Case Study QF Computational Fluid Dynamics

by Lydia Denton, Courtney Boulter and Gilberto Osornioi

In the realm of Building Physics analysis, Computational Fluid Dynamics (CFD) modelling of stadiums can present a distinct set of challenges. In the case of the Qatar Foundation (QF) stadium, Buro Happold (BH) was assigned the task of providing the client with a comprehensive CFD analysis of the thermal comfort conditions on the field of play (FOP) and in the spectator seating areas.

Hosting the FIFA World Cup event in the Middle East poses a wide array of challenges, with the most significant being the mitigation of the effects of extreme heat throughout the year. In 2022, the FIFA World Cup was rescheduled for the winter months to ensure more comfortable conditions. Nevertheless, even during this period, temperatures average around 29.5°C throughout November.

The design team determined that meeting the cooling requirements for the bowl and pitch would imply substantial financial and carbon costs. As a result, the mechanical team at Buro Happold collaborated closely with the architectural design team to devise a series of solutions involving "micro-environments". This endeavour demanded the expertise and experience of the analysis team at Buro Happold, led by Gilberto Osornio Nieto PhD. Specifically, a series of Computational Fluid Dynamics studies, in conjunction with wind, Mean Radiant Temperature (MRT) and radiation calculations, were essential to address the design and performance challenges.

Aspire Zone Foundation for Sports Excellence (ASTAD) took the initiative to establish a Supreme Committee (SC), entrusted with the formulation of comprehensive guidelines. These guidelines amalgamated crucial weather data and imposed stringent criteria and prerequisites that demanded strict compliance. Furthermore, Peer Reviewers from Qatar University, appointed by the SC ASTAD, had meticulously scrutinized all models and subjected them to a series of rigorous tests, constituting a stringent peer review process. This thorough evaluation and testing phase posed a demanding challenge for any Computational Fluid Dynamics (CFD) modeler. Achieving a favourable outcome under such scrutiny marked a noteworthy accomplishment.

A complex dynamic model was used to review the solar exposure impact during operation. For the November tournament, notable reductions were achieved in heat accumulation and the extent of solar coverage on the tiers. In contrast, addressing the solar coverage challenge in the Legacy configuration during the remaining months of the year proved difficult. The CFD was used to improve the performance of the ventilation system through the modelling of several scenarios; that included the fine-tunning of air flow rates, distribution over the bowl and changes to the oculus design The final design included air supply distribution under the spectator seating which funnelled cool air down to seating and standing areas. The flow of air was distributed specifically to deal with solar loads at the legacy configuration to provide a comfortable thermal sensation with the right temperature and humidity for spectators, while at the same time recirculating and recovering to save energy.

The field of play posed its own unique set of considerations. The primary challenge that emerged for both the field of play and a section of the lower tiers pertained to the so-called scouring or scooping effect. This phenomenon involves the entrainment of wind through the oculus, triggering a recirculation pattern that displaces the cooler air supplied to the lower tiers and the bowl, ultimately replacing it with warmer air. Addressing this intricate effect necessitated the incorporation of wind aerodynamics across the building's exterior and the oculus.

The analysis undertaken served as a pivotal guide for design decisions concerning the dimensions of the oculus and its apertures, as well as the geometrical factors instrumental in mitigating the scouring effect under prevalent wind conditions. This intricate analysis required a boundary width exceeding 2 kilometres. An initial iteration illustrating the scouping effect is presented in below.

The CFD model of the stadium incorporates calculations for wind, solar, and thermal physics to track the airflow's impact within the bowl. It assesses resultant temperatures, standard effective temperatures, and wet bulb temperatures in accordance with SC criteria. The model also evaluates the effectiveness of implementing "micro-environment" solutions. The main challenge lay in accurately simulating the distribution of airflow from the provided cooler air slots and monitoring how this correct airflow distribution improves temperature and humidity levels, especially in areas influenced by direct or delayed solar radiation and external warm wind effects.

These conditions had to adhere to the stringent Standard Effective Temperature (SET) targets stipulated by FIFA regulations. Additionally, they needed to align with the comfort criteria standards established by the Qatar Supreme Committee and adhere to CFD modeling guidelines.

The team faced an unprecedented challenge of creating a suitable mesh. The large aspect ratio, with external boundary cells spanning up to 30m and inlet diffusers situated below the tiers 10cm in size, as well as the size of the mesh was a considerable challenge. Gilberto Osornio Nieto, Associate at Buro Happold, commented “It is the largest mesh made in a simulation over the past 20 years at Buro Happold, above 100 million grid cells!” BH needed a software that could handle these challenges to get the job done in an efficient and accurate manner.

Not only that, but the escalated intricacy of modelling, resulting from the expansion levels on the surface, the intricate geometry of the skin, variations in porosity, and the incorporation of solar radiation, led to heightened complexities in setup conditions and simulation challenges that demanded greater computational resources.

The meshing software employed by the team was Harpoon, developed by Sharc Ltd. Harpoon expertly addressed and resolved the challenges. It adeptly accommodated the substantial aspect ratio disparities and demonstrated its remarkable mesh flexibility. Furthermore, the produced mesh exhibited notable stability in various scenarios, including intricate calculations involving radiation and moisture. Large Eddy Simulation (LES) time-dependent calculations were executed over a 20-minute timeframe, at intervals of either one or half a second. This incorporated the integration of thermal energy, with the model running until achieving complete moment convergence.

he meshing process was carried out using a Hex/Prism mode, incorporating a boundary layer for both the ground and the stadium's exterior surface. In one specific scenario, porosity was applied to the stadium's skin, involving four cells within the skin's thickness. This was crucial for capturing the wind effects and how the wind interacts with the roughness of the skin. The maximum grid cell size was 32 meters at the outer edges of the 2-kilometer domain, and it gradually reduced to as small as 0.05 meters at the inlet diffusers located at the tiers. This finer mesh was necessary to accommodate four cells per thickness of the linear diffuser. Notably, Harpoon proved to be particularly suitable and efficient in terms of both time and capacity, enabling the generation of a stable mesh.

The QF stadium models, made by the BuroHappold Engineering team, underwent a rigorous evaluation. Each model went through scrutiny and testing at Qatar University. The analytical work was commended by the peer review team. The mesh generated in Harpoon showed significant refinement and expansion across the inlets and the complex skin of the stadium. The inflation layer across the skin was able to capture the Coandă effect of the wind reattachment across the geometry of the roof.

After conducting extensive computational work that utilized a cluster of high-performance computing nodes comprising up to 520 cores, the results demonstrated successful compliance with comfort standards as outlined in the Supreme Committee guidelines. Under the 'Legacy' mode, these conditions were met for 91% of the seating during all times and for 99% starting from 4 pm onwards. This achievement provided valuable insights for the design process, the modelled scenarios illustrated the shading at different periods before and after kick off in different specific areas. Adjustment of airflows and supply distribution was proposed to align with the design objectives and ensure adherence to both FIFA and Qatar's Supreme Committee regulatory requirements.

The utilization of Harpoon played a pivotal role in the success of the analysis process. The speed and reliability of this meshing software were absolutely crucial in tackling the complexities of this computational problem-solving endeavor. This unprecedented project stands as a remarkable achievement within BuroHappold. Peer reviewers from SC for Delivery and Legacy Qatar conducted a thorough examination of all the CFD studies and expressed their full endorsement of the work. They commended these studies as some of the most robust, well-executed, and successful stadium assessments they had ever evaluated.