Published online by Cambridge University Press: 01 June 2020
Direct numerical simulations are performed to study the flow regimes at the wake behind a strongly streamwise rotating sphere, covering the range of rotation parameters $0\leqslant \unicode[STIX]{x1D6FA}\leqslant 3$ and laminar and transitional Reynolds numbers $Re=250$, 500 and 1000. The wake dynamics is investigated in terms of flow topology, dominant modes and force coefficients. A higher wake complexity is found for growing values of the rotation parameter $\unicode[STIX]{x1D6FA}$ for all the Reynolds numbers investigated. In particular, at low and intermediate $Re$, successive bifurcations entail the development of periodic, quasi-periodic and irregular regimes, constituting a classical scenario of route to chaos, through the destabilization of different structures associated to incommensurate frequencies, which have been analysed by means of flow decomposition techniques. At low $Re$ and high rotation rates, the flow is governed by double-threaded structures due to the destabilization of helical symmetries of azimuthal wavenumber $m=2$, which are not dominant at larger $Re$. Interestingly, at intermediate values of $\unicode[STIX]{x1D6FA}$ and $Re=500$, a bistable dynamics is observed whereby the wake undergoes a random switching between a modulated quasi-periodic regime and an irregular regime, which is associated to a sudden increase of the drag coefficient, on account of the development of a double-celled recirculating bubble. Finally, for $Re=1000$, the flow is already chaotic at $\unicode[STIX]{x1D6FA}=0$, and the evolution with the rotation rate of the flow dynamics is simpler, with wake regimes being characterized by the rotation and massive shedding of vortex loops, that are a continuous deformation through axial rotation of the irregular wake behind the static sphere.
Wake behind a streamwise rotating sphere (Re=250, Ω=0.4)
Wake behind a streamwise rotating sphere (Re=250, Ω=1.2)
Wake behind a streamwise rotating sphere (Re=250, Ω=1.6)
Wake behind a streamwise rotating sphere (Re=250, Ω=2.2)
Wake behind a streamwise rotating sphere (Re=250, Ω=3.0)
Wake behind a streamwise rotating sphere (Re=500, Ω=0.0)
Wake behind a streamwise rotating sphere (Re=500, Ω=0.4)
Wake behind a streamwise rotating sphere (Re=500, Ω=0.6)
Wake behind a streamwise rotating sphere (Re=500, Ω=1.1)
Wake behind a streamwise rotating sphere (Re=500, Ω=1.2)