Thus the original void fraction correlations evaluated here are assigned
positive drift velocity values for vertical upward orientation
and negative values for downward orientation. The correlations of
Gomez et al. [22], Nicklin et al. [23], Rouhani and Axelsson [24], Bonnecaze
et al. [25] and Sun et al. [26] were developed for vertical upward
flow but could be used for the downward orientation by
changing the sign of drift velocity from positive to negative