# F-DEMPack Tutorial 2: Annular pipe

## Introduction

Before starting with this tutorial, the user is strongly encouraged to follow the D-DEMPack Tutorial 2: Conveyor belt to get a feeling of how the problem type works, and in particular the DEM section. This tutorial will focus mainly in the Fluid section and its particularities.

## Geometry

The process should start by creating a new geometry from scratch, by modifying an existing one or by opening a finished one. It is assumed that the user already knows how to do this procedure so no details will be given about it.

In this tutorial, a simple geometry was created for the sake of simplicity. The idea is to understand all the steps involved in the problem type. The user must start by downloading the file File:F DEMPack2 Tutorial 6.gid.zip, which contains the geometry and mesh of the proposed geometry. This file has already created the groups that will be used in the simulation.

The geometry of study consists of an annular tube through which a flux of an intermediate viscosity mud passes. An inlet creating DEM particles with time is located in the base of the annular pipe.

### DEM group

In this sample case, the following five groups were created: Dem_inlet, Fluid, Inlet, No_slip and Outlet. The lower circle (surface) would be the Inlet, the smaller one just on top of it was the DEM_Inlet, the circle at the top would represent the Outlet, the No_slip would be the walls of the cylinder and, finally, the volume named Fluid would constitute the mass of fluid.

## Materials

The Materials section in F-DEMPack contains the material data for both DEM and fluid elements. See the next figure for an overview of that section:

The DEM part is identical to D-DEMPack, and a full explanation of every aspect in it can be found in the corresponding links given above. In this case, though, an additional section for the Fluid part exists, where the user can set the values of some fundamental fluid properties, as for example density, viscosity, bulk modulus or rheological characteristics.

### DEM-FEM wall group

The parameters in this section are identical to those in the D-DEMPack problem type, so no extra information is necessary in this case.

### Inlet DEM group

See D-DEMPack Tutorial 2: DemPack 2.0 for details. It is important to note that it is still not possible to create neither clusters nor nanoparticles from an inlet entity. This aspect of the code is still under development.

### DEM Initial Conditions

This section is also identical to its counterpart in D-DEMPack, so no further explanations are needed.

## Materials

The Materials section in F-DEMPack contains the material data for both DEM and fluid elements. See the next figure for an overview of that section:

The DEM part is identical to D-DEMPack, and a full explanation of every aspect in it can be found in the corresponding links given above. In this case, though, an additional section for the Fluid part exists, where the user can set the values of some fundamental fluid properties, as for example density, viscosity, bulk modulus or rheological characteristics.

## DEM-Fluid Interaction Settings

Most of the interaction parameters between the DEM spheres and fluid are inside the General Application Data section, whose overview is given next:

Some of the parameters in this tree are very straightforward, as for example the duration time, the output delta time, the number of threads to use in the simulation or the gravity vector. Others, though, carry a higher difficulty and are mostly related to the way the two subdomains interact. A deeper explanation of those parameters can be found in F-DEMPack2 manual. The previous screenshot has been given to the user as a reference as well as a guide for choosing some default values that give good results in this particular case. This tree also includes the Results section, which is pretty straightforward.

## Fluid

This section contains the information about the properties of the fluids, the different existing fluid elements and, when necessary, their assigned conditions. It also has some parameters in relation to the settings of the fluid solver. The figure that follow shows an overview of this section:

The screenshot shows the chosen parameters in this example. The user is encouraged to play a little bit to those values and see the results. In this case a monolithic solver was chosen to get more accurate results but no turbulence model was considered necessary. The linear solver parameters require a much deeper understanding of the underlying theory and the associated numerical methods and will not be discussed here, check for more information. The default parameters in the problem type gave good results in this particular case. Finally, the user can enter the desired computational time step, which does not have to be too small as long as stable simulations are obtained.

### Properties

In this section, the user can create a Property related with each of the fluids in the problem. In order to do this, the corresponding fluid must have been previously created in the Materials section. The next picture shows the process:

In this particular case, the Water default material was assigned to the Property1, which is good enough in this sample simulation.

### Elements

The user must specify the fluids that will be present in the problem, so the next step is to assign the desired groups to the Fluid Elements. The process is the same as in other tutorials. A screenshot is given next:

To finish the elements assignation, though, an additional step is necessary. The user must specify the FEM element type to be used in the fluid mesh and the corresponding Property. The next figure shows this:

In this case, the only available element in 3D is the tetrahedra, while the chosen property was number 1, corresponding to the Water material.

### Conditions

We finish the process by assigning the necessary conditions to their corresponding groups.

#### Initial Conditions

The fluid solver needs some initial conditions in the fluid to solve the problem. The next figure shows the section:

For this simulation, an initial vertical value of 1m/s was assigned to the mass of fluid inside the tube. In this case, no initial pressure was necessary.

#### Boundary Conditions

• Inlet velocity

An inlet velocity is necessary in this simulation for a flow to exist in time along the interior of the tube. See the next picture:

A vertical constant flow of 1m/s entering the bottom opening of the tube was chosen. No Outlet Pressure was needed in the simulation.

• No-Slip

The user must tell the program if there exists any relative velocity between the fluid and the solid boundary. The capture that follows shows the section:

For this example, a no-slip behaviour was given to the walls. If there had been additional DEM-FEM entities, an Is-Slip behaviour could have been given to them.

## Meshing and Running

If the user has succesfully assigned to the corresponding groups all the previous Properties, Elements and Conditions, the only thing that is still missing is the computational mesh. As already said, all the messing characteristics has been previously set in this problem, so the user has been able to focus in the Fluid aspects of the problem type. So to mesh, hit Ctr-G and enter a value of 0.02 and press OK. A global view of the resulting mesh should be similar to this one:

On the other hand, the initial DEM spheres mesh should look like the following:

While the inlet surface mesh should be very similar to this:

## Results

Once a mesh is obtained and the file saved, the simulation can be run. To start the computations, the user must only hit F5 and the simulation will begin. The next four captures show the resulting simulation at different times:

The next figure shows the stationary pressure field in the fluid:

The user can also make a cut in the mass of fluid and obtained a 2D view of the velocity field in the fluid: