The spatial modulation of a forced triangular jet

Abstract

An investigation of the possibility of controlling the evolution of jets into the far field is presented. Driven by practical concerns the study examined a highly turbulent jet flow. To enhance controllability of the flow evolution the virtues of non-circular nozzles and active flow excitation were combined. The study examined an air jet, Reae = 8ooo, average turbulence intensity 1.8% issuing into stagnant room air out of a triangular nozzle, which had piezoceramic actuators mounted on the flat sides. The evolution of the jet flow field was examined over the range of o <~XID <~ 3o.Small amplitude, single mode, excitation with frequency as the varying parameter was found to be ineffective for controlling the far field evolution. In contrast, excitation of the jet with non-integer and counter propagating azimuthal modes yielded marked changes in the jet streamwise evolution. The most notable changes in the far field were the transition of the cross section from round to elliptical, increased jet cross sectional area based on half centerline velocity contour, asymmetric threedimensional flow, and an increase in the entrainment rate. The entrainment of ambient air by the jet increased slightly more than twofold for non-integer and counter propagating azimuthal modes, compared with the unexcited jet, and only a 5o% increase in the entrainment for single, integer, mode excitation.While excitation of the jet with modes m = o and I resulted in symmetric evolution of the jet in the x-y-t space, excitation at non-integer and counter propagating modes resulted in time-dependent asymmetric motion. The near field induced jet column motion is controlling the far-field evolution of the examined jet.

On starting square jets and forced plumes

“…More recently, strong instabilities of a jet column were observed when a square jet was forced using azimuthal modes excitation effected via amplitude modulation, although detailed analysis about the effects of nozzle geometry on the jet dynamics was not provided (Wiltse and Glezer 1993). In later work, spatial mode excitation had been implemented on triangular jets and elliptic jets with various initial velocity profiles (Vandsburger and Ding 1995;Ding 1995). It was shown that the initial velocity profiles have significant influence on the effects of such combined excitations in terms of the flow characteristics associated with nozzle geometry.…”

“…(Sforza et al 1966, Trentacoste & Sforza 1967. Later studies of jets emerging from noncircular nozzles with corners showed that the introduction of sharp corners in the nozzle can increase significantly the fine-scale turbulence at the corners relative to the flat segments of the nozzle (Schadow et al 1988, Toyoda & Hussain 1989 and enhance mass entrainment (Vandsburger & Ding 1995). In some cases, the small-scale turbulence was produced by threedimensional flows, which were formed at the corner regions inside the nozzle (Su & Friedrich 1994).…”

“…rectangular (Sfeir 1975, Husain & Hussain 1983, Quinn 1989, Austin 1992, square (Quinn 1992, Grinstein et al 1995, Vandsburger & Ding 1995, triangular (Schadow et al 1990, Mi et al 2000, lobed (Wang et al 2001, Zaman 1994 and elliptic (Dhanak & Debernardinis 1981, Ho & Gutmark 1987. Koshigoe et al (1989) conducted linear stability analysis on different initial conditions that might promote the non-uniform growth behavior underlying axis switching.…”

Shock structures of underexpanded non-circular slot jets

“…The sharp corners lead to the generation of high-frequency turbulent eddies, and the edges shed large-scale coherent structures, thus resulting in higher spread along the edge than that along vertex [9]. The small-and large-scale mixing is essential in combustion systems [10], and to improve the entrainment rate [11]. Further, the triangular jets introduce ''high-instability modes into the flow via the non-symmetric mean velocity and pressure distribution around the nozzle'' as discussed by Schadow et al [12].…”

Flow structure and turbulence in near fields of circular and noncircular jets

“…One point that is not fully investigated in previous studies is the e¨ect of corner radii in noncircular jets and this point is indicated to be a further research topic by Quinn [4]. The existence of sharp corners in the nozzle can increase the ¦ne-scale turbulence at the corners relative to the §at segments of the nozzle as indicated in [5,6], and enhance mass entrainment signi¦cantly in [7].…”