Automat­ing the Gen­er­a­tion of a Struc­tured Mesh for CFD Analysis

For the fully-auto­mated shape opti­miza­tion with CFD, one key task is to robustly automate the geometry and mesh gen­er­a­tion as well as the actual CFD flow analysis. Within one of our ongoing research projects, GAMMA, we recently had the chal­lenge to automate the gen­er­a­tion of a struc­tured mesh. 

Why Struc­tured Meshes?

Compared to an unstruc­tured mesh, struc­tured meshes allow you to tremen­dously speed up the flow analysis, and they gen­er­ally give you a better result accuracy. Both are great when con­sid­er­ing the inves­ti­ga­tion of large design sets with a high analysis quality. However, in the context of opti­miza­tion, gen­er­at­ing an unstruc­tured mesh can be auto­mated more easily — just hand over the geometry and off you go. This is typ­i­cally not the case for struc­tured meshes. Why? 

The Chal­lenge with Struc­tured Meshes

Well, the question is: How should your struc­tured mesh actually look for an arbi­trary shaped and feature-filled geometry? When con­sid­er­ing e.g. volutes for tur­bocharg­ers as given in our research project, there is a little feature in the geometry that is respon­si­ble for most of the headache — the tongue area. Here is a picture:

The tongue area is the location where the remain­ing geometry is merged. And then it gets a bit com­pli­cated for struc­tured meshes. For the main geometry, the struc­tured meshes can be defined quite well. Typ­i­cally, you have a good idea what it looks like. But how should it auto­mat­i­cally pass from the main geometry into the outlet area while having this tongue feature in between? There is some user infor­ma­tion missing. 

Support Geometry

If you think about how to generate a struc­tured mesh in such a flow volume, you quickly end up with the common idea of pro­vid­ing some helpful infor­ma­tion to your meshing system. You simply cannot do it auto­mat­i­cally in a gen­er­al­ized way for all types of complex geome­tries. And this is what we did: Instead of only handing over a new design can­di­date to the meshing system, we also set up a para­met­ric support geometry in CAESES® that serves as a guide for the block struc­tures. It inscribes some sort of logic how the mesh will be orga­nized. This support geometry is then trans­ferred to the mesh process in addition to the remain­ing volute geometry. The next picture shows an example of such a support geometry setup (click on it to enlarge it):

By means of this addi­tional infor­ma­tion, it is now possible to automate the struc­tured meshing process for such a complex geometry. Since these inner curves are para­met­ric objects and part of the volute model itself, they also change when you modify the design vari­ables of the volute. This is also possible for the non-GUI (batch) run where the geometry is created through a script within an opti­miza­tion, e.g. con­trolled by external opti­miza­tion tools. This makes it directly suitable for use in HPC envi­ron­ments which is also a require­ment in this research project. 

Gen­er­al­ized Solutions

Setting up the support geometry is an addi­tional task (plus creating the automa­tion script that copes with it). However, if you do it in a smart way for one class of geome­tries, then you have a good chance to recycle it for future projects with similar geome­tries. A related example of such a support geometry is the periodic flow volume of tur­bo­ma­chin­ery blades. This is some­thing the user sets up once and can then re-use it in all future projects, even though the blade shapes might look slightly different.

Download Tech Brief

A short summary of the CAESES® meshing automa­tion capa­bil­i­ties can be found in this tech brief (PDF). 

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