COMSOL 6.4 - Release

Use the node to release grains into the model from a selected set of domains. The release times, initial values of the grains’ degrees of freedom such as positions, velocities and temperature, and initial value of any auxiliary dependent variables can be specified.

The grains are released sequentially at each release time. For each grain, a release position and radius are first determined based on the options provided in the and the sections. An attempt is made to release the grain at this position, and the attempt is considered successful only if the new grain does not overlap with any existing grains or wall elements. Alternatively, a maximum amount of allowed overlap can also be specified.

If the attempt is deemed successful, the grain is released at the release position. Alternatively, an unsuccessful attempt can be followed by a number of attempts at nearby release positions. Once the maximum number of attempts is reached, the grain is discarded, and the next grain is attempted.

Select a : (default), , , or .

Enter (SI unit: s), or click the button (

) to select and define a range of specific times. At each release time, grains are released with initial position and velocity as defined in the following sections.

Enter the , along with the (SI unit: s) and the (SI unit: s). In addition, select whether the should be or . When is selected, an array of length of uniformly spaced release times is generated. This array starts with the and ends with the exactly. The release times are reproducible each time the solution is computed.

When is selected, an array of random numbers of length , with a minimum lower limit of the and maximum upper limit of the is generated. In this case, a release time of exactly the first and last time values is extremely unlikely.

Enter the along with the (SI unit: s) and the (SI unit: s). In addition, select whether the should be or . When is selected, an ordered array of length of normally distributed release times is generated. The release times are more closely spaced around the value entered for the and the spacing of the release time drops off according to the value of the . The release times are reproducible each time the solution is computed.

When is selected, the normal or lognormal distribution is generated by pseudorandom sampling, so there may be some statistical error.

Select a from the list: (the default) or .

For the grains are released at random positions within the selected entities. If grains are released at multiple release times, the initial positions are uniquely generated for each release time. By contrast, for the grains positions are the same at each release time.

The grain positions are determined in the following way. First a random mesh element is selected for each grain with a probability proportional to the element size, so that grains are more likely to be released from larger mesh elements than smaller elements. After the mesh element is selected, random local coordinates are chosen within the element, then converted to global coordinates. A grain is then attempted to be placed at this location.

For the grains are positioned in the selected domains by sampling from a user-defined spatial distribution. Enter a value or expression for the ρ (dimensionless). The default is 1.

The field is an expression; the resulting grain distribution approximately has a density that is proportional to this expression. The resulting distribution looks a bit random, and it depends on the underlying mesh.

Select a between 1 and 5 (the default is 5), which determines the integration order that is used when computing the number of grains to release within each mesh element. The higher the accuracy order, the more accurately grains will be distributed among the mesh elements.

The (default 0) must be a nonnegative integer. When the refinement factor is 0, each grain is always assigned a unique position, but the density is taken as a uniform value over each mesh element. If the refinement factor is a positive integer, the distribution of grains within each mesh element is weighted according to the density. Further increasing the increases the number of evaluation points within each mesh element.

Enter values or expressions for the components of the initial grain velocity v0 (SI unit: m/s) based on space dimension. The defaults are 0 m/s.

This section is only shown when the checkbox is selected in the physics interface section. Enter a value or expression for the initial grain temperature Tg,0 (SI unit: K). The default value is 293.15 K.

Enter values or expressions for the components of the initial angular velocity in the body-fixed frame ω0 (SI unit: rad/s).

This section is used to specify the total number of grains and the distribution of the grain properties to be released per release time. Select a : (default), , , or .

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When is selected, enter the number of grains to be released for each grain type corresponding to a node. The default value for the first row in the table is 1, and it is 0 for all other rows.

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When one of the , , or options is selected, enter the (default 1). Then enter the values of the corresponding fractions of the distribution for each grain type. The default value for the first row in the table is 1, and it is 0 for all other rows. The fractional values are normalized internally so that their sum is 1 even if the provided values do not add up to 1.

The values of the distribution can be numerical values or parameters defined in the node.

	The grains are released sequentially, and each grain is associated with a Grain Properties node with a probability dictated by the user-specified distributions. Further, the grains are only released if the overlap with existing grains and wall elements is acceptable. Therefore, the total number of grains being released (and the distribution of their properties) at release time should be viewed as maximum values (ideal distributions), and may not always exactly match the user-specified values. The number of grains being released and their distributions may therefore also be different for each release time specified in the Release Times section. The user should therefore test the released grain populations to ensure an appropriate population is attained. Furthermore, for performance reasons, it is advised to ensure that the Number of grains per release is set as close to the actual number of grains being released as possible.

	The algorithms used for the contact detection are described in Contact Force: Linear Elastic Model section in the Theory for the Granular Flow Interface.

This section is available if an Auxiliary Dependent Variable has been added to the model. For each of the (rp) added to the model, enter an initial value (rp0). The default value is 0.

Enter a nonnegative value or expression for the (SI unit: m/s). The default is 0 m. This value is used to control the amount of overlap that is acceptable between a grain being considered for release and existing grains or wall elements.

Enter a value or expression for the (default 10). This value controls the number of release positions that are attempted for each grain.

If is selected from the list in the physics interface section, the text field is available. Enter the seed value of the pseudorandom number generator (PRNG) used by this feature. The default value is 1. The PRNG is used to generate random numbers for sampling grain release times, positions, grain properties, and initial values of auxiliary dependent variables.