A general discussion is given of modelling techniques based on dimensional analysis and similarity. Particular attention is focused on the modelling of two-phase effects, where it is neither always obvious which are the predominant forces nor whether considering only two forces will give reasonable similarity. Three examples—air-entraining flows, high, siphons, and bubbly flows—are considered and some suggestions of suitable model scaling laws are made. These include We/Re for air entrainment and the cavitation number for high siphons. The results of model tests on air-entraining vortices at the Kariba Hydroelectric Scheme are presented as an application of the discussion.
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