Glider CFD analysis

CFD analysis of a glider

In this tutorial we are going to define a CFD simulation of a glider flying horizontally at a speed of 90 Km/h (55 mph). This tutorial is providing a step-by-step guide to the user to start using SimWorks. Please also find a detailed installation guide

Glider CFD analyis - simworks tutorial
Pressure field on the surface of a glider. Simulation carried out with SimWorks.

The glider CFD analysis shows in detail the pressure field on the glider surface and why this kind of aircraft is particularly efficient.

Please find a video tutorial below and follow the steps below to complete the CFD simulation of a glider.

Before starting the tutorial simply download SimWorks if you have not already and the tutorial geometry

Create new file

If the entities have not already been defined the first step is to create a new project (1), a new geometry (2) and new simulation (3):

  • Select the Geom → Click on the New simulation icon

In the Simulation manager window a new simulation will appear. Also the Simulation editor window will be populated with all the data relevant for the simulation in analysis.

Load the geometry

The next step is to select and load the relevant geometry in the Geometry viewer. Currently only .igs files are supported (more formats will be introduced in the next version 20.12). 

  1. Click on the Browse geometry icon in the simulation editor
  2. Select an .igs file from the navigation window
  3. The full address of the file will be displayed in the Geometry field in the simulation editor window
  4. Leave the Geometry units field selected to m (meters) and click on the Load geometry icon to actually load the geometry
  5. Confirm the simulation name in the input dialog box, in our case we can name it as “Glider”

Define the outer domain

Now that the geometry is loaded in the Geometry viewer an external outer domain is added automatically around the geometry. The outer domain (OD)  is essential in external aerodynamics simulations to correctly define the CFD problem and the relevant boundary conditions. The initial domain dimensions will be 3 times the max dimension in each direction of the imported geometry:

  1. Select the Regions tab in the Simulation editor and define the Box dimensions as 60 40 8
  2. Define the Origin as -12 0 0

This way the front face of the OD will be roughly 2 chords away from the geometry while leaving around 5 chords behind the geometry to correctly simulate the glider wake:

Base mesh parameters

From the Mesh tab in the Simulation editor window it is possible to define all the relevant parameters for the Mesh. The most important parameter is the base size which defines the biggest cell allowed dimension. The point in mesh defines which portion of volume will be actually meshed and has to be inside the OD but outside of the glider, its dimensions are defined only for visual purposes and will be visualised as a red point in the Geometry viewer.

  1. Define the base size as 0.5 which means a max cell dimension of 0.5 m
  2. Define the point in mesh location as 5 0 0 and its size as 0.5, the point will be shown in the Geometry viewer, verify that is inside the OD but outside of the geometry
  3. In the Surface mesh option 7 define both the Surface level and the Edge level as 5

The Surface mesh options define the different mesh groups of the simulation, which means geometrical assemblies which share the same mesh parameters. The first 6 define the properties of the OD faces, while the number 7 in this example defines the actual glider geometry. In the following tutorial we will see how to define new surface mesh groups and how to use one other very useful mesh feature called refinement boxes.

Boundary conditions definition

It is now time to define the actual boundary conditions, in this case we will define a velocity of 25m/s normal to the inlet face of the OD (corresponding to 90 km/h) and a constant pressure of 0 on the exit face. In this case the exit pressure will correspond to the ambient pressure. SimWorks implements a pre-processor to visualise the boundary conditions directly on the Geometry viewer, to access to it just click on the Layering menu which shows a number of possible visualisation layering modes, the one we are interested in for this tutorial is the Boundary conditions types

  1. Open the Layering menu from the Geometry viewer
  2. Select the Boundary conditions types visualisation

3. With the  Boundary conditions types visualisation mode active click on the front face of the OD a popup window will appear. From here it is possible to select Velocity normal as BC and type 25, this will define a speed of 25 m/s normal to the inlet face

4. Once those parameters are defined the simulation editor entries will be automatically updated with the values entered. It is also possible to modify the BC directly from this menu

5. At the same time the geometrical faces will be coloured with the relevant colour defined in the Geometry viewer displayed menu

6. Add the boundary condition for the exit as pressure outlet with value 0

Write the simulation dictionaries

Once all the parameters are defined it is time to export the simulation setup in the form of OpenFoam dictionaries which are ready to start the meshing and the running phases of the simulation. At the same time in the background the original geometry (igs) will be meshed to an STL file which will be used by the mesher, for this reason the operation can take a few minutes depending on the geometry complexity and the global mesh size. Please make sure that you double checked the max cell dimension since if too small this operation can freeze the code. We will leave all the setup parameters as std we will investigate the available options in the next tutorials.

  1. Write the dictionaries clicking on the Run setup icon in the Simulation manager window
  2. Depending on the setup this operation can take a few minutes, once completed the progress bar will show 100%

Mesh the geometry

From the Simulation manager window it is possible to mesh the geometry and track the progress of the meshing phase. Depending on the geometry the meshing phase can take some time, on an average machine completing the mesh of our tutorial will take 6-7 minutes.

  1. Click on the Mesh geometry icon
  2. The Live data window will then appear showing the mesher output, here it is possible to check in detail the mesh evolution
  3. For convenience also the progress bar in the Simulation manager window is progressively updated
  4. Once the meshing phase is completed with a right click on the simulation field in the Simulation manager window a customised menu will appear, selecting Fields → Load the mesh will be loaded in the post-processor

5. The Field viewer window will open showing the resulting mesh both on the part surface and on the coordinates planes X, Y and Z. To activate or deactivate the planes or the 3D surface just click on Show parts and Show sections

6. Visualise the X planes as an example 

7. Hide the Y and Z planes

8. Select the X direction and scroll through the planes with the arrows or press the arrow up and down on the keyboard to move through the planes

Run the simulation

Finally we are ready to run the simulation

  1. Click on the Run simulation icon in the Simulation manager window
  2. The Live data window will be populated with the solver feedback as it was done in the meshing phase. In this case also the Residuals and Forces tabs will be populated with the live results of the simulation
  3. Finally in the same manner as already done with the mesh, right click on the simulation and select Fields → Load to load the results in the Fields window

4. Now it is possible to select which results to show from the relevant dropdown menu, for instance let’s select p, which is the pressure. Also set the min and max values to -200 and 200 Pa and click on the Update legend button

5. Again it is possible to activate / move the sections planes as already done in the meshing phase to visualise the actual flow field around the glider

SimWorks - Glider surface pressure field

It is possible to analyse and output the full pressure (p in Pa) field and the velocity field (U in m/s) and all the velocity components Ux, Uy and Uz. A further tutorial will introduce the user to the definition and analysis of the Pressure Coefficient (Cp) and the Total Pressure Coefficient (Cp0).

It is also possible to open and analyse the CFD simulation results in much greater detail using Paraview which is a free and open source software by opening the .foam in the case directory. So in our case inside the glider folder there is a subfolder called case and here we can find the glider.foam file:

Congratulations your simulation is now complete! Please find additional info at the following links about the gliding flight and about the glider aerodynamics and compare the results of the previous exercise with the ones presented in those articles.

Glider geometry

It is possible to download the glider CAD at the beginning of this tutorial, please also see below the geometrical details of the analysed geometry: 

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