Wake characteristics of a sphere performing streamwise rotary oscillations

Three-dimensional computations have been carried out to investigate flow past a sphere performing rotation and rotary oscillation about streamwise axis. The effect of streamwise rotation has been analysed for fixed Reynolds number of 300, with non-dimensional rotational rate taking values 0, 0.5, 1, 1.5 and 2. The transitions in the near wake have been depicted with the help of time-averaged path lines, iso-λ2 surfaces and vortex core transformation. This study is further extended by considering forced rotational oscillations of the sphere for two different amplitudes of oscillation and reduced oscillation frequency ranging from 0.5 to 2. Effect of these parameters on flow structure has been described using λ2-surfaces and streamlines at different locations in the flow field. The response of mean hydrodynamic forces to variation in amplitude and frequency of oscillation has also been reported. Temporal behaviour of wake has been disclosed through Hilbert spectral analysis of time dependent force coefficient signals. The growth and evolution of wake nonlinearities have been elaborated with the help of marginal spectra, Hilbert spectra and distribution of non-stationarity.