Three-component flow velocity measurements with stereoscopic picosecond laser electronic excitation tagging.

Nonintrusive three-component (3C) velocity measurements of free jet flows were conducted by stereoscopic picosecond laser electronic excitation tagging (S-PLEET) at 100 kHz. The fundamental frequency of the burst-mode laser at 1064 nm was focused to generate the PLEET signal in a free jet flow. A stereoscopic imaging system was used to capture the PLEET signals. The 3C centroids of the PLEET signal were determined by utilizing simultaneous images from two cameras placed at an angle. The temporal evolutions of the centroids were obtained and used to determine the instantaneous, time-resolved 3C velocities of the flows. The free jets with various inlet pressures of 10-40 bars exhausting into atmospheric pressure air (i.e., underexpanded free jet with large pressure ratios; Reynolds numbers from the jet ranged from 39,000 to 145,000) were measured by S-PLEET. Key 3C turbulent properties of the free jets, including instantaneous and mean velocities, were obtained with an instantaneous measurement uncertainty of about ${\rm{\pm 10}};{\rm{m/s}}$, which is about 2% of the highest velocities measured. Computation of higher-order statistics including covariances related to turbulent kinetic energy and the Reynolds stress component was demonstrated. The 3C nonintrusive and unseeded velocimetry technique could provide a new tool for flow property measurements in ground test facilities; the measured high-frequency turbulence properties of free jet flows could be useful for turbulence modeling and validations.