# Negative pressure room simulation

## Negative pressure room CFD simulation

In this tutorial, a Computational Fluid Dynamics (CFD) simulation of a negative pressure room is described. The pressure inside the room is maintained lower than that of the nearby rooms to prevent air, and potential contaminants, to escape and spread in other spaces. With a CFD simulation is possible to visualise in detail the air velocity field in a negative pressure room.

In this tutorial we will complete a CFD simulation of a negative pressure room where we assume that the doors of the room are closed. Therefore, as the room is in isolation with respect to rest of hospital floor, we can use a simplified geometry of the room itself only.

This tutorial is carried out using SimWorks Manager Computational Fluid Dynamics (CFD) software, just download it and contact us to get a free evaluation licence.

As already done in previous tutorials we will define a new project and carry out an analysis of the negative pressure room.

1. Define a new project called Hospital project and a geometry called Floorplan and finally a new simulation called Closed doors
2. Load the igs file and leave the unit to m making sure that the Outer domain is disabled (you can disable it before or after loading the geometry)
3. Finally, select from the Part layering menu the Part names to display the different parts forming the hospital floorplan and select the Roof part and click on the Hide current selection icon in the Geometry viewer window

4. Now the internal of the floorplan is visible, it is time to define the point in mesh with coordinates 4, 4, 1.5 and its size to 0.2

5. Define a mesh base size of 0.25 and reduce the Surface mesh 7 refinement level to 2

## Base setup

1. For this problem we want to consider density variations with temperature and activate the buoyancy to correctly simulate the effect of gravity on the flow. To do this, select Buoyancy in the Thermal field in the Setup Define the Reference temperature as 298.15 K (25°)
2. Define the Number of processes as 4 and the End iteration as 1000

## Boundary conditions definition and run

1. Select the two exits NegativePressureRoomOutlet_1 and NegativePressureRoomOutlet_2
2. Create a new part group and define it as a Pressure Outlet boundary condition with pressure 0.0 (ambient pressure)
3. Do the same for the NegativePressureRoomInlet_1 and define a boundary condition of type Volumetric Flow Rate of 2 (m3/s) with a temperature of 288.15 K (15°)
4. Complete the Run setup, Mesh and Simulation run phases as already done in the previous tutorial

## Post-Processing

Once the simulation of the positive pressure room has finished, we can load the results in the Fields viewer.  Right click on the entry in the Simulation manager window and select Fields –> Load

1. Select the p_rgh as a field, this is the absolute pressure (in Pa) which includes the effect of the gravity on the flow
2. Define the range from 101315 to 101316 Pa, bear in mind that the atmospheric pressure is 101325 Pa therefore we achieved a negative pressure of -10 Pa
3. There is as expected a minor increase in pressure in the proximity of the inlet as expected
1. Now select the velocity U as variable to plot
2. Select the planes normal to Y and scroll to the inlet intersection, leave only the inlet and the outlets visible. It is now evident that a jet of cold fresh air is coming from the inlet and dropping towards the centre of the room

## Conclusions

The two outlets have been placed in the back of the room while the inlet has been placed next to the entrance door (not modelled in this geometry) to provide a sealing effect on the room itself. This way the fresh flow entering the room from the inlet located next to the entrance will provide a curtain of clean air next to the entrance itself minimising the chance of contaminants mixing with the external air when the room is opened.

Looking at the results in the previous section it is evident that the plume of fresh air is limited in size and does not have enough momentum to actually seal the door area. To improve this, it would be advisable to increase the inlet mass flow rate, which would lead to a stronger jet to cover. Moreover, the inlet which is currently modelled as a simple rectangular, could be improved with the addition of a diffuser to direct the plume in a more effective way. An alternative solution could be to add a further inlet on the opposite wall.

The simulation of the negative pressure room has shown that CFD is an invaluable tool to understand the flow structures which will affect the room even before the design stage. Once the flow structures are properly simulated, it is easier to  the design modifications required to achieve the required objective are. See the article on negative and positive pressure rooms to get more info about this topic.

## Disclaimers

The results and the description provided above where defined to describe physical behaviours under certain circumstances. They should not be considered a medical guidance and do not account for environmental variants such as humidity or wind.