A turbomachine assembly includes annular stator (59) surrounding rotor (44) circumscribed and rotatable about centerline axis. One or more non-axisymmetric partial arc annular circular stiffeners (60) are disposed around and on stator and may be helical. Non-axisymmetric partial arc annular circular stiffeners may extend about one third to one half of a circumference, be hollow, and located over one or more blade stages of rotor. One or more annular mid-span flanges (70) may be attached to stator and axially positioned within the hollow partial arc annular circular stiffeners. One or more masses or externals may be attached to the stator or to at least one of the one or more annular mid-span flanges between ends of partial arc annular circular stiffeners. Annular stator and rotor may have matching frequencies causing rotating stator waves rotating with rotor. The stator may be fan casing surrounding fan rotor or labyrinth seal land (92) surrounding labyrinth seal teeth.