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.