# Preprocessor he APM Preprocessor reads the mesh of the body and assigns a type to each element. The mesh must satisfy the following requirements: * The mesh must be watertight (unless using the [symmetry option](#symmetry)) * The surface normals of the body must point outward i.e. into the flow * If the mesh includes a wake, the wake normals should point in the negative z-direction * The mesh must be in a body-fixed coordinate system i.e. the x-axis must point forward, the y-axis must point to the right wing and the z-axis must point down * The mesh must be in STL file format More information about the format of the mesh file is available in the [mesh file](/user-guide/the-mesh-file.md) section. The image below shows an unstructured mesh of a fixed-wing UAV. ![Unstructured mesh of a fixed-wing UAV](/files/-MVXeHS7XjiYUEXVOXE2) {% hint style="info" %} The mesh should be in a body-fixed coordinate system, should be watertight and the mesh normals should point outward i.e. into the flow. {% endhint %} After running the APM Preprocessor each element of the mesh is assigned a type. A list of the element types is given in the table below. | Type | Description | | ---- | -------------------------------------------------------------------------------------------------- | | 1 | Body element | | 2 | Upper trailing edge element (normal points in negative z-direction) | | 3 | Lower trailing edge element (normal points in positive z-direction) | | 4 | Upper intersection edge element (at locations where the wake elements intersect the body elements) | | 5 | Lower intersection edge element (at locations where the wake elements intersect the body elements) | | 6 | Element belonging to a flat surface (wing tip, tail tip, fuselage boom base, etc.) | The image below shows the element types for the fixed-wing UAV. ![Element types of the fixed-wing UAV.](/files/-MVcIm1ZRfwxtttC2HRA) To find trailing edge elements the APM Preprocessor compares the angle between the normals of two adjacent elements to the `trailing_edge_angle` option in the **.conf** file. If the angle is greater than the one specified by the `trailing_edge_angle` option the elements are marked as trailing edge elements. Trailing edge elements are used by the APM Solver to enforce the Kutta condition. If the mesh contains a wake, as in the fixed-wing UAV example, the APM Preprocessor will find all elements which have more than 3 neighbours. Such elements exist along the trailing edges and at the location where the wake meets the body. Elements belonging to flat surfaces (e.g the wing and tail tips and the fuselage base) should be type 6. The APM Preprocessor is unable to detect type 6 elements automatically. You need to manually specify elements of type 6. For the fixed-wing UAV example, the elements at the fuselage base must be type 6. To set the type of all elements belonging to the base you need to determine the identifier (ID) of at least one element belonging to the base. You can easily determine the identifier by opening the **.stl** file in a data analysis and visualization application such as ParaView. {% hint style="info" %} Run the APM Preprocessor without specifying identifiers for type 6 elements. Open the created **.vtu** or **.dat** files and find the identifier of at least one element belonging to a flat surface. Add the identifier to the tip\_panels option in the **.conf** file and re-run the APM Preprocessor. {% endhint %} After you obtain the identifier of at least one element belonging to the base, add it to the `tip_panels` option in the **.conf** file. You can add multiple identifiers if you have multiple flat surfaces in your model. Make sure the identifiers are separated by a comma. ``` tip_panels = id1,id2,id3... ``` After the APM Preprocessor runs it will create a **.mdl** file. The format for the **.mdl** file is specific to the Aviumtechnologies Panel Method and contains the element vertices, types, and neighbours. The APM Solver uses the **.mdl** file to solve for the potential flow around the body. ## Symmetry option To make use of the symmetry option, you need to mesh only half of your geometry. Your geometry must lie entirely in the -y or the +y direction. The preprocessor will mirror the mesh, including the panel types, across the x-y plane accordingly. The image below shows half of the mesh of a fixed-wing UAV.

Unstructured half-mesh of a fixed-wing UAV

## MATLAB script for plotting .mdl files ``` clear all; close all; clc; structured=0; model=dlmread('.mdl'); N_vertices=model(1,1); N_faces=model(1,2); model=model(2:end,:); for i=1:N_vertices vertices(i,1)=model(i,1); vertices(i,2)=model(i,2); vertices(i,3)=model(i,3); end model=model((N_vertices+1):end,:); for i=1:N_faces faces(i,1)=model(i,1)+1; faces(i,2)=model(i,2)+1; faces(i,3)=model(i,3)+1; if structured == 1 faces(i,4)=model(i,4)+1; end r1=vertices(faces(i,1),:)'; r2=vertices(faces(i,2),:)'; r3=vertices(faces(i,3),:)'; if structured == 1 r4=vertices(faces(i,4),:)'; end if structured == 1 r=1/4*(r1+r2+r3+r4); n=cross(r3-r1,r4-r2); else r=1/3*(r1+r2+r3); n=cross(r2-r1,r3-r1); end n=n/norm(n); centres(i,1)=r(1); centres(i,2)=r(2); centres(i,3)=r(3); normals(i,1)=n(1); normals(i,2)=n(2); normals(i,3)=n(3); end model=model((N_faces+1):end,:); for i=1:N_faces types(i,1)=model(i,1); end figure(1); hold on; patch('Faces',faces,'Vertices',vertices,'FaceVertexCdata',types,'FaceColor','flat','EdgeColor',[0,0,0]/255,'Clipping','off'); quiver3(centres(:,1),centres(:,2),centres(:,3),normals(:,1),normals(:,2),normals(:,3),1,'Color',[0 143 255]/255,'Clipping','off'); grid on; axis equal; axis off; rotate3d on; set(gca,'Zdir','reverse'); set(gca,'Ydir','reverse'); colormap(jet); colorbar; camproj('perspective'); view([135,25]); drawnow; ``` --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://docs.aviumtechnologies.com/user-guide/preprocessor.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.