Elastic
The Elastic subnode is available from the context menu (right-click the Collisions parent node) or from the Physics toolbar, Attributes menu. Use it to model momentum-conserving collisions between model particles and background gas molecules.
Collision Frequency
Select an option from the Specify list: Cross section (the default) or Collision frequency.
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For Collision frequency, enter a value or expression for the Collision frequency ν (SI unit: Hz). The default is 2 × 106 Hz.
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For Cross section enter a value or expression for the Collision cross section σ (SI unit: m2). The default is 3 × 10-19 m2.
For either choice:
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Select the Number density specification: From parent (the default) or User defined. For User defined, enter a Background number density Nd (SI unit: 1/m3). The default is 1020 1/m3. If From parent is selected, the number density is inherited from the Collisions node.
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Select the Molar mass specification: From parent (the default) or User defined. For User defined, enter the Background gas molar mass Mg (SI unit: kg/mol). The default is 0.04 kg/mol. If From parent is selected, the background gas molar mass is inherited from the Collisions node.
If the molar mass of the background gas is similar to the molar mass of the particles then the particles tend to form a Maxwellian velocity distribution function very rapidly. If the mass of the particles is very different to that of the background gas then the particles tend to have a non-Maxwellian distribution function.
If the geometry is axisymmetric and Include out-of-plane degrees of freedom is selected in the physics interface Advanced Settings section, enter a value or expression for the Cutoff radial coordinate for out-of-plane momentum rc (SI unit: m). The default value is 0. If a particle undergoes a collision while its radial coordinate is less than the specified cutoff, the azimuthal component of its reinitialized velocity is set to zero. Use this setting to prevent particles from being subjected to inordinately large centrifugal forces if they undergo collisions very close to the axis of symmetry.
Collision Statistics
Select the Count collisions check box to count the number of times each particle collides with the background gas. The collision counter is only incremented by this instance of the Elastic node, not by any other collision type or by other Elastic nodes. To define a variable that counts all types of collisions, select the Count all collisions check box in the settings window for the parent Collisions node.
New Value of Auxiliary Dependent Variables
This section is available if an Auxiliary Dependent Variable has been added to the model. For each auxiliary dependent variable, select an option from the New value of auxiliary dependent variable list: From parent (the default) or User defined. If From parent is selected, the value of the auxiliary dependent variable is reinitialized according to the settings for the parent Collisions node.
If User defined is selected, select the Assign new value to auxiliary variable check box to assign a new value to the auxiliary dependent variable when an elastic collision occurs. Then enter the new value or expression in the field. For example, if there is an auxiliary variable, psi, then enter a value for psinew in the field. So, to increment the value of psi by 1 when a particle undergoes an elastic collision, enter psi+1 in the text field for psinew.