CFD Simulations of the Static Airflow Resistivity of a Perforated Solid: Effects of Size and Flow Velocity
AbstractFor acoustic absorbers, the static airflow resistivity is the parameter which has the greatest impact on their acoustic absorption coefficient in the linear amplitude regime. Therefore, its measurement according to ISO 9053 (or ASTM C522) should be conducted with care. This ISO standard gives specifications on the size and mounting of specimens, their location in the measuring cell, minimum and maximum flow velocities, calibration specimens and the measurement procedure. An important requirement is to ensure a stable linear flow so that the resistance is independent of the velocity. Additionally, it specifies the use of a calibration test specimen to ensure proper operation of both hardware and software. The suggested calibration specimen consists of straight cylindrical pores whose value can be calculated theoretically. However, no other specification is given for designing the calibration specimen. Since the airflow resistivity of a perforated low-porosity solid can behave nonlinearly with velocity, it is important to present some additional guidelines for their design. This work presents CFD simulations on the flow resistivity of a cylindrical solid containing a single perforation subjected to an air flow velocity ranging from 0.5 mm/s to 10 cm/s. The simulations replicate a commercial airflow resistivity meter. The results of the simulations are compared with the theoretical formula and the experimental measurements. The results highlight the importance of the size of the perforation (diameter and depth) and the flow velocity to ensure that the measurement aligns to the theoretical value.
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