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CHAPTER 4. HYDRODYNAMIC MODELS
• Air compression effects, such as waves breaking on vertical
walls, can only be in similitude if the model is conducted
in a partial vacuum (Le Méhauté 1976). Consequently,
empirical corrections have been developed. (This topic is
discussed in Chapter 5.)
• Material strengths are seldom reproduced in a scale model
unless the model material is carefully chosen to reproduce
strength properties. This is often difficult because model
materials often become so fragile that they cannot withstand the relatively rough handling during model construction. For example, concrete armor units are usually scaled
according to their mass density, and the model units can
withstand many times the stress loading that would shatter the prototype units.
• Studies where elastic forces are important must conform to
the Cauchy similitude. A good example is the elastic loads
generated by ship mooring lines as the ship is moved by
wave action. Le Méhauté (1976) also pointed out that ship
mooring fenders must be very smooth or oiled so that frictional scale effects are reduced. (Mooring line similitude is
discussed in Chapter 5.)
There are a number of papers discussing scale effects specific to certain
situations. Examples include scale effects in modeling wave attenuation
in rubble channels (Bishop 1986), and scale effects related to modeling
solitary waves (Gbbel 1984). The important point to remember is that the
modeler is responsible for identifying potential scale effects in the model,
and then quantifying the effects in terms of the influence it has on the
primary phenomenon being investigated. As pointed out by Le Méhauté
(1990), dissipative, capillary, elastic, and compressible scale effects (and
other scale effects) can eventually be quantified.
4.2.3 Short-Wave Model Boundary Layer Similitude
The development of similitude criteria from the continuity equation and the
equations of motion resulted in the conflicting requirements of maintaining
both the Froude and Reynolds number. For coastal models, the overwhelming option is to preserve Froude number and suffer the consequences of not
having similar viscous forces in the model. This results in viscous scale
effects, which were discussed in an earlier section.
CHAPTER 4. HYDRODYNAMIC MODELS
• Air compression effects, such as waves breaking on vertical
walls, can only be in similitude if the model is conducted
in a partial vacuum (Le Méhauté 1976). Consequently,
empirical corrections have been developed. (This topic is
discussed in Chapter 5.)
• Material strengths are seldom reproduced in a scale model
unless the model material is carefully chosen to reproduce
strength properties. This is often difficult because model
materials often become so fragile that they cannot withstand the relatively rough handling during model construction. For example, concrete armor units are usually scaled
according to their mass density, and the model units can
withstand many times the stress loading that would shatter the prototype units.
• Studies where elastic forces are important must conform to
the Cauchy similitude. A good example is the elastic loads
generated by ship mooring lines as the ship is moved by
wave action. Le Méhauté (1976) also pointed out that ship
mooring fenders must be very smooth or oiled so that frictional scale effects are reduced. (Mooring line similitude is
discussed in Chapter 5.)
There are a number of papers discussing scale effects specific to certain
situations. Examples include scale effects in modeling wave attenuation
in rubble channels (Bishop 1986), and scale effects related to modeling
solitary waves (Gbbel 1984). The important point to remember is that the
modeler is responsible for identifying potential scale effects in the model,
and then quantifying the effects in terms of the influence it has on the
primary phenomenon being investigated. As pointed out by Le Méhauté
(1990), dissipative, capillary, elastic, and compressible scale effects (and
other scale effects) can eventually be quantified.
4.2.3 Short-Wave Model Boundary Layer Similitude
The development of similitude criteria from the continuity equation and the
equations of motion resulted in the conflicting requirements of maintaining
both the Froude and Reynolds number. For coastal models, the overwhelming option is to preserve Froude number and suffer the consequences of not
having similar viscous forces in the model. This results in viscous scale
effects, which were discussed in an earlier section.