wake control and VSP aero Viewer intepretation

[zach....@fhengineering.com]

I was playing around with different wake settings with a very simple wing at a CL of ~ 0.9.  I would like to better understand how wake settings change the plotted wakes. 

Sorry this is far from a well done study ( took a glance at results like CL and span efficiency and didn't notice anything major changing) - I just changed the wake nodes and hit run and then plotted in the Viewer.  My major finding was that the number of nodes changed the wake quite a bit - fewer nodes meant a faster wake rollup.  I'm not sure if this should be the case in the solver, or if its a graphical effect that is or is not true to the mathematical solution.  Any VLM I ever wrote had wakes on the X axis only.  Probably a Dave question, but I'll throw some images out here.

 

It seems, on first thought, increasing nodes should give me a better defined wake, but the basic shape will be mostly dependent on the flight condition.  The answer with 8 nodes "looks" better to my eye.  I was concerned maybe I just needed more iterations with more nodes, but this does not seem to be the case - checking convergence history seems to support this.  It really looks like the "pitch" of the wake is set by the # nodes.  I'd be curious to know if the correct answer is for a wake rollup much further downstream (ie: more nodes = more accurate answer, fewer nodes = premature rollup).  What determines the length of the wake shown in VSPaero Viewer?  Possible to extend it?  (I tried far field distance)

Thoughts?

[Brandon Litherland]

Far Field Distance will allow the wakes to go much farther.  The default of -1 sets it to, I believe, 1 semispan aft of the TE.  I'd have to experiment with far distance a bit to actually figure out what the parameter is setting in a physical sense but I've seen values of 50 - 100 go to a good distance aft.

Run each of these out to Far Dist = 100 and watch what the wakes do.  I'm betting the wake just wasn't going far enough downstream.

[zach....@fhengineering.com]

Thanks for the reply.

It doesn't appear that increasing the far field distance affects the wakes drawn in the VSP Aero Viewer 6.0.3.  I tried -1, -10 and -100

[zach....@fhengineering.com]

Quick follow up to this thread.  What happens when a user specifies zero trailing wake nodes in the VSP aero GUI?  The printout in the progress text reports

"Number Of Trailing Vortices: 90", but I'm not sure if this is the same thing as trailing wake nodes.  I'd suspect the number of trailing nodes would be a power of 2, like 64 wake nodes.