# Why multiple geometry to compute drag ?

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 July 16, 2023, 07:25 Why multiple geometry to compute drag ? #1 New Member   Kevin Join Date: Jun 2023 Posts: 3 Rep Power: 2 Hello everyone ! From this video : https://www.youtube.com/watch?v=qQWT...Y4Hc8&index=29 I want to understand why at 2:34 he has 3 geometry to compute the drag of the cybertuck ? I'm a complete newbie in this field. Why does he needs "air" ? Because in my beginner understanding the box that he creates is already made by air. Can someone explain it to me ? Thanks a lot

 July 16, 2023, 15:13 #2 Senior Member   M Join Date: Dec 2017 Posts: 609 Rep Power: 11 By 3 geometries, are you refering to the air, CyberTruck and street visible to the left? These appear to be predefined surface regions he uses to define the domain. You can see at 5:41 the "air" is the top boundary face. The naming is a bit counterintuitive, as it suggests it is the material "air". The street is just a plane surface below the truck - I am not sure how Engys treats it, its not neccessarily needed. The CyberTruck obviously is the outer walls of the vehicle. Regarding the fluid: the fluid properties are set at 5:16. The mesh itself does not have any material, its just cells enclosing a volume. He could have simulated with water, if he changed the material properties at this point.

 July 17, 2023, 05:05 #3 New Member   Kevin Join Date: Jun 2023 Posts: 3 Rep Power: 2 HI ! thank you for your answer. What I still don't understand is why does he needs the "air" and street boundary, if they're already bondaries conditions that we can set for wall (to replace the street boundary), and patch (for the air boudary). Thank you so much for making me understand.

 July 17, 2023, 13:25 #4 Senior Member   M Join Date: Dec 2017 Posts: 609 Rep Power: 11 These regions are not necessary for the case, but he uses them to define the upper and lower boundary anyway. The air and street faces are positioned in such a way, that they clip away the upper or lower face of the blockMesh. Instead, a boundary is created at this position with the name of the respective region. If you look closely at 5:25, you can see he has 7 boundaries remaining after meshing: ffminx, ffmaxx, ffminy, ffmaxy and the pre-defined regions air, street and hull. Now these ff-faces are just the outer faces of the blockMesh. Because the face ffmaxz was cut by "air", it is not available, instead he has the air as the top boundary. The face ffminz was cut away by the street and now he has this as a boundary. The whole case would have worked perfectly fine without these regions, he would have just ended up with ffmaxz and ffminz instead of air and street and could have defined boundary conditions for them. Yann likes this.