Numerical Analysis of Unsteady Three-Dimensional Flow in a Propeller Fan Using Lattice Boltzmann Method

F1 - Lattice Boltzmann Methods (i)

Reduction of turbulent flow noise generated from fans is a crucial issue. The direct approach in computational aeroacoustics is a powerful tool for such a problem. Lattice Boltzmann Method (LBM) is a promising approach to directly simulate flow and acoustic fields with low Mach number. In fact, there have been many engineering applications of LBM to aeroacousic simulations in the automotive industry. On the other hand, only a few applications to turbomachinery such as fans have been reported. The goal of this study is to establish the prediction method of turbulent flow noise radiated from low speed fans using LBM. The present paper provides the validation result of LBM for a complicated flow field around a propeller fan.
In the present simulation, solid boundaries of the rotor and the shroud were calculated by a simple immersed boundary scheme. The computational grid around the propeller fan was generated by the Building-Cube Method (BCM). The grid was refined locally near the solid boundaries based on Cartesian grid. The multi-scale model was introduced into LBM to allow the calculation with such grids. The number of grid points amounts to 1.7 billion in total. Although the grid resolution was still not enough, the direct numerical simulation (DNS) was attempted. In the present simulation, any subgrid scale models were not introduced. In order to suppress the numerical instabilities which occurred in a part of the domain with coarser grid, the low pass filtering operation was used.
It is important to capture the three-dimensional vortex structure near the rotor tip which is called tip vortex, because the tip vortex dominates the flow field around the propeller fan. In regard to vortex structures, the result of LBM was compared with the result of detached eddy simulation (DES) which solved the Navier-Stokes equations using a body-fitted grid. The structure of tip vortex calculated by LBM agreed well with the result of DES. Furthermore, pressure fluctuations on blade surfaces were compared with the experimental results. The highest pressure fluctuation was observed on the suction surface near the leading edge of the tip region in the both results. The simulation showed that the fluctuation was caused by the leading edge separation vortices. The pressure fluctuations caused by the tip vortex and the corner separation were also captured in the both results. These results validated the applicability of LBM to the numerical analysis of unsteady three-dimensional flows in the propeller fan.