Visualization of Continuous Stream of Grid Turbulence Past the Langston Turbine Cascade

This paper describes a direct numerical simulation visualization study on the migration and distortion of a continuous stream of grid turbulence passing through a representative turbine cascade with and without being segregated by the blade leading edge. For the nonimpinging case, as the turbulent stream starts to enter the turbine cascade, there is a coarsening and reorientation of the small-scale and random grid-turbulence structures into short vortex tubes; inside the cascade, these short vortex tubes are further stretched into relatively long quasi-streamwise vortices. For the impinging case, very long streamvvise vortices on the pressure surface are observed that correspond nicely to the colored-fingers phenomenon reported in previous turbine heat transfer experiments. The spanwise locations of the observed vortices vary with time. The origin of such turbine pressure-side streamwise vortices is revealed using a relatively complete sequence of images documenting their life cycle. For the impinging case, the short vortex tubes formed at the stagnation are gradually distorted into the shape of a hockey stick. The long stick is nearly parallel to the blade pressure surface, whereas the short end is bent toward the freestream. This process can be well explained by invoking results from previous fundamental studies on simple stagnation flows and on the stretching and rotation of material lines by the mean strain-rate field under the assumption of rapid distortion. Taken together with earlier investigations on turbine distortion of migrating planar wake and migrating isolated turbulent block, it is quite likely that Taylor-Goertler centrifugal instability may not be an important player in the formation of pressure-side elongated streamwise vortices in turbomachinery applications.