碩士 / 國立成功大學 / 水利及海洋工程學系 / 86 / The importance of directional spectrum has aroused a
great interest inthis research. It is the purpose of this
study to discuss local features ofdirectional wave off Taishi
coast in the winter monsoon season and to evaluatethe validity
of Mitsuyasu''s empirical formula of directional energy
spreading. A spatial array of wave gauges installed on an
observation platform hasbeen arranged to measure the wave
directionality and the Bayesian ParameterEstimation Method
(BPEM) was adopted to determine the main direction and
thedirectional spreading parameter for each frequency.Wind
measurements were alsomade on the platform.The statistical
properties of waves and winds show thatthe main directions
of waves at peak frequency don''t coincide with the
winddirections. The well known Snell''s law was then used to
justify that waverefraction may be the main reason of wave
direction deviated from winddirection. As expected that
wave directions estimated from both BPEM and Snell''s law at
low frequency components always tend to shift toward the
normalto the isobaths as water depth decreases. However,
there exists a slowlychanging disagreement of numerical
results with observed incidence. The morecomplete conditions
and numerical scheme should be considered to explain
thediscrepancy.
The Mitsuyasu''s formula for directional spreading function was
justifiedto be representative of wave field. The normalized
directional spreadingparameter S/Smax in the formula was derived
and plotted as a function of f/fp.The results shows the same
tendency as previous observations that directional concentration
has maximum value at the spectral peak and decreased at the
bothsides of spectral peak. However, as mentioned by Hasselmann
et al. (1980) thatthe maximum directional spreading
parameterSmax didn''t always happen at peak frequency for our
collected field data. Also, the data scatter compared to
theMitsuyasu''s formula indicates the complexity of S. It was
then assumed and proved that S should be related to wave age,
which is the ratio of the wavephase speed to the wind speed.
The linearregression was performed on the datato have a modified
Mitsuyasu''s formula in consideration of the stage of
wavedevelopments. The bigger the wave age , the more rapid
decrease in spreading.Nevertheless, the major portion of the
wave energy, which is around the peakfrequency, may not be
completely related to the local wind condition. The relation
between the parameter Smax and the waveage, as suggested
by Mitsuyasu''s formula, may be improper. The field data were
shown that the wavesteepness is more related to Smax than wave
age does. In general, the values of Smax decrease as the
values of the wave steepness increase. The
directional response of ocean waves to changes in wind
direction isan important aspect for reliable wave predictions
of sea states during stormevents or moving frontal systems.
However, information on local main wavedirection to changing
wind direction is scarce. In this study we selected adataset of
changing sea states followed after a moving frontal system.
Asexpected,the high-frequency waves align rather quickly with
the wind direction, whereas the low-frequency wind-sea
components tend to lag behind the winddirection and take
several hours to align completely with the wind. In
theearlier stages ofwind direction changing, the directional
spectrum becomesbroader and less peaked. A new higher-frequency
peak grows gradually in thenew wind direction after wind shift
for duration. The combination of gradualgrowth of energy in
the new wind direction and decay in the old direction results
in a smooth changing of the mean wave direction for the high-
frequencycomponents. As growth continues, most of wave
components align in the new winddirections and a highly peaked
form of new wind-sea spectrum dominates.
| Identifer | oai:union.ndltd.org:TW/086NCKU1083007 |
| Date | January 1998 |
| Creators | Mai, Chiao-Hung, 麥喬閎 |
| Contributors | Kao Chiao-Chuen, Laurence [.H. Chuang, 高家俊, 莊士賢 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 63 |
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