A void fraction covariance in the expression for area-averaged local relative velocity has been assumed to be one by current and past researchers. Similarly, in a multi-bubble group approach, void covariances for each bubble group appear in the group area-averaged relative velocity expressions. The covariance terms have been analyzed with a substantial database from literature including upward flow in pipe diameters of 1.27 cm to 15.2 cm, downward flow in pipe diameters of 2.54 cm and 5.08 cm, and upward flow in an annulus (Dh = 1.9cm) under adiabatic, boiling, and condensing conditions. Simple relations are proposed to specify the covariance in order to improve the prediction of area-averaged local relative velocity. The correlations were found to agree well with the experimental data for the flow configurations and conditions analyzed.

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