4 Turbulence Scale Effect in
Distorted Models
Introduction
Although numerical modeling of free surface flows has shown astonishing
growth in the past two decades, one of the few remaining difficulties is
simulating turbulent flow processes such as flow around solid bodies, flow
separation, and flow entrainment. There are specialized numerical models in
developmental stages that can address very localized and specific turbulent
processes; but this capability is not generally available in the numerical models
typically used to investigate coastal engineering problems.
For many years engineers have used small-scale physical models to study
and design coastal engineering projects. The models are useful for understanding
and quantifying flow patterns in the vicinity of projects and for predicting
changes to flow patterns brought about by project modification. Hydrodynamic
forces on solid objects can also be accurately determined using small-scale
physical models. Physical modeling of hydrodynamic phenomena is only
possible because we have an understanding of how to scale the models. With a
properly scaled model, measured and observed responses in the model can be
reliably translated to full-scale values.
General Similitude Requirements
Complete similarity between a physical model and its real-world counterpart
(prototype) requires:
a. Geometric similitude. Ratios of all corresponding linear dimensions
between prototype and model are the same (i.e., model is a small-scale
replica).
b. Kinematic similitude. The ratio between components of all vectorial
motions for the prototype and model are the same for all fluid particles at
all times.
c. Dynamic similitude. Ratios of all vectorial forces between the prototype
and model are the same at all times.
Generally, these three similitude requirements are reasonably met for all free-
surface flows in coastal and riverine environments by geometrically undistorted
Froude-scaled models. The term "geometrically undistorted" means that the
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Chapter 4 Turbulence Scale Effect in Distorted Models
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