Defining boundary conditions for a periodic structure

Hello everyone,

I was wondering if you can help me with this. I'm trying to model a piezoelectric stack containing a number of disks as an actuator in comsol. For lower numbers of disks I can manually select the boundaries for which I want to apply the boundary conditions. However, for larger numbers of disks (e.g. 700 disks), this is not viable. For my case, I'm intending to apply an Electrical Potential condition on the upper surfaces of the disks, and a Ground condition on the lower surfaces of the disks. I'd be glad to hear your suggestions.

Thanks.