Symscape

Simulate the flow of air over a NACA 4415 airfoil. Compare the simulation results with experimental data.

Goals

In this tutorial, you will learn how to:

• Import a file into SymLab
• Extrude an airfoil to create a wing
• Specify physical conditions for a simulation. In this case, you will specify the fluid (as air) around the wing, the velocity of the air flowing over the wing, and the properties of the wing's outer surfaces.
• Attach a wake to the wing
• Specify the accuracy of the results you require
• Generate a color map (contours)
• Create an XY plot
• Compare the results of your simulation with experimental data

Assumptions

1. You have activated the SymLab Builder and SymLab Panel Flow add-ons, or SymLab Professional.
2. You are familiar with SymLab essentials.
3. You have downloaded the naca-4415-airfoil.brep and the cp_0_15.csv files.

Import the Brep File into SymLab

In the Menu Bar, select File->Import.... Select Geometry as the file type and navigate to the location of the naca-4415-airfoil.brep file you downloaded. Double-click on naca-4415-airfoil.brep to import the NACA 4415 airfoil into SymLab.

To see the airfoil, rotate the view (left-click-and-drag the mouse in the View Window) until the airfoil is in the approximate orientation shown below.

Create a 3D Wing

To create a wing from the airfoil, you will extrude the airfoil in the z-direction. However, to cluster mesh points around the leading edge of the wing, you will first scale the airfoil in the y-direction. Then, after the extrusion, you will rescale the wing in the y-direction to restore the original airfoil profile.

You will scale the airfoil relative to the origin. By default, transforms are performed relative to the midpoint of the bounding box that contains the geometry, not relative to the origin. To scale the airfoil relative to the origin, you will first need to create axes at the origin. In this tutorial, the origin is 1/4 of the chord length (0.25m) along the airfoil.

To create the axes at the origin, select the Geometry Tool Palette. Select the Others->Axes tool. Drag and drop the Others->Axes tool onto the View Window. Double-click sim in the Select dialog and select Done to create axes.

You may need to scroll down through the Geometry Tool Palette list to find the Axes tool.

To scale the airfoil in the y-direction (to enhance mesh resolution around the leading edge of the airfoil), select the Transforms->Scale tool in the Geometry Tool Palette and set the parameters in the Properties Panel to [1 2 1]. Press Enter on the keyboard to apply the changes to the Properties Panel.

Drag and drop the Transforms->Scale tool onto the axes you created in the View Window. Select Set Axes. Right-click on an edge of the airfoil in the View Window and select Done to scale the airfoil in the y-direction.

To extrude the profile of the NACA 4415 airfoil to create a 3D wing, select the Others->Extrude tool. In the Properties Panel, check that the Direction is set to +Z and the Length is set to 5m. Drag and drop the Others->Extrude tool onto an edge of the airfoil in the View Window. Double-click face in the Select dialog and select Done to extrude the airfoil in the z-direction. Click on the Zoom , Spin , and/or Pan icons in the toolbar and then left-click-and-drag the mouse in the View Window to see the whole wing.

To restore the original profile of the airfoil, select the Transforms->Scale tool, set the parameters in the Properties Panel to [1 0.5 1], and press Enter. Drag and drop the Transforms->Scale tool onto the axes in the View Window, and select Set Axes. Right-click on an edge of the airfoil in the View Window and select Done to scale the airfoil in the y-direction.

Specify the Properties for the Simulation

Select the Physics Tool Palette. Drag and drop the Substances->Air tool onto the background of the View Window. Double-click sim in the Select dialog and select Done to set air as the fluid around the wing.

Drag and drop the Conditions->Linear Velocity tool onto the background of the View Window. Double-click sim in the Select dialog and select Done to set a velocity of air around the wing.

To set the value of the linear velocity of the air, right click on the View Window, double click on sim, and select Properties. In the Properties Panel, select the Simulation tab . Expand the Physics: Linear Velocity property (click on the + to the left of Physics: Linear Velocity), and then expand Velocity. Set the velocity of air in the X direction to be 10m/s and press Enter on the keyboard.

Define the Symmetry Plane for the Wing

The experimental results that you will compare with your simulation results at the end of this tutorial are for a 2D airfoil. To better simulate a 2D airfoil, you can use a symmetry condition to double the span of the wing. This allows you to get results faster and using less computing resources compared to simulating a longer wing without symmetry.

Drag and drop the Conditions->Symmetry tool onto the background of the View Window. Select Done to specify that your simulation contains a symmetry plane.

To specify the actual symmetry plane for your simulation (the original airfoil section), return to the Properties Panel, and click on the Simulation tab. Expand the Physics: Symmetry property and check that Planes is set to Z.

Set the Properties of the Outer Surface of the Wing

Drag and drop the Conditions->Wall tool onto the leading edge of the wing. Double-click face_2 (or whichever face is first in the list) in the Select dialog. To select the 2nd face, select Select/Deselect and then right-click on the leading edge. Double-click face_1 (or whichever face is second in the list) in the Select dialog. Select Group to create a group of the upper and lower faces of the wing, and then select Done to create walls on the upper and lower surfaces of the wing.

A wall is a solid surface through which fluid cannot flow.

Note that the original airfoil section is set to be a symmetry plane. Also note that, in this tutorial, we are not interested in the details of the simulation at the wing tip, hence we will leave it open so as to simplify the calculation.

Select the Group tab in the Properties Panel, change the Name of the group to Wing, and press Enter.

Attach a Wake onto the Trailing Edge of the Wing

To simulate lift using a panel method, you must identify where the flow leaves the trailing edge of the wing by applying a wake model.

Drag and drop the Conditions->Wake tool onto the trailing edge of the wing. In the Select dialog, double-click on edge_3 (or whichever edge is the trailing edge in your example) and select Done to add the wake.

Specify the Accuracy You Require for Your Results

Drag and drop the Conditions->Accuracy tool onto the leading edge of the wing. Double-click Wing in the Select dialog and select Done to set the accuracy of the calculation on the upper and lower faces of the wing.

To set the value of the accuracy, select the Group tab in the Properties Panel, and select High for the Accuracy.

Calculate the Pressure Coefficient on the Wing

Select the Results Tool Palette. Drag and drop the Scalar Fields->Pressure Coefficient tool onto the leading edge of the wing in the View Window. Double-click Wing in the Select dialog and select Color Map to create contours of pressure coefficient on the wing.

The pressure coefficient is a dimensionless number that relates pressure changes to the surrounding field pressure.

Symlab will perform the simulation. When the calculation is complete, you will see the wake appear in the View Window.

Use the Zoom, Spin, and/or Pan functions in the toolbar to see the full wake.

Ideally, the wake should extend 10 or more chord lengths behind the wing. SymLab allows you to specify both the number of wake elements and the time step for the elements in order to vary the total length of the wake. To specify the number of elements and the time step, right-click on the View Window, double-click on sim in the Select dialog, and select Properties. In the Properties Panel, select the Simulation tab, then expand the Substance: Air, Solver: Panel Flow, and Wake properties. To increase the number of wake elements, enter a larger number for Iteration. A larger number of elements will provide more accuracy for your simulation, but the simulation will take longer to perform. Time is the time step per wake element, i.e., the time between the start of each new row of elements.

Plot Pressure Coefficient on the Airfoil Versus Distance Along the Chord

The leading edge of the airfoil does not correspond to the origin. In fact, 1/4 of the chord is located in the negative x-direction. To plot the pressure coefficient versus distance along the chord, you need to take the following steps:

1. Create a scalar called 1/4 Chord (0.25m).
2. Add 1/4 Chord to the x-position value to find the distance along the chord.
3. Create an XY plot of pressure coefficient versus distance along the chord.

Create a Scalar Called 1/4 Chord (0.25m)

In the toolbar, click on the New icon and select Result.

In the Create Result dialog, select the Constant tab. For Units, select Length.

Click Create to create a scalar.

In the Results Tool Palette, select Scalar Variables->Scalar (the scalar variable you just created). Click on it again to make its name editable, and change Scalar to 1/4 Chord.

In the Properties Panel, set the Value of 1/4 Chord to 0.25m and press Enter on the keyboard.

Create Chord

In the Create Result dialog, select the Binary tab. Select Add from the list.

In the Results tool palette, select Vector Fields->Position. Verify that X is selected as the Scalar in the Properties Panel. Drag and drop Vector Fields->Position onto the left-hand target in the Create Result dialog. Select Scalar to specify the x-position as the left-hand variable for the equation.

Drag Scalar Variables->1/4 Chord from the Results tool palette and drop it onto the right-hand target in the Create Result dialog.

Click Create to create the scalar field chord. In the Results tool palette, select Scalar Fields->(Position:X + 1/4 Chord), rename it Chord, and press Enter.

Close the Create Result panel.

Create an XY Plot of Pressure Coefficient Versus Distance Along the Chord

Drag and drop the Scalar Fields->Pressure Coefficient tool onto the original airfoil section in the View Window. Select face3->Edges (or whichever face is the original airfoil section in your example) in the Select dialog (the edges of the face will be highlighted in the View Window), click OK, and select XY Plot to create an XY plot of pressure coefficient versus x-position.

To plot the pressure coefficient versus distance along the chord, drag and drop Scalar Fields->Chord onto the background of the Pressure Coefficient Plot View Window. Select X Axis to display values of distance along the chord along the x-axis of the XY plot.

Compare Experimental Data and SymLab Simulation Pressure Coefficients

To compare the pressure coefficient values from your SymLab simulation with the experimental values, you need to plot both data sets on the same XY plot.

Import the Experimental Pressure Coefficient Data

In the menubar, select File->Import.... Select the location of the plot data you saved at the beginning of this tutorial. For the file type, select Plot Series (*.csv). Double-click on cp_0_15.csv to import it into SymLab.

The plot series will be visible in the Results tool palette, under Imported.

Plot the Imported Data and the SymLab Simulation Data on the Same XY Plot

Select the Pressure Coefficient Plot View Window. Drag and drop the Imported->cp_0_15.csv tool onto the View Window and select Done to plot the experimental data on the same XY plot as the simulation data.

Click on the View Window to select it. In the Properties Panel, expand the Y Axis property. Check the Reverse checkbox and click anywhere in the View Window to reverse the direction of the y-axis.

In the View Window, select ExpLower and ExpUpper at the bottom of the window. (First, click on ExpLower, then hold down the Control (or ctrl) key on your keyboard and select ExpUpper. Both words should now be highlighted.) In the Properties Panel, select None for the Line type, and select Circle Open for the Symbol.

In the View Window, select the two words that say "edge" at the bottom of the View Window (edge_4 and edge_6 in this example) so that both words are highlighted. In the Properties Panel, select None for the Symbol.

Notice the good agreement between the computational and experimental data.

For a more detailed study of this airfoil, see our NACA 4415 Airfoil Calculation.