A quantitative laser sheet image processing method for the study of the coherent structure of a circular jet flow

A quantitative two-dimensional digital image processing technique is successfully developed to enhance qualitative flow visualization and to obtain quantitative results. The technique is applied to study some less known properties of the coherent structural interaction and evolution mechanism of a low Reynolds number circular jet flow under high level acoustic excitation. Before processing the quantitative data, many inherent errors and uncertainties of the instruments and the system are first discussed and corrected. In this research, the uniformity and the traceability of the flow marker are carefully tested, and the distortion of the imaging system and the fan-shape of the laser sheet are calibrated. Through the image processing technique, the spreading of the jet, the trajectory and the convective velocity of the vortex can be analyzed rapidly and simultaneously. By analyzing the constant jet fluid concentration contour, the mechanism of vortex roll-up and entrainment, which has been ambiguous by traditional pointwise measurements, are more solidly confirmed. Also, the detailed tearing process of the vortex and the evolution mechanism of partial pairing, which can not be clearly detected in the conventional flow visualization pictures, are made clearly visible and carefully delineated.A version of this paper was presented at the 11th Symp. on Turbulence, University of Missouri-Rolla, 17–19 Oct. 1988