100
CHAPTER 4. HYDRODYNAMIC MODELS
in theory, provide better prototype-to-model correspondence. Costa’s proposed scaling would be difficult to implement in many cases, and therefore,
it is often not practical.
Instead, engineers have learned to live with these scale effects. Because
the scale effects can be quantified, model results can be examined in a critical manner by the engineer who uses experience and judgment to interpret
and correct the results (Le Méhauté 1990).
In short-wave hydraulic models scaled according to the Froude Criterion, the non-similitude of viscous forces and surface tension forces can lead
to scale effects involving wave reflection, wave transmission, wave energy
frictional dissipation, and wave breaking dissipation. Engineers have long
recognized these scale effects, and in some cases, have developed empirical methods to minimize these effects in the model. Nevertheless, clearly
defining the study objective and recognizing important scale effects in the
physical model are necessary if an intelligent analysis is to be made of a particular problem (Whalin and Chatham 1974). The important scale effects
are discussed in the following sections.
Wave Reflection. The reflection of short waves from steep beaches or
structures in a model will sometimes be greater or less than the wave reflection that occurs in the prototype, depending on the specific circumstances.
Waves reflected from smooth walls tend to be smaller in the model than
in the prototype (Le Méhauté 1976) because of increased friction in the
small scale model (the surface is relatively rougher than in the prototype).
Conversely, reflection coefficients for riprap in the model are higher than
experienced at full scale (Le Méhauté 1976) because the flow through the
riprap is influenced by viscous effects in the model, and consequently the
structure behaves as if it is less porous than in the prototype. This is corrected in the model by increasing the size of the riprap above that called
for by the geometric length scale.
for short-wave harbor models, reflection from outer breakwaters is usually not significant, and reflections from the basin boundaries are minimized
with absorbers. If wave reflection from structures must be correctly simulated because of its importance to the physical processes, the usual procedure is to determine the “true” reflection of the structure using a larger
scale (1.10 - 1:20) test in a wave tank with the stones sized according to
the geometric scale. This larger scale assures that flow into the structure is
fully turbulent like in the prototype. Then the model is geometrically reduced to the scale used for the basin tests, and wire mesh screens are placed
on the structure to reduce the reflection to the desired value (Hudson, et
al 1979; Oumeraci 1984).
CHAPTER 4. HYDRODYNAMIC MODELS
in theory, provide better prototype-to-model correspondence. Costa’s proposed scaling would be difficult to implement in many cases, and therefore,
it is often not practical.
Instead, engineers have learned to live with these scale effects. Because
the scale effects can be quantified, model results can be examined in a critical manner by the engineer who uses experience and judgment to interpret
and correct the results (Le Méhauté 1990).
In short-wave hydraulic models scaled according to the Froude Criterion, the non-similitude of viscous forces and surface tension forces can lead
to scale effects involving wave reflection, wave transmission, wave energy
frictional dissipation, and wave breaking dissipation. Engineers have long
recognized these scale effects, and in some cases, have developed empirical methods to minimize these effects in the model. Nevertheless, clearly
defining the study objective and recognizing important scale effects in the
physical model are necessary if an intelligent analysis is to be made of a particular problem (Whalin and Chatham 1974). The important scale effects
are discussed in the following sections.
Wave Reflection. The reflection of short waves from steep beaches or
structures in a model will sometimes be greater or less than the wave reflection that occurs in the prototype, depending on the specific circumstances.
Waves reflected from smooth walls tend to be smaller in the model than
in the prototype (Le Méhauté 1976) because of increased friction in the
small scale model (the surface is relatively rougher than in the prototype).
Conversely, reflection coefficients for riprap in the model are higher than
experienced at full scale (Le Méhauté 1976) because the flow through the
riprap is influenced by viscous effects in the model, and consequently the
structure behaves as if it is less porous than in the prototype. This is corrected in the model by increasing the size of the riprap above that called
for by the geometric length scale.
for short-wave harbor models, reflection from outer breakwaters is usually not significant, and reflections from the basin boundaries are minimized
with absorbers. If wave reflection from structures must be correctly simulated because of its importance to the physical processes, the usual procedure is to determine the “true” reflection of the structure using a larger
scale (1.10 - 1:20) test in a wave tank with the stones sized according to
the geometric scale. This larger scale assures that flow into the structure is
fully turbulent like in the prototype. Then the model is geometrically reduced to the scale used for the basin tests, and wire mesh screens are placed
on the structure to reduce the reflection to the desired value (Hudson, et
al 1979; Oumeraci 1984).