3-DIMENSIONAL SUBHARMONIC WAVES DURING TRANSITION IN THE NEAR-WAKE REGION OF A CYLINDER

Detailed quantitative measurements of three-dimensional subharmonic wa ves have been obtained with a scanning laser anemometer in the near-wa ke region of a circular cylinder. A spatial decomposition technique ha s been used to separate the secondary three-dimensional flow from the primary Strouhal vortex flow. The appearance of subharmonic frequencie s in the spectrum of the secondary instability field at Reynolds numbe rs of 300 and above provides experimental evidence consistent with the scenario of a period-doubling route to turbulence. The basic flow str ucture of the secondary field responsible for producing the subharmoni cs is shown to be pairs of counter-rotating streamwise vortices, which alternate the direction of rotation every Strouhal period. (C) 1996 A merican Institute of Physics.