4. Similarity laws and empirical forecasting methods

4.1 Laws of similarity

Similarity laws are useful in many ways. They make it possible to predict the aeraulic and acoustic performance of a fan at a given speed and impeller diameter, based on measurements taken at other speeds and diameters. This significantly reduces the number of tests required to determine the performance of a range of fans of different sizes at different speeds. Manufacturers use these laws not only to draw up their catalogs, but also to predict the flow-pressure curves and noise levels of large fans based on tests on reduced-scale models.

These laws can also be used to compare the performance of several fans of the same type, with identical diameters and rotational speeds, enabling us to identify the influence of blade geometry and...