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Vortex shedding in the wake of a cylinder in uniform flow can be suppressed via the application of a porous coating; however, the suppression mechanism is not fully understood. The internal flow field of a porous coated cylinder (PCC) can provide a deeper understanding of how the flow within the porous medium affects the wake development. A structured PCC (SPCC) was three-dimensionally printed using a transparent material and tested in water tunnel facilities using flow visualisation and tomographic particle image velocimetry at outer-diameter Reynolds numbers of 
$Re = 7 \times 10^{3}$ and 
$7.3 \times 10^{4}$, respectively. The internal and near-wall flow fields are analysed at the windward and mid-circumference regions. Flow stagnation is observed in the porous layer on the windward side and its boundary is shown to fluctuate with time in the outermost porous layer. This stagnation region generates a quasi-aerodynamic body that influences boundary layer development on the SPCC inner diameter, that separates into a shear layer within the porous medium. For the first time via experiment, spectral content within the separated shear layer reveals vortex shedding processes emanating through single pores at the outer diameter, providing strong evidence that SPCC vortex shedding originates from the inner diameter. Velocity fluctuations linked to this vortex shedding propagate through the porous layers into the external flow field at a velocity less than that of the free stream. The Strouhal number linked to this velocity accurately predicts the SPCC vortex shedding frequency.
Theoretical predictions and numerical simulations are used to determine the transition to bubble and conical vortex breakdown in low-Mach-number laminar axisymmetric variable-density swirling jets. A critical value of the swirl number 
$S$ for the onset of the bubble (
$S^*_B$) and the cone (
$S^*_C$) is determined as the jet-to-ambient density ratio 
$\varLambda$ is varied, with the temperature dependence of the gas density and viscosity appropriate to that of air. The criterion of failure of the slender quasi-cylindrical approximation predicts 
$S^*_B$ that decreases with increasing values of 
$\varLambda$ for a jet in solid-body rotation emerging sharply into a quiescent atmosphere. In addition, a new criterion for the onset of conical breakdown is derived from divergence of the initial value of the radial spreading rate of the jet occurring at 
$S^*_C$, found to be independent of 
$\varLambda$, in an asymptotic analysis for small distances from the inlet plane. To maintain stable flow in the unsteady numerical simulations, an effective Reynolds number 
$Re_{eff}$, defined employing the geometric mean of the viscosity in the jet and ambient atmosphere, is fixed at 
$Re_{eff}=200$ for all 
$\varLambda$. Similar to the theoretical predictions, numerical calculations of 
$S^*_B$ decrease monotonically as 
$\varLambda$ is increased. The critical swirl numbers for the cone, 
$S^*_C$, are found to depend strongly on viscous effects; for 
$\varLambda =1/5$, the low jet Reynolds number (51) at 
$Re_{eff}=200$ delays the transition to the cone, while for 
$\varLambda =5$ at 
$Re_{eff}=200$, the large increase in kinematic viscosity in the external fluid produces a similar trend, significantly increasing 
$S^*_C$.
