topology optimization of wing rib

Hello 

so i'm working on my fyp which is about topology optimization of a wing rib. i've been working with Optistruct for a while and Im pretty familiar with the software and how to setup the optimization, but i still don't understand some parts

for now i'm just practicing on my model and i'm trying to work with different objectives and constraints to understand what can be a good objective and function for my optimization. 

here is my model that i'm working on now (i used 2D automesh) and for this case, i have chosen Min vol-fraction for my objective and displacement constraint on the node that the force is applied to(please tell me if this is a good objective and constraint) . When it comes to constraints, i don't understand what are the upper and lower limits and what do they correspond to.

For this case when I run the optimization, I do get a feasible design, but I still don't know how to view my results and final model. 

Im following the steps in the tutorial to find out how to view my results, and when im trying to View an Iso Value Plot of Element Densities, it is just removing material from the middle of my model and there fore not getting any reasonable structure. I also dont get what that current value of the density threshold is too as the number is different from every optimization i do. 

 

I would really appriciate any help you can provide as im running late for my project and i need to get answers asap. 

cheers

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