Experimental Extraction of Linear Disturbance in a Two-Dimensional Turbulent Jet and Comparison to a Linear Stability Theory

In a two-dimensional turbulent jet, there exists a lateral vibration of the high-speed jet core. In the present study, a periodic initial disturbance is introduced into fully developed turbulent jet from a slot mounted at a duct exit in order to control this vibration. The streamwise and lateral velocity fluctuations are measured with an anemometer with an X-type probe, and are filtered using a phase ensemble average technique based on the periodic initial disturbance. This vibration enables extraction of the fluctuation of the synchronized disturbance and restruction of its velocity field. The experimental result shows that the streamwise variation of the extracted amplitude draws a characteristic growth curve that has the exponential growth at the upstream and slow decay parts at the downstream. It is worth noticing that within a certain amplitude of the initial disturbance the growth curves are identical as well as the spatial distribution of the extracted fluctuations. Furthermore the disturbance amplitude is directly proportional to the intensity of the initial disturbance. A linear stability theory with parallel flow assumption well captures futures of the lateral disturbance. This apparent linearity of the disturbance keeps until the average velocity and the total velocity fluctuation starts to vary with sufficient amplitude of the disturbance. The existence of the linear mode in the turbulent jet implies that the large-scale disturbance can be regarded as an incoherent set of the linear modes.