Changing height of DesignSpace during optimization

Hi, I have to optimize the natural frequencie of a component and got for this task a nearly unlimited design space. Only at the top and bottom of the part has to be Connections (plates), so there are nondesign spaces.

 

To get the optimum in the topology optimization the design space is as big as possible, but with this optimization Optistruct calculate only the optimum in this design space, because the two nondesign spaces at the top and bottom are fixed.

 So there is also the possibility that the optimum is somewhere between. To consider that my idea is to use two optimizations like topology and shape.

 

I have not found a solution how to change the height of my design space with shape optimization. I think Optistruct have to get a start value for example 40mm and an increment like 2mm so it can calculate all Options in any height. Furthermore for this example OptiStruct have to mesh the Designspace every time it Change the height again and automaticaly because otherwise it press the Elements to much togehter.

Is that possible or do you have any other ideas?

 

Thank you very much!