An underwater acoustic stealth metasurface model based on acoustic black hole theory

Abstract

The application relates to an underwater acoustic stealth metasurface model based on acoustic black hole theory, which comprises a base body, a plurality of horizontally and continuously distributed curved surfaces are arranged on the upper end face of the base body, the base body is made of metal, and the impedance of the base body is greater than 30 times the impedance of water; the curved surface satisfies L = 2(x-w) 2 wherein x is a horizontal axis coordinate value, and lambda is the wavelength of an incident sound wave; an embedded structure is embedded into the curved surface of the base body and combined with the base body to form a rectangular structure, the embedded structure is made of flexible material, and the impedance of the embedded structure is close to the impedance of water. The application has the advantages of compact and reasonable structure, convenient operation, etc., the device designs the metasurface by embedding a local non-uniform structure, realizes surface wave conversion of underwater plane sound waves, achieves the effect of underwater acoustic stealth, has simple structure, and adjusts sound wave conversion by adjusting the relationship between the structure width w and the wavelength lambda, so that the number of change parameters is small.

Description

Technical Field

[0001] The present invention relates to the technical field of stealth cloaks, and in particular to an underwater acoustic stealth metasurface model based on acoustic black hole theory. Background Art

[0002] In the field of underwater acoustics, as sonar detection continues to evolve toward lower frequencies, reducing acoustic scattering from underwater vehicles and improving their stealth performance have become pressing challenges. Traditionally, the concept of underwater stealth technology has been to reduce acoustic scattering by absorbing incident sound waves through absorbing materials. For example, anechoic tiles, commonly used in submarines, offer excellent sound absorption properties at higher frequencies. However, due to their limited thickness, it's difficult to extend the absorption frequency range to lower frequencies, resulting in poor noise reduction at these low frequencies.

[0003] The introduction of acoustic black holes has opened up new avenues for the development of acoustic stealth. An acoustic black hole refers to a phenomenon in which the bending wave velocity in a structure gradually decays as its thickness decays exponentially. Ideally, the bending wave velocity can be reduced to zero, achieving zero acoustic reflection. As early as 1989, Krylov et al. studied the manipulation of bending wave propagation using a wedge-shaped one-dimensional beam structure (Krylov V V. Conditions for validity of the geometrical acoustics approximation in application to waves in an acute angle solid wedge [J]. Journal of Soviet Physics Acoustics, 1989, 35:176–180). Wang Bohan et al. studied the vibration of a thin plate embedded with an acoustic black hole. The results showed that adjusting the thickness change and power law of the acoustic black hole region within a certain reasonable range will change the local modal parameters (WANG Bohan, YANG Deqing, XIA Lifu. Study on numerical simulation method for vibration characteristics of shell embedded with acoustic black hole[J]. Chinese Journal of Ship Research, 2019, 14(4): 30-39).

[0004] Furthermore, most current research on acoustic structures focuses on airborne acoustics, while relatively few theoretical and experimental studies on underwater acoustic regulation exist. Therefore, designing underwater acoustic structures with simple structures and good stealth performance is an urgent problem to be solved.

[0005] To this end, we propose an underwater acoustic stealth metasurface model based on acoustic black hole theory. Summary of the Invention

[0006] In response to the shortcomings of the above-mentioned existing production technology, the applicant provides an underwater acoustic stealth metasurface model based on acoustic black hole theory. By embedding local inhomogeneous structures to design the metasurface, the surface wave conversion of underwater plane sound waves is realized, thereby achieving the effect of underwater acoustic stealth.

[0007] The technical solutions adopted in the present invention are as follows:

[0008] An underwater acoustic stealth metasurface model based on acoustic black hole theory is characterized by comprising:

[0009] The substrate has a plurality of horizontally continuously distributed curved surfaces on its upper end surface, the substrate is made of metal, and the impedance of the substrate is greater than 30 times the impedance of water;

[0010] The surface satisfies L=2(xw) 2 , where x is the horizontal axis coordinate value and λ is the wavelength of the incident sound wave;

[0011] The embedded structure is embedded in the curved surface of the substrate and is combined with the substrate to form a rectangular structure. The embedded structure is made of a flexible material, and the impedance of the embedded structure is close to the impedance of water.

[0012] Furthermore, the width w of the embedded structure 1 is λ, the height h of the embedded structure 1 is λ / 2, the distance t between the embedded structure and the base floor is 0.025m, and the distance p between the embedded structure and the rightmost and leftmost sides of the base is 0.015m to 0.02m.

[0013] Furthermore, the substrate is made of steel, and its acoustic impedance is more than 30 times that of water.

[0014] Furthermore, each of the embedded structures forms a period, and the acoustic metasurface includes at least 3 periods.

[0015] Furthermore, the embedded structure is made of rubber, whose impedance is infinitely close to that of seawater.

[0016] The beneficial effects of the present invention are as follows:

[0017] The present invention has a compact, reasonable structure and is easy to operate. The device is designed by embedding a localized inhomogeneous structure to design a metasurface, realizing the surface wave conversion of underwater plane sound waves, achieving the effect of underwater acoustic stealth, and has a simple structure. At the same time, the device adjusts the sound wave conversion by adjusting the relationship between the structure width w and the wavelength λ, with few variable parameters. When the structure width w is the same as the wavelength λ, the angle of the scattered sound wave by the metasurface satisfies θ r=acrsin(λ / w), sound wave reflection angle θ r =90°. However, according to the generalized Snell's law with diffraction order correction, due to the diffraction effect of the periodic structure on the incident sound wave, when the acoustic loss is not large, the sound wave energy localized by the surface-bound mode will form different diffraction orders in space, causing the acoustic stealth to fail. Therefore, to achieve the complete conversion of the incident sound wave into a surface wave, it is necessary to reduce the diffraction effect of the periodic structure on the incident sound wave. Under ideal conditions, the black hole theory can reduce the sound wave speed to 0 at the tip, thereby greatly reducing the diffraction effect of the periodic structure on the incident sound wave, so that the sound wave energy localized by the surface-bound mode no longer scatters into space to form different diffracted waves. When the wavelength λ is not equal to the structure width w, the embedded structure 1 no longer reduces the sound wave speed to 0 at the tip, but still has the characteristics of a metasurface and can produce abnormal reflection phenomena within a certain frequency range. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic diagram of the principle of the present invention.

[0019] Figure 2 This is a model size identification diagram of the present invention.

[0020] Figure 3 is the incident sound pressure field corresponding to Example 1 of the present invention.

[0021] Figure 4 This is the scattered sound pressure field corresponding to Example 1 of the present invention.

[0022] Figure 5 : is the sound pressure diagram of the reflected sound wave of Example 1 of the present invention (along the y-axis direction, x=0m).

[0023] Figure 6 Graph showing the sound pressure level of reflected sound waves according to Example 1 of the present invention (along the y-axis, x=0 m).

[0024] Figure 7 This is the total sound wave pressure diagram of Example 2 of the present invention.

[0025] Figure 8 This is the far-field radiation pattern of the scattered acoustic wave according to embodiment 2 of the present invention.

[0026] in:

[0027] 1. Embedded structure; 2. Matrix. DETAILED DESCRIPTION

[0028] The specific embodiments of the present invention will be described below with reference to the accompanying drawings.

[0029] See Figure 1-6As shown in FIG, this paper invents an underwater acoustic stealth metasurface model based on acoustic black hole theory. The model includes a substrate 2 and a non-embeddable structure 1 embedded in the substrate 2.

[0030] like Figure 1 and Figure 2 As shown, the substrate 2 in this embodiment has a plurality of horizontally continuously distributed curved surfaces on its upper end surface. The substrate 2 is made of metal, and the impedance of the substrate 2 is greater than 30 times the impedance of water.

[0031] The surface satisfies L=2(xw) 2 , where x is the horizontal axis coordinate value and λ is the wavelength of the incident sound wave;

[0032] like Figure 2 As shown, the embedded structure 1 in this embodiment is embedded in the curved surface of the substrate 2 and is combined with the substrate 2 to form a rectangular structure. The embedded structure 1 is made of a flexible material, and the impedance of the embedded structure 1 is close to the impedance of water.

[0033] Specifically, the width w of embedded structure 1 is λ, and the height h of embedded structure 1 is λ / 2. The basis for the realization of the metasurface is a phase variation covering 0-2π within a period. The relationship between the phase of the reflected wave of the metasurface unit and the acoustic wave satisfies φ = 4πh / λ, that is, when the height of the metasurface unit is equal to half the wavelength, the phase φ = 2π. Reflected in embedded structure 1, the unit height of embedded structure 1 continuously changes along the x-axis, with the highest position height h being λ / 2 and the lowest position h being 0. The entire structure is approximately equivalent to an ideal metasurface periodic structure with uniform impedance variation (similar to the metasurface structure with multiple uniform dielectric arrangements in the previous patent), with a phase coverage of 0-2π.

[0034] When the width w is equal to the wavelength λ, the sound wave can be completely converted into a surface wave. When the width w is not equal to the wavelength λ, the structure still satisfies the metasurface properties.

[0035] In this embodiment, the material of the embedded structure 1 is preferably rubber. The impedance of rubber is infinitely close to that of seawater through adjustment of its mixture.

[0036] like Figure 1 and Figure 2 As shown, the distance t between the embedded structure 1 and the base 2 is 0.025m, the distance p between the rightmost and leftmost sides of the embedded structure 1 and the base 2 is 0.015m to 0.02m, and the base 2 is made of steel, and the impedance of steel is 46.8×10 6 Pa·s / m, and an acoustic impedance more than 30 times that of water, which can physically serve as a rigid isolation surface. The embedded structure 1 is an embedded structure, each embedded structure 1 forms a period, and the acoustic metasurface includes at least 3 periods.

[0037] Under the condition of vertical incidence, the thickness of the embedded structure 1 from right to left decays exponentially. As mentioned in the background technology, the acoustic black hole means that when the thickness of the structure decays exponentially, the bending wave velocity in the structure will gradually decay. Under ideal circumstances, the bending wave velocity can be reduced to 0, and zero reflection of the sound wave can be achieved.

[0038] The present invention uses the generalized Snell's law θ under vertical incidence conditions r =arcsin(λ / w), where λ is the wavelength of the sound wave. When the medium width is equal to the wavelength of the sound wave, the reflection angle is 0. The acoustic black hole effect prevents the sound wave from diffracting on the metasurface, thereby confining the sound wave to the near field of the metasurface and converting the vertically incident plane sound wave into a surface wave, thus achieving the acoustic stealth phenomenon.

[0039] Example 1:

[0040] When the acoustic wave frequency f = 3750 Hz, the background medium is water, and the acoustic wave is directed horizontally and perpendicularly onto the acoustic metasurface, the metasurface structural parameters are w = 0.4m, h = 0.2m, and t = 0.025m. Simulation calculations were performed using COMSOL Multiphysics software. The upper boundary of the acoustic metasurface is located at x = 0. The acoustic wave is incident on the acoustic metasurface at x = 0 and is reflected from the upper boundary at x = 0. Figure 3 is the incident sound pressure field corresponding to f=3750Hz, and the incident sound wave pressure is 1Pa. Figure 4 This is the scattered sound pressure field corresponding to f=3750Hz after numerical simulation. It can be seen that for vertically incident plane waves, the far-field acoustic wave scattering of the metasurface is small, and the plane acoustic wave is converted into a surface wave. Figure 5 and Figure 6 Figure 2 shows the reflected and reflected sound pressure levels along the y-axis at x = 0m. It can be seen that in the near-field close to the acoustic metasurface, sound waves overlap, resulting in a high sound pressure level. In the far-field, however, the scattered sound pressure is approximately zero, a 12dB decrease compared to the incident sound pressure level. Therefore, the metasurface can achieve underwater acoustic invisibility with a simple structure.

[0041] Example 2:

[0042] When the acoustic wave frequency f = 4250 Hz, the background medium is water, and the acoustic wave is directed horizontally and perpendicularly onto the acoustic metasurface, the metasurface structural parameters are w = 0.4m, h = 0.2m, and t = 0.025m. Simulation calculations were performed using COMSOL Multiphysics software. The upper boundary of the acoustic metasurface is located at x = 0. The acoustic wave is incident on the acoustic metasurface at x = 0 and is reflected from the upper boundary at x = 0. Figure 7is the total sound pressure field corresponding to f = 4250Hz after numerical simulation, Figure 8 is the far-field radiation pattern of the scattered sound wave. The theoretical reflection angle calculated according to the generalized Snell's law is 61.98°, and the Figure 8 The reflection angle obtained by numerical simulation is 63°, and the angle error is 1.02°, which proves that the simulation value is highly consistent with the theoretical value. Therefore, the metasurface can realize arbitrary control of the angle of underwater reflected sound waves through this structure.

[0043] The present invention has a compact, reasonable structure and is easy to operate. The device is designed by embedding a localized inhomogeneous structure to design a metasurface, realizing the surface wave conversion of underwater plane sound waves, achieving the effect of underwater acoustic stealth, and has a simple structure. At the same time, the device adjusts the sound wave conversion by adjusting the relationship between the structure width w and the wavelength λ, with few variable parameters. When the structure width w is the same as the wavelength λ, the angle of the scattered sound wave by the metasurface satisfies θ r =acrsin(λ / w), sound wave reflection angle θ r =90°. However, according to the generalized Snell's law with diffraction order correction, due to the diffraction effect of the periodic structure on the incident sound wave, when the acoustic loss is not large, the sound wave energy localized by the surface-bound mode will form different diffraction orders in space, causing the acoustic stealth to fail. Therefore, to achieve the complete conversion of the incident sound wave into a surface wave, it is necessary to reduce the diffraction effect of the periodic structure on the incident sound wave. Under ideal conditions, the black hole theory can reduce the sound wave speed to 0 at the tip, thereby greatly reducing the diffraction effect of the periodic structure on the incident sound wave, so that the sound wave energy localized by the surface-bound mode no longer scatters into space to form different diffracted waves. When the wavelength λ is not equal to the structure width w, the embedded structure 1 no longer reduces the sound wave speed to 0 at the tip, but still has the characteristics of a metasurface and can produce abnormal reflection phenomena within a certain frequency range.

[0044] The above description is an explanation of the present invention, not a limitation of the present invention. The scope of the present invention is defined in the claims. Any modifications may be made within the scope of protection of the present invention.

Claims

1. An underwater acoustic stealth metasurface model based on acoustic black hole theory, characterized in that: include: The substrate (2) has a plurality of horizontally continuously distributed curved surfaces on its upper end surface, the substrate (2) is made of metal, and the impedance of the substrate (2) is greater than 30 times the impedance of water; An embedded structure (1) is embedded in the curved surface of the substrate (2) and is combined with the substrate (2) to form a rectangular structure. The embedded structure (1) is made of a flexible material, and the impedance of the embedded structure (1) is close to the impedance of water. The width w of the embedded structure (1) is λ, the height h of the embedded structure (1) is λ / 2, λ is the wavelength of the incident sound wave, the distance t between the embedded structure (1) and the floor of the substrate (2) is 0.025m, and the distance p between the rightmost and leftmost sides of the embedded structure (1) and the substrate (2) is 0.015m to 0.02m; The surface satisfies L=2(xw) 2 ; The unit height of the embedded structure (1) changes continuously along the x-axis. The highest position height h is λ / 2, the lowest position h is 0, the phase covers 0-2π, and the thickness of the embedded structure (1) decays exponentially from right to left.

2. The underwater acoustic stealth metasurface model based on acoustic black hole theory according to claim 1, characterized in that: The substrate (2) is made of steel, and its acoustic impedance is more than 30 times that of water.

3. The underwater acoustic stealth metasurface model based on acoustic black hole theory according to claim 1, characterized in that: Each of the embedded structures (1) forms a period, and the underwater acoustic stealth metasurface model includes at least 3 periods.

4. The underwater acoustic stealth metasurface model based on acoustic black hole theory according to claim 1, characterized in that: The embedded structure (1) is made of rubber, and its impedance is infinitely close to that of seawater.

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