Study On Wave Diffraction Effect Of Liquid Surface And Detection Of Underwater Low Frequency Sound Source

Problems of water wave are closely linked to the national construction, economic development and environmental improvement, especially ocean strategy attaches great importance to by the governments. Laser detecting liquid surface wave plays an important role in the field of surface wave. The sound source information of the surface and liquid following is obtained by detecting the surface conditions. Laser diffraction detecting technology applied to the surface and underwater low-frequency acoustic signal detection has non-destructive, rapid, non-contact, mobility and other characteristics. This article main contents are as follows:(1) Based on the theory of fluid dynamics, the liquid surface wave grating generated by the water disturbance and vibration of underwater acoustic source is researched. The sound and light effects of liquid surface are analyzed using the principle of wave optics. When the surface of water forms surface wave, this surface wave can be seen approximately as a reflex type sinusoidal phase grating and the diffraction effects occur when incident laser is modulated.(2) Based on the acousto-optic diffraction theory, the light diffraction characteristics of low-frequency liquid surface waves is investigated using the method of optical. An expression is given about the non-corresponding orders and the analytical expression is simulated by means of the numerical method, which explains the mechanism of the non-correspondence effect. In the experiment, the steady and visible diffraction pattern was observed, and the fringe spacing has obvious asymmetry. With the increasing of incident angle, the distinct non-correspondence of the positive and negative diffraction orders is found. That is to say, the number of the positive diffraction pattern is more than the negative one. Our studies show that:the non-correspondence effect is related to the incident angle. With the increasing of incident angle, the non-correspondence effect is more obvious. The negative diffraction order has maximum. For the orders exceeding this value, the negative diffraction pattern disappeared.(3) Based on the acousto-optic diffraction theory, the mechanism of laser diffraction and interference fringes in the water surface wave modulation is analyzed theoretically, and the interference and diffraction pattern is obtained experimentally under different excitation intensity. The relationship of the width of the stripe pattern of the interference and diffraction and the amplitude of surface wave is studied. The relationship of interference pattern width and amplitude of surface wave is given. The rule that the diffraction pattern width increases as the amplitude of surface wave increases is found by linear fitting for diffraction pattern width with amplitude of surface wave, which explains theoretically that the relationship of the width of the diffraction pattern and the amplitude of surface wave.(4) The laser detection system of elevating underwater low frequency acoustic signal is established. The diffraction pattern of a high-contrast is observed experimentally, which provides a new idea for measuring underwater acoustic signal with light diffraction method. We found that the relationship of the depth of the underwater sound source and the amplitude of excitation surface wave is first increasing then decreasing by measuring experimentally the relation of underwater acoustic frequency, depth and the amplitude of excitation surface wave. When the depth of sound source into the water is fixed, the surface wave amplitude with sound source frequency increasing presents first increasing and then decreasing. Combined with the three relations, the excitation frequency for the maximum amplitude of surface wave decreases as the water depth increases, and the change of the frequency near the water is very big. The frequency gradually slows as the depth in water increases. Different frequency corresponds to different depths.