Recently, the design of cooling air circuits for hydro power generators has transitioned towards utilization of computational fluid dynamics (CFD) simulations, increasingly supplementing or even replacing the previously favored network calculations. To validate the calculations for individual generators, common practice involves direct measurements of the pressure drop and volume flow characteristics using scaled models of the corresponding generators. This paper delves into the analysis of generator model ventilation flow and derives a set of similarity numbers to dimensionlessly represent this flow problem. The set of similarity numbers specified herein aids in determining the key considerations when scaling a generator for a ventilation model test. The focus of these similarity numbers lies on pressure loss characteristics, with no exploration of heat transfer aspects due to their usual exclusion from ventilation model tests. Two of these similarity numbers, although not appearing in existing literature, are predicted to significantly impact generator cooling flow. To evaluate and discuss this impact, CFD simulations of a generic generator cooling flow section are conducted. These CFD simulations primarily illustrate the importance of the newly introduced similarity numbers while maintaining adherence to the state of the art.