Small Gaseous Jets Visualized by Means of a Laser Speckle Based Background Oriented Schlieren Technique.

Abstract. Small size synthetic jets have been investigated by means of a Particle Image Velocimetry (PIV) technique and the obtained results compared with the visualizations obtained with the recently introduced laser Speckle Based Background Oriented Schlieren technique (Speckle BOS). The latter technique have been adopted in order to increase the sensitivity of the standard Background Oriented Schlieren (BOS) technique and visualize the density inhomogeneity (essentially the inhomogeneity of the refractive index of the gas) induced by a gaseous isothermal jets injected in a quiescent gaseous environment. Since the isothermal and low speed jets, the induced effects on the surrounding environment are so small as to require the adoption of a visualization technique as sensitive as the interferometry techniques, so the Speckle BOS technique showed a high sensitivity with a remarkable simplicity of execution of the reliefs. The gaseous flows studied are substantially two: the first was constituted by a synthetic jet (not stationary) generated by the use of a small speaker normally used in the earphones of the headphone audio; while the second jet is formed by steady state conditions butane jet, discharged by means of a cylindrical duct with an internal diameter of 2.5 mm. The latter jet, having a different refractive index than ambient air, has been used to investigate the ability of optical resolution of the adopted speckle BOS technique.