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

Since the tracer gases are invisible, they were replaced by smoke when visualizing the flow structures experimentally, in real-time. Smoke was then released into one of the rooms, where it stayed for a while, letting the temperature difference to vanish. The smoke was then neutrally buoyant. Since flow-driven transport phenomena dominate over diffusion, after the door-opening smoke was carried into the other room by the air flows in the same manner as the tracer gases. Similarly, simulated smoke would be a useful illustrative tool to simulate the flow structures predicted by LES. Moreover, comparison of simulated smoke videos and real camcorder shots would be an interesting method to qualitatively validate the CFD results. Similar propagation of smoke and tracer gas makes it possible to use the same passive scalar to represent them both in the LES simulation. When penetrating deeper into the smoke, into areas of larger concentration of smoke droplets, the light scattering becomes more and more effective. This means that the smoke becomes less and less transparent. This effect can be discretized by first dividing the smoke into a few nested volumes with different concentrations. Next, let us approximate that all the light scattering is taking place on the limiting surfaces between these volumes. Then only a few isosurfaces of tracer mass concentration need to be drawn and a negative correlation between the concentration and the transparency of the surface needs to be defined. This principle is illustrated in and they are shown with a frequency of 25 frames per second, we get a simulated smoke video in natural speed, easy to compare with an experimental one. An example is seen in S2 Video. It is a simulation combining two separate smoke experiments. Smoke escaping from the isolation room into the anteroom is coloured white, whereas smoke going the opposite way is coloured yellow, corresponding to a yellow light source. This video is based on a single CFD LES simulation, having different passive scalars dosed into each room in the initial state (corresponding to two tracer gases or two separate smoke experiments). Table 1 lists the isosurfaces and their transparencies, common to both passive scalars, used in preparing the video. There are no strict rules for the selection of these quantities. The only requirement is that different layers should be seen through each other in a natural-looking way. The video corresponds to situations where the rooms are entirely lit. It displays some basic flow structures, such as the jet through the gap below the door starting to open, the door vortices, and the wake of air the nurse is dragging behind her. It is comparable with the latter part of S3 Video by Tang et al. [22] , showing the same features except that there is not a similar gap below the door.