Similitude and Dimensional Analysis by Engr. Murtaza zulfiqar. Definition and uses of Similitude.
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Usually we find it impossible to determine all essential facts for a given fluid flow by pure theory alone, and so we must often depend on experimental investigations. However, we can greatly reduce the number of tests needed by systematically using dimensional analysis and the laws of similitude or similarity.
For these enable us to apply test data to other cases than those observed.
The similarity laws enable us to experiment with a convenient fluid such as water or air, for example, and then apply the results to a fluid that is less convenient to work with, such as hydrogen, stream, or oil. Also in both hydraulics and aeronautics , we can obtain valuable results at a minimum cost from tests made with small-scale models of the full size apparatus. The laws of similitude enable us to predict the performance of the prototype, which means the full-size device, from tests made with the model.
A few examples of where we have used models are ships in towing basins, airplanes in wind tunnels, hydraulic turbines, spillways of dams, river channels, and the study of such phenomena as the action of waves on beaches, soil erosion and the transport of sediment.