While a machine operates at the design point, the flow along all internal components is smooth, pressure distributions are homogeneous and no flow detachments occur. The circumferential velocity component at the turbine runner exit is small, thus the flow in the draft tube is perfectly stable.
The more we depart from this optimum condition, the more the irregularity of pressure distribution in some components will be accentuated. Also, the intensity of interaction between the stationary and rotating components increases. At the same time the swirl component of runner exit flow becomes more important, thus rendering the flow in the draft tube unstable; this gives rise to low-frequency pulsations, in particular at partial load.
If the turbine has to operate at still lower load – in the extreme case down to speed-no load – then an increasing amount of hydraulic power from the working fluid is dissipated inside the machine; the internal flow turns increasingly chaotic, resulting in severe stochastic loads on the machine components.