On the generation of swirling jets: high-Reynolds-number rotating flow in a pipe with a final contraction

We investigate the generation conditions of a high-Reynolds-number swirling jet experiment, based on a rotating honeycomb device and using a final contraction. Using hot-wire measurements, we first show that for high swirl levels, the flow at the jet exhaust may exhibit fully developed turbulence in the whole plane. By analysing the fluctuation levels obtained for several values of the contraction ratio, ranging from 4 to 18.4, we prove that this turbulence does not result from upstream-propagating disturbances initiated in the jet, but originates in the pipe flow upstream of the exit plane. Using stereo particle image velocimetry, we then measure the flow in the constant-cross-section pipe located between the rotating honeycomb outlet and the contraction. This investigation is supplemented with simplified numerical simulations of the mean flow. The pipe flow dynamics is found to result from the interplay of a rich variety of complex phenomena, which are independent of the contraction ratio in the range considered here. In the near-wall region, centrifugal instability occurs in the form of intermittent azimuthal vortices, starting from moderate swirl levels and persisting for all higher levels. As the flow exiting from the honeycomb has a swirl level high enough to reach the subcritical regime, a complex mean flow organization is observed, dominated by the presence of large-amplitude axisymmetric Kelvin wave trains. Gradients in the resulting flow lead to the appearance of generalized centrifugal instabilities in an annular region in the rotational core, starting in the early subcritical regime. As the swirl level is further increased, large-scale, high-amplitude axisymmetric and simple spiral perturbations add to the global dynamics, leading to an overall very high fluctuation level. Consideration of the turbulent spectra in the jet exit planes suggests that the simple spiral coherent structure could be the resonant response of the flow to the periodic excitation by the rotating honeycomb. Overall, the study illustrates why a swirling jet experiment should exclude the use of a final contraction in order to guarantee smooth flow conditions in the exit at high swirl.