P51D Mustang Mk. III by M. Cel (http://hangar.openvsp.org/vspfiles/420)
meshed in OpenVSP and analyzed in flow5.
flow5 beta 14 introduces the option to carry out analyses on planes defined by imported stereolithography meshes. As the acronym suggests, STL models are essentially intended for the exchange of models for 3d printing. However, there is apparently no other universally accepted file format for surface meshes, so that the STL format has become the usual standard. The advantage of the format is that it is ubiquitous and versatile. It is therefore the format recognized by flow5 to import the surface meshes.
The only information contained in the STL file is the position of each triangle's vertices and its normal. It does not contain any meta data, or any information on mesh connections. All the additional information required to define a geometry and run an analysis needs therefore to be reconstructed in flow5, either automatically or manually as explained hereafter.
Since flow5 is not intended to be CAD software, it does not offer any significant tool to correct, heal or improve the mesh. It is therefore important that the external STL mesh should be of a quality acceptable as-is for a panel analysis, i.e.:
The information about the model which is not present in the STL file but which is required for the analysis is the reference wing area, the mean aero chord length and the span length. These dimensions are necessary for the calculation of the aerodynamic coefficients.
They should preferably be defined for the airplane, and if necessary adjusted for each analysis.
The analysis requires that the mesh elements be connected, i.e. that each triangle "knows" with which other triangles it shares its edges, and that each node "knows" of which triangles it is a vertex. This information is not contained in the STL file and is reconstructed in flow5.
These connections are necessary to compute the pressure coefficients on the panels.
The only other required information which is missing in the STL format is the location of the trailing edges and the identification of the triangular panels on the top and bottom sides of the trailing edges.
The definition of the trailing edges is required to implement the Kutta condition, without which the wake cannot be defined and the lift and induced drag cannot be calculated.
Since the STL format contains no information about the airfoils or about the fuselage, neither the viscous drag of the wings or of the fuselage can be estimated. The analysis will therefore only be of the inviscid type, although additional user-defined extra drag can be added to the results. An option to add AVL-type parabolic drag has been introduced in v7.01 beta 15.
All the types available for flow5 planes are also available for STL type planes. However, without viscous information and viscous drag, type 3 "speed polars" will not be representative of the physical flight.
Since the fuselage is not identified in the STL file, the contribution of its pressure forces cannot be excluded from the calculation of the moments. This can be a problem in cases where the wake panels shed by the wings interact with the fuselage. To limit these issues, the use of a vortex particle wake is recommended.
A good starting point can be Tom's videos which show in detail the process to generate an STL mesh in OpenVSP and to import it into flow5:
A project file containing examples of STL meshes can be downloaded here: STL_beta_14.fl5.This step-by-step guide uses an STL model generated from the Blended Wing Body model built by Michael Kruger and which can be downloaded from the VSP hangar.
Use the menu option Plane/Import/From an STL file to import a plane from an external mesh.
The length unit is not part of the STL format. If it is known, select the length unit with which the file was written, otherwise select a large unit, typically meters. The model may be scaled down at a later stage if necessary.
Reading for instance in millimeters a file written in meters will lead to a model of very small size. In the case illustrated in the image above, the size of the imported model is 1.5 mm. Triangles will have a very small area which could cause issues when scaling up the model. In such a case, it is recommended to restart the import process using meters.
The only check made during the import process is the verification and eventual correction of the triangles' orientation.
Once imported successfully, the program will show the plane editor, as illustrated below.
Use the interface inside the red circle to define the reference area, chord length and span length.
The T.E. panels must be identified to meet the following constraints:
flow5 will identify as T.E. panels any pair of connected triangular panels such that the angle between their opposite normals is less than the specified angle.
In the 3d view controls, de-select "Surfaces" and activate "Panels".
Specify a value for the T.E. angle. Start preferably with a small max. value such as 5 to 10°, and increase it progressively.
Click "Guess T.E.".
The set of T.E. panels will likely require manual correction.
Click first on the button "Top T.E. panels" to activate their selection. Then click on the panels in the 3d view to select or deselect them one by one.
If "Guess opposite" is selected flow5 will attempt to identify the T.E. panel on the bottom side for each selection, using the max. specified T.E. angle.