The 2D CAD layout of the basement will be converted into a 3D geometry using 3D modelling software. The location of the fresh air & exhaust will also be marked in the 3D model. The exhaust will be labelled as velocity outlet boundary condition (meaning this label will have a constant velocity and move air out of the basement). The ramp will be labelled as pressure inlet (meaning this label will behave as having ambient pressure and will move air inside the basement depending on the internal negative pressure).
Flow Physics of the Basement: Now let’s imagine what will happen when we turn on the exhaust fan in this car parking basement. Flow from the ramp will develop & move towards the exhaust and after some time the flow becomes fully developed. When we examine the fully developed flow, we can identify multiple flow phenomenon. The first and most prominent flow phenomenon is the main flow current (MFC), this MFC will originate from the fresh air and move towards the exhaust and will have high velocity. The second phenomenon of interest is the re-circulation region, these re-circulation regions will often be found in the periphery of the MFC and are one of the unfavourable flow phenomenon. The third phenomenon of interest is the stagnation regions; these are regions that have low or no air movement. Now that we have classified the flow profile based on their characteristics, we can see what happens when cars enter the basement.
When cars enter the basement, they emit contaminants through their exhaust. When the contaminants are emitted inside the MFC they are transported out of the basement as the MFC moves all the contaminants towards the exhaust. When the contaminants are emitted in a re-circulation region, it will be sucked to the centre of the re-circulation region. This is because of the centrifugal force present in the re-circulation region. Larger the re-circulation size and velocity magnitude the stronger the centrifugal force and higher the concentration of the contaminants at the centre of this region. Any contaminates that are emitted in the stagnation region, they simply stays there and gets accumulated when more contaminants are emitted here.
So it is clear that the contaminants are accommodated by the stagnation & re-circulation regions. If we are able to remove these Undesirable Flow Phenomenon’s (UFP), then we have removed locations of high contaminant concentration inside the basement.