Duct-borne Noise & Vibration Onboard Maritime Vessels For Underwater Radiated Noise Management.

Overview

Noise & Vibration studies are one of the most important aspects of designing any dynamic system. Any machinery which has components having high inertia, rotating components, friction, losses and other random forces create excitations at various locations for a system. Ducts are an excellent carrier of various types of vibrations which are created by these excitation forces and carry them to various locations nearby. Thus, if excessive noise levels are generated and transmitted to different ship locations, there are possibilities of detrimental effects such as hearing disabilities for crew and passengers onboard, failure of equipment’s due to resonance, noise emission to surroundings, violation of stealth requirements etc.

This project focuses on primarily understanding the sources of noise & vibrations onboard marine vessels, transmission of these vibrations to various locations on ships via ducts and attenuation of the generated noise. Using the guidelines established by various classification societies such as American Bureau of Shipping, Bureau Veritas, Lloyd’s Register etc., a software package written in Python is developed to calculate the theoretical attenuation across the duct system for a case under consideration. A harmonic acoustic model is created in ANSYS software to replicate this noise & vibration scenario onboard ships for various duct subsystems.

After the simulation is carried out, a comparison of the output of the Python package and ANSYS model is carried for attenuation of noise levels. This provides a basis for establishing the accuracy of the simulated model. It is found that the error is minimal and the 2 output results are in sync. Thus, this enables us to further improve these models for various ships and ensure the noise levels are restricted within the required limits

Key highlights
  • HVAC system noise is a significant issue in ships, and current regulations lack specificity for equipment-generated noise.
  • Utilizes ANSYS software for creating a harmonic acoustic model, allowing for detailed analysis of various duct components.
  • Highlights the critical role of meshing in finite element analysis (FEA) for accurate simulations, discussing methods like tetrahedral and hexahedral elements.
  • Discusses the significance of realistic boundary conditions for obtaining meaningful simulation results in FEA.
  • Examines simulation results for straight ducts, turns, branches, plenums, and duct openings, comparing theoretical values with simulation outputs.
  • Acknowledges discrepancies between theoretical and simulated values, attributing them to software licensing limitations and meshing constraints.
  • Explores end reflection loss in duct openings and introduces the concept of silencers, highlighting the trade-off between pressure loss and low-frequency attenuation.
Key Challenges
  • Lack of specific regulations for noise generated by ship equipment poses a challenge in defining acceptable noise levels and compliance standards.
  • The intricate nature of duct systems, involving turns, branches, and plenums, complicates the prediction and control of noise transmission through these pathways.
  • Constraints in software licensing and meshing capabilities, particularly in the case of the student version of ANSYS, may impact the precision and scope of simulations.
  • Achieving an optimal mesh for accurate simulations can be challenging, especially when dealing with complex geometries, leading to potential convergence difficulties.
  • Addressing low-frequency models specifically for simulations can be challenging due to limitations, impacting the accuracy of results in certain frequency ranges.
  • Balancing the desire for low-frequency attenuation with the need to minimize back pressure in silencer design involves a trade-off that requires careful consideration.
  • The transition between low-frequency modelling for plenums and high-frequency modelling for rooms introduces challenges in accurately predicting sound attenuation.
  • Achieving a balance between the desired level of accuracy in simulations and the time and computational resources invested is an ongoing challenge in FEA modelling.
Major Opportunities
  • Opportunities to enhance regulatory frameworks for ship equipment noise, ensuring clearer standards and proactive noise control measures during design.
  • Development opportunities for effective preliminary design tools to estimate noise impact in HVAC systems early in the ship design process.
  • Potential for advancements in simulation software, like ANSYS, offering improved capabilities, features, and enhanced meshing options for more precise simulations.
  • Integration opportunities for advanced meshing techniques or software upgrades to address challenges related to mesh quality, ensuring accuracy and efficiency.
  • Research opportunities to enhance low-frequency modelling techniques, overcoming software limitations for more comprehensive simulations.
  • Opportunities to optimize silencer design, striking a balance between low-frequency attenuation and back pressure for improved overall system performance.
  • Opportunities for collaboration among experts in acoustics, structural dynamics, and fluid dynamics to enhance overall understanding and control of noise in duct systems.

"Use of computer simulations in ANSYS software for sound propagation inside air ducts, providing advantages in terms of cost reduction and faster, accurate results compared to experiments."

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