Step 1 The Geometry

Basically, in 2D we are only working with surfaces and not solid bodies therefore the majority of the geometry will use sketches to create surfaces which ANSYS can then use. To start with there is a check list of everything that needs to be done during the modelling stage:

Determine the geometry of the airfoil/ hydrofoil to be used

Create a coordinate file of the profile

Create a new plane at the centre of the turbine

Import the airfoil line geometry (this will import at the origin)

Create a new plane and rotate 90 degrees in z direction using axis transform Create a circular pattern of the airfoil profile three times and rotate about the rotated planes new x axis.

Create a new sketch at the turbines centre plane and create a circle from the centre of the turbine to outside of the airfoil profile

Click generate and start a new sketch

Create a rectangle and make it big enough so that the walls will not interfere with the turbine

Create another circle exactly the same as the previous one ( a T should appear specifying tangency)

Click generate Generate one surface from edges for all three airfoils

Generate surfaces from sketches for other the two sketches

Create a Boolean and subtract airfoil surfaces from the circle surface

Almost all turbines use different airfoil/ hydrofoil profiles but the most common is the symmetrical NACA 4 digit series such as the naca0018. Using a profile generator, create a coordinate file with roughly 60-70 points; no more than this are necessary. Personally I used the NACA 4 digit series profile generator at http://www.ppart.de/aerodynamics/profiles/NACA4.html for symmetrical profiles and Airfoiltools http://airfoiltools.com/airfoil/naca4digit for unsymmetrical cambered profiles, both are very effective. Once a profile is selected youre going to need get it in the right format so that Designmodeler can read it. I suggest you run through this tutorial at http://www.mne.psu.edu/cimbala/Learning/ANSYS/Workbench_Tutorial_Airfoil.pdf made by Penn State University and offers an excellent tutorial over the basics of airfoil flow. Once you have your airfoil data file open ANSYS WORKBENCH and drag a fluid flow (Fluent) from the analysis systems toolbar into the workspace. Click on geometry and wait for Designmodeler to open.

Once Designmodeler has opened select the XY plane and then click on the new plane button on the top toolbar. Select from coordinates from the drop down type menu and enter the negative radius of your turbine for the y direction and the positive half chord length for the x direction, click generate. The reason we do this is I could only get ANSYS to import the airfoil at the origin and therefore I had to move the turbines origin to accommodate. Next select the newly created plane and click on new plane icon again, select rotate about x axis under the Transform 1 (RMB) heading and enter a value of 90 degrees, click generate. This will allow you to select the y axis when creating a pattern for the airfoils.Next go to concept>>3D curve and browse coordinates file for the airfoil coordinate file you created. This should import the airfoil line at the origin. We then create a pattern: Create>>Pattern and select the airfoil line and also change linear to circular. Select the axis of rotation to be the y axis we rotated by selecting that plane from the modelling tree and selecting the y axis (note if you have followed step by step this should already be active in the graphics window). Change instances to the number of blades required and click generate.

Once the array of blades has been created, create a new sketch in what should be plane 4 or the origin of the turbine. Sketch a circle from the origin roughly twice the diameter of the turbine (although not important it just has to be a little bigger to account for accuracy of sliding mesh). generate this sketch and create another sketch in the same manner this time drawing a large rectangle which will be the main fluid zone also create another circle the exact size as the first, note that when sketch a T should appear specifying tangency of the two circles. Click generate and then move to concept>> surface from edges. Select all three of the airfoils and click apply, then generate. The next step is to create the fluid surfaces so select concept>>surface from sketches, and select the first of the two sketches, change add material to add frozen. Click generate and repeat with the remaining sketch. With all surfaces created the next step is to remove the airfoils from the circle surface to effectively create a wall zone. Select Boolean from create>>Boolean and change type to subtract. Select the circle as the target and all of the airfoils as the tool bodies. With this completed you should have a circular surface domain with airfoils cut out all inside a larger rectangular surface domain if so then move to the next step if not start again or run through the steps individually checking if everything is correct. Save project and exitStep 2 - MeshingThe next step is the most critical and is what caused me the most problems however, it is also the simplest as I will tell you what to select and how to select them so you hopefully dont run into the same problems I faced. Update the project and double click the mesh cell in workbench. Once loaded the geometry should appear within the meshing utility. To zoom fit click the z axis in the lower right corner. Select the mesh from the utility tree and then select both the rectangle and the circle. Then on the top toolbar, select mesh control>>method. Change quadrilateral dominant to triangle and click generate repeat the but select mesh control>>refinement (this is not necessary but gives a much finer mesh) either keep at refinement 1 or change to 3 depending on required mesh densitUsing the selection tools (green faced cube icons) along the toolbar (namely vertex, edge, face and body) select the edge tool and then select the edge of the circle. This will bring up two layer boxes in the bottom left corner of the graphical window and means that at the point selected there is two edges which is correct, one for the rectangle and one for the circle. At this point using CTRL select both of these boxes (highlighted in red when selected) and go to mesh controls>>sizing. Change the type to number of divisions or if you prefer, specify the element length. Change the value to whatever is necessary. This will give a much neater mesh at the interface between the two zones. Once completed click mesh>> generate mesh and wait for your mesh to be displayed hopefully it should look neat. You may also specify edge sizings on the airfoil edges if necessary.The next step is to create named selections of each of the key points, these are: the inlet, outlet, hydrofoils, interface, rectangle surface and circle surface. Using the selection tools from the previous step select the body icon and select the rectangle domain, right click and name rectangle_surface, repeat with the circle domain using circle_surface for the name. Note that at this step it is important that the body select tool is used as fluent will not produce surfaces bodies. Then change to the edge selection and name the leftmost and rightmost vertical line inlet and outlet respectively. Zoom in on the turbine area containing the airfoils and select one of the airfoil edges. The two box symbol should reappear in the left corner and should be in two different colours these colours represent the surface on which the edges are located. Select the rectangle with the colour of the circle domain and then name selection airfoil or hydrofoil. Repeat with the two remaining airfoils and save. Lastly select the edge of the circle area, the two boxes in the left corner will reappear and its the one the same colour as the rectangle area you need; name this circle_edge. Hopefully if set up correctly the majority of the work is done however, this is where most of my problems occurred so if things dont work out in the next step either start again from scratch (sometimes easier and helps you speed up your Designmodeler skills) or follow the guide step by step checking for errors. Step 3 Fluent

Setting up fluent depends on what type of simulation you are running and therefore it is important that you understand the fluid dynamics of your problem. For the purpose of this tutorial, fluent will be set up for a hydro turbine in a free stream of 2.5m/s and a turbulent viscosity of 0.8. The pressure based solver will be used as I am assuming the flow to be incompressible due to the low speed. The transient solution will be used due to the simulation being time dependant.Before starting, I advise checking that the boundary condition zones match (i.e. zone type and number of zones not necessarily the names) with the bulleted list below if not a problem lies in the naming of the zones in ANSYS Meshing.

To start off, update the workbench file and open fluent. A start-up box will appear showing several options. I set mine to double precision, two dimensional, running 4 parallel processes. Running parallel processes greatly speeds up the solution time of the problem especially when used on a network though, depending on your license, this option may not be available. After clicking ok, fluent will load the geometry and then display it in the graphics window. Initially you can check the mesh and run quality checks to determine the characteristics of the mesh. In General, set the solver to pressure based and transient. Move to models and select viscous laminar, this will allow you to change the viscous model of the problem for this tutorial well use laminar. Select materials>>fluid>>air>>edit>>fluent database and scroll down to find H2O liquid. This will add water to the list of fluid materials. Next, move to cell zone conditions and click circle surface, here we will set up the sliding mesh behaviour of the turbine blades. Change material name to water and check the mesh motion tick box. Depending on your turbines diameter and half chord length used in the geometry enter these for the rotational axis origin, making sure the y-direction is negative. The turbine used is set to spin at 12.5 rad/s a TSR of 2.5. After that, click ok and move to rectangle surface changing only the material to water and leaving the rest the same. Move to boundary conditions and ensure the list of zones is as follows:

Circle_edge Hydrofoil1, 2 and 3

Inlet

Outlet

Interior Rectangle_surface

Interior - Circle_surface

Surface body - Rectangle surface

Surface body - Circle surface

If these zones are not present check the naming of the sections in ANSYS Meshing. When we named certain section it automatically assigned the correct zones to the sections however a few need to be changed for the interface. First, check each of the hydrofoils are walls, then move to inlet and edit. Change the velocity specification method to velocity components and magnitude and enter a value of 2.5m/s for the x-velocity, click ok. Next move to circle_edge and change the type to interface and do the same with surface body circle surface. Edit outlet and check if gauge pressure is zero. All the zones should be correctly set up, so now move to the interfaces options. Creating the interface allows fluent to treat the two areas (circle and rectangle) as an interface whereby the fluid is allowed to flow across the domains. First, select change/create and then enter a name for the interface in the mesh interface box. Select circle_edge in interface zone 1 and surface body circle surface for interface zone 2. As the turbine is periodic and the solution repeats after a given time during the quasi-steady state, the interface options should be defined as periodic boundary conditions. Next change the periodic boundary condition options from translational to rotational and uncheck the auto compute offset tick box. Click create and the interface should be now shown in the interfaces list. At this point it is possible to check whether or not the sliding mesh is set up correctly. In order to do this click preview mesh motion and enter a small time step such as 0.1 and number of time steps to 5. Click preview and if zoomed in on the turbine the turbine should be shown to rotate. Sometimes if the rotational origin is set up incorrectly this will show and the turbine will rotate off centre then an error will occur.After checking the mesh motion go to the reference values tab and review the information. The reference values used here are what ANSYS uses to calculate values such as moment coefficient lift and drag values etc. therefore you can set these to reflect the dimensions of your turbine. From experience I think length is the span of the turbine and for a 1m diameter this is roughly 0.68m. You can leave these as they are and just enter the set values in your equations later on. Compute the reference values from the inlet. Next move to solution controls and change the turbulent viscosity to 0.8.

In order to obtain accurate results we need to change the convergence criteria to 1e-5. Go to monitor>>residuals, print, plot and change each of the convergence check criteria to 1e-5. Next we can set ANSYS up to export the moment coefficient produced on each blade edit the moment option. Select each of the hydrofoils on the right hand pane and change the moment centre to the centre of the turbine. Check write to write a moment file (note if per zone is selected it will write/ plot the moment coefficient for each of the hydrofoil zones). You can also check plot so that the moment coefficient will be plotted in the graphics window whilst the solution is running however if you need to speed this is not necessary and slows the solution, you can also choose to not plot the residuals.In order to start the solution, the problem needs to be initialised so move to solution initialisation and compute from inlet. Then move to run calculations and enter a desired time step size and number of time steps. Baring in mind that initially the turbine is spinning the water in the circular domain, and therefore the number of time steps has to be enough for the domain to settle and the solution to reach a quasi-steady state. Enter the number of iterations required per time step; various cfdonline forums have tended toward 10-20 iterations however, do your experiment for time available and accuracy of results.

All settings are now complete so all that is left to do is click calculate. Calculations usually take up to 5 hours running on a quad core i5 machine, for a simulation time of 5 seconds, roughly anyhow.