Revealing the details of vortex core precession in cyclones by means of large-eddy simulation

We investigate the three-dimensional, time-dependent behaviour of coherent flow structures inside two cyclone separator geometries, a Stairmand cyclone and a swirl tube, operating in the turbulent regime. For this, large-eddy simulations (LES) with a standard Smagorinsky subgrid-scale model are carried out on the single-phase flow employing a nonorthogonal curvilinear hexahedral mesh. Reynolds numbers (Re) are in the range 9.10⁴ to 4.10⁵. The simulations successfully capture the quasi-periodic precessing vortex core (PVC) phenomenon. The spectra derived from the temporal signals of different flow variables viz. static pressure, tangential velocity, axial velocity and radial velocity in the Stairmand cyclone exhibit two distinct frequencies at all Re, whereas the spectra for the swirl tube show a single peak. These spectra strongly resemble those reported in the experimental literature. The periodicity pertaining to the inner vortex about its rotational axis is also analysed. Next, the effect of Re on the two distinct spectral peaks have been evaluated for the Stairmand cyclone. Lastly, we present the effect of Re on mean and fluctuating fields and their variations along the geometrical axis of the cyclone.