434
CHAPTER 7. LABORATORY WAVE GENERATION
Table 7.2: Estimation of Spurious Wave Amplitudes Generated by a Directional Wavemaker (from Burcharth, et al. 1986).
L/b
Primary Wave Amplitude / Spurious Wave Amplitude
1.00
1.25
1.50
1.75
2.00
2.50
3.00
4.00 5.00
0.25
7.18
6.78
6.45
6.18
5.94
5.56
5.25
4.78 4.43
0.50
14.5
13.7
13.0
12.4
12.0
11.2
10.6
9.59 8.89
0.75
22.0
20.7
19.7
18.8
18.1
16.9
15.9
14.5
13.4
1.00
30.0
28.2 26.7
25.5
24.5
22.8
21.5
19.5
18.0
1.25
38.7
36.2 34.2
32.6
31.2
29.0
27.2
24.6 22.7
1.50
48.6
45.1
42.4
40.2
38.4
35.5
33.3
30.0 27.6
1.75
61.0
55.7 51.9
48.9 46.5
42.7
39.8
35.7 32.7
2.00
90.0
70.8 64.0
59.4
55.9
50.8
47.1
41.8 38.2
2.25
90.0
90.0 90.0
75.6
68.7
60.7
55.4
48.6 44.1
spurious waves. Sand and Mynett (1987) stated that spurious waves could
be minimized to an acceptable level by assuring the ratio of wavelength
to wave board segment width (L/b) is greater than 2.0 for the shortest
wavelength to be generated by the directional wavemaker.
Burcharth, et al. (1986) used Sand’s (1979) equations to construct a
table that can be used to estimate the magnitude of spurious waves that
occur when generating oblique regular waves. Burcharth, et al.’s table
is reproduced in Table 7.2. The rows represent increasing values of the
parameter L/b and the columns give the ratio of primary wave amplitude
to spurious wave amplitude. Tabulated values are propagation angle, 0, of
the primary wave expressed in degrees.
For a finite-length directional wavemaker, Takayama (1984) found good
agreement between his theoretical prediction and measured oblique waves,
provided the wave board segment length was small relative to wavelength.
Spurious waves formed by the wavemaker were stated to be a function of
wavemaker length, segment length, wavelength, and propagation angle.
Harkins, Dalrymple, and Losada (1990) performed a numerical sensitivity analysis of the directional wave generator with the individual segments being modeled by “top hat” functions consisting of the difference
between two Heavyside step functions. They examined the effects of stroke
CHAPTER 7. LABORATORY WAVE GENERATION
Table 7.2: Estimation of Spurious Wave Amplitudes Generated by a Directional Wavemaker (from Burcharth, et al. 1986).
L/b
Primary Wave Amplitude / Spurious Wave Amplitude
1.00
1.25
1.50
1.75
2.00
2.50
3.00
4.00 5.00
0.25
7.18
6.78
6.45
6.18
5.94
5.56
5.25
4.78 4.43
0.50
14.5
13.7
13.0
12.4
12.0
11.2
10.6
9.59 8.89
0.75
22.0
20.7
19.7
18.8
18.1
16.9
15.9
14.5
13.4
1.00
30.0
28.2 26.7
25.5
24.5
22.8
21.5
19.5
18.0
1.25
38.7
36.2 34.2
32.6
31.2
29.0
27.2
24.6 22.7
1.50
48.6
45.1
42.4
40.2
38.4
35.5
33.3
30.0 27.6
1.75
61.0
55.7 51.9
48.9 46.5
42.7
39.8
35.7 32.7
2.00
90.0
70.8 64.0
59.4
55.9
50.8
47.1
41.8 38.2
2.25
90.0
90.0 90.0
75.6
68.7
60.7
55.4
48.6 44.1
spurious waves. Sand and Mynett (1987) stated that spurious waves could
be minimized to an acceptable level by assuring the ratio of wavelength
to wave board segment width (L/b) is greater than 2.0 for the shortest
wavelength to be generated by the directional wavemaker.
Burcharth, et al. (1986) used Sand’s (1979) equations to construct a
table that can be used to estimate the magnitude of spurious waves that
occur when generating oblique regular waves. Burcharth, et al.’s table
is reproduced in Table 7.2. The rows represent increasing values of the
parameter L/b and the columns give the ratio of primary wave amplitude
to spurious wave amplitude. Tabulated values are propagation angle, 0, of
the primary wave expressed in degrees.
For a finite-length directional wavemaker, Takayama (1984) found good
agreement between his theoretical prediction and measured oblique waves,
provided the wave board segment length was small relative to wavelength.
Spurious waves formed by the wavemaker were stated to be a function of
wavemaker length, segment length, wavelength, and propagation angle.
Harkins, Dalrymple, and Losada (1990) performed a numerical sensitivity analysis of the directional wave generator with the individual segments being modeled by “top hat” functions consisting of the difference
between two Heavyside step functions. They examined the effects of stroke