The growth of large scales at defect sites in the plane mixing layer

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Abstract
The evolution of vortex structure in the vicinity of a pattern defect or dislocation, generated experimentally by forcing a high Reynolds number mixing layer, is studied using a new two-dimensional wavelet transform called Arc. This transform localizes spectral information in physical space – as all wavelets do – but is not direction-specific in wavenumber space. Various types of forcing, including forcing at the fundamental and subharmonic wavenumbers, produce a range of mixing-layer responses. The most significant finding is that a dislocation site acts as a nucleus and initiates a rapid, localized evolution to larger scales. The area of the localized ‘patch’ grows approximately as the square of downstream distance. Defect-initiated patches bear generic similarities to the disturbed regions in cylinder wakes – commented upon recently by many researchers – and in particular to the Λ-structures described by Williamson (1992).