A technique is introduced for extracting the coherence time scale of Lagrangian coherent structures (LCS) in aperiodic flows from experimental, light intensity time series. The technique employs digital photography to record the transport of passive tracers in a chaotic instantaneous flow. Coherent eddies are detected by time-averaging the instantaneous light intensity field on finite-size temporal windows. The optimal size of the time-average window (the coherence time scale of the LCS) is extracted from the light intensity field using statistical scaling arguments based on the central limit theorem. The technique is applied to visualize LCS in the vicinity of a surface-piercing rectangular block mounted at one corner of a rectangular open channel.
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