Helyx-3Dwing Activity PDF

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3D UAV WING ACTIVITY: HELYX & OPENFOAM M10AEE – Computational Fluid Dynamics Dr H. Medina

1 INTRODUCTION The purpose of this activity is to both introduce the functionally to Helyx as a support tool for the setup of OpenFOAM simulations and to explore the application of Computational Fluid Dynamics (CFD) to 3D geometries. For this activity, a 3D simulation of the flow over a simple UAV wing will be used as an example. It is suggested that you work in groups consisting of 2 members.

In the next sections, the general steps needed in order to successfully set up this simulation will be briefly described. These steps start with the choice of geometry file, and include: mesh generation, assignment of boundary conditions, selection of appropriate solver settings, running the simulation and post-processing using the parallel-capable visualisation software ParaView Notes:   

For this activity you will need to have Helyx-OS installed in your PC in addition to OpenFOAM (with paraFoam complied or using the ParaView binaries available from kitware’s website) This activity requires you to have access (and read) the OpenFOAM user guide The activity will be supported with in-class explanations/demonstration i.e. this guide is not all-inclusive

2 ACTIVITY STEPS 2.1 Geometry The geometry to be used consist of a symmetric 3D UAV wing. The geometry has already been cleaned for it to be used with OpenFOAM i.e. the CAD file has been checked to ensure that small features are removed and that the resulting .stl file is closed. The cleaned .stl surface that will be used as the basis for this activity is available for download from Moodle (wing_sharp_TE-ASCII.stl).

2.2 Mesh Generation (snappyHexMesh utility operated within Helyx) Domain Definition In order to generate the mesh, launch Helyx-OS (if you have not already done so). Under the “base mesh” menu generate a custom box (or domain) with the following properties:

Min Max Elements

-0.5 2.0 60

-0.5 0.5 25

0.0 1.0 25

Table 1 Domain box definition

Importing the geometry and selecting feature edges Now, we can use the “Geometry” section to import the .stl file containing our UAV wing geometry definition. Simply, click on “STL” and select the file wing_sharp_TE-ASCII.stl. SnappyHexMesh has the ability to allow the user to define where sharp edges in the geometry are located to enable the mesh generation algorithm to capture these features. A feature edges file has already been prepared using ParaView (and demonstrated during the lecture). Click the “lines” option and select the featureEdges.vtk file which contains information about the sharp edges in the wing geometry.

Create a refinement box (around UAV wing) In order to improve the accuracy of our CFD simulation at a reduced cost, it is necessary to create a refinement region around the UAV wing. For this purpose, a box-shaped refinement region will now be generated. Still within the “Geometry” module, click on the “Box” button. Create a box with the following properties:

Min Max Level Mode

-0.25 0.75

-0.2 0.2 2 inside

0 0.5

Table 2 Refinement box properties

Create a refinement box (wing tip) Similarly, due to the increased property gradients expected near the tip of the UAV wing. It is recommended to also have local refinement in this region. Using the same process used for creating the previous refinement box, create a refinement box near the wing tip. Use the following properties for the box:

Min Max Level Mode

-0.05 0.75

-0.04 0.065 4 inside

0.35 0.45

Table 3 Wing-tip refinement box properties

Select the coordinates of the Material Point The snappyHexMesh utility requires a material point to be defined. The material point is a point in the geometry where cells are to be kept i.e. a point inside the domain. Notice that the material point

must fall within a cell (not a cell face) of the algorithm will fail to compute. Use the following material point coordinates: Material (x y z)

Point

-0.0552

0

0

Table 4 Material Point coordinates

2.3 Boundary Conditions On the patches or boundary tab, the boundary conditions can be selected. By now you should be familiar with how boundary conditions are deduced at outlets, inlets and walls. For simplicity, the inlet boundary conditions can be set as: (assuming we will use a k- ε model)

Property Value

U (m/s) 0.75

k ε (m2/s2) (m2/s3) 0.065 0.45

Table 5 Inlet boundary conditions

2.4 Solver Settings Using the solver setting tab, select a steady-state simulation for incompressible flow (is this assumption correct? Remember to ensure your simulation settings matches the flow physics). This will ask OpenFOAM to solve our simulation using the SIMPLE algorithm to solve the RANS equations in steady-state. Select the RNG k- ε model for this exercise. However, you are encouraged to test other models as a home exercise to gain familiarity with other turbulence models.

2.5 Running the Simulation Finally, you can set up the run-time and writing option in Helyx. Feel free to decide what parameters you want to employ for your simulation. At this stage, it is recommended to use the default relaxation factors and to use upwind schemes to ensure stability at the cost of accuracy.

2.6 Post-Processing (ParaView) As demonstrated in our lecture, ParaView is a very powerful post-processing tool. Although, the use of this software is not within the scope of this activity, you are strongly encouraged to explore the features available in ParaView as a home exercise (ideally using a less computationally demanding simulation that you can develop yourself e.g. why not try to simulate the flow over a simple cylinder?)...