Detecting Liquid Droplets or Gas Bubbles

When you analyze the behavior of a liquid or a gas in post-processing, it can be useful to identify grid-resolved liquid droplets or gas bubbles in the VOF or MMP field.

The Blob Detection phase model allows you to detect such droplets and bubbles (identified as "blobs" of material). The model identifies the cells in which the volume fraction of a particular phase is greater than or equal to a threshold volume fraction that you specify.

A single blob is formed of a number of cells, which can be several cells or hundreds of cells. However, once a blob is detected, it is treated as a spherical particle with properties such as mass, velocity, volume, and diameter. These properties are exposed as field functions and are available for use in scenes, reports, plots, threshold parts, and tables.

You can visualize the blobs in scenes. When you select blob clouds or threshold parts that contain blobs, the blobs in the selected parts are shown in the scene like particles. The position of each particle is the center of mass of the corresponding blob. You can also color the particles with the available blob properties. See Field Functions.

To detect liquid droplets or gas bubbles:

  1. For the phase that you want to detect, open the Phase Model Selection dialog and select Blob Detection from the Optional Models group box.

    See Blob Detection Model Reference.

A blob cloud is the group of blobs that are defined by a specific volume fraction threshold. You can have any number of blob clouds, and specify a different threshold volume fraction for each. The first blob cloud is created automatically when you activate the Blob Detection model.

  1. For each blob cloud, right-click the [phase name] > Models > Blob Detection > Blob Clouds node and then click New.
    The corresponding Blob Cloud [number] sub-node is added.
For each blob cloud, set the appropriate threshold volume fraction. Every cell that has a volume fraction of the relevant phase greater than or equal to the specified threshold is included in the blob cloud. These blobs represent the bubbles of gas or the droplets of liquid that exist in the VOF or MMP field.
  1. Select the Blob Cloud [number] node and set the Volume Fraction Threshold Value.
    The default volume fraction is 0.5.

For each phase, select the regions in which the Blob Detection model is used. By default, when the Blob Detection model is activated in a phase, it is used in every fluid region.

  1. For each phase in the region, select the Regions > [region name] > Phase Conditions > [phase name] > Physics Conditions > Blob Detection Option and set the Blob Detection Option to Active.
    If you want to deactivate the Blob Detection model in a particular region, set the Blob Detection Option to Inactive.
  2. For Mixture Multiphase (MMP) simulations, to activate the Adaptive Interface Sharpening (ADIS) convection scheme, select the [physics continuum] > Mixture Multiphase node and set Convection to Adaptive Interface Sharpening (ADIS).
    The Blob Detection model requires the use of the ADIS convection scheme for MMP simulations to maintain sharp interfaces between the Eulerian phases.