### 5.5.8. Interphase Turbulent Dispersion Force

The topics in this section include:

- Favre Averaged Drag Model

- Lopez de Bertodano Model

#### 5.5.8.1. Favre Averaged Drag Model

CFX implements a model for turbulent dispersion force, based on the Favre average of the interphase drag force [90].

| (5–86)

Here, is the momentum transfer coefficient for the interphase drag force. Hence, the model clearly depends on the details of the drag correlation used is the turbulent Schmidt number for continuous phase volume fraction, currently taken to be .

is a user-modifiable CEL multiplier. Its default value is unity. These defaults are appropriate for flows where the particle relaxation time is short relative to turbulent timescales, that is, for a low turbulent stokes number (see, Turbulent Stokes Number). This is true for dispersed phases that are light relative to the continuous phase, for example bubbles. However, for dispersed phases that are significantly heavier than the continuous phase, it is only true for very small particles. The default values will overestimate the turbulent dispersion force for large, heavy particles. In this case, better agreement with experiments can be achieved by reducing the value of , or by making it a decreasing function of turbulent Stokes number that is equal to unity when , and tending to zero as . If such information is not available, it is recommended that you ignore the turbulent dispersion force for particles of large Stokes number.

#### 5.5.8.2. Lopez de Bertodano Model

The model of Lopez de Bertodano (1991) [20] was one of the first models for the turbulent dispersion force:

| (5–87)

Unfortunately, it is not possible to recommend universal values of for this model. values of 0.1 to 0.5 have been used successfully for bubbly flow with bubble diameters of order a few millimeters. However, values up to 500 have been required for other situations. See Lopez de Bertodano [21] and Moraga et al. [91].

This model is included in CFX for historical back compatibility with CFX. However, the relatively more universal Favre Averaged Drag model is recommended for all situations where an appropriate value of is unknown.

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