The local structure and regime transition in a gas-liquid bubble column are studied via multi-resolution analysis of the local gas holdup signal.  We show that the flow in a bubble column is characterized by the<BR>

¥  energy content of the measured gas holdup signal in the time-frequency plane, and<BR>
¥  intermittence of the gas holdup signal, as characterized by its multifractal nature.<BR>

The measured signal is decomposed into its contributions in various regions of time and frequency using wavelet-based techniques.  We show that the energy distribution in time and frequency provides greater insight into the physical processes underlying the gas-liquid flow, and may be used for on-line regime identification.  The intermittence of the measured signal allows characterization of the transition of the flow from the homogeneous to the heterogeneous flow regime.  Both, the time-frequency distribution of energy, and the intermittence of the local gas holdup signal are extracted via techniques derived from wavelet theory.  Thus, wavelet analysis provides a rigorous framework for unifying statistical, spectral and fractal analysis methods.