A　comparison　of　the　mechanical　performances　of　radial　plate　and　concentric　clutches　is　made　on　the　basis　of:　torqueisize,　acceleration,　control　ratio　and　maximum　casing　temperature-for　both　a　single　clutch　pair　and　multi　plate　models,　all　with　double　acting　surfaces.　The　various　embodiments　examined　herein　assume　a　constant　rotor　and　case　thickness,　negligible　inertial　effects　of　the　fluid　and　the　clutches　are,　in　the　main,　always　slipping　steadily.　Electrical　heating　is　not　considered.　CFD　is　used　for　the　computations　and　is　verified　against　analytical　solutions　and　experimental　data　where　possible.　The　fluid,　which　may　be　MRS　or　ERF　is　taken　to　be　a　Bingham　plastic　with　temperature　invariant　plastic　viscosity　and　yield　stress.　Where　acceleration　is　calculated　the　initial　value　(when　full　excitation　is　just　applied　to　an　already　slipping　clutch)　is　given.　Input　and　output　shafts　are　not　included　in　the　comparisons　though　they　may　have　some　effect　on　acceleration　and　heat　transfer:　Casing　temperatures　are　taken　to　be　uniform,　as　they　approximately　will　be　in　the　steady　state,　as　highly　conductive　materials　of　construction　being　used.　Through　these　simplified　models　some　idea　of　this　relative　overall　performances　of　the　two　clutch　forms　can　be　gleamed.　Perhaps　more　importantly　the　CFD　method　is　shown　to　be　useful　for　detailed　investigation　of　more　specific　situations.　FLUENT　[l]　was　chosen　as　the　operating　programme　with　sub-routines　to　handle　the　non　Newtonian　nature　of　the　fluid.　No　regulation　of　the　input　drive　speed　due　to　loading　is　accounted　for.