Professor Kozo Fujii recently shared preliminary results of a large-eddy simulation of an axial fan that were computed on overset structured grids. Decomposition techniques were then used to compute the fan’s acoustic signature. A journal article and conference presentation are currently in-work and will contain more detail on the simulations and a broader range of results. 

by Prof. Kozo Fujii
Professor, Department of Information and Computer Technology
Tokyo University of Science
Professor of Emeritus, JAXA

Large-eddy simulations of a general axial small fan with the 6th order compact differencing scheme (Reference 1) were conducted using the supercomputer SX-ACE at Cyber Science Center of Tohoku University, Japan. The fan diameter and the impeller’s tip velocity-based Reynolds number was 1.6×10⁵. The flow around the fan was initially quiescent and the flow was created by the rotating impellers.

Figure 1 – Instantaneous axial flow velocity distributions, iso-surfaces of velocity gradient
tensors, and structured over grids used in this study.

Figure 1 shows instantaneous resultant axial flow distributions and iso-surfaces of second invariant of velocity gradient tensor around the impellers and the casing as well as the structured overset grids used in this project.

Figure 2 – Proper orthogonal and dynamic mode decomposition for the pressure
fluctuation generated by the fan. Pressure fluctuation was associated by acoustic
pressure.

Currently, the source in the fan noise has been explored based on the numerical results with several feature extraction techniques (Figure 2). We used the structured grid and the overset grid technique to deal with three-dimensional geometry of the fan. Total number of overset grid is twenty-four and total number of grid point is approximately 81 million.

Reference

1. S. K. Lele, “Compact finite difference schemes with spectral-like resolution,” Journal of Computational Physics, vol. 103, no. 1, pp. 16–42, 1992.