مطالعه آزمایشگاهی تاثیر امواج درونی بر روی انتشار امواج صوتی

نویسندگان

1 دانشجوی پژوهشگاه ملی اقیانوس شناسی و علوم جوی

2 مدیر گروه موسسه ژئو فیزیک دانشگاه تهران

3 هیات علمی پژوهشگاه ملی اقیانوس شناسی و علوم جوی

4 مدیر گروه فیزیک دریا دانشگاه بابلسر

چکیده

برای محاسبه‌ی فشار اکوستیکی در دریا با استفاده از روش‌های معمول، دانستن نحوه‌ی توزیع میدان چگالی و به تبع آن نحوه‌ی تغییرات سرعت صوت در محیط بسیار مهم است. عوامل محیطی بسیاری بر توزیع میدان چگالی در دریا تاثیرگذارند که بسته به شرایط محیطی و جغرافیایی شدت و ضعف هر یک از آن‌ها متفاوت می‌باشد. یکی از این عوامل امواج درونیاست که ساختار آنباعث تغییرات زمانی مکانی میدان چگالی و به تبع اثر گذاری روی نحوی انتشار امواج صوتی در اقیانوس می شود.هدف از این مطالعه بررسی آزمایشگاهی تاثیر امواج درونی ناشی از نوسان یک استوانه در یک کانال چینه بندیشده، بر روی انتشار امواج صوتی می-باشد. در این مطالعه ابتدا با استفاده از روش‌های معمول امواج درونی در محیط تشکیل داده شد و با استفاده از فرستنده و گیرنده صوتی تاثیر امواج درونی بر روی انتشار امواج صوتی بررسی شد. همچنین علاوه بر نتایج عملی شبیه سازی با یک نرم افزار انتشار صوتی صورت گرفت که نتایج آن مطابقت خوبی با نتایج آزمایشگاهی داشت. نتایج آزمایشگاهی نشان داد که امواج درونی در حالاتی باعث جمع‌شدگی و پراکندگی پرتوهای امواج صوتی می شود و در حالاتی قدرت سیگنال صوتی تا 54 درصد را کاهش دهد. در محیطهای دریایی نیز به دلیل وجود این امواج تغییرات سیگنال صوتی می تواند رخ دهد.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

A laboratory study of the effect of internal waves on acoustic propagation

نویسنده [English]

  • Hamed Deldar 1
چکیده [English]

For calculating the acoustic pressure due to sound propagation at sea using usual methods (pressure variations signals), knowing the density distribution and consequently, changes the speed of sound in the environment is very important. Many environmental factors affect the distribution of the density at sea, depending on environmental conditions and geographical location and the weaknesses of each of them are different. One of them is internal waves which usually cause temporal and spatial changes and consequently affect the acoustic wave propagation in the ocean. Internal waves can be generated by tidal currents over sea floor sloping that is very common in the stratified oceans. Results of study in the some researches showed that internal waves can effected on sound waves in two ways: 1-Internal waves can be decrease sound level up to 25 dB due to sound mode coupling in an exact frequency. 2- Internal waves can fuscous and defocus sound waves because of sound speed fluctuation.
The purpose of this study is a laboratory investigation of internal waves caused by fluctuation of a cylinder in a stratified glass channel with 3 meters long, 0.5 meter width and 1 meter height, on the sound waves propagation. In this study, using the double bucket and filling box method for generating stratification that stratification can be measured by one pair of salinity and temperature meters fixed on a rail moved up and down. Using the usual methods of setting up internal waves and using acoustical transducers in 53 KHz frequency, internal wave's effects on the propagation of sound waves, were investigated. In this study with usual optical method (Synthetic Schlieren) internal waves generated in the tank can be detected. In this method Internal wave generated in the glass tank change optical index of water layers and cased deviation of Straight lines designed on the back of tank. Laboratory results showed that sound waves can be focused and defocused due to the normal modes of internal waves. Some 9 experiments were done mainly in cases withvertical linear density stratified fluid. As the modal structure of internal waves in the water tank change due to the waves, constant density surfaces change slopes, hence changing the sound ray's paths and the amount of signals reaching the receivers. Similar results of numerical simulation also show similar behavior in the strength of the acoustic signal. Numerical simulation modeled by AcTUP v2.2L software that use KERAKENC method based on normal mode method. The acoustic signal can be weakened up to 54 per cent depending on the degree of sound ray divergence. We can conclusion that in the laboratory tank in this study internal waves effects on sound waves by focusing and defocusing and not by mode coupling.
Similar behaviors can be expected in the open ocean as the existence of internal waves is ubiquitous. For this goal dimensionless numbers should be use. Bowen (1993) showed that for simulating a sound waves interaction with a phenomenon in laboratory scale we can use ka = k'a'. With this formula we can compare laboratory results with real results on oceans.

کلیدواژه‌ها [English]

  • Internal waves
  • propagation of sound waves
  • focusing and defocusing of sound waves

مراجع

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فیزیک زمین و فضا
دوره 43، شماره 1
اردیبهشت 1396
صفحه 181-192

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