Assimilation of EFD and CFD in Analysis of Complex Internal Flow

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An analysis method for complex internal flow fields has been developed by combining experimental fluid dynamics (EFD) and computational fluid dynamics (CFD). This EFD/CFD hybrid analysis was applied to unsteady three-dimensional separated and vortical flow phenomena in transonic and low-speed axial flow compressor rotors.At near-stall operating condition in a transonic axial flow compressor rotor, the spiral-type breakdown of the tip leakage vortex is caused by the interaction between the vortex and the shock wave. The vortex breakdown has the nature of self-sustained flow oscillation and gives rise to the large fluctuation of the tip leakage flow field, in terms of shock wave location, blockage near the rotor tip and three-dimensional separation structure on the suction surface. It was found that the breakdown of the tip leakage vortex leads to the unsteady flow phenomena near the rotor tip, accompanying large blockage effect in the transonic compressor rotor at the near-stall condition.A transient flow phenomenon of rotating stall inception in an axial flow compressor rotor has been investigated by the EFD/CFD hybrid analysis. Instantaneous distributions of the casing wall pressure were obtained experimentally by ‘Synchronous Field Measurement' using time interpolation. Unsteady three-dimensional separated and vortical flow structure was captured by unsteady Navier-Stokes flow simulation. To elucidate the unsteady flow phenomenon at the stall inception, vortex structures and separation topology were identified by the critical point theory. It was found that the spiral-type breakdown of the tip leakage vortex occurred at near-stall condition, and the cause of rotating stall was the leading edge separation on the rotor. Furthermore, the stall cell was found to consist of a tornado-type separation vortex linking from blade suction surface to the casing at rotating stall inception.