Effect of sound source predicted by large eddy simulation on aerodynamic sound prediction of a box fan

Abstract

In recent years, to predict the performance and the aerodynamic sound of box fans with high accuracy, numerical analysis has been necessary for the transitional boundary layer on the blades. In this study, two types of box fans with a baseline fan and a high-load fan, where the boundary layer that develops on the suction surface of the rotor blade transitions to turbulent flow, were targeted for large eddy simulation (LES) analysis and acoustic analysis using different grid resolutions to predict the far-field sound. The box fan in this study has an impeller of 180 mm outer diameter and five blades, and the Reynolds number based on the diameter and the tip velocity is 3.38 × 105. The decoupled method with LES for incompressible flow and computational aeroacoustics (CAA) based on a dipole sound source was performed. The computational grid resolutions had 25 million grid points (Case 1), 52–56 million grid points (Case 2 and Case 4), and 420–450 million grid points (Case 3 and Case 5). The performance of the fans and the sound pressure level (SPL) were compared with each case and experiment. The performance was in good agreement with the experiment in higher grid resolution cases. Boundary layer transition on the blade surface was predicted as the main source of aerodynamic sound generated from the box fan. Increasing the grid resolution improved the prediction accuracy of the SPL in the frequency range corresponding to the scale of the turbulent eddies captured by the grid resolution.

Keywords

• large eddy simulation
• computational aeroacoustics
• lighthill analogy
• box fan