Numerical Modeling of Air-Grass Flow and BPF Noise in Lawn Mowers

Abstract

The noise of domestic machines including lawnmowers be comes an urgent issue. As the technology matures, designers need better tools to predict performance and efficiency of these machines across a wide range of operating conditions and find optimal ways to reduce noise. Computational fluid dynamics is an increasingly powerful tool which enables designer to better understand all features of unsteady flow in these machines and to find optimal designs providing higher energetic characteristics, better cutting quality and lower pressure pulsation, vibration and noise. Cutting quality linked with evacuation of grass is a key lawnmower characteristic. Due to this fact application of two-phase (air-grass) lawnmower flow model is inevitable in a prediction procedure. The modeling procedure comprises determination of lawnmower average aerodynamic characteristics and CFD-CAA analysis by acoustic-vortex method to predict sound power data. This method is based on splitting the equations of compressible fluid dynamics into two modes — vortex and acoustic Computational approach applied for the vortex mode flow is a “moving body”-technique: The problem is solved in the absolute frame of coordinates and computational grid changes during the blade passing. Computations can be made in 4 stages: 1) Computation of the incompressible medium with getting average values of energetic parameters; 2) Computation of the incompressible medium for definition the source function of inhomogeneous acoustic-vortex wave equation; 3) Solution of the acoustic-vortex wave equation; 4) Computation of 2-phase flow. In the 3rd stage the pressure pulsation field can be represented like a sum of acoustic and vortex oscillation. Wave equation is solved relatively to pressure oscillation using an explicit numerical procedure. Zero pulsatory pressure is an initial condition for solution of the wave equation. The local complex specific acoustic impedance is used to define boundary conditions for the acoustical part of the pressure field. Thus the numerical procedure gives pressure pulsations field and sound power data on blade passing frequencies (BPF). For the 4th stage computations effective grass particle parameters are determined with accounting the stubble effect on flow parameters and particularities of grass particle interaction with rigid surfaces. Results of a lawnmower air-grass flow (grass particle trajectories and concentration) and corresponding BPF sound power data prediction are presented as an example of modeling procedure application.