Unsteady Reynolds-averaged Navier-Stokes computations of transitional wake/blade interaction

The interaction between wakes generated by a moving array of cylindrical bars and the transitional boundary layer over a low-pressure turbine blade located downstream of the bars is studied by means of an unsteady Reynold-averaged Navier-Stokes (RANS) method that incorporates a recent low-Reynolds-number, nonlinear eddy-viscosity turbulence model. The context of the study is the modelling of unsteady rotor/stator-interaction phenomena. A number of issues pertaining to the ability of the unsteady RANS approach to simulate this interaction within a phase-averaged framework are investigated, including numerical accuracy and resolution, the consequence of vortex shedding from the bars, the characteristics of the wake within the blade passage, and the ability of the model to return the transitional response of the boundary layer to the dynamics of and the turbulence transported by the wakes. The results of an initial investigation, in which both the moving bars and the blade are included in the computation, show this approach to be ill-suited to a phase-averaged framework. As an alternative, the wake conditions just behind the bar are extracted from a precursor simulation in which the wake is subjected to averaging before being prescribed, and this approach is shown to be both appropriate and effective. The evolution of the wake in the blade passage is well predicted with this method. Moreover, the unsteady wake-induced transition process is also well reproduced in comparison with experiment, but there are indications that the relative roles of the (essentially inviscid) wake dynamics and the turbulence within the wake in effecting transition and relaminarization in the boundary layer differ somewhat from those gleaned from the experiment.