Physics Setup

Launch Fluent

To launch FLUENT, double click Setup under Project Schematic.

Check the option "Double Precision". This will make the simulation more accurate, since Fluent will use twice more bits in the calculations.

You can leave the rest as Default. Now press OK to launch FLUENT.

The FLUENT window layout is basically divided into 4 zones:

1) The left most column contain all the possible operations that one may want to perform to setup a simulation, like setting the boundary conditions, selecting the fluid used, and which simplifications to make.

We usually go from the first "General", one by one, until "Run Calculation". However in this tutorial we will have to break a little bit this symmetry to specify what is called "Interface Zones" (not (yet) shown in the list of options).

Here are just the main options; the actual "knob tweak" is made right next to it. Note that once you click in one item from the menu, a grey column appear right next to it showing more detailed options.

2) Sub-options for each main option selected from the left most column. Here we actually modify the way Fluent will behave.

3) The graphical window, showing at first launch the mesh. You can navigate on that using the left mouse button to Pan the mesh.

Drawing a box using the middle mouse button from left to right (either top or bottom) will zoom in, while drawing a box from right to left will zoom out.

The right mouse button will not often be used for our relative simple simulations. It displays the "surface group" the "id" and the "name" of the surface containing the point you clicked.

4) A mesh box where Fluent will print its operations, messages, warnings, iterations, results (as requested), etc.

Enough of talking! Let's start the CFD Simulation!

Check Mesh

It is always a good practice to check mesh, specially if you are importing mesh that wasn't created by you. It is not frequent to get matching errors, particularly when meshes with multiple Cell-Zones like our case.

In order to do that, you can click on Mesh > Check under the upper tab with options, next to File.

You can also note that, when Fluent is first launched, the second menu box (described previously) contains the options for "General". There, on the first option you can see under "Mesh", the option for Check. You can see this highlighted in the above figure.

Lastly, you can type "mesh/check" and hit Enter in the messages box (number 4 from previous description).

Create mesh interfaces

As briefly discussed, we need to start by creating the interfaces between the different zones of mesh.

To account for the rotation of the mesh, we couldn't simply have created one single mesh. Instead, we had to create sub regions, or Zones of meshing.

The mesh created is "non-conformal", i.e. the nodes does not match across the interface), so we need to tell Fluent that the adjacent cell across that interface share information.

To make the "Interfaces" option appear, first go to "Boundary Conditions". Select the first Zone on the list, "blade_bot_in" and change the type to "Interface". Click OK the popped window to keep the default name.

Note that "Mesh Interfaces" option appear on the main menu to the left.

Go ahead and set the other interfaces Boundary Conditions. There is no quick way to do that, as Fluent does not allow you to Copy Interfaces Boundary Conditions. So, proceed as before and assign the following Zones to Interface:

blade_bot_in

blade_bot_outer

blade_right_in

blade_right_outer

blade_top_in

blade_top_outer

hub_inner

hub_outer

Now we're ready to finally create the interfaces!

In our case, each pair of Interface Boundary Condition will be an Interface Zone. So there will be 4 zones in total.

Highlight "Mesh Interfaces" and click "Create/Edit..."

In the first box, "Mesh Interface", write "blade_bot". That's the name of the interface. No select as Interface Zone 1, blade_bot_in. For Interface Zone 2 select blade_bot_outer. Click "Create". The window will not close by it self, go ahead and close it.

Repeat this and create the remaining 3 interface zones.

Be careful to select the correct in-outer pairs!!

Note: you don't have to close the Mesh Interface window all the time. But be aware that after selecting the first zone, a warning will pop saying that the second zone is already assigned to other interface. Just close the message and select the correct Interface Zone 2.

By the end of this step, you window should look like this.

Set Solver informations

Now, resuming...

Select the first option under "Setup": "General".

Here we can change key information about the solver, like whether it's steady or transient, planar or axisymmetric, etc.

For our problem, you can leave the default options, i.e., Pressure-Based, Steady and Planar.

Lastly, click in "Units" and change the "angular-velocity" unit to rpm.

Set the Model

Here is where we tell FLUENT all simplifications for the model it can assume. For instance, here you specify if the model is Inviscid, Laminar or Turbulent, if you should consider the Energy Equation (for supersonic flows), and other options.

For us, we are considering turbulence, and we will use the k-epsilon Realizable model.

Highlight "Model" and double click the third item in the list, "Viscous - Laminar". Select "k-epsilon (2 eqn)". Change "k-epsilon Model" to Realizable. Retain the rest as default. Click Ok.

Materials

We will use the default properties for air. Go ahead and check if they are correct.

Highlight "Materials" and double click "air" under "Fluid". 

Density: 1.225 kg/m3

Viscosity: 1.7894e-05 kg/m-s

Cell Zone Conditions

Here we specify to fluent the material of each meshed zone (usually correct by default, unless we create a new material).

Also, we set if we're solving a moving mesh problem or moving frame of reference problem (our case!). We can also set here the centroid of each zone and the angular velocity of that zone.

Highlight "Cell Zone Conditions". Note that our problem is made out of 5 zones:

• blade_bot: corresponds to the mesh around the bottom blade
• blade_right: similar as above, for the right blade
• blade_top: again, for the top blade
• fluid-surface_body: corresponds to the big circular mesh around the main geometry of the turbine.
• inner: corresponds to the zone between the blades, inside the hub.

These are the same zones described previously, when creating the mesh interfaces. We will have to edit parameters for each of them.

fluid-surface_body

Highlight fluid-surface_body and click "Edit...".

No options should be selected here, and the material must be set to air. This is the default. You can verify that and click Ok.

inner

Highlight inner and click "Edit...". Here we will set that the inner portion of the hub is spinning, adding more terms to the equations to account for local acceleration, even though the mesh is not actually moving.

Right under "Material Name", select the first option "Frame Motion". We want to specify the angular velocity and the origin about which the mesh turning.

From the geometry, the centroid is the global origin (0,0). Verify that that's inputted.

From the problem statement, the turbine is spinning at 40rpm, so go ahead and input 40 to "Rotational Velocity". This is the absolute velocity. Note that "absolute" is selected under "Relative Specification". 

The rest you can keep as default. Click Ok.

FIGURE 11

blades

Highlight blade_top and click "Edit...". 

Again, select "Frame Motion" option. Now, in order to make the blades rotate and keep the same relative position to the hub, select the "inner" Cell Zone under "Relative Specification". The relative velocity will be zero.

We need to do some basic geometry to find the rotating center.

MAYBE it's to big? Video...? 

ADD MATCH CONTROL TO MESH? (see wind blade tutorial) 
Import mesh as a wbpz file instead o msh? 

To explain a little bit about the MFR method, see Numerical Results part, where I wrote something very short.

Cell zone conditions:

name zones, centroid, angular velocity. do for top blade

make table with values for other blades

Boundary Conditions:

-farfield 1: velocity inlet....

-farfield 2: pressure outlet...

etc

Go to Step 5: Numerical Solution

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