Jet flow structure under the perturbation of two unsteady microjets

The manipulation of turbulent jet mixing has great potential benefits in various industrial applications. This work reports an experimental investigation on the active control of a turbulent round air jet using two unsteady radial microjets. Two microjets were placed at diametrically opposite locations upstream of the nozzle exit. The Reynolds number was 8, 000. The flow rate ratio C_m and excitation frequency ratio f_e/f₀ of the microjets to the main jet were varied over the ranges of 0-16% and 0-1.4, respectively, where f_e was the excitation frequency of unsteady microjets and f₀ the preferred-mode frequency in the uncontrolled jet. It has been found that, given f_e/f₀ ≈ 1, jet mixing can be greatly improved with a small C_m. The flow physics is explored based on flow visualization technique in two orthogonal diametrical planes through the geometric axis of the flow, as well as a number of cross-sectional planes normal to the axis. It has been found that strong entrainment is predominant in the injection plane of microjets. In contrast, rapid spreading occurs in the orthogonal non-injection plane. A close examination of flow visualization photographs unveils 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.