Articles techniques et présentations

In this paper, we report FEM studies of the effect of unwanted interfering structural vibrations on the performance of an underwater Acoustic Vector Sensor (AVS) performed in the COMSOL Multiphysics simulation tool. An AVS is a compact sensor that measures a four-dimensional vector quantity consisting of the acoustic particle velocity of the water medium in three dimensions and the scalar pressure. It is primarily used to sense the acoustic intensity vector and estimate the direction of one or more radiating acoustic sources in the medium. The acoustic particle velocity of the medium is generally measured indirectly using a tri-axial accelerometer which is also sensitive to the mechanical vibrations transmitted to the sensor through the suspending structure. It is essential to understand the effects of these transmitted vibrations on the AVS, especially when it is mounted on a moving platform like an autonomous underwater vehicle (AUV). This is because it will reduce the effective SNR, introduce deviation in the measurement of the direction of acoustic sources by the sensor, and may impose extra challenges when these vibrations overlap with the frequencies of the desired acoustic source signal whose direction must be found. Thus it is vital to study how potential vibration patterns are transmitted from the suspending platform to the accelerometer embedded in the AVS. It allows us to design appropriate isolation mechanisms to reduce mechanical coupling between the AUV platform so that the unwanted vibrations from the platform do not interfere with the AVS' measurements.

To formulate this problem in COMSOL Multiphysics, we will integrate the Acoustics and Structural Mechanics modules to model the interaction of different physical phenomena. We create a 3D geometry of the AVS and its suspending structure, specifying material properties for accurate mechanical behavior representation. Then we set up the physics. For structural vibration analysis, we employ Solid Mechanics, and for sensor measurements influenced by the background pressure field, we introduce the Pressure Acoustics Module. We also define appropriate boundary conditions; for example, for simulating far-field conditions, we utilize a Perfectly Matched Layer (PML), which acts like an absorbent material for acoustic waves. Subsequently, suitable meshes that discretize the geometry to ensure accurate results are generated. Further, two distinct studies are merged: one that performs a random vibration analysis of the mounting structure along with the AVS, using power spectral density (PSD) as the input load model applied to the structure, and another that focuses on frequency domain analysis to compute the pressure and acceleration variation measurements at the sensor. For random vibration analysis, a predefined study available in COMSOL Multiphysics is being used, which consists of the eigenfrequency analysis that generates a reduced order model (ROM) based on which further computations are done.

The paper shall present the results of the simulation study in COMSOL and provide insights into how to arrive at a suitable design of the AVS that reduces the interfering vibrations.