The Collision Avoidance control determines the approach that is to be taken in areas of proximity to avoid collisions that may occur from marching inflated surface meshes from opposite sides into each other. For 2D inflation, the Collision Avoidance control detects geometry limitations in faces that would otherwise cause inflation mesh to overlap or cross face boundaries, or result in a space that is so small that it leads to bad quality mesh for the remaining region of inflation mesh. When Collision Avoidance is set to Layer Compression or Stair Stepping, the value of the Gap Factor control, along with the local mesh size, will determine how much space will be adequate for the remaining region of inflation mesh. The following options are available: None – The None option does not check for layer collisions. Selecting this option speeds up inflation layer computation time; however, it can result in an invalid mesh and mesh failures as a result. For these reasons, this option is not recommended. For 2D inflation, if a collision/proximity limitation is detected during layer creation, creation of inflation layers stops with the previous layer. (Inflation stops completely; contrast with Stair Stepping below.) Layer Compression – The Layer Compression option compresses inflation layers in areas of collision. In these areas, the defined heights and ratios are reduced to ensure the same number of layers throughout the entire inflation region. Generally, this option is best for avoiding the creation of pyramids in the mesh. Layer Compression is the default only when the Physics Preference is set to CFD and the Solver Preference is set to Fluent; otherwise, the default is Stair Stepping. For more details of these methods, check out the help section here: help/wb_msh/msh_Coll_Avoid_Inf.html