Large Eddy Simulation of Air Escape through a Hospital Isolation Room Single Hinged Doorway—Validation by Using Tracer Gases and Simulated Smoke Videos

Saarinen, Pekka E.; Kalliomäki, Petri; Tang, Julian W.; Koskela, Hannu

The simulation was performed with ANSYS CFX software using an incompressible, timeresolved large eddy simulation (LES) solver with a LES WALE subgrid model. The time step was chosen to be very short to ensure that the flow would normally travel no more than one mesh node spacing during one step, i.e., the maximal Courant number stayed below 1. The length of the time step was a constant 2 ms, until after 22.3 s of simulated time. Then after the door and nurse movements had ended, it was increased to 4 ms. The computational mesh was a pure tetra mesh comprising 11.7 million nodes (control volumes, corresponding to 68.7 million elements). In order to reduce the mesh size, a refined volume of approximately 6 m × (1-2.4) m × 2 m (greater width near the volume swept by the door), or some 20% of the overall volume of the rooms, was created in the path of the moving nurse and door. A dense mesh is inevitable in this volume not only to resolve the air currents, but also because the immersed solid method applied uses the underlying mesh to resolve the surfaces of the moving objects. In the refined volume, a typical mesh element diameter was of the order of 1 cm, and the volumes of the finite volumes were 1.5×10 −6 -2.5×10 −6 m 3 . Away from the refined area, the volumes increased to 1.5×10 −3 -4×10 −3 m 3 . The discretization schemes used were second order backward Euler (transient scheme) and central difference (advection scheme). The two rooms together formed a closed system surrounded by wall boundary condition, and in the initial state the relative pressure as well as all the velocity components were set to zero everywhere. A 2 cm high gap was left below the door to avoid convergence problems in the beginning of the door-opening phase. This detail differed from the mock-up, where this gap was sealed by a moving seal that was automatically lifted when the door started to move. The two tracer gases were modelled as passive scalars (CFX additional variables) and at zero time there was only one type of gas in one room.