Digital Wind Tunnel


Personal Simulation Works

Postmarc is a graphic postprocessor for Cmarc and Pmarc-12 output files. It runs under Windows and provides a simple, intuitive, and self-documenting user interface. Explanation of Postmarc View Options Postmarc provides rotatable and zoomable views of the body and wake, with color mappings of pressure, velocity, and Mach number; vector arrows representing local velocities; pressure distributions along any cross-section; and on- and off-body streamlines and wakes. On-body streamline display includes boundary layer analysis. Data may be displayed for a single output file or for the difference between two output files for the same body (for example with different angles of attack or dimensions). Rectangular and cylindrical scan volumes can be displayed in several ways, including spectrum-plotted planes cutting through the flow field at multiple locations. Color images may be printed on compatible color printers. Although Postmarc was written to be intuitive, some function keys and arrow keys are used for model rotations and framing.



Wake Display Both the initial geometry and animation of time-stepped wakes may be displayed.


wakantn.gif This example shows the wake curl up off a wing tip at 5 degrees angle of attack in a GIF89A animation.




Contouring control

Contouring control The first button in the contouring group is the contouring/display setup form. In this form the user sets options pertaining to the display.

To begin with, the user can set the number of colors to be used. The maximum number is 256, but large palettes may slow display generation on some systems. Generally, between 20 and 50 colors provide a good compromise between detail and speed.

The spectrum range may be set by the user or automatically. The useful range of pressures is from about -1.5 to 1.0, though the band from -0.4 to 0.4 may be of greater practical interest, and is the default. The useful range of velocities is from 0 to 1.5. Mach number depends on the flight speed of the model and the value for the speed of sound provided in the input deck.

Automatic selection is useful only if the input data are free of spurious highs and lows. This is seldom the case. It is handy, however, for finding the desired range, for instance when an actual velocity, rather than 1.0, has been entered for VINF in the Cmarc input file.

Spectrum plots may be smoothly contoured ("Fringe contours" on) or each panel may be solidly colored. Fringe contours process more slowly, but produce a more realistic and intuitively meaningful display.

Panel edges may be displayed or omitted.

Symmetrical half-models can be mirrored about any of the three orthogonal planes. Display time increases for full models, since the panel count is doubled. The reflection plane of interest is usually the XZ.

Cp, pressure coefficient

Cp, Pressure coefficient The spectrum plot option depicts pressure distribution on the surface of the body.


Surface flow vectors

Surface flow vectors The vector ("XYZ") button causes the surface flow velocity for each panel to be displayed as an arrow on a hidden-line depiction of the body. The direction of the arrow indicates flow direction and its color represents the speed of the flow. The length of an arrow does not represent the magnitude of velocity; arrow are sized to fit within their panels.

To enhance the visibility of arrows, the color of the model can be set by selecting View > Solid color from the pull-down menus. RGB values close to zero make a dark or black model, ones close to 255 make a light model.

Pressure Distributions

Pressure distributions Once a color "spectrum plot" has been drawn, the user may "drag" a line anywhere along an orthogonal view of the geometry and a 2D graph will be generated of the Cp (pressure coefficient), Vm (velocity magnitude) or M (mach number), whichever was contoured, on both near and far surfaces along the length of the line. Jaggedness in plots reflects the discontinuous nature of the data, which is due to the paneling.

On-body streamlines and boundary layer analysis

On-body streamlines and Boundary Layer Analysis The paths of streamlines can be displayed in relation to the body. On-body streamlines originate from the centroids of specified panels. Off-body streamlines originate from points specified in the input file. Lines are color-coded to indicate local flow velocity.




On-body streamlines and Boundary Layer Analysis In this example, the streamline is marked at the location that laminar flow transitions to turbulent flow. An "x" marks the location. After the transition occurs, turbulent separation occurs at the end of the streamline. This information is displayed by text.



Off-body streamlines

Off-body streamlines Off Body streamlines allow displaying of coefficient or pressure, velocity magnitude, or Mach number. Streamlines can be arbitrarily defined through user defined points.



Off Body streamline locations can be cursor-selected for detailed study of areas of special interest



Postmarc will map the differences in Cp, Vm, and Mach between two models. The models must be identically paneled, but can differ in dimensions and/or in flight conditions.

The second icon on the tool bar allows you to select a second file to add to or subtract from the first. After you make a selection, all commands related to pressure, velocity, and Mach number (including pressure distribution graphs, but not including XYZ plots) relate to the two files together. The appearance of the icon changes, becoming a triangle. To turn off differencing, click on the triangle.

The default operation is second file minus first file. You can select first-minus-second or even first-plus-second by clicking on File > Second file operator.

Rectangular and cylindrical velocity scans

recttn.gif cylintn.gif Flow velocity and direction may be displayed at grids of off-body points defined in the input file.


POV Ray rendered model

m2povtn.gif Postmarc includes output of POV Ray files. Some manual editing is required for a complete file. Simple light sources and view information is included in the POV Ray file written by Postmarc. The view information is taken from the current view in Postmarc.

F15 VRML file M2 VRML  File Postmarc writes VRML files for your browser (such as CosmoPlayer). Both geometry and contour plotting results can be written in VRML format. A VRML plug-in is required to view these two VRML examples.


Streamline Function Capability in Postmarc

On Body Streamlines Viscous Drag Form Postmarc can compute streamlines for any or all panels, and for any boundary layer parameter, on the basis of a single Cmarc run. The information used by Postmarc is stored by Cmarc in a file with the extension .PM. A Monte Carlo type scheme has been implemented in order to efficiently (and automatically) calculate enough on-body streamlines to cover virtually every panel. This allows Boundary layer information to be available on every panel (if separation has not occured) which allows a viscous drag estimate.

Manage Panel Crossing Selecting streamlines in Postmarc is as easy as cursor picking panels or choosing the automatic algorithm to generate streamlines that cross all panels. When one or more streamlines has been identified, the Manage Panel Crossing dialog box is used to select only cursor-picked panels or ALL panels for processing. Processing of boundary layer data is optional. If you want a full boundary layer analysis, you must select Include Boundary Layer Calculation. If you do not supply values for Reynolds number and kinematic viscosity, Postmarc automatically looks in the .PM file (from Cmarc) for the values.

On Body Streamlines Display of boundary layer characteristics can be done on the streamlines or panel averaged fringe contour plots.



Skin Friction Coefficient Boundary Layer Thickness Laminar Transition Point Friction coefficient may be mapped over the entire model in the form of a spectrum plot by selecting the Cf button in the spectrum plot dialog. Other boundary-layer characteristics, such as thickness, shape factor, laminar transition point , and so on, can be displayed on individual streamlines or as collections of streamlines covering the entire model or panel averaged.

A demonstration version is available for download.

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Dave Web Works