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TunnelSim Open Return Version 1.0g


This is a beta 1.0f version of the TunnelSim Open Return program. If you find errors in the program or would like to suggest improvements, please send an e-mail to

Due to IT security concerns, many users are currently experiencing problems running NASA Glenn educational applets. The applets are slowly being updated, but it is a lengthy process. If you are familiar with Java Runtime Environments (JRE), you may want to try downloading the applet and running it on an Integrated Development Environment (IDE) such as Netbeans or Eclipse. The following are tutorials for running Java applets on either IDE:

Other problems may occur while running the applet due to it being outdated. Until it is updated, HERE is another application that demonstrates the simulator.

This page contains an interactive Java applet to explore the design and operation of an open return wind tunnel. All of the information presented by the applet are available within the Beginner's Guide to Wind Tunnels. You should start with the slide describing the design of a wind tunnel.

TunnelSim - Open Return

With this software you can investigate the design and operation of an open return wind tunnel by changing the geometry and flow conditions in the tunnel.

There are two versions of TunnelSim; a version for open return tunnels and another version for closed return wind tunnels. This web page contains the on-line student version of the open return program. It includes an on-line user's manual which describes the various options available in the program and includes hyperlinks to pages in the Beginner's Guide to Wind Tunnels describing the math and science of wind tunnels. More experienced users can select a version of the program which does not include these instructions and loads faster on your computer. You can download these versions of the program to your computer by clicking on this yellow button:

Button to Download a Copy of the Program

With the downloaded version, you can run the program off-line and do not have to be connected to the Internet.


This program is designed to be interactive, so you have to work with the program. There are a variety of choices which you must make regarding the display of results by using a choice box. A choice box has a descriptive word displayed and an arrow at the right of the box. To make a choice, click on the arrow, hold down and drag to make your selection. The current values of the design variables are presented to you in boxes. By convention, a white box with black numbers is an input box and you can change the value of the number. A black box with yellow numbers is an output box and the value is computed by the program. To change the value in an input box, select the box by moving the cursor into the box and clicking the mouse, then backspace over the old number, enter a new number, then hit the Enter key on your keyboard. You must hit Enter to send the new value to the program. For most input variables you can also use a slider located next to the input box. Click on the slider bar, hold down and drag the slider bar to change values, or you can click on the arrows at either end of the slider. At any time, to return to the original default conditions, click the red Reset button at the upper right of the program.

If you see only a grey box at the top of this page, be sure that Java is enabled in your browser. If Java is enabled, and you are using the Windows XP operating system, you need to get a newer version of Java. Go to this link:, try the "Download It Now" button, and then select "Yes" when the download box from Sun pops up.

If you experience difficulties when using the sliders to change variables, simply click away from the slider and then back to it. If the arrows on the end of the sliders disappear, click in the areas where the left and right arrow images should appear, and they should reappear.


The program screen is divided into five main parts:

  1. On the top left side of the screen is the View Window. The view window includes a graphic of the wind tunnel that you are designing and a slider which control the graphic. Details of the window are given in the Graphics section of this page.
  2. On the upper right side of the screen is the Control Panel. The control panel holds several buttons which invoke the input panels located in the middle right. The open tunnel has a bellmouth, a test section and a diffuser. The selected section is colored yellow. You can also select the units to be used in the calculations by using the choice button.
  3. On the middle right side of the screen is the Input Panel. Various input panels are displayed in this window. You select the input panel by using the section buttons on the control panel. Details of the input variables are given below.
  4. On the lower right side of the screen is the Output Panel. The output can be presented as variable values in the test section of the wind tunnel. Details of the output variables are given below.
  5. On the lower left side of the screen is the Graphics Window. In the graphics window, you can display bar charts of the flow variables across the wind tunnel. Details of the flow variables are given in the output section.


The View Window contains a schematic drawing of the wind tunnel that you are designing and some controls for the schematic drawing. The walls of the tunnel are shown in blue while the selected section is colored yellow. There are two views of the tunnel, one from the top and one from the side. Flow through the tunnel goes from left to right. The borders of the various sections of the tunnel are denoted by eight, numbered white lines. Values of the flow variables at these locations are given in the bar graphs below the view window. You can move the picture within the window by moving the cursor into the window, holding down the left mouse button, and dragging the tunnel to a new location. You can also "Zoom" in close to the tunnel by using the green slider at the left of the window. If you get lost, pushing the yellow Find button will return the tunnel to the initial position.


There are two major outputs from the program. Conditions in the test section are given at the lower right and the variation of variables at eight stations through the tunnel are given by bar graphs at the lower left. You select the variable to output on the bar graphs by using the four labeled buttons above the bar graphs. The selcted variable is shown in yellow and the value of the variable is shown at the top of the bar.

  1. The default bar graph is the Velocity. The dimensions for the velocity are noted in the test section conditions at the lower right. The velocity output is limited to 350 ft/sec because of compressibility limits in the analysis.
  2. You may also display the Static Pressure through the tunnel. As with the velocity, the dimension of the variable is given at the lower right. The total pressure through the tunnel is a constant and equal to the room static pressure which, by default, is sea-level standard pressure (14.7 psi).
  3. You may select the Mach number variation through the tunnel. Mach number is dimensionless and limited to a maximum of nearly 0.3 .
  4. The final selection is the cross-sectional Area value at the eight stations shown in the view window. The dimensions for the area are the square of the linear dimension shown in the input panel (square feet or square meters).

The values of the flow variables in the test section are shown at the lower right. The velocity and static pressure are related through Bernoulli's equation because the analysis is one dimensional and no work is done on the flow through the duct. The Reynold's number and Mach number are calculated based on sea-level standard day conditions, the local flow velocity, and the diameter (height) of the test section. The airflow through the tunnel is the product of the velocity and cross-sectional area of the test section. With the air density assumed to be constant through the tunnel, the conservation of mass provides that the airflow is constant through the tunnel. The Diffuser Angle is the slope of the diffuser wall expressed in degrees. In the diffuser, the static pressure is increasing as one moves downstream. The adverse pressure gradient causes boundary layer separations in real diffusers if the diffuser angle is much greater than 7 degrees. In this analysis a warning light is displayed at the lower right whenever the diffuser angle exceeds 7 degrees. Another warning light is displayed if the Mach number in any section exceeds 0.3 because the analysis neglects compressibility effects.


The input variables are located on input panels that are displayed at the middle left. You can select the input section to display by using the buttons on the control panel. You can choose to vary the Bellmouth, the Test Section, or the Diffuser.

For any section, there are two groups of input variables; flow variables and geometric variables. The flow variables include the static pressure, the velocity and the airflow as described in the Output Variables of the previous paragraph. On the input panel, changing the value of any one of the flow variables will affect the value of the other flow variables. The geometric variables include the length of the section and the cross-sectional shape and dimensions. Using the choice box, you can choose either a square, circular, or rectangular cross-section. For the Bellmouth, the cross-section is specified at the entrance of the wind tunnel. For the Test Section, the cross-section is assumed to be constant from the end of the bellmouth to the beginning of the diffuser. For the Diffuser, the cross-section is specified at the exit. As you vary the size of the side, diameter, or width and height, the program calculates the area and uses the mass flow equation and Bernoulli's equation to determine the velocity and pressure.


The Education Programs Office will continue to improve and update TunnelSim based on user input. Changes from previous versions of the program include:

  1. On 16 July 09, version 1.0g was released. This version corrected some display problems that occured when changing flow conditions. A blinking banner was also introduced for the boundary layer separation and compressible flow errors.
  2. On 18 June 09, version 1.0f was released. This is the first released version of the program.

Guided Tours


Button to Display Wind Tunnel Index Button to Display Aerodynamics Index
Beginner's Guide to Wind Tunnels
Beginner's Guide to Aeronautics


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Editor: Nancy Hall
NASA Official: Nancy Hall
Last Updated: May 13 2021

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