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MolFlow+ user guide For version 2.4.1 – 3 June 2014 Marton ADY, Roberto KERSEVA

MolFlow+ user guide For version 2.4.1 – 3 June 2014 Marton ADY, Roberto KERSEVAN CERN MolFlow+ User Guide cern.ch/test-molflow page 2 1. About Molflow ................................................................................................................................................... 3 2. Basic concepts .................................................................................................................................................... 3 2.1. Working with Molflow ....................................................................................................................... 3 2.2. Elements of the geometry ................................................................................................................. 4 2.3. Post-processing within Molflow ....................................................................................................... 5 3. MolFlow+ functions one by one .................................................................................................................... 7 3.1. Interface overview .............................................................................................................................. 7 3.2. Camera controls ................................................................................................................................. 8 3.3. Selecting facets ..................................................................................................................................... 9 3.4. Viewers ................................................................................................................................................ 13 3.5. Viewer parameters ........................................................................................................................... 14 3.6. Importing geometry .......................................................................................................................... 21 3.7. Setting up the simulation - facet parameters .............................................................................. 24 3.8. The simulation panel ........................................................................................................................ 28 3.9. Global settings ................................................................................................................................... 30 3.10. Processing results .............................................................................................................................. 34 3.11. Advanced handling ............................................................................................................................ 45 3.12. Geometry Editor ............................................................................................................................... 48 3.13. Periodic boundary conditions (teleport) ..................................................................................... 65 3.14. Superstructures ................................................................................................................................. 67 3.15. Desorption map / Outgassing map ............................................................................................... 70 4. Theory ................................................................................................................................................................ 73 4.1. Pressure calculation .......................................................................................................................... 73 5. Computing aspects .......................................................................................................................................... 74 5.1. Random generator ............................................................................................................................ 74 5.2. Area calculation ................................................................................................................................. 75 5.3. Ray tracing, collision finding ............................................................................................................ 75 5.4. Graphics, OpenGL, SDL Framework ........................................................................................... 75 5.5. Shared memory, worker subprocesses ....................................................................................... 75 5.6. Auto-updater ...................................................................................................................................... 76 5.7. 7-Zip compression ............................................................................................................................ 77 5.8. Website ............................................................................................................................................... 79 CERN MolFlow+ User Guide cern.ch/test-molflow page 3 1. About Molflow Molflow+ is a Windows program that allows you to calculate the steady-state pressure in an arbitrarily complex geometry when ultra-high vacuum condition is met. The name comes from molecular flow, the condition when the mean free path of molecules is so long compared to the geometry size that collisions can be neglected. In this case, particles fly independently, which makes this physics particularly suitable for Monte Carlo simulations. The first version of this program was written by Roberto KERSEVAN in 1990 (see this reference article). In 2007 a new version called MolFlow+ was written in ESRF (Grenoble, France), that uses the original algorithm in a modern environment: it takes advantage of OpenGL acceleration for the 3D display and it can use multiple cores for the calculations. In 2012, the further improvements were added in CERN, and also a website was created. Geometries can be imported from most CAD programs that support the STL file format, and the development is still underway, with current focus on being able to simulate time-dependent processes. 2. Basic concepts This chapter is a general overview of the program and of the expressions later used. 2.1. Working with Molflow Using Molflow to simulate a vacuum system will in most cases consist of four distinct steps: 1) Importing the geometry. To begin, we need a geometry in which particles will fly. This will most likely originate from a CAD program (Autodesk Inventor, Solidworks, Catia, etc.). There is a standard STL file format (STereoLithography or Standard Tessellation Language) that all CAD programs can export to. Molflow is able to open these files and merge (collapse) the list of triangles to a geometry delimited by polygons, referred to as facets. 2) Setting up simulation parameters. Once we have the geometry, we have to define physical properties of its boundaries through facet parameters. Such a parameter is the temperature or the reflective property of the facet. Also, in this phase we execute the important step of defining where the particles come from (outgassing and desorption) and also the location and properties of pumps. 3) Setting up what to measure. After the second step, our simulation is ready to run. However, we want to derive useful results of the whole process. Although some physical quantities are measured by Molflow out of the box (average pressure on each facet, etc.), we will most likely want to obtain formulas that calculate a given quantity, profiles that show the evolution of a quantity along the length of a facet, or textures that show us the pressure directly in the form of colors on our geometry. 4) Running the simulation. Once we have set up everything, Molflow will trace the trajectories of particles in our system from their insertion until they are being pumped away. Every second, displayed results are being updated. The longer you run a simulation, the smaller the statistical fluctuation becomes. CERN MolFlow+ User Guide cern.ch/test-molflow page 4 2.2. Elements of the geometry Vertex A vertex is a point in the 3D space. In Molflow, they are interesting for two reasons: - All facets have an outline that consists of vertices (vertexes of a given facet are referred to as its indices) - Some geometry operations (such as creating a facet) require creating and selecting vertices first Facet A facet and its normal and U, V vectors Facets are polygons that are the elementary constituents of the geometry. They have indices, ie. the vertices they consist of, and they have a normal vector defining the direction they are looking to. They also have a local U,V coordinate system, where the U vector points from the first to the second index, and the V vector is perpendicular to it. The normal vector’s direction can be swapped, and the U,V vectors can be rotated by the user. Significance of facets is that their physical parameters can be specified one by one. Therefore a facet can serve as a pump, as a gas inlet, as a teleport gate for boundary conditions and it can also have profiles and textures. CERN MolFlow+ User Guide cern.ch/test-molflow page 5 Structures (advanced) Structures, also called superstructures are a set of facets that are specified by the user to be distinct from the rest of the geometry. Structures are connected by link facets. They were introduced to Molflow to make geometry handling easier (for example, you can restrict visibility or facet selectability to a given structure only), but their real role is to speed up simulation: ray-tracing algorithms will only look for collisions within the current structure (instead of with every facet in the geometry). Please note that whereas the computing time can be reduced by using structures, the time to set them up can be long. Experience shows that unless the geometry consists of a lot of facets (in the order of hundreds of thousands), not splitting the geometry to structures might be just fine. 2.3. Post-processing within Molflow To interpret simulation results (notably the pressure at certain points of the geometry), facets in Molflow can have profiles and textures. Profiles Profiles are quick measuring tools if the user wants to see the pressure along a facet. When a profiling is enabled on a facet, it is divided into 100 equal parts along the U or V vector, and for each slice the pressure is calculated. The advantage is that the pressure evolution can be visualized within Molflow, and several profiles can be plotted on the same Profile Plotter window for comparison. The disadvantage is that it only works on rectangular facets, and sometimes the 100 element split is not enough. CERN MolFlow+ User Guide cern.ch/test-molflow page 6 Textures Textures are tools to locally visualize the pressure on given facets with the help of a color scale that the user can customize. To calculate them, the user has to create a Mesh first, which is the division of a facet to squares (cells) of arbitrary size. Then the pressure is calculated individually for each of these squares and can be displayed with colors chosen in the Texture Scaling dialog. Whereas adding textures to existing facets is sufficient in many cases, it is possible to create transparent facets whose only purpose is to hold a texture. These transparent facets serve as tools to visualize the pressure in the middle of the geometry, for example. Transparent facet to measure the pressure in the middle of a tube Apart from the visual appearance, it is possible to read and export pressure values cell by cell using the Texture Plotter. CERN MolFlow+ User Guide cern.ch/test-molflow page 7 Texture Plotter: cell values can be read and exported Formulas Molflow offers the user to set up custom formulas, which are expressions containing internal values as variables. One could, for example, derive the transmission probability for a pipe from the number of particles on its two sides. These formulas are then updated in real-time as the simulation is running. 3. MolFlow+ functions one by one 3.1. Interface overview When you open MolFlow, you will see an empty geometry window, with the menu bar on its top and panels on the right. Most functions will be unavailable because there is no geometry loaded until you import a file or use the Test -> Quick Pipe command (ALT+Q). CERN MolFlow+ User Guide cern.ch/test-molflow page 8 The MolFlow window and functions of different panels 3.2. Camera controls Once you have the geometry in MolFlow, you must be able to move the camera so you can access facets in every part of the geometry. The most important things to note are:  Selecting a facet: left-click  Scrolling: move mouse with middle button pressed (no 3-button mouse? Hold ALT and the left button to scroll)  Rotating: move mouse with uploads/Ingenierie_Lourd/ molflow-user-guide.pdf