Response of the shear layers separating from a circular cylinder to small amplitude rotational oscillations

Abstract

The responses of the shear layers separating from a circular cylinder in water to small amplitude rotational oscillations of the cylinder were investigated experimentally in the Reynolds number (Re) range 250 to 1200 [J. R. Filler, Ph.D. dissertation, Washington State University (1989)]. Magnitude and phase of the response (in terms of velocity fluctuations in the shear layers) were measured using a hot film anemometer connected to a phase-sensitive detector. The hot film was placed in the shear layer approximately one diameter downstream from the cylinder. Synchronous velocity oscillations were found to be roughly linear in the driven rotational amplitude. At lower Re, cylinder oscillations are shown to excite the common Strouhal (or Karman) mode of vortex shedding. Near the Strouhal frequency there is a rapid 180° phase jump characteristic of a driven oscillator. Above Re≈500, a secondary mode of vortex shedding or instability is also excited associated with the instability of the free shear layers. Frequencies of maximum response in this range agree with frequencies of maximum amplification predicted analytically by others for free shear layers. The response phase varied with frequency as expected for waves convected on a shear layer. [Work supported by ONR.]