Computational fluid dynamics (CFD) is the science of predicting fluid flow, heat transfer, mass transfer, chemical reactions and related phenomena by solving the mathematical equations which govern these processes using a numerical process. Today, the real-world applications of CFD can be found in the built environment, chemical & petrochemical, defense & security, food & beverage, manufacturing, marine & offshore, mining & mineral processing, oil & gas, pharmaceutical, renewable energy, semiconductor and water & wastewater industries.
An example where CFD simulation is applied in the built environment is the study of wind-driven rain (WDR). Many people have not heard of the term ‘wind-driven rain or WDR’ until they realise their insurance policy does not cover damage to their properties caused by WDR. Wind-driven rain, as the name implies, is rain droplets propelled by wind blowing from various directions. WDR concerns public and private buildings because it can damage these buildings and reduce the occupants’ comfort and safety. A good example is above-ground train stations which in many cases were designed to rely on natural ventilation to increase passengers’ thermal comfort. When it rains, stations in which the design only addressed natural ventilation but not WDR mean rain droplets carried by the winds will ingress into the station. An approach to minimise if not prevent the ingress of WDR is to employ CFD performance-based assessment.
The objective of performing WDR simulations is to determine the effectiveness of a building’s weather protection features whilst still maintaining the effectiveness of its natural ventilation capability. Performing WDR simulation using CFD will address these questions: Is the weather protection effective against WDR? If rain droplets can penetrate the interior spaces, where are the exact locations, what is the extent, distance and quantity? Which façades are affected the most? Even if the weather protection is effective against WDR does it compromise the natural ventilation of the station? How effective are the louvres in allowing sufficient air to enter the interior space whilst keeping the rain droplets out? Thus, the objective of WDR simulation is to minimise the ingress of WDR into the interior space of the station in order to increase the comfort, enhance the safety of the passengers and prevent rainwater-induced damages to the station.
To simulate WDR, typically four rain droplet sizes, each with its own terminal velocity and drag coefficient, will be modelled and simulated. The reason for simulating rain droplets of various sizes is because each of these rain droplets responds differently to wind. For example, smaller rain droplets tend to be carried by the wind and follow its direction whereas larger heavier droplets, due to their mass, tend to travel based on their own trajectories, which are mostly straight lines and are less affected by the mean flow of wind. Prevailing wind in eight directions distributed radially at a 45o angle, obtained from Bureau of Meteorology, will be modelled and simulated. An approaching train will also be factored into the numerical model since it can pull the rain droplets into the interior space of the train station. It is well established that, when simulating natural ventilation, it is important to include neighbouring buildings because these buildings can block the wind and alter the wind direction. However, in WDR, consideration should be made to model the building of interest by itself without neighbouring buildings to simulate the highest wind velocity if the existing neighbouring buildings are pulled down in the future and they should also be modelled and simulated with neighbouring buildings as well because these buildings can alter the direction of wind and thus the rain droplets trajectories. From here, the results from the worst of the two scenarios can be employed to fine tune the station’s design.
Modelling & simulation of wind-driven rain via CFD can help the design engineer or architect to achieve a balance between the ingress of WDR into the station and passengers’ experience from using the station. Most importantly, if conducted as part of the building envelope assessment during the design phase, it can prevent costly rework of the station post-construction.
Some large firms may want to purchase the CFD simulation software and employ a specialist to run simulation, improve designs and produce reports. Others will likely find it more feasible to engage external CFD consultants as they may only have to run simulations three or four times a year.
CFD consultants will require two to four weeks to gather the necessary information, develop the 3D model, run the simulation and write up a report. However it is carried out, CFD simulation is a powerful investment in a project, with positive outcomes for all stakeholders.
Simulation showing interaction between rain droplets and the station
Simulation to study the effectiveness of triple-bank louvre against WDR
Jimmy Lea P/L