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