TURBULENCE GENERATION MECHANISMS IN COMPRESSOR BOUNDARY LAYERS

Turbomachinery flows are highly unsteady and feature complex interactions between stationary and rotating blades coupled with high freestream turbulence effects. This unsteadiness can lead to a variety of transition mechanisms. A detailed investigation of turbulence production under various levels of freestream unsteadiness in attached and separated boundary layers may guide the definition of "laminar" and "turbulent" kinetic energies for the development of highly accurate transition models based on laminar kinetic energy concept. For this purpose, high-fidelity scale-resolving simulations of a compressor blade-row exposed to turbulent inflow conditions were studied. The conditions were chosen such that the suction side boundary layer was either predominantly attached or separated. Using a turbulent event recognition technique a local laminar-turbulent discretization of the flow was obtained providing the reference value of the local boundary layer intermittency. Then, the contribution of turbulence production associated with different flow scales was computed for the attached and the separated boundary layers. Comparing the local levels of turbulence production with the local value of the boundary layer intermittency function allowed classifying the individual flow features as "laminar" and "turbulent" depending on their role in transition. It is shown that the fluctuations originating from vortex shedding in separated boundary layers should be classified and included in the turbulent kinetic energy budget. On the other hand, low frequency shear layer fluctuations and streaky structures are seen to mostly contribute to the laminar kinetic energy budget. Data presented in this work is expected to provide insight into the turbulence generation mechanisms in turbomachinery flows and can be adopted for the future construction of physics-based transition models.