COHERENT VORTEX STRUCTURES IN THE WAKE OF A SPHERE AND A CIRCULAR DISK AT REST AND UNDER FORCED VIBRATIONS

The near-wake of a circular disk was investigated in the Reynolds number range 1·5 × 104Re3 × 105 at low values of Mach number. The decomposition of the large-scale wake structure into azimuthal modes via cross-spectral analysis of signals either of two hot-wire or two microphone probes shows that the wake is dominated by three instability mechanisms: axisymmetric pulsation of the recirculation bubble at very low frequency f1(S1 = f1D/Uo ≅ 0·05); antisymmetric fluctuations induced by a helical vortex structure at a natural frequency fnSn = fnD/Uo≅ 0·135); and a high frequency instability (S3 = f3D/Uo = 1·62) of the separated shear layer. The mechanisms are highly coherent in space but random in time. If the disk is forced with a frequency fe to oscillate in the "nutation" mode with small detuning fe - fn to the (helix) frequency fn of the wake, the situation changes drastically. Whereas the "pumping" and the high frequency instability remain nearly unaffected, the helix structure locks in, now being completely stabilized in space and time at a coherence level γ ≅ 1 and exclusively dominated by the azimuthal mode m = 1. Clearly this establishes a feedback mechanism which, under favorable conditions, may force the body to nutate, as has been observed for free-falling spheres or non-spin stabilized bullets. Oscillation of the disk at small amplitudes in modes other than the nutation mode was ineffective in altering the wake structure. For comparison, the wake of a sphere was analysed, although not so intensively. Except for the fact that its wake is perceptibly more "Reynolds number sensitive" than that of the disk, both wake structures are very similar. © 1990 Academic Press Limited.