Some laser measurement techniques used in aero engine research

Holographic flow visualization and laser anemometry are described. Flow within a fan blade passage can be visualized in three dimensions by recording two superimposed holograms on the same film, using a double pulse laser. Interferometric fringes formed by the images represent a shearing of the density field with respect to itself, caused by rotor movement during the separation of the double pulse laser. Shocks, overtip vortices, wakes, shock interaction with the boundary layer, and separated boundary layers can be observed. The anemometer measures the time taken for particles to cross between two focused laser beams separated by a known amount. The backscattered light from each beam is recorded by two photomultipliers and the discriminated events from the two beams are cross correlated. Correlograms of events for selected angles of orientation of the two spots at gated positions in rotor passage provide a spatially localized, but time averaged, value of flow velocity.