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Title: Axisymmetric jet manipulated using two unsteady minijets
Authors: Yang, H
Zhou, Y
Keywords: Flow control
Mixing enhancement
Issue Date: 2016
Publisher: Cambridge University Press
Source: Journal of fluid mechanics, 2016, v. 808, p. 362-396 How to cite?
Journal: Journal of fluid mechanics 
Abstract: The manipulation of a turbulent axisymmetric jet is experimentally investigated based on two unsteady radial minijets. The Reynolds number is 8000. The mass flow rate ratio Cm of the two minijets to that of the main jet and the ratio fe=f0′ of the excitation frequency fe to the preferred-mode frequency f0′ in the natural jet are examined. The decay rate K of the jet centreline mean velocity exhibits a strong dependence on Cm and fe=f0′ and is classified into three distinct categories in terms of required Cm, achievable enhancement in K and flow physics involved. Great effort is made to understand the flow physics associated with the first category of the manipulated jet, under which K can be immensely improved with a very small Cm. Detailed measurements are conducted upstream and downstream of the nozzle exit using hot-wire, flow visualization and particle imaging velocimetry techniques. Whilst strong entrainment is predominant in the injection plane of the minijets, rapid spread occurs in the orthogonal non-injection plane. Three types of coherent structures are identified, i.e. the contorted ring vortex, two pairs of streamwise vortices and mushroom-like counter-rotating structures sequentially 'tossed' out radially in the non-injection plane. Their interactions account for the large rise in K. The unsteady disturbance of the minijets is found to play a key role in the formation and interaction of these vortices, which are distinct from those formed under the manipulation of steady minijets and other techniques. A conceptual model of the flow structure under manipulation is proposed.
ISSN: 0022-1120
EISSN: 1469-7645
DOI: 10.1017/jfm.2016.634
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