Geometric Similarity — Some Applications in Fluid Mechanics

Fluid mechanics is essentially an experimental subject, and similarity laws in one form or another are its natural background. Various types of similarity are possible: in the first place there is geometric similarity between the model and the prototype. If the ratio of any two corresponding dimensions in the two systems is a constant then there is geometric similarity. The ratio is called the scale ratio. At this stage it is worth pointing out that complete similarity, even in the geometric sense, is not often possible. Such factors as surface roughness or particle size may be very difficult to match. In the case of, say, river models practical considera‐tions necessitate separate vertical and horizontal scales.