146
CHAPTER 4. HYDRODYNAMIC MODELS
Wave Reflection. Long-period waves, as previously mentioned, reflect
more of their energy when they meet solid bodies or boundaries; and hence,
reflections are more apt to be an important aspect in long-wave physical
models. For geometrically undistorted models, most of the discussion given
previously for short-wave model wave reflection scale effects applies to longwave models as well. If reflection from a structure is a critical aspect in
the model, the best procedure is to conduct a 2-d test of the structure
cross-section at a scale large enough to minimize scale effects arising from
nonturbulent flow through the structure. This is followed by small scale
tests with wire mesh placed over the structure to induce turbulence and/or
stone sizes increased slightly to provide reduced reflection that is similar to
that measured in the larger model.
Boundaries and solid structures in geometrically distorted long-wave
models have steeper slopes than in the prototype. The steeper slopes cause
greater reflection which can bias the model results. For porous structures,
the testing technique mentioned above would use the results from largerscale 2-d tests of an undistorted version of the structure to adjust porosity
and turbulence in the smaller-scale distorted structure so that correct reflection characteristics are obtained.
Another reflection consideration for geometrically distorted long-wave
models is the cumulative reflection from bottom slopes as waves propagate
from deep to shallow water. If the prototype bottom slope is gentle and
model distortion is minimal, this isn’t a significant scale effect. However,
if the prototype bottom slope is steep, or contains bars with steep seaward
slopes, then reflections can be significant. This may cause problems if the
model distortion is too large (Whalin and Chatham 1974).
Wave Transmission. The wave transmission scale effect due to incorrect
simulation of the flow regime through porous structures can be corrected for
geometrically undistorted long-wave models using the techniques developed
for short-wave models (see Figure 4.4 and the Eqns. 4.36 - 4.40).
Le Méhauté s (1965) method for sizing model stones for correct similitude of wave transmission is represented in the nomogram given in Figure 4.6 for the case of geometrically distorted long-wave models.
The only difference between Figure 4.6 and Figure 4.4 (undistorted models) is in the ordinate axis and in the definition for the factor
The
ordinate, Lp/Lm (which is the same as A^), on Figure 4.4 has been replaced
on Figure 4.6 with
2/3
[
1 2/3
^x(Az)1/2]
(4.118)
CHAPTER 4. HYDRODYNAMIC MODELS
Wave Reflection. Long-period waves, as previously mentioned, reflect
more of their energy when they meet solid bodies or boundaries; and hence,
reflections are more apt to be an important aspect in long-wave physical
models. For geometrically undistorted models, most of the discussion given
previously for short-wave model wave reflection scale effects applies to longwave models as well. If reflection from a structure is a critical aspect in
the model, the best procedure is to conduct a 2-d test of the structure
cross-section at a scale large enough to minimize scale effects arising from
nonturbulent flow through the structure. This is followed by small scale
tests with wire mesh placed over the structure to induce turbulence and/or
stone sizes increased slightly to provide reduced reflection that is similar to
that measured in the larger model.
Boundaries and solid structures in geometrically distorted long-wave
models have steeper slopes than in the prototype. The steeper slopes cause
greater reflection which can bias the model results. For porous structures,
the testing technique mentioned above would use the results from largerscale 2-d tests of an undistorted version of the structure to adjust porosity
and turbulence in the smaller-scale distorted structure so that correct reflection characteristics are obtained.
Another reflection consideration for geometrically distorted long-wave
models is the cumulative reflection from bottom slopes as waves propagate
from deep to shallow water. If the prototype bottom slope is gentle and
model distortion is minimal, this isn’t a significant scale effect. However,
if the prototype bottom slope is steep, or contains bars with steep seaward
slopes, then reflections can be significant. This may cause problems if the
model distortion is too large (Whalin and Chatham 1974).
Wave Transmission. The wave transmission scale effect due to incorrect
simulation of the flow regime through porous structures can be corrected for
geometrically undistorted long-wave models using the techniques developed
for short-wave models (see Figure 4.4 and the Eqns. 4.36 - 4.40).
Le Méhauté s (1965) method for sizing model stones for correct similitude of wave transmission is represented in the nomogram given in Figure 4.6 for the case of geometrically distorted long-wave models.
The only difference between Figure 4.6 and Figure 4.4 (undistorted models) is in the ordinate axis and in the definition for the factor
The
ordinate, Lp/Lm (which is the same as A^), on Figure 4.4 has been replaced
on Figure 4.6 with
2/3
[
1 2/3
^x(Az)1/2]
(4.118)