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We don't get realistic glide ratios with our settings. And we have no clue what to do

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Old   December 11, 2018, 10:52
Default We don't get realistic glide ratios with our settings. And we have no clue what to do
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Dear CFD Collegues,

at the moment i try to evaluate a Delta 2 Lippisch-type ground effect glider like this one in the video (https://www.youtube.com/watch?v=NvxzGERqDXI).

We are relatively new to CFD and donít have any advice anymore to get realistic values.
First, we want to find out whether our simulation is plausible. So we report the glide ratio (lift coefficient to drag coefficient, CA/CW). This Value ranges between 3 and 6, but we think it should be much more.
We tried a lot different meshes of the type polyhedral-mesh and trimmed mesh. The number of cells are around one to ten million. We used mostly the k-epsilon model with steady and incompressible conditions, but we tried other models too. We used the reference area that results when you look up below the plane. We used this area for both coefficients. Additionally we use 20 prism layers for a high resolution of the boundary layer on the glider.
We use the following settings for CA (lift coefficient) and CW (drag coefficient):

There we just varied the speed and set the actual reference area (when we changed the angle of attack as an example) and of course the specific subtract, all other options remained unchanged.

For the prism layer Mesher propeties we set the default ones.

We tried to round or chamfer the edges and we always use the option "surface wrapper" to correct bad parts of the model.

Even if we only simulate the wing of the glider we do not get better results.
With four degrees angle of attack. The result of the glide ratio is around 6.5.
And we hope, that you are able to help us.
Attached Images
File Type: jpg Mesh around wing.jpg (155.2 KB, 9 views)
File Type: png Trimmed_Mesh_most_possible_cells_10mio.PNG (30.5 KB, 9 views)
File Type: png CA and CW-settings.png (64.4 KB, 8 views)
File Type: jpg polyhedralmesh with physics.jpg (54.0 KB, 11 views)
File Type: jpg wing_wtih_Results.jpg (136.3 KB, 9 views)
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drag coefficient, glide ratio, lift coefficient

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