The  noise  of  domestic  machines  including  lawnmowers becomes 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..

Downloads: S.Timuchev, A.GGamarnik, A.Tsipenko: Numerical modeling of air-grass flow and bpf noise in lawn mowers, 2007 year, PDF, 726KB