Chapter 3
Principles of Similitude
“The practical application of similitude in hydraulic model testing is based on recognition of the fact that there is not complete
similitude, but rather several imperfect similitudes which can be
exploited as required.” ...Jacob E. Warnock (1950)
There are numerous problems in fluid mechanics that can be solved using theoretical and/or numerical models. However, there are many more
problems that require solution using an empirical approach based on experimental data. Coastal engineers need a working knowledge of experimental
modeling techniques so they can (1) interpret and utilize the results of
other investigators, and (2) plan and execute their own experiments in the
laboratory (Munson, et al. 1990).
The basis of all physical modeling is the idea that the model behaves
in a manner similar to the prototype it is intended to emulate. Thus, a
properly validated physical model can be used to predict the prototype
under a specified set of conditions. This important concept allows us to
perform model studies to obtain information that will aid in the design of
the prototype, and therefore, avoid costly mistakes. By the same token, a
wise engineer doesn’t assume that model studies will provide answers to all
questions, and the engineer recognizes that it is wasteful to conduct a model
study if the results can be adequately predicted by theory (Langhaar 1951).
Finally, we must keep in mind that there is a possibility that physical model
results may not be indicative of prototype behavior, so it is important to
minimize this possibility by careful model design and validation, careful
testing of the model, and careful interpretation of model predictions.
This chapter reviews the physical basis on which we justify conducting
hydraulic models at reduced scale, and it provides the necessary background
for deriving appropriate scaling relationships for hydraulic physical models.
Principles of Similitude
“The practical application of similitude in hydraulic model testing is based on recognition of the fact that there is not complete
similitude, but rather several imperfect similitudes which can be
exploited as required.” ...Jacob E. Warnock (1950)
There are numerous problems in fluid mechanics that can be solved using theoretical and/or numerical models. However, there are many more
problems that require solution using an empirical approach based on experimental data. Coastal engineers need a working knowledge of experimental
modeling techniques so they can (1) interpret and utilize the results of
other investigators, and (2) plan and execute their own experiments in the
laboratory (Munson, et al. 1990).
The basis of all physical modeling is the idea that the model behaves
in a manner similar to the prototype it is intended to emulate. Thus, a
properly validated physical model can be used to predict the prototype
under a specified set of conditions. This important concept allows us to
perform model studies to obtain information that will aid in the design of
the prototype, and therefore, avoid costly mistakes. By the same token, a
wise engineer doesn’t assume that model studies will provide answers to all
questions, and the engineer recognizes that it is wasteful to conduct a model
study if the results can be adequately predicted by theory (Langhaar 1951).
Finally, we must keep in mind that there is a possibility that physical model
results may not be indicative of prototype behavior, so it is important to
minimize this possibility by careful model design and validation, careful
testing of the model, and careful interpretation of model predictions.
This chapter reviews the physical basis on which we justify conducting
hydraulic models at reduced scale, and it provides the necessary background
for deriving appropriate scaling relationships for hydraulic physical models.