On the flow unsteadiness and operational characteristics of a novel supersonic fluidic oscillator

Abstract

A novel supersonic jet oscillating method is investigated both experimentally and numerically. A rectangular primary supersonic jet is issued into a confined chamber with sudden enlargement. Secondary control jets are issued from the top, and bottom backwards-facing step regions formed due to sudden enlargement. The secondary jet also expands in the confined chamber shrouding the primary jet from the top and bottom sides. The primary jet is oscillated in the transverse direction by blowing the secondary jets in the streamwise direction in a pulsating manner with a phase shift. The out-of-phase secondary jet blowing causes the primary jet to periodically adhere to the upper and lower part of the confined chamber, causing flapping of the primary jet and acting as a supersonic fluidic oscillator. The supersonic jet oscillation characteristics are experimentally investigated using shadowgraph type flow visualization technique and steady and unsteady pressure measurements. Quantitative analysis of the shadowgraph images using the construction of $y-t$ and $y-f$ plots reveals the presence of periodic jet oscillation with a discrete dominant frequency similar to the secondary jet excitation frequency. The existence of linearity between the excitation frequency and the flapping jet frequency on the low-frequency ($0.66-6.6$ Hz) side is first proven experimentally. Later, the high-frequency ($16.67-5000$ Hz) operation extent of the supersonic fluidic oscillator is further demonstrated using unsteady computational studies owing to the existing experimental facility's limitations. It is found from the computational studies that there exists a limiting oscillation frequency for the present fluidic oscillator (nearly $4.486$ kHz with the particular geometric size and the injection momentum considered in the present study).