of more closely spaced bars and a transition zone 72 where the inner and outer zones meet the depths of the shallow grooves 56 and the deep grooves 60 will preferably vary with position at the inner diameter 29 as shown in fig3 the difference in depth between the shallow and deep grooves will be at its minimum for example the shallow grooves may be eighty one percent of the depth of the deep grooves at the transition zone 72 the depth of the shallow grooves may decrease while the depth of the deep grooves remains constant to a ratio of for example seventy one percent between the depth of the shallow and the deep grooves at the outer diameter 33 as shown in fig5 the ratio may be fifty seven percent the depth of the deep grooves at the outer diameter may remain constant or it may be decreased to increase the rigidity of the bars 50 the grooves 56 60 may be formed to have gradually decreasing depth as they extend radially outwardly or they may be formed to have step like discontinuous changes in depth at zone changes throughout the disk the depth and or the width of the grooves can be varied as required for optimizing the refining and flow requirements for the specific application for example it may be.