بررسی اثر جریان نفوذی روی افت‌و‌خیزهای سیگنال آکوستیکی در آزمایشگاه

نویسندگان

1 دانشجوی دکتری، گروه فیزیک دریا، دانشگاه علوم و فنون دریایی خرمشهر، ایران

2 استاد، گروه فیزیک فضا، موسسه ژئوفیزیک دانشگاه تهران، ایران

3 دانشیار، گروه فیزیک دریا، دانشگاه علوم و فنون دریایی خرمشهر، ایران

4 استادیار، موسسه ملی اقیانوس‌شناسی، تهران، ایران

5 استادیار، گروه فیزیک دریا، دانشگاه هرمزگان، ایران

چکیده

از دیدگاه آکوستیکی، اقیانوس به‌طور گسترده متغیر است. وجود جریان نفوذی (برای مثال، خروجی خلیج فارس)، امواج داخلی و تلاطم ریزمقیاس، مشخصه افقی لایه‌بندی سرعت صوت آشفته می‌‌کند و باعث افت‌و‌خیزهای زمانی و مکانی انتشار صوت می‌‌شود. در این تحقیق علاوه بر شبیه‌سازی جریان نفوذی شوری در آزمایشگاه، نحوه افت‌و‌خیزهای سیگنال آکوستیکی که با 10 عدد بسته موج سینوسی در بسامد 120 کیلوهرتز ایجاد شده را با جابه‌جایی منبع در بالا، پایین و داخل جریان نفوذی، مورد اندازه‌گیری و تجزیه و تحلیل قرار گرفته شده است. نتایج حاصل نشان می‌‌دهد که چنانچه منبع و گیرنده در داخل لایه جریان نفوذی نصب شود، دامنه سیگنال در لحظات گوناگون ورود پلوم آب نمک، کاهش می‌‌یابد ولی چنانچه منبع در بالا و پایین این لایه جریان نفوذی نصب شده باشد، دامنه سیگنال افزایش می‌‌یابد (قانون اسنل). با اِعمال فناوری‌‌trace envelop روی سیگنال‌های آکوستیکی ضبط شده، مشخص شد که شکل و فاز سیگنال با گذشت زمان تغییر می‌‌کند. این تحقیق نشان داد که جریان نفوذی، پدیده مهمی است که روی افت‌و‌خیزهای سیگنال‌‌های آکوستیکی تاثیر می‌‌گذارد. براساس محاسبات باور و همکاران، پهنای جریان خروجی (نفوذی) تنگه هرمز کمتر از 10 کیلومتر است، در نتیجه می‌‌توان نتایج این شبیه‌سازی آزمایشگاهی را بر طبق رابطه مقیاس‌سازی آکوستیکی  (a ضخامت پدیده و k عدد موج ) با آزمون‌‌ میدانی در محدوده جریان نفوذی تنگه هرمز در بسامد 5ر1 هرتز متناسب در نظر گرفت.

کلیدواژه‌ها


عنوان مقاله [English]

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

نویسندگان [English]

  • Mohammad Aakbarinasab 1
  • Abbas Ali AliakbariBidokhti 2
  • Masoud Sadrinasab 3
  • Vahid Chegini 4
  • Mohammad Mahdi Mehdizadh 5
1 Ph. D. Student of Physical Oceanography Khoramshahr Marine science and Technology University, Iran
2 Professor, Space Physics Department, Institute of Geophysics, University of Tehran, Iran
3 Associate Professor, Khoramshahr Marine science and Technology University, Khoramshahr, Iran
4 Assistant Professor, Department of Marine Engineering, Iranian National Center for Oceanography, Tehran, Iran
5 Assistant Professor, Physical Oceanography Hormozgan University, Iran
چکیده [English]

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=g'×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.

کلیدواژه‌ها [English]

  • Stratified intrusions
  • Sound propagation
  • Trace envelope
  • Signals fluctuations-burst