External Forces and Moments Reference

You can add additional forces and moments using the External Forces and Moments node right-click menu.

For a 6-DOF body, Simcenter STAR-CCM+ provides the following forces:

Unless otherwise stated, forces and moments are defined with respect to the position of the body. The body position depends on the selected body motion option. For free motion, multi-body motion, four-DOF, maneuvering motion, and equilibrium motion, the body position is the location of the center of mass. For planar motion carriage, one-DOF rotating, and one-DOF translating, the body position is specific to the motion option.

You can activate or deactivate a force or moment using the Enabled property, which is available for all force and moment nodes except for those representing a coupling or a constraint. You can deactivate coupling and constraint forces through their respective coupling or constraint node.

Predefined Forces and Moments

Gravity Force

Represents the gravity force acting on the 6-DOF body ( ${\mathbf{\text{f}}}_g$ in Eqn. (4879)). When this force is enabled, activate the gravity model in at least one physics continuum associated with the 6-DOF body. The body is associated to one or more physics continua via the regions of the boundaries used in its body surface.

Other Model Related Forces and Moments

Fluid Force and Moment

Activated automatically through the Segregated Flow or Coupled Flow model selected in the physics continua of the 6-DOF boundaries' regions. Cannot be activated without a flow model.

It represents the force and moment exerted on the 6-DOF body by the surrounding fluid. It includes the contribution from both pressure and shear forces (see Eqn. (4879)). The available properties are:

• Enable Smoothing—when activated, the fluid force and moment are averaged over a specified time interval before they are applied to the body. Simcenter STAR-CCM+ uses a moving weighted average as a low pass filter; you set the number of time-steps that are included in this moving average using the Number of smoothing time steps property.
• Use Static Pressure—when deactivated, Simcenter STAR-CCM+ uses the default working pressure. When activated, Simcenter STAR-CCM+ uses the static pressure.

DEM Force and Moment

Activated automatically when the Discrete Element Model (DEM) model is selected either in a mesh-based (flow based) or a meshfree workflow (using the Meshfree DEM model) in the physics continua of the 6-DOF boundaries' regions. These boundaries can include the body surfaces of a Continuum DFBI body.

It represents the forces and moments that act on the 6-DOF body due to interaction with DEM particles (see Eqn. (4888) and Eqn. (4889)).

The coupling between the 6-DOF body and the DEM model is explicit. Due to the explicit coupling, you are advised to use an appropriately small time-step (see Controlling the DEM Time-Step). You can visualize the time-averaged DEM contact force acting on the 6-DOF body through the Average DEM Total Force field function.

Electromagnetic Force and Moment

Activated automatically through either the Finite Element Magnetic Vector Potential model or the Transverse Magnetic Potential model selected in the physics continua of the 6-DOF boundaries' regions. See also: Finite Element Magnetic Vector Potential Model Reference and Transverse Magnetic Potential Model Reference.

It represents the force and moment that act on the 6-DOF body due to electromagnetic fields (see Eqn. (4350) and Eqn. (4352)). The electromagnetic force and moment are equivalent to the Magnetic Force and Magnetic Torque reports. For a correct calculation of the force and torque, the 6-DOF body must be surrounded by a force-free region (see Reports).

This type of external force and moment is available when the physics continuum contains either the Finite Element Magnetic Vector Potential model or the Transverse Magnetic Potential model (see Finite Element Magnetic Vector Potential Model Reference and Transverse Magnetic Potential Model Reference.

Virtual Disk Force

Represents the forces and moments applied at the center of mass of the 6-DOF body calculated from a specified virtual disk. This force can only be added by users when the physics continua of the 6-DOF boundaries' regions include the Virtual Disk model.

The available property is:

• Virtual Disk— select a pre-defined virtual disk.

For more details, see Virtual Disk Force and Moment.

User-Defined Forces and Moments

Damping Force

Represents a damping force acting on the body, opposite to the body velocity (see Eqn. (4887)). The available properties are:

• Damping Constant—specifies the linear damping constant $a_{d,f}$ in Eqn. (4887). A large value can lead to numerical instabilities. In this case, reduce either the damping constant or the simulation time-step.

Damping Moment

Represents a damping moment acting on the body, opposite to the body angular velocity. The magnitude of the moment, which always acts to decelerate the body, is proportional to the angular velocity (see Eqn. (4886)). The available properties are:

• Damping Constant—specifies the angular damping constant $a_{d,f}$ in Eqn. (4886). A large value can lead to numerical instabilities. In this case, reduce either the damping constant or the simulation time-step.

Force

Represents a user-defined force (see Eqn. (4879)) acting on the 6-DOF body at a specified location. This type of force generates a moment (see Eqn. (4880)) around the center of mass of the body.

The available properties are:

• Force—defines the external force as a vector, with an appropriate unit of force.
• Position —specifies the coordinates of the point, on the 6-DOF body, at which the force is applied.
• Coordinate System 1—specifies the coordinate system in which the point position is defined.
• Coordinate System 2—specifies the coordinate system in which the force is defined.

Force CM

Represents a user-defined force (see Eqn. (4879)) acting on the 6-DOF body at its center of mass. This type of force does not generate any moment about the center of mass.

The available properties are:

• Force—specifies the vector that defines the external force, with the appropriate unit of force.
• Coordinate System—specifies the coordinate system in which the force is defined.

Moment

Represents a user-defined moment acting on the 6-DOF body (see Eqn. (4880)). You can define the moment components in any coordinate system—the moment always acts with respect to the body position.

The available properties are:

• Moment—defines the external moment as a vector, with an appropriate unit of moment.
• Coordinate System—specifies the coordinate system in which the moment is defined.

Propulsion Force

Like Force, the Propulsion Force represents a user-defined force (see Eqn. (4879)) acting on the 6-DOF body at a specified location. You define the Propulsion Force by specifying its magnitude and direction, rather than vector components. This type of force generates a moment (see Eqn. (4880)) around the center of mass of the body.

The available properties are:

• Position —specifies the coordinates of the point, on the 6-DOF body, at which the force is applied.
• Thrust—specifies the magnitude of the force as a constant scalar value, with an appropriate unit of force.
• Direction—defines the direction in which the force acts as a vector.
• Coordinate System 1—specifies the coordinate system in which the point position is defined.
• Coordinate System 2—specifies the coordinate system in which the direction is defined.

Torsional Spring Moment

Represents the effect of a torsional spring acting on a rotating body (see Eqn. (4885)). You can only add the torsional spring moment to bodies with one rotational degree of freedom. Such bodies include:

• Bodies with One-DOF Rotating Motion (see One-DOF Rotating Motion). In this case, you specify the axis of rotation on the motion node. The moment is defined with respect to the rotation axis origin.
• Bodies undergoing free motion, with only one rotational degree of freedom selected and all other degrees of freedom deactivated (see Free Motion). In this case, the axis of rotation is parallel to the axis of the laboratory coordinate system around which the body is allowed to rotate. The moment is defined with respect to the center of mass.

The available properties are:

• Spring Constant—specifies the spring constant of the torsional spring ( $k$ in Eqn. (4885)).
• Relaxation Angle—specifies the angle at which the spring moment becomes zero ( ${\alpha }_r$ in Eqn. (4885)). The angle is measured with respect to the initial orientation of the body.

Body Coupling Forces

Body couplings such as linear spring and catenary couplings induce forces that act on the 6-DOF body (see Body Connections).

When you define body couplings, Simcenter STAR-CCM+ automatically adds corresponding forces under the External Forces and Moments node of the relevant body. These forces are read-only.

The name of the force reflects the type of coupling and its name. For example, the force induced by a linear spring coupling named Linear Spring 1 is Linear Spring Force [Linear Spring 1].

For this type of forces, the Coupling Element property displays the name of the body coupling associated with the force.

Constraint Forces

The following forces are created automatically:

Constraint Force [Freeze Motion Components]

Created automatically when you activate multi-body motion. Represents the additional forces that Simcenter STAR-CCM+ calculates to prevent motion of degrees of freedom that you deactivate on the Multi-Body Motion node (see Multi-Body Motion).

Constraint Force [Contact Constraint n]

Created automatically when you create a body constraint. Represents the forces required to impose a specific contact constraint on the motion of the body. The constraint can be applied to the entire body surface or to selected sections. See also Setting Up Body Constraints.

You can report these forces using a 6-DOF Body Force or a 6-DOF Body Moment report.