Energy dissipation across the wave spectrum - Breaking and Dissipation of Ocean Surface Waves - page 305

for a simple case of monochromatic waves in the tank. Mechanically-generated gently
sloped (

075) waves were run which therefore involve no additional
forcing superposed over wave orbital motion.
A single set of measurements was conducted for waves of 0

= ak = 0 . 035-0 .

667 Hz frequency, with the
wave amplitude gradually being changed from 2 cm up to 4 cm, then down to 2 cm and
up to 4 cm again. For such a frequency and water depth of approximately d

.

=

1m, the

waves are in a finite depth environment ( kd 9), and the wave orbits are somewhat

elliptical. For simplicity, the vertical amplitude of the wave orbit was chosen to estimate
the Reynolds number. Traces were injected into the water in the centre of the tank at 10 cm
depth below the surface (exp

= 1 .

( − kz ) = 0 . 83), far away from the bottom, surface and wall

boundary layers.
At the lower margin of a 0

= 0 . 02m, WRN (7.70) is Re wave = 1150 and the orbital

motion was expected to be fully laminar. At the other end of a 0 = 0 . 04m, Re wave = 4600

and fully turbulent motion was expected. Transition was anticipated at a 0 ≈ 0 . 03-0 . 035m

where Re wave = 2600-3500. Note that, according to (7.72) , at different distances from the

surface z transition is expected at different surface wave amplitudes a 0 .
The experiment is illustrated in Figure 7.21 .At a 0 02m, the motion remained

clearly laminar, and patterns of injected ink, while moving along the orbits, stayed un-
changed for minutes. At a 0

= 0 .

= 0 . 03m, some vortexes became visible which eroded the

a < 2cm

a ≈ 3 cm

a > 4 cm

Figure 7.21 Wave-tank experiment with wave-induced turbulence. f = 0 . 667 Hz, d = 1m, inkis
injected for waves of amplitude a 0 = 2 cm (top), a 0 = 3 cm (middle), a 0 = 4 cm (bottom)