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CHAPTER 4. HYDRODYNAMIC MODELS
Finally, Coulomb friction scale effects must be considered in situations
where two solid bodies are in contact. The forces necessary to overcome the
Coulomb friction are relatively larger in the model than in the prototype.
This can be simply illustrated (Le Méhauté 1990) by placing two differentsized paper clips on a surface, and then slowly tilting the surface until one
of the paper clips moves. The larger clip (prototype) will always overcome
the friction before the smaller clip (model), provided the surface is uniform
and the relative area of contact between the paper clips and the surface is
the same.
A good example of the Coulomb friction scale effect in coastal physical
modeling is in model studies of concrete armor unit stability. Model armor
units can withstand more intense wave action before being dislodged than
their prototype equivalents made of the same material. This scale effect
can be decreased by fabricating model armor units out of a different material from the prototype units (while still retaining the necessary mass and
geometric scaling) so that Coulomb friction can be more easily overcome in
the model.
Wave Breaking. The process of wave breaking on a beach or a coastal
structure is of major importance to coastal engineering physical models,
and it is important that wave breaking in the model produces the same
hydrodynamic response as in the prototype. In breaking waves, entrained
air bubbles are larger in the model because the size is determined by surface
tension. Also the depth of air entrainment will be greater in the model.
Miller (1972) investigated the effect different surface tension forces had
on various breaking wave parameters. He examined waves in “normal”
water and in water where additives reduced the surface tension to about
half the “normal” value. Breaker parameters were determined from 16-mm
film images. Miller concluded that fluid surface tension played an important enough role that it should be included in any theoretical treatment
of breakers. He also suggested that waves in small scale experiments may
suffer a scale effect and studies be made to address this issue.
Le Méhauté (1976) stated that the process of energy dissipation during
wave breaking will be in similitude, even if the fine details of the flow process
are different. The momentum equations, which provided the scaling for
short-wave models, express the total rate of energy dissipation as a balance
of the external forces irrespective of the internal dissipation mechanisms.
So even if the proportions of energy dissipated in the model by various
mechanisms are different than in the prototype, the total energy budget
remains in similitude by virtue of the momentum theorem (Le Méhauté
1990).
Stive (1985) provided experimental confirmation of Le Méhauté’s con
CHAPTER 4. HYDRODYNAMIC MODELS
Finally, Coulomb friction scale effects must be considered in situations
where two solid bodies are in contact. The forces necessary to overcome the
Coulomb friction are relatively larger in the model than in the prototype.
This can be simply illustrated (Le Méhauté 1990) by placing two differentsized paper clips on a surface, and then slowly tilting the surface until one
of the paper clips moves. The larger clip (prototype) will always overcome
the friction before the smaller clip (model), provided the surface is uniform
and the relative area of contact between the paper clips and the surface is
the same.
A good example of the Coulomb friction scale effect in coastal physical
modeling is in model studies of concrete armor unit stability. Model armor
units can withstand more intense wave action before being dislodged than
their prototype equivalents made of the same material. This scale effect
can be decreased by fabricating model armor units out of a different material from the prototype units (while still retaining the necessary mass and
geometric scaling) so that Coulomb friction can be more easily overcome in
the model.
Wave Breaking. The process of wave breaking on a beach or a coastal
structure is of major importance to coastal engineering physical models,
and it is important that wave breaking in the model produces the same
hydrodynamic response as in the prototype. In breaking waves, entrained
air bubbles are larger in the model because the size is determined by surface
tension. Also the depth of air entrainment will be greater in the model.
Miller (1972) investigated the effect different surface tension forces had
on various breaking wave parameters. He examined waves in “normal”
water and in water where additives reduced the surface tension to about
half the “normal” value. Breaker parameters were determined from 16-mm
film images. Miller concluded that fluid surface tension played an important enough role that it should be included in any theoretical treatment
of breakers. He also suggested that waves in small scale experiments may
suffer a scale effect and studies be made to address this issue.
Le Méhauté (1976) stated that the process of energy dissipation during
wave breaking will be in similitude, even if the fine details of the flow process
are different. The momentum equations, which provided the scaling for
short-wave models, express the total rate of energy dissipation as a balance
of the external forces irrespective of the internal dissipation mechanisms.
So even if the proportions of energy dissipated in the model by various
mechanisms are different than in the prototype, the total energy budget
remains in similitude by virtue of the momentum theorem (Le Méhauté
1990).
Stive (1985) provided experimental confirmation of Le Méhauté’s con