A plenum for an apparatus for transferring energy between a rotating element and a fluid, the plenum comprising: a through hole extending horizontally through a wall of the plenum ; a plurality of inlet guide vanes disposed proximate a peripheral edge of the through hole , the plurality of inlet guide vanes comprising a first group of inlet guide vanes having a symmetrical profile, a second group of inlet guide vanes and a third group of inlet guide vanes, wherein each inlet guide vane of the second group and third group has a cambered profile, wherein each inlet guide vane of the second group has the same cambered profile, and wherein each inlet guide vane of the third group has a different cambered profile from each other inlet guide vane of the third group , wherein the inlet guide vanes of the first group are disposed proximate a top and a bottom of the peripheral edge of the through hole ; wherein the inlet guide vanes of the second group are disposed proximate a first horizontal side of the peripheral edge of the through hole and the inlet guide vanes of the third group of are disposed proximate a second horizontal side of the peripheral edge of the through hole opposite the first side; the plenum further comprising a ring disposed at least partially within the through hole , wherein the plurality of inlet guide vanes are coupled to the ring ; wherein each of the plurality of inlet guide vanes is rotatably coupled to the ring , and wherein each of the plurality of inlet guide vanes rotates about an axis of rotation of each of the plurality of inlet guide vanes ; characterized in that the inlet guide vanes of the second group have a comparatively weaker cambered profile than the inlet guide vanes of the third group . a through hole extending horizontally through a wall of the plenum ; a plurality of inlet guide vanes disposed proximate a peripheral edge of the through hole , the plurality of inlet guide vanes comprising a first group of inlet guide vanes having a symmetrical profile, a second group of inlet guide vanes and a third group of inlet guide vanes, wherein each inlet guide vane of the second group and third group has a cambered profile, wherein each inlet guide vane of the second group has the same cambered profile, and wherein each inlet guide vane of the third group has a different cambered profile from each other inlet guide vane of the third group , wherein the inlet guide vanes of the first group are disposed proximate a top and a bottom of the peripheral edge of the through hole ; wherein the inlet guide vanes of the second group are disposed proximate a first horizontal side of the peripheral edge of the through hole and the inlet guide vanes of the third group of are disposed proximate a second horizontal side of the peripheral edge of the through hole opposite the first side; the plenum further comprising a ring disposed at least partially within the through hole , wherein the plurality of inlet guide vanes are coupled to the ring ; wherein each of the plurality of inlet guide vanes is rotatably coupled to the ring , and wherein each of the plurality of inlet guide vanes rotates about an axis of rotation of each of the plurality of inlet guide vanes ; characterized in that the inlet guide vanes of the second group have a comparatively weaker cambered profile than the inlet guide vanes of the third group . <EOS>