Leveraging the Full Power of CFD Simulations in the Urban Realm
Creating density in cities is necessary to support the growing population. However, the tall and supertall buildings that provide this density create impacts at the pedestrian level. This presents a complex design challenge that must be met with practical solutions: to create density while maintaining a pleasing urban realm.
In November 2022, the Council on Tall Buildings and Urban Habitat (CTBUH) held a conference in Chicago, Illinois called Tall Excellence: Seeking the Ideal in Vertical Urbanism. The aim was to discuss new challenges facing tall buildings and densifying cities – and how the challenges are forcing us to rethink our approach to tall buildings in cities to create more sustainable urban spaces.
As part of this conference, one of RWDI’s leading microclimate specialists, Duncan Phillips, gave his presentation Top to Bottom: Reducing Uncertainty of Tall Buildings Via Full Environmental & Microclimate Analysis. It focused on updating the status of computational fluid dynamics (CFD) simulation in the urban realm and how, in many instances, urban realm modeling isn’t currently leveraging its full power and potential. This was a follow-up to his 2018 CTBUH presentation and subsequent paper, where he presented on computer modeling for the built environment.
We spoke to Duncan to get the most important takeaways from his presentation.
Q: Why should we simulate the urban realm?
Duncan: Building in the urban realm is becoming increasingly expensive. Understanding how different design elements will impact the surrounding urban environment and vice versa can be invaluable.
Running simulations allows designers to make smarter decisions with more information earlier. When done in the early design stages, simulations can reveal the most effective design options and where adjustments are needed. Building projects can save money by avoiding additional costs of changing design elements in later stages, since that is usually more costly, and avoid post-construction fixes if a need for a design modification isn’t caught before construction.
Q: What are the benefits of early design simulations?
D: Ultimately, a well-tested design delivers a better product – this is true of widgets and buildings. When city planners and building designers take care to design productive, public spaces, it offers, for example, an increased opportunity to engage in biophilic design. This means we use the design of the spaces to connect occupants and passersby more closely to the natural world through elements such as natural landscape features, natural ventilation, and natural lighting.
Leveraging biophilic design means a significant indirect increase in climate resiliency for the urban realm, often through reduced energy demand from buildings as well as better storm water retention. Well-designed open spaces in the urban realm also offer increased commercial value and even economic benefits for businesses, such as increased foot traffic. It even creates better air quality and reduced urban heat island (UHI) effects in the long term. Simulations can help demonstrate improved microclimates and can be used to quantify the benefits and justify any additional cost or schedule impacts.
Q: How are current applications of simulation technology falling short?
D: Generating a picture of the wind speed and direction or assessing the winds at grade is fairly standard. Obtaining the correct results is crucial – and a critical part of achieving this is having a qualified person run the simulations. However, to truly leverage the full power of CFD simulations, designers need to take a holistic approach. This means including additional simulations in a fuller analysis to reduce uncertainty and harness this potential.
A truly holistic approach should include assessing winds at other levels than just at grade, such as terrace, mid-terrace, and rooftop terrace, as well as during all seasons and at all times of day. It should also involve simulations exploring whether low wind periods make conditions stagnant, predicting the distance that wind can drive rain under canopies and other semi-outdoor spaces, as well as thermal comfort analyses. Thermal comfort analyses should assess urban heat island (UHI) and balance wind-driven rain with thermal comfort and urban ventilation.
Q: What is the biggest challenge in the simulation and modeling process?
D: Although the various types of CFD have their benefits and uses, they do not deliver equal results in all scenarios. In several scenarios, for instance, to truly access the power of CFD, non-standard methods are crucial.
As an example, for the most part, buildings are non-aerodynamic shapes, which aerodynamicists refer to as “bluff bodies.” Airflow abhors going around bluff bodies. When air comes to a sharp corner, instead of turning quickly and hugging the surface, it turns more slowly in a smoother fashion, separating from the surface of the building creating a separation bubble. The pressure is lower inside the separation bubble, which can be both good and bad – quality design takes advantage of this. As airflow travels along the face or rooftop of a building, it recovers and reattaches to the surface.
Getting the full, accurate picture when predicting the size and strength of this bubble, as well as when it will reattach, requires non-standard CFD, such as large-eddy simulation (LES). LES does a better job representing the turbulent field, therefore providing increased accuracy when modeling the effect of wind flow separating from and reattaching to the surface.
Q: What does the future of CFD look like?
D: Wind tunnel testing and CFD will continue to intertwine and better complement one another over time. As they do so, we will be analyzing more complicated scenarios, such as the potential impacts of typhoons and hurricanes.
Simulation will take on resiliency issues. For example, simulating the impact from severe storms, such as wind-driven rain (WDR) from typhoons and hurricanes, will contribute to making buildings safer. Simulating WDR from a hurricane is different from a common storm because of the physics involved. Different wind profiles, more wind directions, and higher speeds change how this is done.
Simulations will be faster: we expect that in the future, estimates of certain urban parameters will be available in minutes. This is something that can be achieved through a variety of means, including faster computing clusters, improved numerics, as well as neural networks and artificial intelligence. In fact, there are now artificial intelligence (AI) generated predictions of urban wind flow that can be developed in a couple of minutes.
It will eventually be possible to accurately simulate the most challenging urban realm physics. For example, predicting dispersion from rooftop stacks or similar point sources has proven to be problematic, as the range of turbulence scales poses a challenge in making accurate predictions, coupled with accurately predicting entrainment into the exhaust’s jet or plume.
Q: What is the number one takeaway from your presentation?
D: Computational methods, like CFD, are here to stay. However, much of the current modeling in the urban realm isn’t leveraging the full potential of simulations. CFD simulations can do so much more than simply generate a picture of the wind speed and direction. The future of CFD is a bright one as its capabilities are set to continue growing.
