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**Dynamic mesh**

In Most CFD project, we design geometry, create a static mesh for this geometry and simulate this in by fix mesh. But sometimes we want to move some boundary or we have deforming shape during times. We must use dynamic mesh method. For example in mots aerodynamic problem we use fix mesh for our simulation but you can imagine there are two airplanes that are getting closer and you want to know effect of fluid flow behavior around airplanes and determine interaction between them during the time. So the relative position them are changing during time and should be modeled using dynamic mesh method. By using this method, mesh size and shape will be changed. If we use re-meshing or layering method, number of element also will be changed. Dynamic mesh method is entirely different from moving and sliding mesh also moving reference frame. In moving mesh whole zone rotate or translate in some direction but in dynamic mesh method boundary start to translate or transform. We can apply a predefined velocity by UDF or profile for boundaries or velocity of an object can be predicted based on fluid flow and gravity force balance. If we want to know the velocity of object based on this balance we should use SIX DOF dynamic mesh method.

Mesh quality during change should be conserved.ANSYS fluent has three different methods for changing mesh, smoothing, re-meshing, and layering.

We are highly experienced in using dynamic mesh in various CFD project and in the following, you can see a summary of our project related to using this module:

- Six DOF dynamic mesh for modeling variation of location and angle of ship and boat in wavy sea situation
- Simulation of the tidal turbine using six DOF dynamic mesh method (calculating rotation speed)
- Releasing of food box form airplane using six DOF dynamic method
- Simulation of globe valve movement during the time
- Train movement in the tunnel by variable velocity function in a urban tunnel
- Simulation of flap movement of an airplane wing
- Falling boxes into the water tank
- Modeling the underwater vehicle in Submerged and non-submerged situation
- Solid fuel combustion and modeling solid height level during burning fuel
- Fluid solid interaction of blood flow and vessel
- FSI simulation blood flow pumping ina human heart
- Fluid solid interaction of airflow around building and finding final deflection and fluctuation (two way and one way FSI)
- Two and four-stroke internal combustion engine engine

## Dynamic Mesh by Fluent

Whenever the problem model was such that the fluid zone required mesh moment changes over time, the Dynamic Mesh technique will be used. In fact, in problems where the specific region of fluid and its boundaries were displaced during the simulation, a moving mesh should be used to momentarily transform and move the cells in the mesh. The mesh Smoothing and Remeshing techniques, provide instant mesh deformation capability. Remeshing is used when the mesh is more sensitive to mesh changes, which can be used to manually specify ranges of maximum and minimum elements in the Maximum and Minimum Length Scale for mesh changes. In the Dynamic Mesh Zone section, areas or boundaries of the model that should be affected by the mesh change must be defined, such as by Deforming to change the boundaries in the area (fluid around a moving object or a boundary-changing object), Rigid Body to moving physical boundary and moving boundaries but not boundaries themselves, from Stationary to fixed body and finally from System Coupling to FSI solution.

Whenever the problem model was such that the fluid zone required mesh moment changes over time, the Dynamic Mesh technique will be used. In fact, in problems where the specific region of fluid and its boundaries were displaced during the simulation, a moving mesh should be used to momentarily transform and move the cells in the mesh. The mesh Smoothing and Remeshing techniques, provide instant mesh deformation capability. Remeshing is used when the mesh is more sensitive to mesh changes, which can be used to manually specify ranges of maximum and minimum elements in the Maximum and Minimum Length Scale for mesh changes. In the Dynamic Mesh Zone section, areas or boundaries of the model that should be affected by the mesh change must be defined, such as by Deforming to change the boundaries in the area (fluid around a moving object or a boundary-changing object), Rigid Body to moving physical boundary and moving boundaries but not boundaries themselves, from Stationary to fixed body and finally from System Coupling to FSI solution.

Generally, the dynamic mesh technique is used when simulating models that require a moving mesh area. In fact, this model enables the spatial variation of the defined areas for fluid flow and allows the mesh of the region at the same time as the spatial location of the points and boundaries of the region changes compatibility with the place at the present moment. The dynamic mesh technique composed of three steps, including determining **dynamic mesh methods**, specifying specific modes with dynamic mesh **option**s and defining the **dynamic mesh zone**.

Dynamic mesh production methods are divided into three modes: **smoothing**, **layering**, and **remeshing**.

The **smoothing** method adjusts the mesh of an area by moving or deforming boundaries, but the number of nodes and connections does not change. In this type of mesh switching, in the **spring-based smoothing** method, the edges between the two nodes of the mesh are known as a mesh of interconnected springs. The initial occupancy of the edges between the two mesh nodes prior to any boundary movement constitutes an equilibrium state of the meshes. Each displacement at a given boundary point produces a partial force for displacement along all the springs connected to the node, that is, the edges between two nodes, such as a pressure spring or a force pull to displace the node. Therefore, for the settings of this section, one must consider the coefficient of spring constant (denoting the value of the spring stiffness and having a value between zero and one) in the **spring constant factor** and the number of iterations in a duplicate equation derived from the state of the spring’s equilibrium in the number of **iterations**. Therefore, the software repeatedly resolves at each time step until the set number of iterations occurs or the solution reaches a convergence step whose convergence criterion is the value of the **convergence tolerance** specified in the convergence tolerance. The **tri in tri zones** uses to applying smoothing mesh to triangular elements in all triangular elements, from **tri in mixed zones** to applying smoothing mesh to triangular elements in mixed element areas, and **all** are used to apply the mesh change method to all elements of the region.

The dynamic **layering** method is used when multilayers of cells are added to or subtracted from the cell layer adjacent to the moving boundary. This can be based on the layer height adjacent to the moving surface, meaning that the cell layer adjacent to the moving border is subdivided or merged with its subsequent cell layer, and in fact requires an ideal height limit. It is determined by the value of the **split factor** and the **collapse factor**, or it can be based on the ratio of the layer height adjacent to the moving boundary to its next layer. In this case, it also has a limit-to-height ratio, which is also dependent on the split ratio and the decay ratio.

The remeshing method is used for situations where the boundary displacement is large compared to the size of the local cells, which causes the cell quality to deteriorate; thus, the mesh becomes invalid. For example, a negative cell volume error may occur and eventually lead to a problem-solving divergence at the next time step. Therefore, to address this problem, it compresses cells that disrupt the **skewness** value or critical size limit and locally remeshes the meshed cells or surfaces. Now if the new cells satisfy the critical value of skewness, the mesh is updated locally with the new cells, otherwise, the new cells are discarded and the same old cells remain. Among remeshing methods, the **local cell** method only affects triangular and tetrahedral cells, the **local face** method can modify the tetrahedral and wedge cells in the boundary layer meshes, the face method applies to triangular and tetrahedral cells, and the **2.5D method** only works on hexagonal meshes or wedge cells given the volume of triangular plane elements. To use these dynamic mesh methods, one must define the maximum and minimum longitudinal scales and the maximum plate and cell density values as the dynamic meshing intervals, as well as the number of meshing times to achieve.

Specific states of motion can be defined in the **options** section and defined for the selected areas and borders in the **dynamic mesh zones** section. These specific modes include **In-cylinder**, **6 DOF**, **implicit update** and **contact detection**.

Six degrees of freedom (**6-DOF**) used for the calculation of external forces and moments (including aerodynamic and gravitational forces and moments) applied to components capable of moving the rigid object. These forces are obtained by numerically integrating the shear stress and pressure around the component surfaces. Extra loading forces such as injectable forces, thrust or propulsion forces and torques produced by tubular springs can also be added to the degree of freedom. This multi-degree of freedom can be applied to many applications by using the Fluent software and by applying a dynamic mesh method. In this case, the defined fields are the components of the mass tensors and moment of inertia. To define the 6-DOF model, the properties of the multi-degree model must be adjusted. Thus, when the dynamic discussion of the components is limited to only one degree of freedom, one degree of freedom can be a **1-DOF translation** or one degree of freedom of rotation (**1-DOF rotation**). Using the **axis**, the **center of rotation** of the object can be selected along the rotational axis, using the center of rotation. The amount of **moment of inertia** entering the body can also be obtained. If we want to follow the movement of the 6-DOF components momentarily, we use **write motion history** and defining a file to store the files of each moving component, which allows us to display results in the **motion zone** by **CFD-post**. If the moves in the six-DOF simulation model depend on the fluid flow, an **implicit update** is recommended and if this option also fails to satisfy the stability of the solution, use the **solver option** in the **dynamic mesh zone**. **Gravitational acceleration** is used to define the amount of gravitational acceleration (equivalent to 9.81m.s-2) and its direction; in other words, the settings in this section are equivalent to the settings for gravitational acceleration in **operating condition**. It is also possible to define a **spring** using **Hooke’s law** which represents the forces and torques applied to the dynamic body, such as for the transient motion of the tensile or compressive spring and for the rotational motion of the spring. For example, to define the ultimate force applied to a spring by the sum of a **preload** in units of Newton-meters and the result of multiplying a spring constant (k) by a unit of Newton-meter per omega in the displacement value of the spring relative to the location. Also, **constrained** activation can apply restrictions on the location in transitional motion and angular position in rotational motion using **max** and **min** values.

In the **dynamic mesh zones** section, areas or edges that fall under the dynamic mesh should be defined along with their dynamic mesh type. Different modes of dynamic mesh definition include the **stationary**, **rigid body**, **deforming**, **user-defined** and **coupling system**.

The **stationary** state indicates that no movement (displacement or deformation) is attributed to a plane or cellular region, and thus the region or boundary in which the dynamic mesh is updated and transmitted by passage. It is not considered a time step. The **rigid body** state indicates that the selected region or boundary has motion but no deformation occurs between the points in it. It should be noted that a defined UDF function or one of the types of motion-defined in the **Fluent** itself, such as six degrees of freedom (6DOF), can be used to investigate the motion attributed to these rigid bodies. Also, **relative motion** is used when one rigid object has a displacement relative to another rigid object. If the UDF code is used to define rigid body motion, there is no need to define the angular velocity and orientation of the rigid body in inertial coordinates, and this data is written to the code; These data must be entered manually. The **deforming** mode indicates that the selected boundary or region undergoes displacement and deformation under moving areas or boundaries in contact with it. In 2D simulations, for deforming edges or walls, the specified geometry can be defined as a **plane** or a **cylinder**, and if no geometry is not available in the model, the **faceted** mode selected. The **user-defined model** is also used to define the position of the nodes in the deformed or displaced region using coding. Finally, the **coupling system** mode is used for situations where the system has inter-region coupling in the fluid model with the solids analysis section.

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