ANALYSING THE WAVE SPECTRUM FOR THE PERSIAN GULF IN ...

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صفحه 175 چهاردهمين كنفرانس ژئوفيزيك ايران . . 2. STUDY AREA Data set used in this study comprises of wave data parameters gathered in six stations in Naiband Gulf, Nakhle-Taghi port, Parak region, Taheri and Kangan port locations, during six months field measurements from August 23 rd 2008 to February 23 rd 2009. The Plan view of the measurement stations locations is plotted in figure #1. Figure 1. Study area in Persian Gulf 3. METHODOLOGY AND DATA PROCESSING 3.1. DATA The data available for this research are obtained by ADCP (Acoustic Doppler Current Profiler) and ADV (Acoustic Doppler Velocimeters) wave and current measurement instruments which are manufactured by the NORTEK (Norwegian) Company and their work mechanism is by sending and receiving under water transmission acoustic signals. The present data collected is driven directly from the installed instrument applied in the six locations during six months and measure waves in 17 minute intervals and the wave parameters are sampled every hour. Some severe storms occurred during the study and all wave parameters are recorded and stored from these events. The time series and power spectrums, derived through Fast Fourier Transform (FFT) became available. Next step was finding bad data or errors that could be detected by means of empirical observations. The pressure signal is utilized to estimate the non-directional, energy spectrum. The total energy distribution in this spectrum is then used to estimate wave height and period. The cross-correlation of the U and V velocity measurements (of the waves’ orbital velocities) provided us with a means to estimate the wave direction at specified frequencies. Time series estimates utilize the “zero-crossing” technique to identify individual waves in a time record. Table 1. Location, water depth and data recording specifications of the marine measurement instruments. If smoothing was not used than values would appear extremely noisy. The smoothing used specified by the number of FFT bins used in processing. A value of 64 is encouraged; however the most suitable of course is dependent on the number of samples recorded for each burst.
صفحه 176 چهاردهمين كنفرانس ژئوفيزيك ايران . . 3.2. DISTRIBUTION AND RANGES The range and average value of wave parameters of the data considered in the study is shown in figure #3 to #5. Figure 2. Variation of mean wave period with significant wave height Figure 3. Variation of peak wave period with significant wave height Figure 4- Variation of peak wave period with mean wave period. 4. RESULTS, DISCUSSIONS AND CONCLUSIONS 4.1. DATA DISTRIBUTION AND RANGE Sea state is generally defined using few parameters derived from the wave spectrum. Significant wave height (H s ) and peak period (T p ) are the most commonly used parameters. Mean wave period associated with high wave is usually obtained by assuming a significant wave steepness. In the present case, waves with H s higher than 0.5m are only considered. It was found that T m02 varies 0.5 0.5 between 2.5H s and 7H s (figure #3). While ISSC (1979) recommendation shows T m02 is in a range 0.5 0.5 of 2.6H s and 3.9H s .The present study shows that T m02 beyond ISSC range. T p was found to be 0.5 0.5 between 3.2H s and 12.5H s (figure #4). T p was found to be varying between the values estimated using the equations given below (figure #5): T p 4.5T m02 4.8, T 9.15 3.95 0.55 2 p T m02 T m02 . And also it was obviated that the relation between peak wave period and mean wave period is: T p 1.51 Tm 02 (2) 4.2. COMPARISONS BETWEEN MEASURED WAVE SPECTRUMS AND STANDARD SPECTRUMS RESULTS The measured wave spectrum has been compared with the aforementioned well-known spectrums. The spectral peak period estimated by the Scott and JONSWAP (Hasselmann) was higher than the measured peak value (sometimes they deviate too much). It was found that the Pierson-Moskowitz spectrum can sometimes estimate the spectral peak value but in prediction of peak wave period and frequency band it is not reliable due to independency of T p in their formulas. It was found that although most of the spectra were with predominant single peak, lots of them have local maximums in higher frequencies as well. Another phenomenon which can be seen in comparisons is that TMA spectrum fits better in deeper stations than the shallow waters although, the whole trend is not acceptable. Generally, the comparisons showed that the measured wave spectrum is more close to JONSWAP spectrum modified by Ochi than the other spectrums. Then it was attempted to justify the modified JONSWAP spectrum formula mentioned in equation #3. 4 2 g 5 f S( f ) exp 4 5 (2 ) f 4 f p 0.07, f
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