Correlated Pressure-Velocity Instability in a Transonic High-Pressure Turbine Blade

The present work describes the coupled pressure-velocity instability in the suction side boundary layer of a transonic high-pressure turbine blade. The application case is the VKI LS89 turbine blade MUR237. Modal decomposition techniques such as the Proper Orthogonal Decomposition (POD) and the Dynamic Mode Decomposition (DMD) are applied to data obtained by large eddy simulations (LES) to inspect the existing correlation between pressure fluctuations induced by travelling pressure waves and the velocity disturbances growing into the boundary layer. To this end, POD and DMD were applied to both velocity and pressure fields from the leading edge up to the end of the blade surface. DMD was computed on a 2D slice oriented parallel to the blade surface inside the boundary layer. Data were ordered along the curvilinear abscissas, so that the most spatially amplified waves were obtained. Interestingly, unstable velocity modes were detected in the front blade suction side where pressure waves dominate. The unstable velocity modes computed upstream of the peak suction highlighted the occurrence of elongated streaky structures showing spanwise fluctuations which are typical of their instability. The comparison of pressure and velocity modes highlighted the occurrence of pressure-related fluctuations in the velocity field at the same time instants at which streak instability is observed. The analysis carried out in this work suggests that pressure waves can provoke localized instability of boundary layer streaks, which then break up further downstream causing transition.