4.3. LONG-WAVE HYDRODYNAMIC MODELS
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as possible, a substantial area must be dedicated to wave absorbers (e.g.,
horsehair mats). This increases the already significant area requirements
of the long-wave model.
Training walls often are used in basin models to stem diffraction of wave
energy into quiescent water adjacent to the region of wave generation. This
helps to simulate conservation of wave energy between orthogonals (Lee
1975). However, training walls are highly reflective and any waves reflected
back toward the area of wave generation could encounter these walls and
be re-reflected.
As in short-wave models, accurate reproduction of bathymetry is required to assure reliable results. In geometrically undistorted long-wave
models this becomes increasingly difficult as the modeled area increases.
Molding bathymetry in geometrically distorted models is considerably easier than in an equivalent undistorted model because depth differences are
more pronounced.
Finally, the small scales and low wave heights utilized in some longwave models make hydrodynamic measurements difficult and care must be
taken to assure that measurement error is not a significant portion of the
observation. The possibility of measurement error is reduced somewhat in
a geometrically distorted long-wave model, but careful experimenters will
always be aware of the problem. Also, when the hydrodynamic characteristics being measured are small, instrument intrusion is a laboratory effect
that must be considered (Le Méhauté 1990).
Scale Effects
Just as in short-wave models, scale effects in long-wave hydrodynamic models result primarily from the scaling assumption that gravity is the dominant
physical force balancing the inertial forces. Scaling based on this assumption (Froude scaling) incorrectly scales the other physical forces of viscosity,
elasticity, surface tension, etc., with the belief that these forces contribute
little to the physical processes. In addition, physical processes driven by
turbulent fluctuations are not in similitude in geometrically distorted longwave models.
The role of the model engineer is to recognize scale effects that potentially exist in the model, to evaluate the relative magnitude of these effects,
and to make modifications to the model or to the results to compensate
for those scale effects. Many of the scale effects that exist in short-wave
hydrodynamic models also exist in long-wave models; however, the relative
importance is sometimes different. The following paragraphs briefly summarize known scale effects for long-wave models along with some of the
empirical methods that are used to minimize these effects.
145
as possible, a substantial area must be dedicated to wave absorbers (e.g.,
horsehair mats). This increases the already significant area requirements
of the long-wave model.
Training walls often are used in basin models to stem diffraction of wave
energy into quiescent water adjacent to the region of wave generation. This
helps to simulate conservation of wave energy between orthogonals (Lee
1975). However, training walls are highly reflective and any waves reflected
back toward the area of wave generation could encounter these walls and
be re-reflected.
As in short-wave models, accurate reproduction of bathymetry is required to assure reliable results. In geometrically undistorted long-wave
models this becomes increasingly difficult as the modeled area increases.
Molding bathymetry in geometrically distorted models is considerably easier than in an equivalent undistorted model because depth differences are
more pronounced.
Finally, the small scales and low wave heights utilized in some longwave models make hydrodynamic measurements difficult and care must be
taken to assure that measurement error is not a significant portion of the
observation. The possibility of measurement error is reduced somewhat in
a geometrically distorted long-wave model, but careful experimenters will
always be aware of the problem. Also, when the hydrodynamic characteristics being measured are small, instrument intrusion is a laboratory effect
that must be considered (Le Méhauté 1990).
Scale Effects
Just as in short-wave models, scale effects in long-wave hydrodynamic models result primarily from the scaling assumption that gravity is the dominant
physical force balancing the inertial forces. Scaling based on this assumption (Froude scaling) incorrectly scales the other physical forces of viscosity,
elasticity, surface tension, etc., with the belief that these forces contribute
little to the physical processes. In addition, physical processes driven by
turbulent fluctuations are not in similitude in geometrically distorted longwave models.
The role of the model engineer is to recognize scale effects that potentially exist in the model, to evaluate the relative magnitude of these effects,
and to make modifications to the model or to the results to compensate
for those scale effects. Many of the scale effects that exist in short-wave
hydrodynamic models also exist in long-wave models; however, the relative
importance is sometimes different. The following paragraphs briefly summarize known scale effects for long-wave models along with some of the
empirical methods that are used to minimize these effects.