COMSOL 6.3 - Boundary Load (Thin Layer)

Add a node to apply tractions or pressure on edges adjacent to boundaries where the node is active.

The is selected by default. The list contains all applicable coordinate systems in the model. Prescribed loads are specified along the axes of this coordinate system.

Force

Select the — , , , , or . For 2D components it is possible to also select or . Then enter values or expressions based on the selection and the space dimension.

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For and , the traction components are given explicitly.

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For , COMSOL Multiphysics divides the total force by the area of the boundaries where the load is active. When working with curved boundaries or local coordinate systems, use this option carefully, as the result is not always intuitive.

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For , enter the and with respect to a point. Select the — , , or . For , the coordinates of the application point is the centroid of the selected boundaries. For , select a geometrical point in the section . For , enter the global coordinates of the , xa.


	After selecting a Load type, the Load list normally only contains User defined. When combining with another physics interface that can provide this type of load, it is also possible to choose a predefined load from this list.


Load Type	variable	SI unit	Geometric Entity Level	Space Dimension (components)
Force per reference area	fA	N/m2	boundaries	3D (x, y, z) 2D (x, y) 2D axisymmetric (r, z)
Force per deformed area	fa	N/m2	boundaries	3D (x, y, z) 2D (x, y) 2D axisymmetric (r, z)
Force per reference length	fL	N/m	boundaries	2D (x, y)
Force per deformed length	fl	N/m	boundaries	2D (x, y)
Total force	Ftot	N	boundaries	3D (x, y, z) 2D (x, y) 2D axisymmetric (r, z)
Resultant	F, M	N, N·m	boundaries	3D (x, y, z) 2D (x, y)

Traction Field

This section is only shown when has been selected as . The distribution of the forces over the selected points is controlled by the settings here.

Select a — or .

When is selected, the force distribution approximately matches the stress distribution over a beam cross section.

For , you can write expressions for the force distribution. For 3D components enter expressions for the six dimensionless vector-valued distribution functions, 1, 2, 3, 4, 5, and 6. For 2D components enter expressions for 1, 2, and 4. Usually, the local coordinates of the loaded region would be used for this purpose, but there is no limitation on the form of the functions. The built-in variables for the local coordinates are named <physics_tag>.<load_tag>.x2 and <phys_tag>.<load_tag>.x3, for example solid.bndl1.x2. The default value is the distribution that is used when is selected as .

The local coordinates, x2 and x3, are defined in the following way:

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The origin is placed at the centroid of the selected points.

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The axis directions coincide with the principal axes of a virtual collection of points with unit mass.

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x2 is the axis around which the virtual principal area moment of inertia is larger.

When is selected, you can also select a — , , or . This function acts as a multiplier to the force distribution functions. When is selected, the weight function w = 1. For , enter a radius rw of a circle, outside of which no load is applied (w = 0). Inside the circle, w = 1. For , enter a weighting expression. Usually, the built-in local coordinates of the loaded region would be used for this purpose, but there is no limitation on the form of the function. A time- or parameter-dependent function can, for example, be used to model a moving load.

The traction distribution functions have the property that the forces are distributed as

where the coefficients ci are chosen so that the given force and moment resultants are obtained.

Symmetry

This section is only shown when has been selected as . If the resultant force is applied in a symmetry or antisymmetry plane, this fact needs to be taken into account when creating the corresponding force distribution.

Select a symmetry type — , , or . When one of the types of symmetry is chosen, specify the symmetry plane by entering a , nsym, and a , xsym, located in the plane.

When specifying a resultant load in a symmetry plane, you still provide the full value of force and moment, as if there were no symmetry in the model. Any components of the given load that do not fulfill the selected type of symmetry will be discarded.