To increase the amount of separation in a centrifuge, various techniques are used to induce a vertical 'countercurrent' flow in the gas. Centrifuges are either 'subcritical' or 'supercritical.' A subcritical centrifuge rotor has a length to diameter ratio such that it runs optimally at an angular velocity below the first fundamental flexural critical frequency. At these critical frequencies, the rotational energy of the spinning rigid body is transferred into large displacements from the axis of rotation, breaking the rotor unless mechanical actions are taken to reduce the displacement amplitudes. A supercritical centrifuge operates above the first critical frequency, and avoids damaging effects associated with resonances by mechanical methods such as damping mechanisms and bellows (flexible joints connecting rotor tubes together that act like a spring).