Shadow walls are automatically created whenever a wall has fluid zones on both sides. It is what is referred to as the so-called thin-wall. Thermal is the standard in the coupled solver (sum = 0 of the heat flux in the heat flux passing. Sides). If you want to set a fully independent wall, run the following TUI command in Fluent: /grid/modify-zone/slit-face-zone Then, enter the thread ID of the wall or wall-shadow, which should separate them. This technique can also be used to divide the periodic, if that is slit-face-zone rather than please enter the slit-periodic.