TARDIS 2005–10, the roundels are built into hexagonal recesses in the walls. Ever since the TARDIS was redesigned at the beginning of the 2010 series, there have been a range of different roundel designs around the console room. These include circular holes that are recessed deep into the walls, hexagonal holes that are lit from behind each face, round indents with brass rings around the outside, and a glass centre that is illuminated blue. In "The Day of the Doctor", when the TARDIS' control room briefly appears as the War Doctor's room, the Eleventh Doctor points out to the Tenth Doctor
Working fluid Working fluid A working fluid is a pressurized gas or liquid that actuates a machine or heat engine. Examples include steam in a steam engine, air in a hot air engine and hydraulic fluid in a hydraulic motor or hydraulic cylinder. More generally, in a thermodynamic system, the working fluid is a liquid or gas that absorbs or transmits energy. The working fluid properties are essential for the full description of thermodynamic systems. Although working fluids have a very large number of physical properties which can be defined, the thermodynamic properties which are often required in engineering design and analysis