# Dynamic Mesh

Source: https://help.sim-flow.com/documentation/panels/dynamic-mesh
Last updated: 2026-10-14

## Introduction

![img 59593f4e42d52](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/img_59593f4e42d52.png)

In the **Dynamic Mesh** panel, models for moving and deforming the mesh are defined. Besides the **Static** model representing the lack of any motion, the currently available dynamic mesh models are:

- **Rigid**
- **Multi-Body**
- **Dynamic**
- **6DoF**

## Rigid Body Motion

![img 59593fde2c10a](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/img_59593fde2c10a.png)

The **Rigid** dynamic mesh type allows moving the mesh according to a prescribed motion function. Optionally, the motion can only be applied to a selected cell zone. Note that this model does not allow for any deformation, just rigid body motion.

### Moving Selected Zone

![img 5959404dd051d](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/img_5959404dd051d.png)

When your mesh contains cell zones, the **Move Selected Zone** checkbox will be active, allowing you to apply motion only to the selected **Cell Zone**. This option is typically used for modeling turbomachinery, where you typically have some static and some rotating parts.

### Motion Functions

![img 59594134c1298](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/img_59594134c1298.png)

The **Motion** input defines what type of motion will be applied to the mesh or the selected cell zone.

#### Rotating Motion

![img 595941a1e6ea0](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/img_595941a1e6ea0.png)

The **Rotating** motion function rotates the mesh about a prescribed axis at a defined speed.

**Inputs:**

- **Origin** [m] - the point around which the rotation should occur
- **Axis** - rotation axis vector
- **Omega** [rad/s] - rotational speed (may vary in time)

#### Oscillating Rotating Motion

![img 5959422e0ef97](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/img_5959422e0ef97.png)

The **Oscillating Rotating** motion defines a simple angular oscillation where the rotation angle is described as:

\(\varphi (t)=A \cdot sin(\omega t)\)

where:

- \(\varphi (t)\) - instantaneous value of the rotation angle
- \(A\) - oscillation amplitude
- \(\omega\) - oscillation speed
- \(t\) - time

**Inputs:**

- **Origin** [m] - the point around which the rotation should occur
- **Amplitude** [deg] - amplitude of the rotation
- **Omega** [rad/s] - rotational speed

Note that the **Amplitude** input also defines the rotation axis.

#### Linear Motion

![img 595942a21d79b](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/img_595942a21d79b.png)

The **Linear** motion simply moves the mesh at a constant velocity.

**Inputs:**

- **Velocity** [m/s] - velocity of the mesh

#### Oscillating Linear Motion

![img 595943052bdc4](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/img_595943052bdc4.png)

The **Oscillating Linear** motion defines a simple linear oscillation where the mesh position is described as:

\(x(t)=A \cdot sin( \omega t)\)

where:

- \(x(t)\) - instantaneous position
- \(A\) - oscillation amplitude
- \(\omega\) - oscillation speed
- \(t\) - time

**Inputs:**

- **Amplitude** [m] - amplitude of the oscillation
- **Omega** [rad/s] - rotational speed

Note that the **Amplitude** input also defines the direction of the motion.

#### Ship Design Analysis (SDA) Motion

![sda dynamic mesh](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/sda-dynamic-mesh.png)

The **SDA** motion is a three degree of freedom motion function, comprising sinusoidal roll (rotation about the X axis), heave (translation in the Z direction) and sway (translation in the Y direction) motions with changing amplitude and phase.

**Inputs:**

- **Center of Gravity [m]**
- **Model Scale Ratio [-]**
- **Max Roll Amplitude [rad]**
- **Min Roll Amplitude [rad]**
- **Heave Amplitude [m]**
- **Sway Amplitude [m]**
- **Damping Coefficient []**
- **Time Period for Liquid [s]**
- **Ship Natural Period [s]**
- **Reference Time Step [s]**
- **Increment [-]**

#### Tabulated Motion

![tabulated dynamic mesh](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/tabulated-dynamic-mesh.png)

Tabulated motion allows importing external motion data.

**Inputs:**

- **Center of Gravity [m]**
- **Motion Data** - external file should contain the overall number of data sets, time and displacement vector for each set

## Multi-Body

![img 595944781eb5a](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/img_595944781eb5a.png)

The **Multi-Body** dynamic mesh type is similar to **Rigid**, but it allows for defining different motion functions in each cell zone. Also, it does not allow for moving the entire mesh directly. Options for this panel consist of a cell zone list where individual motion functions can be defined.

### Applying Motion Functions

![img 5959450a4e8b9](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/img_5959450a4e8b9.png)

To apply a motion function to a cell zone, you need to click on the cell zone name in the **Zones** list. Options for the selected zone will be displayed below the list. In the displayed options you just need to select the **Motion** function similarly to what is done for the **Rigid** model.

## Dynamic Motion

![img 59594562842a7](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/img_59594562842a7.png)

The **Dynamic** mesh type allows you to move or deform boundaries. Boundary motion can be described upfront as well as it can be computed dynamically based on the current solution state. The definition of boundary motion is done, like in the case of other models, in the [Boundary Conditions](https://help.sim-flow.com/documentation/panels/boundary-conditions) panel.

### Motion Kinematics

![img 595945cee1bad](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/img_595945cee1bad.png)

The **Kinematics** input specifies how the boundary motion is defined. There are two options:

- **Displacement** - you define how the displacement of the boundary changes in time
- **Velocity** - you define how the velocity of the boundary changes in time

### Mesh Deformation Solver

![img 59594659902c6](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/img_59594659902c6.png)

The **Solver** input defines which technique should be used to interpolate boundary motion into internal mesh deformation. There are currently two solvers available:

- **Laplacian**
- **SBR Stress**

#### Laplacian Displacement Solver

The **Laplacian** solver translates boundary deformation into internal mesh deformation by solving the Laplace equation:

\(\nabla\cdot(D\cdot\nabla\Delta)=0\)

where:

- \(\Delta\) - mesh deformation or velocity
- \(D\) - deformation diffusivity

#### Solid-body Rotation Stress Solver

The **SBR Stress** solver calculates mesh displacement by solving the cell-center solid-body rotation stress equations of the form:

\(\nabla\cdot D(\nabla \Delta + (\nabla \Delta)T -tr(\nabla \Delta))=0\)

where:

- \(\Delta\) - mesh deformation
- \(D\) - deformation diffusivity

Note that this solver can only be used when displacement kinematics is selected.

### Directional Motion

![img 59594751370f6](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/img_59594751370f6.png)

You can enable the **Directional Motion** input if you want to constrain motion to a specific direction. Then you will have to select one of the directions of the global coordinate system.

### Motion Diffusivity

![img 595947b2b3f92](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/img_595947b2b3f92.png)

**Diffusivity** controls how much boundary motion influences the internal mesh. Where the diffusivity is high, the deformations at the boundary strongly influence the internal mesh. When the diffusivity is low, only the mesh near the boundary is deforming. This input is used by both **Laplacian** and **SBR Stress** solvers. Currently, available diffusivity models are:

- **Uniform**
- **Directional**
- **Inverse Distance**
- **Inverse Volume**

#### Uniform Diffusivity

![img 595947de33120](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/img_595947de33120.png)

The **Uniform** diffusivity model assumes that it is constant inside the domain. Equations solved by the **Laplacian** and **SBR Stress** are such that when diffusivity is uniform, its value is not important, therefore you do not have to specify a value.

#### Directional Diffusivity

![img 595948098d149](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/img_595948098d149.png)

The **Directional** diffusivity model allows for diffusivity to be different in every direction of the global coordinate system.

**Inputs:**

- **Diffusivity Directions** - value of diffusivity in each direction of the global coordinate system

#### Inverse Distance Diffusivity

![img 59594840f34d0](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/img_59594840f34d0.png)

In the **Inverse Distance** diffusivity model, the diffusivity is proportional to the inverse of the distance from the selected boundaries.

**Inputs:**

- **Source Boundaries** - boundaries, distance from which is taken to calculate the diffusivity

#### Inverse Volume Diffusivity

![img 5959486abf0cc](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/img_5959486abf0cc.png)

The **Inverse Volume** model calculates diffusivity by taking the inverse of the volume of the cell for which the diffusivity is calculated.

## Six Degrees of Freedom

![img 595948eb1c14f](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/img_595948eb1c14f.png)

The **6 DoF** solver will predict the trajectory of a moving body using the aero or hydrodynamic forces and inertial properties assigned to a given boundary. During the analysis, the mesh will adjust to the new position of the body by moving nodes in the deformation region defined by the distance from the body. Additionally, you have the possibility to apply various constraints to the boundary motion.

### Moving Boundaries

![img 5959497ead158](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/img_5959497ead158.png)

The **Moving Boundaries** input specifies which boundaries should be moving according to the solution of motion equations. Additionally, you will have to specify **Inner Distance** and **Outer Distance** from the boundaries. Mesh nodes within the **Inner Distance** will move exactly like the boundaries. Mesh nodes outside the **Outer Distance** will not move at all.

### Boundaries Properties

![6dof mass prop](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/6dof-mass-prop.png)

To be able to solve the motion equations, the solver requires mass properties of the moving boundary.

**Inputs:**

- Mass [kg] - mass of the body
- Center of Mass [m] - coordinates of the center of mass
- Mom of Inertia [kg m^2] - moment of inertia
- Velocity [m/s] - body velocity

### Solver

![6dof solver](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/6dof-solver.png)

Two solvers are available:

- Newmark
- CrankNicolson

Additionally, we can set the **Relaxation** parameter to control the convergence process.

### Constraints

![6dof constraints](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/6dof-constraints.png)

It is possible to constrain or restrain boundary motion.

#### Translation Constraint

We distinguish 3 types of **Translation Constraint**:

- **Linear** - body can move along one direction - rest DoFs are fixed

![6dof translation constraint line](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/6dof-translation-constraint-line.png)

- **Plane** - body can move on the selected plane which is defined by the normal vector

![6dof translation constraint plane](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/6dof-translation-constraint-plane.png)

- **Point** - translation relative to the selected point is fixed

![6dof translation constraint point](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/6dof-translation-constraint-point.png)

#### Rotation Constraint

**Rotation Constraints** allow fixing the body rotation. There are two types of constraint:

- **Axis** - rotation around the selected axis is fixed

![6dof rotation constraint axis](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/6dof-rotation-constraint-axis.png)

- **Fix Orientation** - all rotation degrees of freedom are fixed

![6dof rotation constraint fixed](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/6dof-rotation-constraint-fixed.png)

#### Linear Restraint

Restraint options:

- **Damper**

![6dof linear restraint damper](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/6dof-linear-restraint-damper.png)

- **Spring**

![6dof linear restraint spring](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/6dof-linear-restraint-spring.png)

#### Angular Restraint

- **Damper**

![6dof angular restraint damper](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/6dof-angular-restraint-damper.png)

- **Spring**

![6dof angular restraint spring](https://help.sim-flow.com/documentation/panels/images/dynamic-mesh/6dof-angular-restraint-spring.png)