Preliminary study of oscillating triangular jets

A visualisation experiment and measurements with a Pitot probe show that fluidic instability of a partially confined triangular jet can produce a continuously oscillating flow. The device which produces this flow consists of a triangular inlet orifice expanding into a short axisymmetric chamber with an exit lip. These oscillations occur over a wide range of chamber lengths and expansion ratios. The amplitude of oscillation and the initial spreading angle of the jet flow are much larger than that of a simple jet flow. The external oscillating jet has preferred azimuthal directions which are aligned with the corners of the triangular orifice. The energy loss coefficient of these jets is significantly lower than what can be achieved from a precessing circular jet. Introduction Oscillating-jet flows can produce turbulence very different from the fully developed turbulence of simple jets. Very large velocity fluctuations produced by oscillating jets are the source of large scale turbulent mixing [8,16], large spreading angles [6,10], large initial decay rates of mean velocity [1,3] and high rates of mass entrainment from the surroundings. Mechanical devices [14,15] can drive large-scale oscillations of the bulk flow but they require high levels of maintenance and have limited durability in harsh industrial environments. In contrast, acoustic devices excite the growth of regular oscillations within the jet shear layers [9,12] but they are also very noisy. Devices such as the fluidic precessing jet (FPJ) use aerodynamic instability to produce large-scale oscillations in the flow. They present fewer practical difficulties than mechanically or acoustically driven devices and they operate successfully in hightemperature environments such as cement [11], alumina, zinc oxide and lime [2] kilns. The success of the FPJ has encouraged interest in other ways of producing fluidically excited flows [4]. Ther jet f mov Osc Figu oscil inter adju of va The the f defin with and inve expa With expa it is the f both exit Exp The 200c regu with wide The diam the straw scree the f deve inlet the j ceili flow 3.5 dow (a) Orifice y Lip