Modeling the damping properties of perforated screens traversed by a bias flow and backed by a cavity at low Strouhal number

Acoustic absorption properties of perforated screens traversed by a bias flow and backed by a resonant cavity are examined at a new absorption regime. The reflection coefficient of these dampers can be canceled at specific frequencies for low Strouhal numbers St ⪡ 1 and Helmholtz numbers of order unity He ∼ 1 . This regime differs from the classical one where resonance is sought at low Helmholtz numbers He ⪡ 1 and large Strouhal numbers St ⪢ 1 . A theoretical analysis is carried out and analytical expressions are derived to determine conditions for optimal absorption in this regime. It is shown that the optimal bias flow velocity and back cavity length can be fixed separately. The former is determined by the plate porosity and the latter controls the peak absorption frequency. This operating regime at low Strouhal numbers enables to increase sound absorption at low frequencies over a larger frequency bandwidth around the peak absorption frequency than the one obtained with a damping system working at high Strouhal numbers. This is confirmed by experiments carried out on a set of dampers in the frequency range 100–1000 Hz. Predictions of the reflection coefficient, including effects of bias flow velocity and cavity depth, show good agreement with experimental data for small pressure perturbation levels. These elements can contribute to the development of robust dampers to control low frequency thermoacoustic instabilities in gas turbines and jet engines.