Published online by Cambridge University Press: 02 November 2016
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.