Analysis and Control of Unsteady Three-Dimensional Vortical Flow Structure in Complex Flow Fields

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A transient flow phenomenon of rotating stall inception in axial flow compressor rotors has been investigated by experimental fluid dynamics(EFD) and computational fluid dynamics(CFD). 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.In the axial compressor rotor with small tip clearance, the tip leakage vortex is so weak as to have little influence on the flow field. The leading edge separation dominates the rotating stall inception, which results in the short length-scale rotating stall. A short length-scale stall is of the part-span type occupying only a part of the annulus. Its cell structure consists of the tornado-l … More ike separation vortex linking the blade suction surface and casing wall boundary layers. The tornado-like separation vortex dominating the short length-scale stall cells has a large blockage effect. As the separation vortex grows and moves toward the pressure surface of the neighboring blade, it increases the incidence of the neighboring blade, thus bringing about a new leading edge separation on the neighboring blade. In the case of small clearance, the stall evolution depends on the stator-rotor gap. However, in the case of large clearance, there is little difference of the stall evolution among three gaps. From the measurement of the pressure and velocity field near the stall point, it seems that breakdown of tip leakage vortex occurs in the case of large clearance for three different gaps. It is supported from the axial velocity distribution at the rotor exit. From the pressure field traces just before the stall, we can find the distinctive flow phenomenon like rotating instability. The tip vortex plays a very important role for stall inception. Less