Impact of opening geometry on the Indoor Environmental Quality in deep, open-plan, naturally ventilated office typologies in temperate climates

Natural ventilation is one means of enriching internal environmental quality (IEQ) whilst reducing the energy consumption of buildings. In an attempt to increase productivity, offices are often deep and open-plan. The typically large energy demands of this type of building indicates the potential to ascertain significant energy savings and a reduction in carbon dioxide (CO2) emissions. Through careful design, the forces associated with natural ventilation can be harnessed and utilised. These small forces mean opening geometry and building form can determine the internal natural airflow patterns, potentially creating unexpected flow characteristics. In these cases, this could lead to a naturally ventilated space overheating, exposing the occupants to thermal discomfort and, in such events, the supply of fresh air may also be reduced.

With respect to an energy conscious environment, some leading architectural firms are beginning to explore the use of high-aspect-ratio (HAR) facade openings to improve the IEQ of these spaces. In addition to this, architects are encouraged by client briefs to design deep, open-plan offices without the typical central core; this being a vital element in any multi-storey building, comprising vertical circulation, services, toilets and lifts. To assist in the ventilation of these spaces, they are often broken-up by impressive atrium/atria. Research suggests, the design of facade openings, atrium and building core may influence the performance of the proposed natural ventilation strategy. The thorough examination of innovative design solutions should address these uncertainties and avoid them where possible. However, there is a lack of detailed evidence and guidance is needed to increase design confidence and the utilisation of optimal office design.

To better understand natural ventilation, computer and physical airflow modelling, such as computational fluid dynamics (CFD) and water-bath modelling (WBM) are used. The use of multiple techniques not only allows the opportunity to validate the results from each, but also increases the amount of highly detailed data in various formats. Nevertheless, such physical models are often under-utilised, difficult to construct and operate, and expensive.

The investigation of the ventilation performance of HAR openings in a generic office environment was carried out using computational models and a novel WBM. Overall ventilation performance was obtained through the installation of the variants of opening geometries into several office configurations with differing core and atrium designs. The utilisation of multiple validated models improved the examination of ventilation performance, thus, providing reliable results to compliment those from the main body of the study. The parametric study was formed of many CFD cases, enabling the juxtaposition of the ventilation performance of all office configurations. The transient nature of WBM and spatial detail of CFD gave the ability to scrutinize issues associated with IEQ; air velocity and thermal stratification and distribution, mean room air temperatures and ventilation flow rates. Therefore, the acknowledgement of the optimal office design allowed the better performing design variants to be highlighted in terms of ventilation performance, also providing a greater understanding of how the design of the space affects IEQ.

To start, an archetypal building, which would be subjected to the alternative design parameters, was established. The WBM's form, enabled multiple experiments to be performed by a relatively cheap and easy to run and maintain WBM, thus, increasing the rigour of the validation process and resultant design guidance. A comparison of the results from both modelling techniques showed them to be working correctly, as errors concerning validation metrics were deemed negligible. Results from the WBM experiments and CFD simulations suggested the supply flow from high-level horizontal (HLH) openings brought warm, potentially stale air down into the occupied zones. Conversely, the flow from mid-level vertical (MLV) openings was shown by the CFD to assist in the formation and lowering of thermal stratification. Nonetheless, significant improvements in ventilation flow rates and cooler air temperatures were possible. Moreover, the ventilation performance of this opening design was independent of the means of air removal.

Additional enhancements were achieved by a transition from a typical to a perimeter core, as the prevalence of unventilated areas and excessive air velocities was reduced. Nevertheless, when specific variants of atrium design were employed into the perimeter core cases, further performance improvements were perceived. The conditions pervading the now open floor plate, were shown to be enhanced by an atrium design closely replicating the building's footprint. It could be said, the optimal core location is on the perimeter furthest away from the atrium. This configuration, when used in combination with MLV fa\c cade openings, was shown to form the best performing.

Substantial advancements have been proposed in the physical modelling procedure of natural ventilation. Further understanding has also been obtained from the results produced, which formed the foundation of the design guidance. The inclusion of these modelling modifications and evidenced guidance throughout the design stage will alleviate performance uncertainties, making the undertaking of naturally ventilated building projects more attractive, simultaneously enhancing the IEQ of low-energy offices.