4.2. SHORT-WAVE HYDRODYNAMIC MODELS
97
this equation to be true, the wave length scale must be equal to the vertical
scale ratio, or
= Nh- Dalrymple (1985) points out two possibilities.
The first is when both horizontal and vertical length scales are the same,
i.e., N-^ = Nh = Nl, where Nl is the undistorted length scale which gives
a geometrically undistorted model. The other possibility is that N^ = Nh,
but
is not equal to the horizontal length scale, i.e., a geometrically
distorted model. The major problem with this assumption is that wave
diffraction around solid boundaries is not properly modeled at the same
time.
Diffraction is highly dependent on the dimensionless ratio of X/L, where
X is a horizontal distance (Whalin and Chatham 1974; Kamphuis 1991).
Therefore, correct scaling of diffraction requires that N^x/l) — 1> which
means that the wavelength must have the same scale as the horizontal
dimension, or N-^ — NxThus, it can be concluded that
Correct reproduction of both refraction and diffraction
in a Froude-scaled, short-wave hydrodynamic physical
model requires that the model be geometrically undistorted.
4.2.2 Short-Wave Model Laboratory and Scale Effects
Laboratory effects and scale effects are the two most important factors
affecting scale model results, and it is critical that the experimenter be
fully aware of their potential impacts.
Laboratory Effects
Laboratory effects in short-wave physical models are primarily related to:
(1) the physical constraints of boundaries on the flow, (2) unintentional
nonlinear effects brought about by using mechanical means of wave and
current generation, (3) and simplification of prototype forcing conditions,
such as representing prototype wave conditions as unidirectional. Laboratory effects in physical models are analogous to problems in numerical
models caused by numerical approximation to the equations, roundoff and
truncation errors, and computer speed, memory, and availability (Kamphuis 1991).
In a two-dimensional wave tank often cross-waves develop if energetic
wave conditions are being generated by a mechanical wave board. Also, the
method of generating waves can create unwanted nonlinear effects, such as
higher harmonics in finite-amplitude regular waves, and spurious long waves
that result from creating irregular waves using a board motion based on a
97
this equation to be true, the wave length scale must be equal to the vertical
scale ratio, or
= Nh- Dalrymple (1985) points out two possibilities.
The first is when both horizontal and vertical length scales are the same,
i.e., N-^ = Nh = Nl, where Nl is the undistorted length scale which gives
a geometrically undistorted model. The other possibility is that N^ = Nh,
but
is not equal to the horizontal length scale, i.e., a geometrically
distorted model. The major problem with this assumption is that wave
diffraction around solid boundaries is not properly modeled at the same
time.
Diffraction is highly dependent on the dimensionless ratio of X/L, where
X is a horizontal distance (Whalin and Chatham 1974; Kamphuis 1991).
Therefore, correct scaling of diffraction requires that N^x/l) — 1> which
means that the wavelength must have the same scale as the horizontal
dimension, or N-^ — NxThus, it can be concluded that
Correct reproduction of both refraction and diffraction
in a Froude-scaled, short-wave hydrodynamic physical
model requires that the model be geometrically undistorted.
4.2.2 Short-Wave Model Laboratory and Scale Effects
Laboratory effects and scale effects are the two most important factors
affecting scale model results, and it is critical that the experimenter be
fully aware of their potential impacts.
Laboratory Effects
Laboratory effects in short-wave physical models are primarily related to:
(1) the physical constraints of boundaries on the flow, (2) unintentional
nonlinear effects brought about by using mechanical means of wave and
current generation, (3) and simplification of prototype forcing conditions,
such as representing prototype wave conditions as unidirectional. Laboratory effects in physical models are analogous to problems in numerical
models caused by numerical approximation to the equations, roundoff and
truncation errors, and computer speed, memory, and availability (Kamphuis 1991).
In a two-dimensional wave tank often cross-waves develop if energetic
wave conditions are being generated by a mechanical wave board. Also, the
method of generating waves can create unwanted nonlinear effects, such as
higher harmonics in finite-amplitude regular waves, and spurious long waves
that result from creating irregular waves using a board motion based on a