Experimental evidence of new three-dimensional modes in the wake of a rotating cylinder

A recent numerical study by Rao et al. (J. Fluid Mech., vol. 717, 2013, pp. 1-29) predicted the existence of several previously unobserved linearly unstable three-dimensional modes in the wake of a spinning cylinder in cross-flow. While linear stability analysis suggests that some of these modes exist for relatively limited ranges of Reynolds numbers and rotation rates, this may not be true for fully developed nonlinear wakes. In the current paper, we present the results of water channel experiments on a rotating cylinder in cross-flow, for Reynolds numbers 200 <= Re <= 275 and non-dimensional rotation rates 0 <= alpha <= 2.5. Using particle image velocimetry and digitally post-processed hydrogen bubble flow visualizations, we confirm the existence of the predicted modes for the first time experimentally. For instance, for Re = 275 and a rotation rate of alpha = 1.7, we observe a subharmonic mode, mode C, with a spanwise wavelength of lambda(z)/d approximate to 1.1. On increasing the rotation rate, two modes with a wavelength of lambda(z)/d approximate to 2 become unstable in rapid succession, termed modes D and E. Mode D grows on a shedding wake, whereas mode E consists of streamwise vortices on an otherwise steady wake. For alpha > 2.2, a short-wavelength mode F appears localized close to the cylinder surface with lambda z/d approximate to 0.5, which is presumably a manifestation of centrifugal instability. Unlike the other modes, mode F is a travelling wave with a spanwise frequency of St(3D) approximate to 0.1. In addition to these new modes, observations on the one-sided shedding process, known as the 'second shedding', are reported for alpha = 5.1. Despite suggestions from the literature, this process seems to be intrinsically three-dimensional. In summary, our experiments confirm the linear predictions by Rao et al., with very good agreement of wavelengths, symmetries and the phase velocity for the travelling mode. Apart from this, these experiments examine the nonlinear saturated state of these modes and explore how the existence of multiple unstable modes can affect the selected final state. Finally, our results establish that several distinct three-dimensional instabilities exist in a relatively confined area on the Re-alpha parameter map, which could account for their non-detection previously.