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Shock vs Shadowgraph


How to Apply Shadowgraph Calculations in Supersonic Flow

Welcome to Part Two of the tutorial about using shadowgraph. There were a couple of great comments (from the previous video, Shadowgraph Technique in Tecplot 360) about shadowgraph not being a schlieren, which is true. There is a synthetic schlieren-type calculation which certainly is something we were considering putting in. We’d love to hear back from the community to find out whether or not that would be valuable to you. To give feedback, use the Contact form.

I will walk through shadowgraph calculations in a little more detail, and how show you how they apply in supersonic flow. And show how they compare to a simple calculation of shock. I’m going to take a look at a very simple case, this is flow over a diamond airfoil shape, at mock, I believe 1.5 (could be 1.6). And as I showed before (Shadowgraph Technique in Tecplot 360), the easiest way to do this is to use the Analyze menu.

  1. First choose Analyze > Field Variables to identify the field variables.
  2. Choose Velocities and click Select… the U and V velocities will auto-populate.
  3. Choose Temperature and Pressure for State Variables.
  4. Click Select… and choose static pressure and static temperature.
  5. Under Analyze menu, choose Calculate Variables… and type in Shadowgraph.
  6. The Calculate on Demand is checked so that if you have a large data file it won’t calculation right away. But this is a very small case, so uncheck Calculate on Demand and press Calculate. And it shows that the calculation was done, and it shows what the shadowgraph calculation yield in terms of max and min.
  7. We will also go ahead and calculate Shock.

To look at Shock vs Shadowgraph in a comparative way, I’m going to rearrange my layout.

  1. Right click and choose Copy.
  2. Paste the frame using ctrl-v.
  3. Show both frames side-by-side by choosing Tile Frames… from the Frames This puts them side by side. You can see that right now they’re not linked.
  4. To link the frames’ size and position, choose Frame Linking… from the Frames Check the X axis range and Y axis range and click Apply Settings to All Frames of this group to link them together.

Shock Features

We are going to look at the shock feature. Right click on the plot, click the Contour icon and select Shock. Right click on the legend and delete it because we don’t really need it.  To make this plot more comparable to a shadowgraph, I’m going to change the contour type. Make the plot symmetric by right clicking on the lower part of the plot, click the Contour icon and select Shock. Open the Contour Details… dialog and in the Coloring tab use GreyScale instead of Small Rainbow. Also choose Continuous instead of Banded. This shows a continuous representation of Shock going from -1.8 to 1.8.

Right click again on your plot and click the small arrow to the right of the Contour icon and choose Shock. Do the same with the lower plot zone.

You can get a quick view of what the shocks coming off the leading edge of the airfoil.

Shadowgraph Features

We want to look at Shock in the context of shadowgraphs. So we will do a similar thing to change the Shadowgraph plot. In the Contour Details… dialog Coloring tab, choose Continuous and use the GreyScale colormap. Select both zones by holding the shift key and clicking on each zone. Then right click, select the small arrow to the right of the Contour icon and select the Shadowgraph function. Reset the values in the shadowgraph function. In the Contour Details… dialog Levels tab, change the levels to -10 to 10. That level picks up the shadowgraph better.

Now you can see the shadowgraph calculation versus the shock for this diamond delta wing. The key feature we are trying to show is the shock coming off the leading edge. And how you use the shadowgraph function.

This is not a replacement for a schlieren, or the synthetic schlieren, but it’s a good way to show shock.  

Thanks for watching!

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