INVESTIGATION OF ENDWALL SECONDARY FLOWS IN A LOW ASPECT RATIO TRANSONIC TURBINE, PART II: PASSAGE OBLIQUE SHOCK

With increasing exit Mach number through the transonic regime, increased losses in a turbine can be incurred due to the increase in shock losses. The transonic regime is specifically of interest as it exhibits both a loss bucket behavior as well as the potential for unique trailing edge shedding modes at midspan. As for the endwall secondary flows, there can be increased losses due to the increase in shock interactions with endwall secondary vortices. In this second part of the two part series, the variation in exit isentropic Mach number varied up to 1.15 where the endwall secondary flow development is compared between numerical predictions and experimental results for a low aspect ratio turbine (AR = 0.66 and Zw = 1.0). Experimental results are produced via testing in the SLU linear turbine cascade and include results of airfoil static pressures. Numerical analysis is completed using Cadence Fine/Turbo (RANS modeling with the SST k-omega turbulence closure model) and validated via the experimental data. Numerical predictions include airfoil static pressure loading, passage secondary flow development, shock interactions within the passage, as well as downstream mixing loss characterization. Results show the impact of the exit isentropic Mach number on the shock-vortex and shock-boundary layer interactions affecting the endwall secondary flow development. With increased exit isentropic Mach number, the flow exhibits a transition in the passage shock behavior having a direct impact on the endwall secondary flow development and shock-induced vortices within the passage. However, this work also shows the presence of a shock-vortex interaction with the passage vortex. As a result, the downstream mixing losses are then compared at six downstream planes (0.2 to 3.0Cx) where numerically predicted energy loss is assessed showing the increased rate of production at higher exit isentropic Mach number. The results of this part show evidence and support for a secondary flow model which details the endwall secondary flow development for an oblique shock structure within the transonic regime.