On Wake Analysis of Flow Past Rotating Downstream Cylinder using Hilbert-Huang Transformation

Two-dimensional numerical simulations have been carried out on flow past two inline circular cylinders with rotating downstream cylinder. Computations are performed for fixed Reynolds number equal to 150 such that the resulting flow field remains laminar and two-dimensional. The inter-cylinder spacing has been chosen equal to 5d ('d' being diameter of cylinder) such that the wake flow is predominantly unsteady. Rotational speed of the downstream cylinder has been varied to investigate its effect on transition in characteristics of temporal wake. This has been achieved by performing Hilbert-Huang transformation (HHT) on time series signals of drag and lift coefficients for the rotating cylinder. Unsteady periodic, unsteady non-periodic and steady transitions in flow behavior have been observed with an increase of rotational speed. Results are presented in the form of vorticity contours, Hilbert spectra and marginal spectra. Degree of stationarity of the signals as measure of nonlinearity has also been quantified. Comparisons are drawn against results from Fourier analysis and it has been shown that HHT is better suited to capture inter-wave and intra-wave modulations indicating nonlinear interactions in the wake.