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Vortex dynamics of stator-rotor interaction

One of the most widely recognized unsteady phenomena taking place in a turbine rotor is the rotor interaction with the stator wake (S/R interaction). For a continuous wake, as modelled in the majority of the S/R interaction studies, the wake characteristics at the rotor inlet are time-independent and successive rotor blades chop off identical wake segments, which are then expected to behave in the same way when passing the rotor passage. This, however, is not true any longer when the wake comprises a sequence of separate, active vortices. Active structures interact with each other and with the boundaries, and  their resultant trajectories differ from those estimated on the basis of pure main flow convection. Moreover, even if steady-state parameters of the vortex wake, such as vortex strength and distance between vortices, are kept constant, the characteristics of particular wake segments chopped off by rotor blades for different phase shifts differ by initial distribution of vortices with respect to the chopped-off section (see the phase shift definition below). Consequently, the deformation of the wake on its way through the rotor passage  can take different courses.

   a)    b)

Phase shift definition (left), and extreme cases of wake deformation (centre, right) leading to the separation of an isolated vortex. a) φ=0,25 b) φ=0,50

This conclusion is of high importance for numerical analyses of S/R interaction done using URANS codes. These analyses are performed, as a rule, for a single realisation, and the question is whether their results are representative for a real process taking place in the turbine rotor. The answer to this question is positive and it was shown that in this case URANS calculations, performed on a sufficiently fine grid and making use of a numerically “averaged” stator wake, properly predict time averaged effects of the real course of the S/R interaction.

  

Velocity fluctuations at the vicinity of the rotor blade trailing edge.

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