Wavelet diagnostics for detection of coherent structures in instantaneous turbulent flow imagery: A review

A review of work over the last decade shows that 2D wavelet techniques applied on flow imagery can provide powerful insights into the nature and lifecycle of coherent structures (the latter through wavelet movies) in turbulent shear flows. The advantage of wavelet techniques in often being able to infer the nature of coherent motion from a single image is emphasized. The techniques are first calibrated by using them on well-known results in the turbulent mixing layer. They are then applied to jets and plumes, and it is shown how off-source heating in such flows can disrupt the coherent structures in the unheated flow. A suitably reduced version of the present method, using discrete wavelet transforms on signals from a finite array of sensors, could be a useful diagnostic tool in near-real-time detection of coherent structures or patterns for the purpose of selecting appropriate control signals to the actuators in a flow-control system.