Luca Deseri — Tue, 16 Sep 2008 01:32:19 -0400

Hello DoQuangHuy

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-------

Luca Deseri, Ph.D.,

D.I.M.S.-Dept. of Mechanical and Structural
Engineering, University of Trento,
via Mesiano, 77 I-38050 TRENTO-Italy

email: deseril@ing.unitn.it

Skype: lucalucaluca
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Thank you I appreciate your

Bernard-Marie Feron — Wed, 16 Jul 2008 10:53:57 -0400

Thank you I appreciate your suggestion.

I've got an excel table showing clearly the comparison, i can transmit it to everyone who wants to.

In another way I can send demo cd's and also free them for 3 months for free so people can try and discover.

Just ASK.

This is just a suggestion.

RoozbehSanaei — Wed, 16 Jul 2008 10:26:36 -0400

Most of users are interested in more popular softwares may be because there is more information about them in the web. also chance of finding person who know that softwares is more. i suggest you to make a comparision table between your software and similar softwares to show you advantages and also disadvantages of this software. what is possible with your software which is not possible with other more popular softwares?

do you need ?

Bernard-Marie Feron — Wed, 16 Jul 2008 09:15:53 -0400

Hi, do you need infos?

Make a comparison table

Roozbeh Sanaei — Wed, 16 Jul 2008 08:05:44 -0400

Roozbeh Sanaei.

Iran University of Science and Technology.

Acoust. Lab.

Yes we are

Bernard-Marie Feron — Wed, 16 Jul 2008 07:15:00 -0400

For the story: Oofelie has been developed in one single C++ monoltihic and object oriented code.

This way was chosen to optimize the strong couplings.

We can answer to most of the demands and also more than ANSYS, but we have no industrial solution for High Frequency.

We also import geometries in Step Iges etc... We can use enven the integrated post treatment or others like ParaView.

The structure allows also other codes to dial with Oofelie Kernel.

Regarding Trainings , we have video tutorials, Chinese, english and Portuguese.

We have pdf with contents of training courses.

We also organize trainings here in belgium.

If you need more, don't hesitate to contact support@open-engineering.com

What about ansys comparison? You seem real competitors

proE — Wed, 16 Jul 2008 07:07:43 -0400

I've been checking on your website, I see that you have a full solution under the same interface, do you have any comparison table between you and ansys?

More further , do you have support and training courses available?

thnx

simple

Bernard-Marie Feron — Tue, 15 Jul 2008 10:53:55 -0400

Just visit our www.open-engineering.com , follow the link : ask free demo cd

academic licenses do exist, classroomkits also, sdk if needed and finally licenses for industry.

just ask !

regards

where and how much

proE — Tue, 15 Jul 2008 10:50:26 -0400

Hi oofelie, I'm reseracher, don't know about your solution.

How to get it and test it?

Do you have any demo license or academic licenses?

stiffness matrix calculation of incompatible elements

Zoeb Kaizar Lakdawala — Fri, 16 May 2008 00:32:22 -0400

I am a student of M. Tech. 2nd year. My project topic for dissertation is "Numerical & Experimental Estimation of Fracture Toughness of Some Materials using Compact Tension Specimen". In this dissertation I also wanted to develop a code which directly gives me the values of stress intensity factor just like ansys.

Actually I have develop an matlab programm (using linear quadrilateral element) which gives me the value of displacement at the cracktip using that i would be finding the displacement extrapolation method i would be calculating stress intensity factors

But the major hindrance in my matlab code is that when i am calculating the stresses at the crack tip there is large deviation in cracktip stresses shown by matlab code and ansys. Looking at the ansys theory reference it was found that apart from four shape function i.e, N1, N2, N3 and N4 they are using two more shape function N5 and N6 whose values are (1-s^2) and (1-t^2) s for x-direction and t for y-direction respectively.

Also such type of shape function is described in R. D. Cook, Bathe and Zienkiewics books which they are calling as non-conformal elements which are used to remove the shear lock effect of linear quadrilateral elements and having co-efficient as a1 a2 for x-direction and a3 and a4 for y-direction. But nothing has been stated about the values of ai (i varies from 1 to 4).

Hence Sir, I would be most thankful to you if you could give me guidelines regarding how to solve such type of FEM problem having non-conformal elements or u could give me reference where Problems are solved using Non-conformal elements I would be most thankful to you

Regards

Zoeb Kaizar Lakdawala
M. Tech (Industrial Process Equipment Design)
S. V. National Institute of Technology, Surat
Mobile : 09904288552