A physics-based analysis framework is developed to characterize low-frequency unsteadiness (LFU) in separated wake flows. LFU is interpreted as an outcome of a temporal imbalance of kinetic energy within the mean recirculation region. A kinetic energy transport equation is derived from the rotational form of the Navier-Stokes equations using vorticity-based quantities, motivated by their direct connection to vortical dynamics and flow structures relevant to LFU. To isolate LFU from dominant wake motions, temporal low-pass filtering is applied together with conditioning based on a local intermittency measure, extracting intermittent fluctuations at frequencies much lower than the primary vortex shedding frequency. The framework is demonstrated for a two-dimensional laminar wake past a normal plate. The filtered drag coefficient exhibits a clear LFU signature and is strongly correlated with the net filtered kinetic energy within the mean recirculation region. Analysis of the kinetic energy budget identifies the Bernoulli energy flux and Lamb-vector transport as the dominant mechanisms governing kinetic-energy charging and discharging, respectively. Strong correlations are observed between the filtered drag and the net Lamb-vector divergence, attributed to vortex merging and intensified roll-up that redistribute the Lamb vector and reduce base pressure. The LFU frequency is estimated using dimensional scaling and a data-driven approach. The analysis highlights the central role of the Lamb-vector divergence in understanding and predicting LFU in separated wake flows. By providing a reduced, physics-based description of LFU in terms of kinetic energy dynamics, the proposed framework offers a useful tool for analyzing LFU in turbulent separated flows, where visual inspection is limited and modal decompositions remain high dimensional.

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• Vortex dynamics
• Vortex flows
• Vortex interactions