# a) Simulation using an Overset Mesh

In this part, the aim is to run a simulation which makes it possible to see what a wind turbine wake actually looks like.

The principle

The principle of an overset mesh is really simple : the mesh is divided in two parts :

• a background one for the entire domain. It needs to be refined enough where the turbine is located.
• a foreground one for the domain close to the turbine. It will be rotating. Obviously, the size of the cells needs to be of the same order as the refined background one.

These two parts share have a small zone in common where the two mesh are superposing : source : doc StarCCM+

More details about this method can be found in the greatly explained StarCCM+ documentation.

# Geometry - Region - Physics Continuum

Geometry

As in the simulation with virtual disks, the wind turbine studied is the Repower MM70 : As it is difficult to create a turbine geometry from scratch in StarCCM+, a possible solution is to import an *.stl file created with a CAD software. It is easy to find one on the internet. The one used here is taken from the website GrabCAD.

Firstly, the *.stl geometry file is imported : Geometry $\rightarrow$ right click on 3D-CAD models $\rightarrow$ New $\rightarrow$ right click on 3D-CAD model 1 $\rightarrow$ import $\rightarrow$ CAD models.

Next, the idea is to modify the bodies to be as close as possible to the geometry of the MM70. Here, mainly Scaling is used. Finally : After this step being done, parts need to be created : here, to simplify, only blades and the nacelle will be represented.

Geometry $\rightarrow$  3D-CAD models $\rightarrow$ 3D-CAD model 1 $\rightarrow$ bodies $\rightarrow$ right click on the body $\rightarrow$ New geometry part.

Now, let us create the domain : right click on Part $\rightarrow$ New Shape Part $\rightarrow$ Block. Be careful to create a domain big enough to avoid problems of limit conditions having an impact on the solution. Split the domain's surface Domaine $\rightarrow$ Surfaces $\rightarrow$ right click on Surface $\rightarrow$ Split by angles. Rename the surfaces created (inlet, outlet, side, floor...).

The next step is to create the moving part : it is defined as To do that, create a cylinder which include the geometry (Part $\rightarrow$ New Shape Part $\rightarrow$ Cylinder). Then, select both part (the geometry part and the cylinder) $\rightarrow$ Boolean $\rightarrow$ Substract parts. Do the same as before to split the surfaces.

The region

Select both the domain and Substract $\rightarrow$ Assign Parts to region : Rename the regions : domain and moving_region.

Now, create an interface : select both region $\rightarrow$ right click $\rightarrow$ Create interface $\rightarrow$ Overset Mesh. Check that the interface node created has the moving region as region-0 and the domain as region-1.

Set the boundary conditions : for the moving_region, everything has a type "wall" excepted the cylinder surface whose type is "Overset mesh". For the domain : The Physics Continuum

To create a physics continuum right click on Continua $\rightarrow$ New $\rightarrow$ Physics Continuum. Now select the models : right click on models $\rightarrow$ Select Models. Select : It is now generally necessary to add a source term : "This source term is required for such cases because the overset mesh method is not conservative and comes with an interpolation error that prevents residuals from converging. STAR-CCM+ automatically calculates the source term." (starCCM+ documentation).
To activate it : right click on Models $\rightarrow$ Edit $\rightarrow$ Segregated Solver It is now possible to put initial conditions for the velocity (10 m/s here) and turbulence intensity (0.12) : It is the good moment to set up the boundary conditions (0.12 as turbulence intensity and 10m/s as entering velocity): The next step is the creation of the mesh

# Mesh

The Mesh

The principle is quite simple : the two regions defined before are meshed separately : the software will automatically deactivate the cell of the background mesh which are not useful. Indeed, the Navier Stokes equations won't be solved in the background cells which are close to the blades geometry (in the area close to the blade geometry, the equations will only be solved in the moving mesh). First, create two mesh continuum : do twice the procedure right click on Continua $\rightarrow$ New $\rightarrow$ Mesh Continuum. Rename one as "moving_mesh" and the other as "background mesh".

On the region domain : region $\rightarrow$ right click on domain $\rightarrow$ Edit $\rightarrow$ Mesh continuum $\rightarrow$ "background mesh". Same procedure for the "moving_region" domain $\rightarrow$ Mesh continuum $\rightarrow$ "moving_mesh".

For both meshes, it is now necessary to select the models to use : righ click on models $\rightarrow$ Polyhedral Mesher and Surface Remesher for the background mesh and Polyhedral Mesher, Surface Remesher, Prism Layer mesher and Surface wrapper for the moving_mesh.

The values used here for the moving mesh are : For the background mesh : It is important that the cell close to the geometry in the background mesh have the same order of magnitude in term of size. To achieve that, let us create a Volume shape : Tools $\rightarrow$ right click on Volume shapes $\rightarrow$ New Shape $\rightarrow$ Block. This block must be at lease the size of the moving region.

Now it is possible to add a volumetric control : right click on Volumetric Controls $\rightarrow$ New $\rightarrow$ Select the shape just created and use Polyhedral Mesher and Surface Remesher with a relative size of 2 (which gives 1m, the base size of the moving mesh).

From this point, one can create the mesh by clicking successively on .

Make the Mesh Moving

To make the mesh moving, Tools $\rightarrow$ right click on Motions $\rightarrow$ New $\rightarrow$ Rotation. Click on rotation $\rightarrow$ Now that the rotation is created, it is necessary to let the software now that we want to assign this Motion the moving_region :

here, Right click $\rightarrow$ Motion $\rightarrow$ Rotation

Visualize the mesh

To get a 3D view of the mesh (like the first image of this page), a "Threshold" derived part is used. It makes it possible for the user to display cell which meet a specific criteria.

Right click on Derived part $\rightarrow$ New part $\rightarrow$ Threshold $\rightarrow$ Right click on Scene $\rightarrow$ New Scene $\rightarrow$ Mesh in the onglet Plot/scene ( ): Finally : Finally, it is possible to set up the simulation.

# Simulation - Vizualisation

Simulation parameters

In the node Implicit Unsteady set up the Time-step. Here, 0.05s is used.

In the node Stopping Criteria the user has to define the conditions which will make the iterations stop : the logical rules are

• OR : if the condition is satisfied, it does the action.
• AND : if the condition is satified, it still iterates until all AND conditions are satisfied or a OR condition is satisfied.

Here, two condition are enabled : In conclusion, the software will do 20 inner iterations for every 0.05s time-step until it reachs 100s.

Now, the idea is to save simulation results, let's say every 0.1s to make an animation for example. Right click on Solution Histories $\rightarrow$ New $\rightarrow$ "load". In fact, the results wanted will be solved in the "load.simh" file.

Right click on load $\rightarrow$ That means : when the simulation reachs 70s (the transcient part is not wanted here) the scalar and vector functions chosen (Pressure, TVR and velocity) in the moving_region and domaine are going to be saved in "load.simh" every Delta time =0.1s until the end of the simulation (100s).

Now that everything is set up, one can initialize and launch the simulation . It could be a good idea to launch starccm+ in background with a batch java function and the "nohup" shell command.

Create a Visualization

Now that the simulation is done, let us visualize the solution : To do so, create a scalar scene : right click on Scene $\rightarrow$ New $\rightarrow$ Scalar. With this scalar node "pressure", the pressure on the wind turbine is seen. Now, to see the wake on this same scene, the Turbulent viscosity ratio is used. Two choice are possible :

• add a TVR isosurface
• add a resampled volume (which is the choice done here)

The second solution will be explained here :

right click on Derived Part $\rightarrow$ Resampled Volume. The idea is to place this volume where the wake should actually be : after the wind turbine. Now in the same scene as before, in the scene/plot onglet ( ) Right click on Displayer $\rightarrow$ New Displayer $\rightarrow$ Scalar. Here, as a part select the resampled Volume and as a scalar the TVR. In the scalar field properties, disable the Auto Range. Add a solution time annotation to the scene by draging Solution Time to the scene : Tools $\rightarrow$ Annotation $\rightarrow$ Solution Time. (This way, it is possible to add logos to the scene as well. Create a new 2D image annotation).

Now, it is easily noticeable that only the solution at the last time is seen. To have access to the solutions stored in "load.simh" : Solution Histories $\rightarrow$ right click on load $\rightarrow$ Create recorded Solution View. In the node Solution View, drag "load" to the scene.Here it is possible to choose the State visualized.

StarCCM+ makes it possible to create a video *.avi of the solution :

Solution Views $\rightarrow$ Load $\rightarrow$ Animation $\rightarrow$ Solution time $\rightarrow$ Solution Time animation. In the Solution Time Animation Properties, choose the length of the video, the starting time and the ending time : In order to record the animation, click on record (the right button here ).

Then be careful to define the same animation length as before : Eventually :