Blob Detection Model Reference

The Blob Detection phase model identifies the cells in which the blob is formed based a threshold volume fraction that you specify. A single blob is then formed of a number of cells, which can be several cells or hundreds of cells. For this reason, to ensure a well-defined mesh for the blobs, you are advised to use to use an adaptive mesh refinement. Further, the adaptive interface sharpening (ADIS) convection scheme is required in Mixture Multiphase (MMP) simulations.


Provided By	[phase name] > Models > Optional Models
Example Node Path	[phase name] > Models > Blob Detection
Requires	In the physics continuum: Material: Multiphase Multiphase Model: Volume of Fluid (VOF) , Mixture Multiphase (MMP) In the Eulerian phase: Material: one of Gas, Liquid, Multi-Component Gas, or Multi-Component Liquid Optional Models: Blob Detection
Properties	Key property for each blob cloud: Volume Fraction Threshold Value. See Blob Detection Properties.
Specific Right-Click Actions	New See Blob Detection Right-Click Actions.
Activates	Physics Models	None.
	Model Controls (child nodes)	Blob Clouds. See Blob Clouds Node.
	Region Inputs	Blob Detection Option See Region Settings.
	Report Options	Element Count, Mass Averaged, Surface-area Average, Volume Average, Volume Integral, Volume Standard Deviation, Volume Uniformity. See Reports.
	Field Functions	Blob Index of [Phase] for Blob Cloud [number], Blob Cell Count, Blob Centroid, Blob Diameter, Blob Inertia Tensor Eigenvalue Ratio, Blob Mass, Blob Radius, Blob Sphericity, Blob Surface Area, Blob Velocity, Blob Volume, Boundary Blob Mark. See Field Functions.

Blob Detection Properties

For each blob cloud:

Volume Fraction Threshold Value

Specifies the volume fraction of the phase above which the cell is included in the blob cloud (that is, considered to be part of a bubble or droplet of the phase).

You can specify different threshold values for each blob cloud.

Blob Detection Right-Click Actions

Available on the Blob Clouds node:

New: Creates a new blob cloud.

Blob Clouds Node

Contains the blob clouds that are defined for the phase. Each blob cloud is the group of blobs that are detected at a specified threshold volume fraction of the phase.

Region Settings

Applies to fluid regions.

Phase Conditions > [phase name] > Physics Conditions > Blob Detection Option

Specifies the regions in which the Blob Detection model is applied.

The Blob Detection Option property is Active by default. To prevent the Blob Detection model from being used in the region, set the Blob Detection Option property to Inactive.

Reports

The following reports are available.

Element Count

The number of blobs in the selected blob clouds or threshold parts.

Mass Averaged

The mass averaged value of the selected scalar field function ( $ϕ$ ) of the blobs in the selected blob clouds or threshold parts.

Figure 1. EQUATION_DISPLAY

\bar{ϕ} = \frac{\sum_{b} m_{b} ϕ_{b}}{\sum_{b} m_{b}}

(309)

where $\sum_{b}$ is the summation over the blobs in the selected blob clouds or threshold parts and $m_{b}$ is the mass of the blob $b$ .

Maximum

The maximum value of the selected scalar field function of the blobs in the selected blob clouds or threshold parts.

Minimum

The minimum value of the selected scalar field function of the blobs in the selected blob clouds or threshold parts.

Sum

The sum of the selected scalar field function values over the blobs in the selected blob clouds or threshold parts.

Surface-area Average

The surface area averaged value of the selected scalar field function $ϕ$ of the blobs in the selected blob clouds or threshold parts.

Figure 2. EQUATION_DISPLAY

\bar{ϕ} = \frac{\sum_{b} π d_{b}^{2} ϕ_{b}}{\sum_{b} π d_{b}^{2}}

(310)

where $d_{b}$ is the diameter of the blob $b$ .

Volume Average

The volume averaged value of the selected scalar field function $ϕ$ of the blobs in the selected blob clouds or threshold parts.

Figure 3. EQUATION_DISPLAY

\bar{ϕ} = \frac{\sum_{b} \frac{π}{6} d_{b}^{3} ϕ_{b}}{\sum_{b} \frac{π}{6} d_{b}^{3}}

(311)

Volume Integral

The volume integrated value of the selected scalar field function $ϕ$ of the blobs in the selected blob clouds or threshold parts.

Figure 4. EQUATION_DISPLAY

Φ = \sum_{b} V_{b} ϕ_{b}

(312)

Here $V_{b}$ is the volume of the cell that contains the center of mass of the blob.

Volume Standard Deviation

Volume weighted standard deviation of the selected scalar field function $ϕ$ of the blobs in the selected blob clouds.

Figure 5. EQUATION_DISPLAY

σ_{ϕ} = \sqrt{\frac{\sum_{b} V_{b} {(ϕ_{b} - \bar{ϕ})}^{2}}{\sum_{b} V_{b}}}

(313)

Here $\bar{ϕ}$ is the volume-averaged value of $ϕ$ .

Volume Uniformity

Volume weighted uniformity index of the selected scalar field function $ϕ$ of the blobs in the selected blob clouds.

Figure 6. EQUATION_DISPLAY

γ_{ϕ} = 1 - \frac{\sum_{b} V_{b} | ϕ_{b} - \bar{ϕ} |}{2 | \bar{ϕ} | \sum_{b} V_{b}}

(314)

Field Functions

When the Blob Detection model is activated in a phase, the following blob property field functions are available. These field functions are available on particles, unless otherwise stated:

Blob Index of [Phase] for Blob Cloud [number]

The unique index that is assigned to each of the detected blob clouds. This field function is available on cells.

Blob Cell Count

The number of cells in the blob.

Blob Centroid

The center of mass of the blob.

Blob Diameter

The diameter is estimated from the blob volume and assumes that the blob has a spherical shape.

Blob Inertia Tensor Eigenvalue Ratio

The ratio of minimum to maximum eigenvalue of the blob inertia tensor.

γ

in Eqn. (2640). You can use this ratio to evaluate the shape of a blob, specifically, how isotropic a blob is. If the blob is isotropic, including a perfect sphere, then

γ = 1

. Otherwise,

γ < 1

.

Blob Mass

The total mass of the relevant phase in the blob.

Blob Radius

The radius is estimated from the blob volume and assumes that the blob has a spherical shape.

Blob Sphericity

The ratio of the surface area of the perfect sphere to the actual surface area of a blob assuming the same blob volume.

ϕ_{i}

in Eqn. (2637). You can use blob sphericity to evaluate the shape of a blob. Blob sphericity values approaching 1 indicate a spherical droplet shape, whereas blob sphericity values approaching 0 indicate a ligament shape.

Blob Surface Area

The surface area is estimated from the blob diameter and assumes that the blob has a spherical shape.

Blob Velocity

The mass averaged velocity of the blob.

Blob Volume

The total volume of the relevant phase in the blob.

Boundary Blob Mark

Indicates blobs that are attached to boundaries of the following boundary types: Velocity Inlet, Mass Flow Inlet, Stagnation Inlet, Outlet, Pressure Outlet, Symmetry, Baffle, Porous Baffle, and Wall.

Internal interface boundaries are not included.

Available values are:

0—the blob is not attached to a boundary.
1—the blob is attached to a boundary.

Due to the effect of the boundary, the blob volume and blob diameter may not be accurate. The blob is not suitable to include in blob statistics or convert to a Lagrangian particle when the Resolved Transition model is used.

This field function is useful when you are visualizing blobs of liquid in hybrid multiphase simulations, for example, gear lubrication, E-machine cooling, and vehicle water management.

Parcel Index

The blob index of the blob.