MicroTunnel Tutorial
- The following tutorial is also available from the Help menu within the program. This edition is the most up-to-date and includes graphical examples. If you have any questions and need technical support, please contact us at the MicroCFD help desk.
How to set up a new test…
1. Select New Test from the File menu (if a previous test is loaded).
2. Set Tunnel Length, flow parameters, gas properties, simulation Time Step and Stop Time, or use the preset values. The Tunnel Length should be about five times the Model Length.
3. Select Load Shape from the File menu to load a basic shape, an airfoil, or a custom designed shape. Shapes that extend beyond the tunnel dimensions are automatically scaled and centered upon loading, but can be resized and repositioned afterwards.
4. The geometric coordinate system is located in the lower left corner, with the x-axis pointing to the right and the y-axis pointing up. The height of the tunnel is ¾ of its length. For example, if the Tunnel Length is set to 4.00 (m), the height would be 3.00 (m), and any shape whose coordinates fall within 0 ≤ x ≤ 4 and 0 ≤ y ≤ 3 can be loaded directly, without scaling.
5. A shape that is not scaled and centered upon loading cannot later be modified. Proceed with Set Shape from the Edit menu, then load another shape, or continue with step 8.
6. For scaled and centered shapes, one can select Modify Shape from the Edit menu to move, resize, or rotate the currently loaded shape. For large step increments use the Page Up / Page Down keys, to maximize or minimize a modifier use the Home / End keys.
7. The position of a scaled shape is now measured from the tunnel center to the center of the shape. The shape center point is the arithmetic mean of all its points. When finished, click Done and select Set Shape from the Edit menu.
8. Place a blue seed pixel within the interior of your set shape through “point & click”. Select Fill Shape from the Edit menu to fill the interior of the shape. Repeat if not completely filled.
9. To set up a multi-element configuration repeat steps 3 through 8. Although one can build a composite shape through merging of individual shapes, shape overlap should be avoided.
10. Select Create Boundary from the Edit menu. For multi-element configurations, all shapes have to be loaded, set, and filled before the boundaries can be created.
11. Select Stability Margin from the Flow menu and increase to Medium or High if the shape is blunt or the Flow Mach Number is below 0.3.
12. To create a boundary layer effect, select Surface Model from the Flow menu and check Rough. Otherwise leave the surface boundary Smooth (consistent with inviscid flow model).
13. For 2-D flow verify that Axisymmetry from the Flow menu is deselected. By checking Ground Effect, the tunnel bottom will act as a ground plane, which also disables axisymmetry.
14. Select Axisymmetry only for bodies of revolution. When checked, the tunnel bottom is the symmetry axis for the revolved shape. Axisymmetry selection disables ground effect.
15. Select Save Setup from the File menu to save your preliminary work.
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How to run a new or saved test…
1. Before running a test, select Automatically… from the Run menu and specify which tasks, if any, you want to be performed every time the graphics are updated (each Time Step):
• Check Save Slides if you only want to save the graphics and flow parameters.
• Check Save Frames if you want to save all data (equivalent to Save Test).
• Check Print Frames, if you want all output sent to a printer (set Landscape Orientation).
2. To start a newly set up test, simply click Start Test from the Run menu. A small window will pop up at the top of the screen displaying the current simulation Run Time and CPU Time.
3. To start a previously saved test, select Load Test from the File menu and select a test to be loaded (ensure that the filename carries the ".tst" extension if typed in manually).
4. If the test was completed (Run Time = Stop Time), select Continue Test from the Run menu, in order to increase the simulation Stop Time and to modify the Time Step. Select Start Test.
5. While a test is running, both the main window and the pop-up clock can be minimized to perform other tasks (a minimal performance sacrifice should be expected).
6. A test in progress can be stopped by selecting Suspend Test from the Run menu (wait for response). In order to continue, select Start Test from the Run menu again.
7. All graphics are updated every Time Step, and plotting itself takes several seconds. Different flow properties can be selected from the View menu.
8. When the computation is completed, select Save Test from the File menu to save the result. If steady state has not been reached yet (graphics are still changing), go back to step 4.
NOTE: Screen Savers use considerable CPU time while the program is computing and should be disabled! Select an automatic shut-off time for your monitor (15-30 minutes) or simply turn it off manually.
How to choose a proper Time Step and Stop Time…
1. The actual integration time step is computed internally based on the CFL condition of the flow and the selected Stability Margin in the Flow menu. Only the simulation time displayed in the pop-up clock advances in actual integration time steps.
2. The Time Step selected by the user should be a multiple of the actual integration time step. It specifies the time elapsed in between graphical updates of the flow.
3. A lower-bound estimate for a Time Step dt is one-tenth the ratio of Tunnel Length L and flow speed V, dt ~ 0.1 L / V, which is equivalent to the time it takes for the air to advance 10% through the tunnel.
4. The flow speed V is given by V = M·a, where M is the flow Mach number, and a is the free-stream speed of sound. For air at room temperature, k = 1.4, R = 287J/kg·K, and T = 288K, the free-stream speed of sound has the well-known value of a = SQR(k·R·T) = 340m/s.
5. For M = 0.9 and L = 4m, V = 306m/s, and dt ~ 0.0013s = 1.3ms, thus any Time Step between one and five milliseconds would be appropriate.
6. In the transonic regime (M = 0.9), steady state within the flow is usually reached after the tunnel has purged itself two to three times. A good initial estimate for a Stop Time would be T ~ 2·L / V, thus T ~ 0.025s in this case.
7. At low Mach numbers (M < 0.3), steady state is reached after the tunnel has purged itself once. For supersonic flow (M > 1.0), one to two times is generally sufficient.
How to interpret the results…
1. After a test is completed, the color map can be changed to a different flow property. Select Property from the View menu and choose Mach #, Density, Pressure, or Temperature from the submenu. Streamlines can also be plotted.
2. All color maps show a non-dimensional scale. The absolute values of local pressure, density, and temperature have been divided by their free stream or ambient values. To obtain the full range of absolute values, in their respective units, select Statistics from the View menu.
3. Also available in the Statistics window is data on lift, drag, and pitching moment. For 2-D flow, all forces and moments, whether dimensional or non-dimensional, are per unit depth. For example, lift and drag are given in units of force per unit depth (N/m).
4. All 2-D aerodynamic coefficients are based on the total length of the model. For a model length L and a free stream dynamic pressure q = ½ Rh·V2, lift, drag, and pitching moment are non-dimensionalized as follows: Clift = Lift / (q·L), Cdrag = Drag / (q·L), Cpitch = Pitch / (q·L2).
5. The pitching moment is computed with respect to the (2-D) aerodynamic coordinate system, located at the tunnel center. A positive pitching moment acts counterclockwise. The line of action of the resultant aerodynamic force is given by the equation, x·Clift - y·Cdrag = Cpitch.
6. For axisymmetric flow, lift and pitching moment are identically zero. The drag is 3-D and is non-dimensionalized by the free stream dynamic pressure, q = ½ Rh·V2, and by the frontal area of the model, A = pi·R2, where R is the model radius, measured from the symmetry axis.
How to create a custom shape file…
1. Shapes are saved in ANSI text format as a sequence of points connected in a closed loop. The following will describe how to create your own shape file using a simple text editor such as Microsoft Notepad. (A word processor in text mode is even more suitable, since it can also display hidden characters such as tabs and return keys).
2. After starting Microsoft Notepad, Select Open… from the File menu.
3. Change the current directory to …\MicroCFD\Shapes\Basic\…
4. Select Files of type: All Files (*.*) in the drop-down box.
5. Select the file Square.shp, and click Open.
6. You should see the following on your Notepad:
• The first row simply contains the string "shp", which is the shape file identifier.
• The second row should read "4", which is the number of points to follow, starting at 0.
• The next five rows contain the data points, with x and y values separated by tabs.
• Points 0 through 3 describe a square; point 4 is identical to point 0.
7. To define your own shape, simply modify the second row (N number of points), followed by the x and y coordinates of points 0 through N, with 0 and N being identical (closing the loop).
8. When done, select Save As… from the File menu and change the current directory back to …\MicroCFD\Shapes\Basic\…
9. Enter the file name Custom.shp. Select Save as type: Text Documents (*.txt) with Encoding: ANSI and Click Save. Older versions of Notepad have no encoding option, but always save in ANSI.
10. For multi-element configurations, separate shape files have to be created, which then have to be loaded one by one during setup.
NOTE: A decimal point must be represented by a period and not a comma. For example, one-tenth should be written as 0.1 and not 0,1. Otherwise your shape file may not load properly.
How to define custom colors…
1. Your screen should be set to a minimum resolution of 1024x768 pixels with a 24-bit color depth or higher (True Color). At a color depth of 24 bits, each RGB (Red, Green, Blue) component is represented by exactly one byte, which ensures proper color rendering of the default colors. At lower settings, some of the default colors can only be presented through dithering, a process of mixing pixels of different colors from a limited color palette (usually 8-bit).
2. The color plots in MicroTunnel will not display properly, if dithered colors are used. Either use a True Color setting in your screen properties, or change the colors to match the system palette. Another reason to change colors is to make them more distinguishable when printed. Sometimes two colors can be clearly differentiated on the screen, yet they may look almost identical on paper.
3. To change colors, select Custom… from the Color menu and adjust each color separately by modifying its RGB components. Gray scales can be created by using equal amounts of red, green, and blue in each color. When done, select Save Colors from the File menu to save your custom colors. Select Close from the File menu for the new colors to take effect. Closing the color window in its upper right corner will leave the current colors unchanged.
4. Although colors can be changed at any time, any previously saved slides remain fixed in their color composition and when reloaded will always display their original colors.
5. MicroTunnel will always start up with its default colors, and custom colors that were saved have to be reloaded each time the application is run.
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