Trouble with 3D panels

[Fokapilot]

I started using Flow 5 recently but still have some cross-reference questions please.

Yes, yes I know 3D panels especially quads are not recommended but I'm happy to take a risk.

1. In XFLR5 I don't always get an option for 3D panels. Any reason for that please? I have always been using VLM2 and only noticed this problem now. What seems to be the reason?

2. In XFLR5 in 3D panels I cannot see an option for "Pick Value', i.e. identify a value by picking it with a mouse pointer. Is this correct?

3. Flow 5 with quads gives nonsense results i.e. Cp of over 30 while less than 4 is expected.

4. Flow 5 with tri give better results but still some errors around the LE

5. The colour bar legend (soft rainbow type) is annoying for someone who has been doing FEA for 40 years. We used to say it is good for management because it looks nice but difficult to identify values :). Is there a way to get "a defined rainbow" please?

[Flying-Geek]

Hi!

1. Yes, this is a limitation of xflr5. It is only possible to analyse standalone wings with the 3D quad panel method, otherwise it may only be used for fuselages. You can read more about it in the guidelines document.

2. Picking a value with the mouse cursor is not possible. However, you can export the operating point by right clicking on it in the object explorer and get pressure coefficient of each panel through that.

3. & 4. Can you share the analyses file? I would need to have a look before I can say what is causing issues. Generally it's also a good idea to have a look at the distribution of doublet densities / vortex strengths to quickly identify problems.

4. You can change the colormap in preferences -> 3D views. However, you can only choose the 2 or 3 colours and there will be a gradient in between them. Having several distinct colours, each for a specific range of values is not possible.

Stefan

[Fokapilot]

Hi Stefan,
Thank you for your quick reply.
Ad1 Yes, I noticed it now, it only works for flying wings. I never did flying wings before.
Ad2. Hmm, that will be tedious...
Ad3 & 4. Don't want to share my file yet, I will come up with some other file. How do I look for the doublet densities/ vortex strengths?

Ad4 But can you make an option for distinctive colours apart from the rainbow please?

Regards
Andrew

[Flying-Geek]

Hi Andrew,

you can display it in the plane view by clicking on gamma:

Adding an option for a discrete colour map is not up to me or within my skills. Maybe André can comment on that. @techwinder

Stefan

[techwinder]

Unfortunately, I don't see that there is enough benefit in implementing the discrete colour map to justify the development work.
André

[Fokapilot]

Hi Andre,
Well, I have been doing FEA and a bit of CFD for 40 years. We (engineers) always considered the rainbow plot to be just a gimmick for management presentations; because it looked nice :) . One may easily loose small, salient feature of the results. Attached is my very old, >30 years analysis. The encircled areas would be smeared and not well visible in a rainbow plot...

Cheers

Andrew

[techwinder]

OK, as a former fellow engineer, now I understand.
There is another consideration though. In the tri-uniform - and recommended - case, the calculated doublet densities and pressures are uniform on each panel. Creating colour splits across panels would be somewhat misleading because not reflecting the calculation.
It may be possible however to create an option to display only those elements with a result above a certain threshold value. I'll give it some thought, but no promises because it still is a lot of development work.
André

[Fokapilot]

Thanks, i trust you.

[Fokapilot]

Hi Stefan,
I have created a flying wing of similar proportions but with different airfoils and washout. I have made 2 versions, one in XFLR5 and one in Flow 5, both are attached.

The results are similar to my original file. Cannot see any singularities in the gamma distribution but I will leave to you to assess it.

The Cp shows a very high maximum values when set to Auto, but a reasonable (?) distribution when in manual.

[Fokapilot]

Hi Stefan,
How do I attach the .xfl and .fl5 files?

[techwinder]

There is a button to "Paste, drop or click to add files" just below the text box.

[Fokapilot]

This function doesn't support .xfl or fl5 files. Do I just change the extension?

[techwinder]

Zip it?

[Fokapilot]

Here they are:

Flying Wing Test XFLR5.zip
Flying Wing Tests.zip

[techwinder]

The Cp distribution is calculated using the gradient of doublet distributions across panels, and is pretty tricky to implement. xflr5 and flow5 do it in slightly different ways.
The local peaks are most likely numerical artefacts and have no physical meaning. Forcing a manual scale as you did is the way to fix the problem.
In your case I suspect that the issue is at the wing tip.
I'll look into it, see if there is a way to make the display more robust.
André

[Fokapilot]

You are correct, it is the wingtip...

[techwinder]

The wing tip is the location of the tip vortex which is a singularity for the velocity field. It means that the velocities calculated in this area are numerically unreliable, and so are the pressures calculated using the velocity gradient.
My recommendation is just to ignore these anomalies of the pressure field.
André

[Fokapilot]

Hi Andre,
After working with the Flow5 for a while I would like to modify my whinging :) about the colour bar. I believe the display of the pressure distribution is OK, since as you mentioned, it is constant over a panel. My only whinging :) would be then about the colour bar i.e. to be with defined colours and not a rainbow.

Another things which I found:

1. The colour bar doesn't limit the value to a maximum in a window. In this picture the maximum Cp is approx -2.92 but the bar displays -2.68. Why is that?

Apologies for a poor resolution but I was trying to get the value displayed and used PrintScreen, don't know how to do it any other way.

Similarly, when one limits the scale to -2, the bar displays -1.76 while the maximum in the window is still -2.92???

2. Could we please get the Pick Value to stay on the screen and therefore allow for saving and printing.

Cheers

Andrew

[techwinder]

I'll look into it and post back.
André

[techwinder]

There is a bug in the labels of the colour scale: the last label is missing. Thanks for reporting the bug.
Discrete colour bar: again, it's quite a bit of development work and I'm not convinced that many people would use the option in the end.
Permanent picked value: OK, I'll put it in the todo list.
André

[Fokapilot]

Hi Andre,
After spending some time on Flow5 I agree; the rainbow bar is OK since the values are constant over each panel.
Cheers
Andrew