4.2. SHORT-WAVE HYDRODYNAMIC MODELS
129
4.2.6
Short-Wave Model Scale Selection
There are a number of factors that go into scale selection for a short-wave
hydrodynamic physical model, and these factors vary widely from one modeling application to the next. In the broadest sense, the engineer must
consider:
• Size of the model facility and wave generation capability
• Flow Reynolds numbers for the proposed scale
• Past experience with physical models of similar problems
• Range of parameters to be considered in the model
• Funding limitations
Numerous laboratories have carried out scale series to investigate best
scales, and they found very large scales are not economical and very small
scales are wrought with scale effects (Le Méhauté 1976). Therefore, the
largest scale may not be the best scale because economics must be an important consideration. Le Méhauté also pointed out that the cost of scale
model experiments conducted in basins increases approximately as the cube
of the increase in model length (fm) chosen to represent the prototype characteristic length (f'p). This is because the model area increases as (^m)2 and
the theoretical duration of the test increases as (fm)1/2 (but the actual duration increases faster because model construction and preparation times
increase as well)13. Le Méhauté (1976) summed up the importance of economic considerations by saying, “Scale selection is a compromise between
economics on the one hand and the technical requirements for similitude on
the other hand. ”
13 This rule isn’t true for very small models, because as model size decreases, there
may be additional expense in reproducing bathymetry and obtaining measurements of
sufficient accuracy.
Short-wave harbor models must balance the larger size requirements
dictated by viscous friction and surface tension scale effects against the
small size requirements of having to reproduce a fairly large coastal area
(and adjacent shoreline) at undistorted scale. This becomes largely an
economic trade-off between size and cost, and often harbor models must
contend with non-negligible scale effects. Also the size of harbor models
must be large enough to reproduce bathymetry accurately and to obtain
reliable wave measurements. An accurate assessment is that
Scale selection will remain an art based on past experience.
And this statement probably can be universally applied to all types of
physical fluid models.
129
4.2.6
Short-Wave Model Scale Selection
There are a number of factors that go into scale selection for a short-wave
hydrodynamic physical model, and these factors vary widely from one modeling application to the next. In the broadest sense, the engineer must
consider:
• Size of the model facility and wave generation capability
• Flow Reynolds numbers for the proposed scale
• Past experience with physical models of similar problems
• Range of parameters to be considered in the model
• Funding limitations
Numerous laboratories have carried out scale series to investigate best
scales, and they found very large scales are not economical and very small
scales are wrought with scale effects (Le Méhauté 1976). Therefore, the
largest scale may not be the best scale because economics must be an important consideration. Le Méhauté also pointed out that the cost of scale
model experiments conducted in basins increases approximately as the cube
of the increase in model length (fm) chosen to represent the prototype characteristic length (f'p). This is because the model area increases as (^m)2 and
the theoretical duration of the test increases as (fm)1/2 (but the actual duration increases faster because model construction and preparation times
increase as well)13. Le Méhauté (1976) summed up the importance of economic considerations by saying, “Scale selection is a compromise between
economics on the one hand and the technical requirements for similitude on
the other hand. ”
13 This rule isn’t true for very small models, because as model size decreases, there
may be additional expense in reproducing bathymetry and obtaining measurements of
sufficient accuracy.
Short-wave harbor models must balance the larger size requirements
dictated by viscous friction and surface tension scale effects against the
small size requirements of having to reproduce a fairly large coastal area
(and adjacent shoreline) at undistorted scale. This becomes largely an
economic trade-off between size and cost, and often harbor models must
contend with non-negligible scale effects. Also the size of harbor models
must be large enough to reproduce bathymetry accurately and to obtain
reliable wave measurements. An accurate assessment is that
Scale selection will remain an art based on past experience.
And this statement probably can be universally applied to all types of
physical fluid models.