|Active control of a turbulent round jet based on unsteady microjets
Jets -- Fluid dynamics.
Hong Kong Polytechnic University -- Dissertations
|Department of Mechanical Engineering
|vi, 121 pages : illustrations ; 30 cm
|The manipulation of turbulent jet mixing performance possesses many potential benefits in various industrial applications. This work reports an experimental investigation on the active control of a turbulent round air jet with two unsteady radial microjets used. The duel microjets were deployed at diametrically opposite positions upstream of the jet exit. The Reynolds number was 8,000. The mass flow rate ratio Cm of two microjets to that of the main jet and the ratio fe/f0′ of the excitation frequency fe to preferred-mode frequency f0′ in the uncontrolled jet with Cm = 0 and fe ≠ 0 were varied over the ranges of 0 16% and 0 1.4, respectively. The dependence of jet mixing on Cm and fe/f0′. was first determined. The decay rate K of jet centerline time-averaged velocity shows a strong dependence on Cm and fe/f0′. The K value is increased, as compared with the control of steady microjets at the same Cm, by more than 80% given fe/f0′ = 1 and Cm = 1.5%, suggesting a significantly improved control efficiency with unsteady microjets deployed. Given fe/f0′, say near unity, the dependence of K on Cm is classified as three distinct types in terms of required Cm, achievable enhancement in K and flow physics involved. In Type I (Cm = 0 2.6%), the significantly increased value of K results from the enhanced strength of large-scale coherent structures. The same magnitude of K as in Type I is also achieved in Type III (Cm = 4.5% 16%), which is ascribed to fully developed turbulence under the excitation of microjets. Type II (Cm = 2.6% 4.5%) is a transition between Types I and Type III and is characterized by a less enhanced K.
Great effort is made to understand thoroughly the flow physics connected with the microjet-controlled jet of Type I, under which jet mixing can be greatly improved with a small Cm. Detailed measurements were conducted in two orthogonal diametrical planes through the geometric axis of the flow and a number of cross-sectional planes normal to the geometric axis using flow visualization and particle image velocimetry (PIV) techniques. Mean velocity contours indicate that strong entrainment is predominant in the injection plane of microjets. On the other hand, rapid spreading occurs in the orthogonal non-injection plane. The vorticity concentrations appear to be tadpole-like in the injection plane, engulfing ambient fluid into the jet core. A close examination of flow visualization photographs displays that a number of structures are sequentially 'tossed' out along the radial direction in the non-injection plane, which is accompanied by a strong ejection of jet core fluid, often in the form of one pair of mushroom-like counter-rotating structures and one pair after another. The finding is distinct from previously reported changes in the flow structure under the manipulation of steady microjets, tabs and other techniques. A conceptual model of the flow structure under the excitation of two unsteady microjets is proposed for the first time. In order to achieve autonomously the optimal control performance, an extremum-seeking feedback control has been developed on the basis of the open-loop control results. It has been found that, given Cm, this closed-loop control technique may obtain automatically and rapidly the optimal value of fe and the desired or maximum K, as achieved in the open-loop control. This control technique is found to be robust and adaptable, that is, the optimal control performance is automatically achieved when the Reynolds number is changed. Based on the dependence of jet shear layer rollup frequency on fe, a separate flow-physics-based feedback control strategy has also been investigated. This technique may again achieve automatically the optimal control performance, and is further characterized by a shorter convergence time, shortened by one order of magnitude compared with the extremum seeking control.
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