Investigating the Separation Bubble Behavior on the Suction Side of a High-Lift Low-Pressure Turbine Blade Using Proper Orthogonal Decomposition

Turbofan engines are widely used in modern aircraft. Low-pressure turbines are the heaviest components of turbofan engines, and reduction of their weights is very effective in improving the specific fuel consumption and overall efficiency of these engines. One of the methods of decreasing the engine weight is to decrease the number of blades which is accompanied by an increase of the blade loading. For this purpose, high-lift airfoils can be used. As the occurrence of flow separation is very probable in high-lift blades, the recognition of the location and size of the separation bubble is important to assess the energy loss of flow. In this research, T106D-EIZ high-lift cascade is simulated by two-dimensional Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with Shear Stress Transport (SST) turbulence model and γ-Re_θ transition model in two Reynolds numbers 200,000 and 60,000 at a constant isentropic exit Mach number of 0.4, which represent a typical flow condition in low-pressure turbine. The results show that when Reynolds number is high, the separation bubble remains small on the suction side and the separated shear layer returns to the blade surface, and the energy loss of flow decreases. On the other hand, at a low Reynolds number, the separation bubble grows and energy loss increases. Separation bubble is not directly detectable in an evaluation of pressure distribution. However, proper orthogonal decomposition of the pressure field provides the capability to identify flow structures including vortex stretching, the onset of flow separation, and flow reattachment. When the separation bubble is long, large vortical structures are formed on the suction surface. Release of these large vortices can increase the profile loss by more than 50 percent.