Goals
The purpose of this tutorial is to introduce the user to modelling flow in high speed external aerodynamic applications.
In this case the flow over a NACA 0012 airfoil at an angle of attach of 1.49° will be simulated and the lift and drag values will be compared to published results. These results were taken with a Reynolds number of 9x106 and a chord length of 1m.*
The airfoil is travelling at Mach 0.7 so the simulation will need to account for compressibility as well as turbulence effects.
To reduce the computational cost, the mesh will be made up of a 2D slice through the airfoil (one element thick).
* NASA TM 81927 Two-Dimensional Aerodynamic Characteristics of the NACA 0012 Airfoil in the Langley 8-Foot Transonic Pressure Tunnel 1981. Harris, C. D.
Start a Workbench project
1.Launch Workbench
2.Save the new project as naca0012 in your working directory
3.Drag a Fluid Flow (CFX) module from the Analysis Systems section of the Toolbox onto the Project Schematic
4.In the Project Schematic right-click on the Mesh cell and select Import Mesh File
5.Set the file filter to FLUENT Files and select NACA0012.msh
-With the mesh file imported the Geometry cell will not be needed so it is removed from the Fluid Flow module.
Note that you could have dragged Component System > CFX onto the Project Schematic, as in the first workshop. The mesh would then be imported after starting CFX-Pre.
Mesh Modification
1.Open CFX-pre by right-clicking on the Setup cell and selecting Edit
-After CFX-Pre has opened the mesh can be examined and it is clear that the scale is incorrect as the airfoil chord is 1000 m rather than 1 m, indicating the mesh was built in mm rather than m. This can be fixed using the mesh transformation options.
2.Right-click on Mesh in the Outline tree and select Transform Mesh
3.Change the Transformation to Scale
4.Leave the method to Uniform and enter a Uniform Scale of 0.001
5.Click OK
6.Select the Fit View icon
-Zoom in further to see the airfoil
The mesh has been built to have a single boundary around the entire outer edge. This needs to be split into inlet and outlet regions. While it is better to create the correct mesh regions when generating the mesh, CFX-Pre can be used to modify the mesh regions.
1.Right-click Mesh in the Outline tree and select Insert > Primitive Region
2.Click on the Start Picking button
3.From the drop down selection menu select Flood Select (see image to the right)
4.In the viewer select any element from the front curved boundary
-The flood fill will select all cells where the change in angle is less than 30°
5.Click in the Move Faces To field and type Inlet
6.Click OK
The remaining section will now be renamed “Outlet”.
1.Expand the Mesh section of the tree so the list of Principal 2D Regions is visible. Note that this list now contains the location Inlet
2.Click on the region pressure far field 1 to confirm it is the region representing the outlet
-It will be highlighted in the Viewer
3.Right-click on pressure far field 1 and select Rename. Change the name to Outlet
Domain Setup
Usually the option to automatically generate domains is active, this can be checked by editing Case Options > General in the Outline tree.
1.Check that Automatic Default Domain is active the click OK.
2.Right-click on Default Domain in the Outline tree and rename it to Fluid
3.Double-click on Fluid to edit the domain settings
This case involves high speed aerodynamics so it is important to include compressibility.
It is important to set the correct operating pressure so that the intended Reynolds number is achieved. The simulation will take place at 288 [K] in air; this allows the speed of sound to be calculated. This can then be converted into a free-stream velocity using the Mach number. Using the definition for Reynolds number the fluid density can be obtained, which can then be used to determine the operating pressure for the simulation, assuming an ideal gas.
In the Fluid domain Basic Settings tab:
1.Set the Material to Air Ideal Gas
2.Set the Reference Pressure to 56867 [Pa]
-Make sure you change set the units
3.Move to the Fluid Models tab
4.Set the Heat Transfer Option to Total Energy
-This is required for compressible simulations
5.Enable Incl. Viscous Work Term
-This includes viscous heating effects
6.Set the Turbulence Option to Shear Stress Transport
7.Click OK
Boundary Conditions
An outlet relative pressure of 0 [Pa] will now be applied. This pressure is relative to the operating pressure of 56867 [Pa].
Absolute Pressure = Reference Pressure + Relative Pressure
1.Right-click on the domain Fluid in the Outline tree and select Insert > Boundary, naming the boundary Outlet
2.Change the Boundary Type to Outlet and check that the location is set to Outlet
3.Move to the Boundary Details tab and set the Mass and Momentum option to Average Static Pressure with a value of 0 [Pa]
4.Click OK
The sides of the domain will use symmetry conditions since this is a 2D simulation.
1.Insert a Symmetry boundary called Sym Left, at the location sym left
2.Insert a Symmetry boundary called Sym Right, at the location sym right
http://www.cadfamily.com/html/Article/Transonic%20Flow%20Over%20a%20NACA%200012%20Airfoil_778_1.htm
http://www.cadfamily.com/html/Article/Transonic%20Flow%20Over%20a%20NACA%200012%20Airfoil_778_2.htm
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