Simultaneous velocity and temperature measurements in gaseous flowfields using the vibrationally excited nitric oxide monitoring technique: a comprehensive study

Abstract

The performance of the vibrationally excited nitric oxide monitoring (VENOM) technique for simultaneous velocity and temperature measurements in gaseous flowfields is presented. Two different schemes were investigated, employing different methods to "write" a transient NO grid in the flow using the 355 nm photolysis of NO(2), which was subsequently probed by planar laser induced fluorescence imaging to extract velocity maps. We find that only one scheme provides full-frame temperature maps. The most accurate velocity measurement was attained by writing an NO pattern in the flow using a microlens array and then comparing the line displacement with respect to a reference image. The demonstrated uncertainty of this approach was 1.0%, corresponding to 7 m/s in a 705 m/s uniform flow. We found that the uncertainty associated with the instantaneous temperature measurements using the NO two-line thermometry technique was largely determined by the shot-to-shot power fluctuations of the probe lasers and, for the flows employed, were determined to range from 6% to 7% of the mean freestream temperature. Finally, simultaneous and local velocity/temperature measurements were performed in the wake of a cylinder in a uniform Mach 4.6 flowfield. The mean and fluctuation velocity and temperature maps were computed from 5000 single-shot measurements. The wake temperature and velocity fluctuations, with respect to the freestream values, were 15% to 30% and 5% to 20%, respectively. The spatial distributions agree with the results of computational fluid dynamics (CFD) simulations. Our results suggest that the VENOM technique holds promise for interrogating high-speed unsteady flowfields.