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TECHNOLOGYfocus CFD for urban design Naghman Khan PhD, Product Marketing Engineer at SimScale GmbH explains how CFD is used to simulate the effects of volatile wind conditions in the proliferating canyons of urban cities Figure 1: CFD simulation results analysing pedestrian wind comfort around the famous 'Walkie Talkie' building in London Figure 2: Tall buildings significantly impact the microclimate of their surroundings and require CFD simulation to capture their impact correctly Computational fluid dynamics (CFD) allows designers and engineers to simulate fluid motion using numerical approaches. A wide range of problems related to laminar and turbulent flows, incompressible and compressible fluids, multiphase flows, and more can be solved using CFD tools. Recent applications in the construction industry have enabled a more advanced treatment of microclimate considerations when designing buildings and cities. In addition to this, the built environment is expected to withstand the pressures of a changing climate which has become increasingly volatile. Fortunately, a building or city's resilience to climate unpredictability can be ensured with testing and simulation to validate designs against external climate factors. Architects and engineers can iterate and innovate faster with easy and accurate CFD simulations on the cloud. WHY USE CFD? CFD is the mathematical modeling of the behavior of fluids using software. Online fluid dynamics analysis allows a designer to create a digital model of a building and simulate how it will respond against atmospheric and environmental variables such as wind velocity and direction. A user can run multiple simulations of their design congruently, testing against various wind profiles, atmospheric conditions such as gusts and storms, and also import CAD models of neighboring buildings to evaluate how a proposed development will impact an existing site. Figure 1 shows the average velocity at the pedestrian head level (1.5 m) for one wind direction. Here, architects and urban planners can quickly identify higher wind velocities around the corners of affected buildings. This cornering effect can have an even more substantial impact when two opposing buildings are subject to it and when the street is parallel to the wind directionand it can be observed on the narrow streets on one of the sides of the building. Uncomfortable areas are the ones with 8 m/s wind velocity and above, which are depicted in the orange and red zones. Simulations like these, considered at the early stages, allow designers to derisk their urban designs from a microclimate perspective. A user can quickly iterate the setup and run comparative analyses. Traditional onpremise fluid dynamics tools take significant computational hardware resources and time to solve a single simulation. In the example above, a study of 16 wind directions on standard CFD software might take 8 hours for each direction, totaling 128 hours. A new generation of CFD solvers, based on lattice Boltzmann method (LBM) codes, can now solve transient CFD simulations an order of magnitude faster, meaning a design team can simulate the same example above at 2 hours for one wind direction (Instead of 8 hours). The GPU accelerated microservices architecture employed by cloud CFD tools also means that simulations can be performed in parallel. Using the same example, all 16 wind directions would run 26 May/June 2021
TECHNOLOGYfocus Figure 3: CFD is used to simulate the impact various types and quantities of trees have on the wind comfort profile of an area Figure 4: Different tree species can be simulated using CFD to evaluate their impact on the local microclimate simultaneously, and the total simulation time would still be 8 hours. An architect or urban designer can simulate dozens of urban layouts in parallel, saving time and cost. THE BENEFITS OF CFD Urban areas have many environmental factors that need accounting for when designing buildings and cities. A city will be subject to uneven heating and cooling as the sun rises and sets, depending on its built form and skyline. Some areas will be shaded, while others are in full receipt of the heat gain from the sun, leading to convective air currents that impact the local microclimate. Wind profiles in urban areas are artifacts of many aerodynamic and atmospheric forces; from surface roughness to turbulence caused by urban canyons, these factors tend to be transient phenomena and are difficult to predict. They require mathematical modeling to understand their behavior and interaction with the built environment. A CFD simulation can accurately model such phenomena, including pedestrian wind comfort, building aerodynamics, urban heat island effect, and the impact of solar shading. Specific types of urban design and building typology also require additional treatment. Tall buildings for example (Figure 2) tend to be sensitive to small changes in atmospheric conditions, such as wind pressure and turbulence. These atmospheric changes create aerodynamic effects, including downwash and corner acceleration, that negatively impact local pedestrian wind comfort. They also significantly impact overall building performance and are not captured in standard building energy design tools used in the industry. Coupling these tools with CFD, however, is a quick way to increase the accuracy of thermal and energy models used for design and compliance. APPLYING CFD - THE CASE OF GREENING STRATEGIES A practical application of CFD can compare urban design layouts and greening strategies (Figure 3). Trees and vegetation are increasingly being used to pedestrianise areas that were initially not intended for people. Trees hinder the effect of wind as it passes through their leafy canopies. A grouping of trees as part of the streetscape or local amenity tends to dramatically impact an areas' character and physical appearance. With this in mind, we can assume varying types of trees offer more or less resistance to the wind flow. For example, the Sycamore tree has less air resistance than a Fir tree (Figure 4), based on its calculated leaf density. This ability to allow fluid (air) to pass through, known as porosity, is measured through experiments, and the results are then defined through a leaf area index and applied in a CFD simulation. SUMMARY CFD is increasingly recognised for its ability to capture advanced building and urban physics and add a level of design assurance/validation to many projects. CFD results provide vivid details and a deeper understanding of how designs perform under various circumstances, allowing more focus on performance at the early design stages for the construction sector and beyond. Cloud-based engineering simulation is unlimited by computing power or accessibility issues, and fosters collaborative working environments for distributed design teams of architects, urban designers, and engineers. This is reflected in recent guidance from local planning authorities. The City of London Corporation, for example, has recently released the wind microclimate guidelines for developers, which specifically mandates the use of CFD during the design stage. It is also seen in standards and planning jurisdictions globally, as the construction industry is forced to face the challenges of an uncertain and changing climate. www.simscale.com May/June 2021 27
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