Holographic Studies of Shock Waves Within Transonic Fan Rotors

Abstract

NASA has funded two separate contracts to apply pulsed laser holographic interferometry to the detection of shock patterns in the outer span regions of high tip speed transonic rotors. The first holographic approach used ruby laser light reflected from a portion of the centerbody just ahead of the rotor. These holograms showed the bow wave patterns upstream of the rotor and the shock patterns just inside the blade row near the tip. Much of the region of interest was in the shadow of the blade leading edge and could not be visualized. The second holographic approach, on a different rotor, used light transmitted diagonally across the inlet annulus past the centerbody. This approach gave a more extensive view of the region bounded by the blade leading and trailing edges, by the part span shroud and by the blade tip. These holograms showed the passage shock emanating from the blade leading edge and a moderately strong conical shock originating at the intersection of the part span shroud leading edge and the blade suction surface. Due to a limited viewing angle, the bow waves upstream of the rotor could not be observed, and only limited details of the trailing edge shocks were obtained. The results of these studies were promising. Reasonable details of the shock patterns were obtained from holograms which were made without extensive rig modifications. These studies indicated several advancements that would give even better results. Larger viewing windows, and holographic plates would permit a wider viewing angle and give much more coverage of the regions of interest. Shorter time delay for double-pulsed holograms is also desirable. This would minimize blade movement and give clearer holograms. With these improvements of technique effective visualization of shock configurations at least outboard of part span shrouds should be possible. Accurate definition of shock configurations will aid in attainment of improved transonic fan and compressor rotors.