Axisymmetric jet manipulated using two unsteady minijets

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$C_{m}$of the two minijets to that of the main jet and the ratio$f_{e}/f_{0}^{\prime }$of the excitation frequency$f_{e}$to the preferred-mode frequency$f_{0}^{\prime }$in the natural jet are examined. The decay rate$K$of the jet centreline mean velocity exhibits a strong dependence on$C_{m}$and$f_{e}/f_{0}^{\prime }$and is classified into three distinct categories in terms of required$C_{m}$, 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$C_{m}$. 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.