I’m doing a problem on minimizing mass of a hollow wind turbine tower. My design variables are the thickness of the wall and internal diameter. I’m having trouble finding constraint functions, since all that I could find (stresses, deflections, buckling) increase in value as both the design variables decrease, so if I pick two of them one will be “shadowed”/dominated by the other constraint. Does anyone have any creative ideas for another constraint, that preferably increases with wall thickness but decreases with internal diameter? Or maybe some other design variables combinations that relate to mass?