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Collision dynamics

1) Collision partner selection

The No-Time-Counter of Bird is the only recommended collision partner selection model (others haven’t been throughly tested). It is implemented as follows in constant/dsmcProperties

// Collision Partner Selection Model
// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

collisionPartnerSelectionModel        noTimeCounter;

In this model, each cell have 8 subcells (2 in each direction) to reduce the mean collision separation.

2) Binary collision

The available binary collision models are:

• VariableHardSphere
• VariableSoftSphere
• LarsenBorgnakkeVariableHardSphere
• LarsenBorgnakkeVariableSoftSphere

The selected model is defined for the entry BinaryCollisionModel and its parameters are given in the subsequent #modelCoeffs subdictionary. Here is an example for the LarsenBorgnakkeVariableHardSphere model, using constant collision numbers

// Binary Collision Model
// ~~~~~~~~~~~~~~~~~~~~~~

BinaryCollisionModel        LarsenBorgnakkeVariableHardSphere;

LarsenBorgnakkeVariableHardSphereCoeffs
{
    rotationalRelaxationCollisionNumber   5.0;
    vibrationalRelaxationCollisionNumber  50.0;
    electronicRelaxationCollisionNumber   500.0;
}

NB: the reference temperature \(T_{ref}\) is set to 273 K by default.

The vibrational collision number can be set to be a function of temperature, in which case the line

    vibrationalRelaxationCollisionNumber  50.0;

should be deleted and replaced by another entry. The available options for a variable vibrational collision number are as follows:

    inverseZvFormulation           "pre-2008"; // (uses the collision temperature)
    inverseZvFormulation               "2008"; // (uses the overall temperature)

and if the entry inverseZvFormulation is not specified, then \(Z_{vib}\) will be defined as in Bird’s 2013 book. The "pre-2008" and 2013 models differ by a coefficient 1/5 and are both using the collision temperature. This is the recommended option and it is shown below

// Binary Collision Model
// ~~~~~~~~~~~~~~~~~~~~~~

BinaryCollisionModel        LarsenBorgnakkeVariableHardSphere;

LarsenBorgnakkeVariableHardSphereCoeffs
{
    rotationalRelaxationCollisionNumber   5.0;
    electronicRelaxationCollisionNumber   500.0;
}

Please also refer to A.1 Species thermophysical properties for the definitions of \(Z_{ref}\) and \(T_{ref, Z_{ref}}\).