Analysis

Updated July 15th, 2020



------------------ WORK IN PROGRESS ------------------

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Far-field vs. on-body forces

This subject is not specific to any particular version, but is insufficently documented and has raised questions.
There are two distinct ways to evaluate lift and drag in panel methods. It can be done either
  1. in a cross-plane far downstream, i.e. the so-called Trefftz plane or far-field plane
  2. by summation of on-body or panel forces
Trefftz_plane
Panel_forces

The first method is usually deemed to give more precise results than the second. It is the default method in AVL, xflr5 and flow5.


To allow comparisons during the beta phase, both the far-field forces and the summed forces can be plotted in the polar graphs. The variables can be found with the extensions "_FF" and "_sum".

Trefftz_plane_option

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Error messages

Triangle analyses

Error message:  "Making the influence matrix... *** numerical error when calculating the influence of panel 0 on panel 3627 ***"

Description

This error message occurs in triangular linear analyses, if one of the triangular panels has an invalid geometry, i.e. has a null area. This occurs when the three vertices are aligned within the working precision, or when at least two of the vertices are coincident, again within the working precision.
The goal is to have the automatic mesher detect and discard these triangles, but some may still remain if generated by early versions of the application, or due to the manual translation of nodes to remove free edges.

Fix

Remesh the fuselage, if necessary with a slightly different mesh size.
Set a size for the fuselage mesh elements close to the size of the wing elements.
Clean the fuselage geometry by removing all the small faces before meshing it.

To identify the faulty elements, use the option "Mesh/Center on panel" in the context menu of the 3d view.
Null_triangle



Error message   "error making doublet derivative for panel 3627"

Description

This error occurs in triangular uniform analysis, if a triangle is not connected to any neighbour triangle.
In such a case, the algorithm cannot calculate a local doublet gradient and outputs the error message. As a consequence, the local pressure coefficients are invalid. However, this will most likely make little difference to the overall lift, drag, and moments.
Results will be generated despite the error message.

Fix

Remove the free edges.




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