Measuring acoustic velocity map in turbulent flow using sub-Nyquist-rate PIV system

The present study provides an experimental method for measuring acoustic velocity components (amplitude and spatial phase shift) in the presence of turbulent flows using either low-frequency particle image velocimetry (PIV) or laser Doppler velocimetry (LDV). The approach leverages compressed sensing (CS) principles to overcome the limitations of classical PIV techniques, such as the need for a reference signal for synchronization, large data size and long measurement duration. Theoretical aspects of CS for extracting acoustic components from PIV and LDV measurements are discussed. The proposed method is applied to both PIV and LDV systems, and the results are compared with microphone measurements. The results show the ability of the proposed experimental method to accurately measure acoustic velocity components at different frequencies and sound pressure levels in the presence of turbulent flow. The presented experimental method offers several advantages, including reduced data size, no need for phase-locking measurements and reduced measurement duration. The actual limitation is the need for a low noise-to-signal ratio (NSR). The effects of high NSR can be mitigated by increasing acquisition time in some cases. The non-intrusive nature of the method makes it valuable for aeroacoustic research. Ongoing research focuses on applying the method to higher-order duct modes and investigating its potential for modal decomposition using optical techniques.