### 6.5.3. Dynamic Drag Models

Many particle drag models assume that the droplet remains spherical throughout the domain. However, this is not always the case and the particle shape may be distorted significantly. In the extreme case, the particle shape will approach that of a disk. The drag coefficient is highly dependent on the particle shape and it is therefore desirable to modify the standard drag laws to account for the effects of droplet distortion.

In CFX, the following models are implemented that modify the drag coefficient depending on the particle distortion:

#### 6.5.3.1. Liu [108]

The drag coefficient is assumed to vary linearly between that of a sphere and that of a disk:

| (6–165)

| (6–166)

y is a measure of the particle distortion. If the droplet is not distorted , then the drag coefficient of a sphere will be obtained. If the particle is maximally distorted , then the drag coefficient of a disk will be obtained. This drag model is only available for the TAB, ETAB and CAB breakup models.

Note: The Liu drag coefficient modification is activated by default for the TAB, ETAB, and CAB breakup models.

#### 6.5.3.2. Schmehl [105]

The droplet deformation due to external aerodynamic forces leads to a change in the drag coefficient of the droplet that is assumed to vary between the two limiting geometries, a sphere and a disk. The drag coefficient of the droplet is given as:

| (6–167)

with:

| (6–168)

| (6–169)

and:

| (6–170)

The droplet aspect ratio, , and the particle distortion, , are related as follows:

| (6–171)

here is 1 for a sphere and 0 for the distorted particle (disc).

This drag model is available for the Schmehl breakup model only. It is activated by default.

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