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
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