Creo Simulate > Creo Simulate > Modeling Structure and Thermal Problems > Idealizations > Shells > Midsurface Shells > Shell Compression > Shell Compression
  
Shell Compression
Creo Simulate processes all solid models as single volumes. However, the software uses a different approach when processing models with paired shells. With shell models, Creo Simulate compresses the model to a midsurface or a set of midsurfaces.
To perform midsurface compression, Creo Simulate requires a pairing scheme for the surfaces you want to model.