CONTROL OF COHERENT STRUCTURE IN COAXIAL SWIRLING TURBULENT JETS

Abstract

The purpose of this investigation is to explore the swirl jet characteristics and the possibility of using artificial means for excitation of shear layers with the application as swirl jet control. For this purpose, a subsonic jet facility and a mechanical excitation device are designed and fabricated for the low speed and plain perturbations. The major system components consist of concentric subsonic nozzles, swirl generators, and the excitation devices with straight lobes. The experiments are carried out at various swirl flow conditions and excitation modes. Three components of mean velocity and turbulence fluctuation measurements are carried out with wave excitation using a stereoscopic particle image velocimetry. The acquired data are presented in cubic plots and two-dimensional contour plots. Furthermore, the numerical analysis is performed to investigate the helical excitation effects on a relatively high speed region. The computed data are presented in two-dimensional contour pictures and the trace plots of particles. Including extracted vorticity, both the experimental and computational results are compared with the baseline at various conditions, and with the values reported in the existing literature. In general, axisymmetric swirling jets are unstable in the near field to all the excitation modes examined. It is shown that the overall response of the swirling jet to excitation is not only dependent on the wave mode number, but also strongly on its sign; meaning the spiral direction of the convex lobes with respect to the swirling jet. This confirms the previous theoretical results. Excitation at both plain and helical perturbations simultaneously affects the flow property distributions in the vortex core and the shear layer at the jet periphery. Especially negative helical wave excitation is considered as the effective way of mixing enhancement for swirling jets compared to the straight lobe perturbation. The preferred mode is the second negative helical excitation in the present work. Consequently, the knowledge gained from this research could benefit the fluid dynamic community by increasing the fundamental understanding of turbulent swirling flows and their effective control for the formation and breakdown of coherent structure.