# Free size optimization and some questions relating to upper and lower limits and design variable parameter chosen in particular what mindim is and how to choose an appropriate one?

## Recommended Posts

Hi! I have performed free size optimization and it ran successfully. However I have some questions relating to some of the upper and lower limits and design variables required to enter. After checking the optimization book provided by altair I still do not completely understand in design variable in the parameter option the mindim is what exactly and how to choose it appropriately? As I have chosen different values and it shows different results. Moreover, I chose Volfrac as the optimization constraint and chose upper limit as 0.2 but I am not sure what this represents? As in volume option upper limit showed the volume in mm^3 but volfrac I am not sure what it represents.  Another main question I have is that when setting load collectors I can add as many but in Loadstep for load and constraint theres only two options. How can I add two loads and one constraint for my analysis? Please let me know I know this is a long one but your help would be much appreciated.  I am attaching my file over here and if you could please take a look to see if my results are appropriate or not?

##### Share on other sites

hi

1. mindim is a minimum member size manufacturing constraint. It penalizes the formation of small members and reduces checkerboarding effect. It is recommended that MINDIM be at least 3 times the average element size for all elements referenced by that DSIZE (or all designable elements when defined on DOPTPRM). The average element size for 2D elements is calculated as the average of the square root of the area of the elements, and for 3D elements, as the average of the cubic root of the volume of the elements.

2. Volfrac is a fraction of design space, expressed in the range between 0-1. Volfrac as the optimization constraint with an upper limit as 0.2 means the optimizer will utilize only 20% volume of the design space.

Moawez Zamir Awan likes this

##### Share on other sites

Hi!

Thanks for the detailed response. I have a question concerning mindim element size. How do I check the area of the elements? In Mesh check elements I can see a wide range of min and max lengths Is there an option to see the area of the elements?

Moreover, for the volfrac setting 0.2 as the upper limit means 20% volume should be retained or something like that? I am a bit confused about utilizing only 20% volume of the deign space.  When I perform with 1 as upper limit I see no difference in my analysis while with 0.1 I see a distinct element thickness analysis.

##### Share on other sites

Is this the way to calculate element area?

##### Share on other sites

The most convenient way to get the average size of elements is with TCL:

Note: in case mindim specified is smaller than the actual average element size, it will be set as the average size.

Yes, setting 0.2 as the upper limit volfrac  means 20% volume should be retained. Setting volfrac as 1 is a trivial case wherein all of the available design space is utilized and the optimized configuration is the same as the initial.