Influence of dual purging jets on interferometric measurement of optical path difference

Abstract

Incompressible large eddy simulations of dual purging jets at three Reynolds numbers of ReL = 1500, 1000, and 500 are performed, providing the database for simulated interferometric measurements subjected to the spatiotemporally varying temperature fields. The temperature is modeled as a passive scalar, and the index-of-refraction field is calculated from the air temperature and pressure using the modified Edlén equation. Based on wave optics, wavefront distortions of laser beams traversing the flow field are computed, and the optical path difference measurement is simulated. Statistical results of the velocity and temperature indicate distinctly different flow patterns at three Reynolds numbers. Spectral proper orthogonal decomposition is adopted to analyze the relationship between coherent flow structures and temperature wavepackets. There is a dominant frequency (St = 0.29) at ReL = 1500 and 1000, which is attributed to the Kelvin–Helmholtz instability, whereas the two jets have notable flapping behaviors at ReL = 500. The wavefront distortions are found to be insensitive to the Reynolds number. The effects of the flow region and tracing distance are examined, and the optical performances of different beam pairs for interferometry are evaluated. The optical beams passing through the outer shear layers of the dual jets and wall jets are distorted severely, resulting in large systematic and random errors in the interferometric measurement.