the surfaces 8 have {HEAD ~ a plurality} of channels 9 of generally U shape which are uniformly distributed around {TAIL ~ the central axis} 4 and disposed so as to be concave in a radially outward direction 
the surfaces 8 have { HEAD ~ a plurality } of channels 9 of generally U shape which are uniformly distributed around { TAIL ~ the central axis } 4 and disposed so as to be concave in a radially outward direction
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH