Continuous wavelet transforms are employed to determine the time-localized frequency content (scalogram) of instantaneous wall pressure signals in upward gas-liquid flow. The flow conditions correspond to well-defined slug flow, well-defined churn flow, and flows near the transition from slug-to-churn flow. Scalograms demonstrate that the frequency content of the pressure signals is time-dependent, and visual observations of the flow conditions suggest that the time-dependent frequencies are related to identifiable physical behaviors of the flow. In well-defined slug flow, the scalograms are characterized by the presence of a dominant frequency throughout the duration of the signal and by frequency shifting events. Scalograms representing well-defined churn flow contain intermittent frequencies, and the energy density in churn flow is spread over a wider range of frequencies than in slug flow. The present results provide evidence that flows near transition alternately display characteristics of both well-defined slug and well-defined churn flows.

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