بررسي اثر جريان نفوذي روي افت‌و‌خيزهاي سيگنال آكوستيكي در آزمايشگاه

Investigation of the effect of outflow intrusion on acoustical signal fluctuations in laboratory

از ديدگاه آكوستيكي، اقيانوس به‌طور گسترده متغير است. وجود جريان نفوذي (براي مثال، خروجي خليج فارس)، امواج داخلي و تلاطم ريزمقياس، مشخصه افقي لايه‌بندي سرعت صوت آشفته مي‌‌كند و باعث افت‌و‌خيزهاي زماني و مكاني انتشار صوت مي‌‌شود. در اين تحقيق علاوه بر شبيه‌سازي جريان نفوذي شوري در آزمايشگاه، نحوه افت‌و‌خيزهاي سيگنال آكوستيكي كه با 10 عدد بسته موج سينوسي در بسامد 120 كيلوهرتز ايجاد شده را با جابه‌جايي منبع در بالا، پايين و داخل جريان نفوذي، مورد اندازه‌گيري و تجزيه و تحليل قرار گرفته شده است. نتايج حاصل نشان مي‌‌دهد كه چنانچه منبع و گيرنده در داخل لايه جريان نفوذي نصب شود، دامنه سيگنال در لحظات گوناگون ورود پلوم آب نمك، كاهش مي‌‌يابد ولي چنانچه منبع در بالا و پايين اين لايه جريان نفوذي نصب شده باشد، دامنه سيگنال افزايش مي‌‌يابد (قانون اسنل). با اِعمال فناوري‌‌trace envelop روي سيگنال‌هاي آكوستيكي ضبط شده، مشخص شد كه شكل و فاز سيگنال با گذشت زمان تغيير مي‌‌كند. اين تحقيق نشان داد كه جريان نفوذي، پديده مهمي است كه روي افت‌و‌خيزهاي سيگنال‌‌هاي آكوستيكي تاثير مي‌‌گذارد. براساس محاسبات باور و همكاران، پهناي جريان خروجي (نفوذي) تنگه هرمز كمتر از 10 كيلومتر است، در نتيجه مي‌‌توان نتايج اين شبيه‌سازي آزمايشگاهي را بر طبق رابطه مقياس‌سازي آكوستيكي (a ضخامت پديده و k عدد موج ) با آزمون‌‌ ميداني در محدوده جريان نفوذي تنگه هرمز در بسامد 5ر1 هرتز متناسب در نظر گرفت.

From the acoustical oceanography point of view the ocean is a sophisticated environment. Existence of outflow intrusion (for instance, outflow of the Persian Gulf), internal waves, and small-scale turbulence perturb the horizontally stratified character of the sound velocity and cause spatial and temporal fluctuations of the sound propagation. In this experimental study, we have investigated signals fluctuations over time (was powered by a 20 MHz/Arbitrary Waveform Generator Model DG 1022 set to generate a 10 cycle sinusoid burst at frequency of 120 kHz with amplitude of 20 volts peak-to-peak) in a pre-stratification environment outside of the intrusion of turbulent plume. All experiments were carried out in a glass tank 2.19 m long, 1.27 m wide and 0.8 m deep. Before the experiments, four transducers (of which three of them are transmitters and the rest of them as receiver) are mounted opposite to each other with a separation of 1.65 m on two iron bars inside the tank. The distance between each transducer is 0.14 m. These holders are facing each other at a distance 0.3m from the tank wall. Prior to the beginning of the experiments with stratification, the acoustic measurements were executed in fresh water. All received signals were sampled at 5 MHz in all experiments. A fourth-order Butterworth band-pass filter was applied to the received voltage time series, with cutoff frequencies at 110 and 130 kHz for the 120-kHz data. In case where a “filling box" stratification (Baines and Turner, 1969) is used, the tank was initially filled with fresh water to a depth of 0.48 m. The water was then stratified using a plume of dense salt solution falling from the end of small tube (a nozzle of 3 mm diameter) placed at 0.47 m from the base with a buoyancy flux of F=gʹ ×Vo= . After the set-up of the “filling box" stratification in the tank (Fig 1), acoustic signals and hydrophysical data were measured simultaneously. Then to produce the outflow intrusions, a source of dyed salt solution with a density less than the previous case (“filling box") with volume flux of was entered into stratified environment. At the start of the experiment with plume intrusion the speed of the nose of the outflow increased with time. The intrusion is also thickened, and eventually split to generate a new tongue of dyed plume water growing beneath the first layer. The dye tracer in the outflow water was slowly adverted upward to replace water entrained into the plume at shallower depths, and eventually reached to the source level. The outflow intrusion is produced at the start of the experiment at the location of the transmitter in the middle of the tank (at the depth of 0.22 m). The dyed outflow water is wedged-shaped with a sloping interface beneath. In different time intervals the acoustic and hydrophysics data are measured simultaneously, and then these signals in different times, based on the place of the plume outflow, are processed. After investigating the output signals, these results are found: when the transmitter and receiver is positioned into the outflow intrusion (dyed outflow) location, the signal amplitude is decreased at different moments of plume intrusion, but if the transmitter is positioned in the upper and lower part of the outflow intrusion, it causes the signal amplitude to increase (Snell’s law). By applying trace envelope techniques on the received signals, shape of signal change was found with time. Thereby, results indicate that outflow intrusion could be important on acoustical signal fluctuations. Results indicate that outflow intrusion could be important in shapes of the received signals. Also we have observed the occurrence of major signal fluctuations over time is accordance with the sound speed vertical structure changes. It is noticed that this phenomenon is also taken place at the outflow of the Persian of Gulf to the Oman sea. The result of such simulation could be used with attention to the acoustic scale rule , (k is the wave number.) where a lab by thickness of this current at outflow of the Persian Gulf which is about < 10 km.