Defining the Grass Blades

You model the grass blades using the Flexible Fiber model.

To account for the influence of air on the grass blades (one-way coupling) for meshfree DEM, you apply user-defined body forces. You model the effect that the air flow caused by the lawnmower blade has on the grass blades by specifying a drag force. Air blows horizontally in the positive z direction with a velocity of 50 m/s and vertically with 2 m/s in the positive y direction. You subtract the particle velocity from the air velocity to obtain the grass blade slip velocity. The drag force is given by:

f d = 0.5 × 1.293 × d G B 2 × | v s |   v s V G B

where d G B is the grass blade diameter, v s is the grass blade slip velocity, and V G B is the volume of the flexible fiber grass particle.

To define the grass blades:

  1. Create a Lagrangian phase for the grass blades:
    1. Right-click the Continua > Physics 1 > Models > Lagrangian Multiphase > Lagrangian Phases node and select New > Free-stream Phase.
    2. Rename the new phase to Grass Blade.
    3. Double-click the Grass Blade > Models node and select the following models, in order:
      Group Box Model
      Particle Type DEM Particles*
      Particle Shape Flexible Fiber
      Material Solid
      Constant Density(selected automatically)
      Optional Particle Forces User-Defined Body Force
      Track Sampling Boundary Sampling
      DEM Boundary Sampling
  2. Create a field function to define the grass blade slip velocity:
    1. Right-click the Automation > Field Functions node and select New > Vector and rename the new node to Grass Blade Slip Velocity.
    2. Edit the Grass Blade Slip Velocity node and set the following properties:
      Property Setting
      Function Name grassBladeSlipVelocity
      Dimensions Velocity 1
      Defintion 
      -$${ParticleVelocity}+[0,2,50]
  3. Create a field function to define the grass blade drag:
    1. Right-click the Automation > Field Functions node and select New > Vector and rename the new node to Grass Blade Drag.
    2. Edit the Grass Blade Drag node and set the following properties:
      Property Setting
      Function Name grassBladeDrag
      Dimensions Force 1
      Volume -1
      Defintion 
      $${ParcelCentroid}[1]>${Grass Blade Height} ?
0.5*1.293*5*${Grass Blade Diameter}*${Grass Blade Diameter}*mag($${grassBladeSlipVelocity})*$${grassBladeSlipVelocity}/${ParticleVolume} :
[0, 0, 0]
  4. Edit the Continua > Physics 1 > Models > Lagrangian Multiphase > Lagrangian Phases > Grass Blade > Models node and set the following properties:
    Node Property Setting
    Boundary Sampling Boundaries Blade
    Flexible Fiber Axial Restitution Coefficient 0.01
    Bending Restitution Coefficient 0.01
    User-Defined Body Force
    User Body Force Method Field Function
    Field Function Grass Blade Drag
  5. Set the grass blade material properties:
    1. Select the Grass Blade > Models > Solid > Rubber node and rename it to Grass.
    2. Edit the Grass > Material Properties node and set the following properties:
      Node Property Value
      Density
      Constant Value 55.0 kg/m^3
      Young's Modulus
      Constant Value 100 MPa
  6. Save the simulation.