Three‐dimensional subharmonic waves during transition in the near‐wake region of a cylinder

Detailed quantitative measurements of three‐dimensional subharmonic waves have been obtained with a scanning laser anemometer in the near‐wake region of a circular cylinder. A spatial decomposition technique has been used to separate the secondary three‐dimensional flow from the primary Strouhal vortex flow. The appearance of subharmonic frequencies in the spectrum of the secondary instability field at Reynolds numbers of 300 and above provides experimental evidence consistent with the scenario of a period‐doubling route to turbulence. The basic flow structure of the secondary field responsible for producing the subharmonics is shown to be pairs of counter‐rotating streamwise vortices, which alternate the direction of rotation every Strouhal period.