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Optimal Design of Spherical Shell of Co-vibration Vector Hydrophone(2)

Views: 0     Author: Site Editor     Publish Time: 2021-09-30      Origin: Site

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The shape of the hydrophone is a standard spherical shape acoustic transducer. The spherical shell of the hydrophone is composed of upper and lower hemispheres. The outer radius of the two hemispheres is 36 mm, the wall thickness of the lower hemisphere is 3 mm, and the wall thickness of the upper hemisphere is 4 mm. A rubber O-ring is used for axial sealing in the middle. In order to minimize the quality of the non-pressure-bearing part of the shell, an American standard O-ring that is thinner than the national standard is selected to reduce the width of the O-ring installation groove. The upper and lower hemispheres are fastened by the threads on the spherical shell, so that there is no need to increase the installation position of the fastening bolts, and it is also to make the non-pressure-bearing part of the shell as small as possible. Because the upper and lower hemispheres are fastened by threads, the alignment position of the two hemispheres is random when tightening. Therefore, 4 spring suspension holes are evenly distributed in the center of the outer surface of the spherical shell instead of two symmetrically distributed on the two hemispherical shells. Loop spring suspension hole. Make the lower hemisphere a little larger and the upper hemisphere a little smaller, so that all the spring suspension holes in the center are located on the lower hemisphere. The vibration pickup sensor uses a three-axis piezoelectric accelerometer. The accelerometer is installed in the center of the spherical shell through a bracket, and the signal conditioning circuit is installed on the other side of the bracket. Note that this "center" is also located in the lower hemispherical shell, so that when the two hemispheres are tightened, no matter what the angle between the upper and lower hemispheres is, it will not affect the alignment of the accelerometer with the direction of the suspension hole. After the assembly is completed, the center of gravity of the entire vector hydrophone should coincide with the center of the spherical shell underwater acoustic transducer as much as possible. The position of the center of gravity of the hydrophone in Figure 1 is automatically calculated by the 3D modeling software, and it is located at the geometric center of the vector hydrophone. The weak area of the designed pressure-resistant spherical shell is the connection between the O-ring groove and the spherical shell and the opening of the piercing part. For the connection between the O-ring groove and the spherical shell, add a large fillet to make the transition smooth to reduce stress concentration. For the opening of the piercing part, on the one hand, increase the thickness of the hole wall to increase the strength of the hole wall, on the other hand, add large round corners at the transition between the hole wall and the inner surface of the spherical shell, and at the transition between the hole wall and the outer surface of the spherical shell Increase the material to smooth the transition and reduce stress concentration. In order to compensate for the strength reduction problem caused by the opening of the upper hemispherical shell, the thickness of the upper hemispherical shell was increased by 1 mm as a whole. In addition, the pressure-resistant steel bolts used for routing through the warehouse have higher strength, equivalent to solid bolts, and support the threaded holes.

 

4.5 Performance simulation of pressure-resistant shell of vector hydrophone

It can be seen from Figure 1 that the designed pressure-resistant spherical shell of the vector hydrophone is no longer an ideal spherical shell. The largest impact on the pressure-resistant performance is the opening of a larger threaded hole in the upper hemisphere. The influence of the hole has increased the thickness of the upper hemisphere by 1 mm. These changes have not been theoretically calculated. The following uses the method of finite element analysis to perform structural static simulation and eigenvalue buckling simulation on the three-dimensional model of the vector hydrophone spherical shell to verify whether the designed vector hydrophone can withstand an external pressure of 30 MPa. The finite element simulation software used is ANSYS Workbench.

 

4.5.1 Structural static simulation

Import the three-dimensional digital model of the vector hydrophone spherical shell into the finite element simulation software, set the shell material to 7075T6 aluminum alloy, and set the contact mode between the upper shell and the plug and between the upper and lower shells to bind mode , The hexahedron method is used to mesh the model, the mesh size is set to a bending function, and the maximum size is set to 0.8 mm. The displacements in the x, y, and z directions are set to 0 on the upper surface of the plug to constrain the translation of the model; a cylindrical surface constraint is set on the outer cylindrical surface of the plug, and the tangential direction is fixed to limit the rotation and rotation of the model. Axial and radial free; apply a pressure load of 30 MPa on all outer surfaces of the hydrophone shell (including the inner surface of the O-ring groove), and perform structural static analysis on it. The stress intensity distribution of the hydrophone shell obtained by simulation is shown in Figure 2. The stress intensity is selected for analysis because it is an equivalent stress based on the third intensity theory, the result is safer, and it is suitable for pressure vessel analysis.


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The stress intensity of the annular bulge caused by the O-ring groove in the middle of the hydrophone shell (which can be considered as a stiffening rib ring) is small; the stress intensity simulation value of the middle part of the upper and lower hemispherical shells of the hydrophone shell is the smallest , Its value is less than 202.7 MPa, here does not include discontinuity and stress concentration, it can be considered as the primary overall film stress intensity, according to formula (6), the theory of the primary overall film stress (that is, the maximum principal stress) of the thin-walled spherical shell The calculated value is 187.8 MPa, which is basically consistent with the simulation results. The stress intensity in most areas of the inner surface of the upper and lower spherical shells is relatively large, and its value is less than 243.2 MPa. The stress at this point belongs to the primary bending stress and meets the limit of less than 1.5 times the allowable stress. There is an annular large stress zone at the junction of the lower hemispherical shell and the central annular protrusion, the stress intensity is about 324.2 MPa, the stress here is the primary stress plus the secondary stress, and its value is less than 3 times the allowable stress, which meets the design requirements. There are local stress concentrations at the place where the top of the upper hemispherical shell is in contact with the plug and a few places in the O-ring groove. The maximum stress is 405.2 MPa, which belongs to the primary stress plus the secondary stress plus the peak stress. This stress will not affect The impact of strength failure mainly affects the fatigue failure of the pressure shell. Therefore, the spherical shell of the vector hydrophone can withstand an external pressure of 30 MPa without strength failure.

 

4.5.2 Eigenvalue buckling simulation

Next, the pressure load on the outer surface of the hydrophone spherical shell model is changed to 1 MPa, and the eigenvalue buckling analysis is performed on the basis of the structural static analysis results. The total deformation of the first-order buckling mode of the spherical shell of the hydrophone is shown in Figure 3.


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It can be seen from Figure 3 that the deformation mainly occurs in the lower hemisphere, because the thinner the spherical shell, the worse the stability. The first-order buckling load factor is 680.35, so the simulation value of the critical instability pressure of the hydrophone spherical shell is 680.35 MPa, which is slightly higher than the circumferential instability critical pressure calculated by the formula of 611.6 MPa. Therefore, the spherical shell of the vector hydrophone can withstand an external pressure of 30 MPa without stability failure.

 

4.6 Vector hydrophone production

The upper and lower hemispherical shells of the vector hydrophone sensor are processed by CNC machine tools. The material is 7075-T6 aluminum alloy, and the surface is anodized to form a dense oxide protective film to improve surface hardness and inhibit seawater corrosion. The completed co-vibration spherical vector hydrophone is shown in Figure 4. After actual measurement, its mass is 274.7 g, and its density is 1.40×103 kg/m3. The outer radius of the vector hydrophone is Ro=36 mm, and substituting into equation (4), the size of this hydrophone supports the upper limit of its working frequency fmax=2653 Hz. For ease of use, round the upper limit of its working frequency to 3000 Hz. At this time, kRo=0.45239, density ratio 0r / r =1.40, substituting equations (1) and (2) into equations (1) and (2) to get v/v0=0.77, the maximum The phase difference is only 0.15°, which meets the application requirements.

 

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5 Vector hydrophone performance test

In order to check whether the acoustic performance and pressure resistance of the designed and manufactured co-vibration spherical vector hydrophone meet the requirements, the hydrophone samples are placed in the standing wave tube for sensitivity and directivity tests, and the static pressure test is performed in the autoclave.

 

5.1 Sensitivity test

The sensitivity of the three-axis piezoelectric accelerometer used in the co-vibration underwater vector hydrophone in this article is Ma=2500 mV/g. The vibration velocity sensitivity of a vector hydrophone is generally expressed by the equivalent free-field sound pressure sensitivity Mp. There is the following conversion relationship between Mp and Ma. Substituting the actual measured value of the hydrophone's average density into equation (3) can be obtained | v/v0|=0.7895, substituting this value into equation (16), the relationship between the theoretical equivalent sound pressure sensitivity of the vector hydrophone and the sound wave frequency can be obtained, as shown by the black solid line in Figure 5. At 500 Hz, the theoretical sensitivity of the vector channel of the vector hydrophone is -187.4 dB (0 dB re 1V/μPa, excluding the amplification factor of the hydrophone's built-in preamplifier), which increases the sensitivity by 6 dB per octave. The vibration velocity sensitivity of the vector hydrophone is tested in a standing wave tube using a comparison method, and the effective frequency band of the standing wave tube is 100~1000 Hz. The measured results of the sensitivity of each channel of the co-vibration spherical vector hydrophone are shown in Figure 5 with the red star points. It can be seen that the measured curves of the sensitivity of the three vector channels are basically consistent with the theoretical curves. The sensitivities of the X, Y, and Z channels at 500 Hz are -188.9, -188.1, and -187.6 dB, respectively. The sensitivity consistency error of each vector channel in the measurement frequency band does not exceed 1.2 dB; the least square method is used to find the slope fitted by the sensitivity curve of the three channels, and the maximum difference between the sensitivity data of the three channels and the corresponding slope is less than 0.8 dB , That is, the sensitivity level instability of the hydrophone is less than 0.8 dB; the sensitivity increases by 6 dB per octave, which is consistent with the theoretical trend.

 

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5.2 Directivity test

 

The three vector channels of the co-vibrating spherical vector hydrophone should theoretically have cosine directivity independent of frequency. The rotation method is used to measure the directivity of the co-vibrating spherical vector hydrophone in the standing wave tube, and the angular interval of the rotation test is 0.4°. The directivity of the X, Y, and Z channels at 100, 500, and 1000 Hz was tested respectively. The results show that the X, Y, and Z channels have good cosine directivity at the three frequency points. The directivity curves of the X, Y, and Z channels at 500 Hz are shown in Figure 6. It can be seen that the minimum pit depth of the X-channel directivity curve is 34.1 dB, and the minimum pit depth of the Y-channel directivity curve is 29.8 dB. The minimum pit depth of the channel directivity curve is 38.9 dB. Since the signal generated by the sound wave on the channel to be measured when the vector hydrophone is at the concave point is extremely small, the rotating system does not stop when the test system is working, and the mechanical vibration and noise of the rotating system are directly transmitted to the vector through the suspension spring. On the hydrophone, the signal generated on the channel to be measured is often much larger than the acoustic signal, so the depth of the pit obtained by the measurement is much shallower than the actual value. Even so, the smallest pit depth in the three vector channels reaches 29.8 dB, which can meet application requirements.

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5.3 Withstand voltage test

The static pressure test of the co-vibrating spherical hydrophone was carried out in the autoclave. According to GB 150.1, for the hydraulic test of an external pressure vessel, 1.25 times the design pressure should be taken as the test pressure. The design pressure of the vector hydrophone is 30 MPa, so the maximum pressure of the pressure test is set to 37.5 MPa. During the test, the pressure mode of the hydrophone glide along the profile of the underwater glider was simulated. First, the pressure was increased to 37.5 MPa at a constant speed, and the pressure was maintained for half an hour, then the pressure was slowly released, and the pressure was increased to 37.5 MPa at a constant speed again, and the cycle was repeated 5 times. There was no sudden pressure drop in the autoclave during the entire pressurization process. The appearance of the two hydrophone samples before and after compression was not damaged, and the weight was the same. Then the acoustic performance of the hydrophone was retested in the standing wave tube. The test results showed that the hydrophone worked normally after suppression, and its sensitivity and directivity were basically the same as before the suppression. It is proved that the co-vibrating spherical vector hydrophone can withstand 37.5 MPa water pressure.

 

6 Conclusion

In accordance with the requirements of pressure resistance and acoustic performance of a large depth vector hydrophone, this paper proposes a design method for the minimum average density pressure spherical shell of a co-vibrating spherical vector hydrophone, which has important theoretical guiding significance for engineering realization. Analyzed and calculated typical deep-sea engineering materials, and selected 7075T6 aluminum alloy as the material for the pressure-resistant shell of the vector hydrophone; adopted the minimum average density pressure-resistant spherical shell design method, through theoretical calculations and finite element simulations, to determine the strength and stability of the shell The design and implementation of a large-depth co-vibration vector hydrophone has passed the 37.5 MPa water pressure test; the external dimensions of the vector hydrophone support the upper limit of its working frequency up to 3000 Hz, and the sensitivity is -188 dB@500 Hz, the sensitivity consistency error of the three channels is less than 1.2 dB, and the sensitivity fluctuations are all less than 0.8 dB. The directivity of the three channels is an ideal figure eight. In the case of mechanical rotation noise, the concave point The depth is also higher than 29.8 dB.


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