The very first study of the development of the turbulent isothermal and incompressible air jet which issues at a constant velocity from a slender annular slot, circumnavigating an open core, into an otherwise quiescent and unbounded environment of the same density, is presented. The geometry of this source is defined by three diameters: the outer diameter of the slot $Do$; the inner diameter of the slot $Di$; and the diameter of the (circular) open core $Dv$. `Slender' refers to a slot for which the inner and outer diameters are approximately equal, i.e. $Di/Do\approx 1$. Our focus lies in understanding the development of the time-averaged flow with distance downstream and the influence of the source geometry on the development of the jet.

Given the absence of information on jets issuing from the sources of interest, the investigation follows an approach reminiscent of the classic investigations into round jets. That is, it begins with the development of a nozzle and experimental set-up which are suitable for studying the slender open-core annular jet. In addition to the experimental measurements, a complementary mathematical model was developed to describe the unique near-field behaviour of the open-core jet. Measurements were acquired using flow visualisation and Particle Image Velocimetry. On examining the streamwise development of the flow, the slender almost fully open-core jet was delineated into four key regions and the characteristic scalings identified. The regions were as follows: a bounded induced-flow region; a near-source planar-jet region; a transitional region; and a far-field round-jet region. Fluid induced through the open core of the nozzle and subsequently entrained into the jet significantly enhanced the near-field dilution of the jet.

Following on from this, the influence of the diameter ratio $Di/Do$ and ventilation ratio $Dv/Di$ on jet coalescence was examined. Over the range of diameter ratios examined ($0.845\le Di/Do\le 0.981$), experimental measurements and the predictions from mathematical modelling indicated that $Di/Do$ significantly influenced the volume flux induced through the core while the coalescing behaviour of the jet and the far-field region remained largely unchanged. Over the range of ventilation ratios examined ($0\le Dv/Di\le 0.90$), experimental measurements demonstrated that $Dv/Di$ controlled the restriction experienced by fluid induced through the open core and significantly influenced the far-field behaviour of the jet. Our findings suggest that jet of interest is then uniquely characterised by the momentum flux $M0$, the diameter ratio $Di/Do$, and the ventilation ratio $Dv/Di$.