# Computational Fluid Dynamics (CFD)

Computational Fluid Dynamics (CFD) plays a major role in solving engineering problems. Due to the enormous development of computing capacity, CFD becomes a major design tool to be used at early stage of a project before final wind tunnel testing. We use CFD to offer considerable advantages in many applications:

1. Pedestrian Level Wind (PLW)
Design of buildings has to follow rules and regulations particularly when it comes to pedestrian comfort. Locations and dimensions of buildings relative to each other have an important effect of wind flow and its effect on the surrounding areas. High-rise buildings can face a high wind speed at the pedestrian level leading to dangerous conditions. CFD can accurately predict the wind flow around buildings (figure 1) and therefore optimize their designs and ovoid any unexpected scenarios.

Figure 1-1: Wind flow around Rogers centre

Figure 1-2: Comparison between prediction and measurements

2. Air pollutant dispersion around buildings
One of the major environmental concerns is the air pollution. Indoor and outdoor air pollution are a main problem faced by HVAC engineers when designing ventilation inlets and outlets of a building. Indoor air pollution occurs when outdoor air pollutants are re-ingested inside by the air exhaust of the same building or by nearby buildings. Thus, the design of stacks and their locations on top of a building is very crucial to determine the amount of concentrations of air pollutants and how this distribution of concentrations will affect the design of a building.  CFD offers the possibility of simulating different designs with different stacks locations to determine the best design in terms of air quality. Figure 2 illustrates the prediction of air pollutant distributions on top of a roof of a building ejected from a stack from the same building.

Figure 2.1: Predicted smoke from a pollutant ejected from a stack

Figure 2.2: Predicted pollutant concentrations ejected from a stack

3. Building ventilation
CFD helps engineers and architects studying the flow and heat distribution in confined spaces such as research facilities, hospitals, waste treatment stations, etc. The main objective of this type of studies is to determine the flow pattern inside the space, and also detection of reverse flow areas which can have a negative impact when they occur (health and safety issues). Another objective is to study the occupant comfort inside the building in conjunction with flow and heat distributions (figure 4).

Figure 4-1: Temperature distribution
inside a supermarket facility

Figure 4-2: Air flow distribution inside a waste treatment facility

The re-entrainment of building exhausts, fumes from laboratory vents, or emissions from idling vehicles into buildings can impair indoor air quality, and negatively affect occupant comfort and safety. Some of the factors that can influence re-entrainment are stack location, gas exit velocity, building design such as the location of operable windows or intakes. Building design, location and orientation to wind are also important.  Similarly, exhausts from building stacks can also be dispersed to other sensitive receptor locations in adjacent buildings, pedestrian areas, or open spaces (figure 5). Exhaust re-entrainment and dispersion studies are used to help identify design measures and features that will help to prevent the entry of exhaust into buildings through air intakes or operable windows and the dispersion of exhaust to pedestrian areas or other sensitive receptors. We utilize numerical tools based on ASHRAE methods and computer simulation models to help design teams identify or modify features that will minimize air quality problems caused by building exhaust re-entrainment or dispersion. Improved air quality is important for occupant safety and comfort and building performance.

Figure 5a: Pollutant spills from under-ground pipe being re-injected into the building through a window

Figure 5b: Pollutant concentrations at a window