Natural ventilation is a method of controlling indoor air quality and thermal comfort in buildings by using natural airflow instead of mechanical systems. One of the challenges with natural ventilation is controlling noise transmission from outside sources, such as traffic or industrial noise while maintaining adequate air exchange or controlling noise transmission from noisy factories to outside in residential zones.
In Australia, the National Construction Code (NCC) Part A5 outlines the acoustic requirements for building elements, including natural ventilation systems. In this post, we will discuss the role of Computational Fluid Dynamics (CFD) in predicting sound transmission in natural ventilation systems, with reference to Australian standards AS 1191 and AS 717.1, and NCC Part A5.
Sound transmission in natural ventilation systems
Sound transmission in natural ventilation systems is affected by various factors, including the type of building construction, the type of ventilation system, the location and orientation of the building, and the surrounding environment. Sound transmission can occur through various paths, including through the building envelope, ventilation openings (wall louvre or roof vents), and building services.
In Australia, the acoustic performance of building elements, including natural ventilation systems, is governed by Australian Standard AS 1191 "Acoustics - Method for laboratory measurement of airborne sound transmission loss of building elements" and AS 717.1 "Acoustics - Rating of sound insulation in buildings and of building elements - Part 1: Airborne sound insulation". These standards provide a method for determining the sound transmission loss (STL) of building elements, which is a measure of the ability of a building element to reduce sound transmission. The STL is expressed as a single number and is used in the NCC Part A5 to assess the acoustic performance of building elements.
The sound transmission loss (STL) formula is:
STL = 10 x log10 (P1 / P2)
where P1 is the sound power incident on the source side of the element, and P2 is the sound power transmitted through the element.
Computational Fluid Dynamics (CFD) and sound transmission
CFD is a powerful tool that can be used to predict airflow and temperature distributions in natural ventilation systems. CFD can also be used to predict sound transmission in these systems by modelling the propagation of sound waves in the building envelope and surrounding environment.
CFD models can be used to analyse the impact of various design parameters on sound transmission, such as the location and orientation of ventilation openings, the size and shape of openings, and the use of sound-absorbing materials. CFD models can also be used to predict the impact of external factors, such as wind direction and speed, on sound transmission.
CFD models can provide valuable insights into the sound transmission characteristics of natural ventilation systems, which can help designers optimise the system for minimum noise transmission while maintaining adequate air exchange, in accordance with the acoustic requirements of AS 1191, AS 717.1, and the NCC Part A5.
An example of acoustic modelling using CFD is modelling the acoustic performance of louvres or vents in natural ventilation systems. Louvres or vents are used to provide a pathway for airflow while attenuating noise transmission from/to the outside environment.
CFD simulations can be used to predict the flow of air through the louvre or vent and the sound transmission through the opening. The geometry of the louvre or vent is defined in the CFD model, and the surrounding environment's airflow and acoustic properties are also considered.
One approach to modelling the acoustic performance of a louvre or vent is to use a multi-physics simulation that couples the airflow simulation with an acoustic simulation. The airflow simulation predicts the flow of air through the louvre or vent, while the acoustic simulation predicts the propagation of sound waves through the opening. This approach allows for the prediction of both the airflow and acoustic performance of the louvre or vent.
Another approach is to use a simplified model that treats the louvre or vent as a porous medium. In this approach, the airflow through the louvre or vent is modelled using Darcy's law, while the acoustic performance is modelled using a transfer matrix approach. This approach is less computationally expensive than a multi-physics simulation but may not capture all of the complexities of the airflow and acoustic performance of the louvre or vent.
Overall, CFD can provide valuable insights into the acoustic performance of louvres or vents in natural ventilation systems, allowing designers to optimise the system for minimum noise transmission while maintaining adequate air exchange.
A third approach is a de-coupled method, which uses a separate CFD simulation for the airflow and acoustic performance of the louvre or vent. In this method, the airflow simulation is performed first to obtain the flow field around the louvre or vent. The results of the airflow simulation are then used as input to an acoustic simulation that predicts the sound transmission through the opening. This approach allows for the use of different solvers for the airflow and acoustic simulations, which can improve the accuracy of the results.
Conclusion:
Sound transmission is a critical consideration in the design of natural ventilation systems in Australia, where the acoustic performance of building elements is governed by Australian Standards AS 1191 and AS 717.1, and the NCC Part A5. Computational Fluid Dynamics (CFD) can be used to predict sound transmission in these systems, in accordance with these standards, by modelling the propagation of sound waves in the building envelope and surrounding environment. CFD models can provide valuable insights into the impact of design parameters and external factors on sound transmission, which can help designers optimize the system for minimum noise transmission while maintaining adequate air exchange.
Comments