Chandra Sekaran
Ansys Employee

To constrain in tangential direction you should rotate the nodes into the local cylindrical coordinate system of that half cylinder. Something like:

local,11,1,x,y,z,    ! where x,y,z is the center of that half cylinder

nsel,s,....           ! select the nodes of that half cylinder or just the top line node

nrotate,all         ! rotate into local cylindrical coordinate system

d,nodeid,uy,0    ! fix tangential motion