Improving indoor airflow assessment through low-cost experimental and numerical methods

Abstract

This thesis explores and evaluates low-cost, efficient assessment methods for predicting indoor airflow and ventilation to support more accessible, performance-based building design. It addresses two aspects of indoor airflow assessment: (1) automating point-wise measurements, and (2) reducing the computational cost of RANS CFD simulations by leveraging different assumptions and geometry simplifications.

The first part of this thesis explores automating point-wise measurements of air velocity magnitude and temperature to reduce their operational cost. To that end, I designed and validated a line-following robot that stops at marked sampling points and conducts measurements. This line-following robot performs measurements with negligible impact on measurement quality and is four times faster than a human operator.

The second part focuses on identifying the most suitable numerical assumptions that balance the accuracy of the results and the computational cost for modelling breathing jets with background airflow ventilation. I conducted a validation study comparing measurements from the robot and the literature, with results from three computational fluid dynamics solvers that employ three different assumptions about the flow: incompressible isothermal, incompressible thermal, and compressible thermal. Compressible thermal flows model the breathing jet most accurately without increasing computational cost.

Therefore, using the compressible thermal solver, I finally propose a framework for indoor furniture’s geometric level of detail (fLOD) to systematically study the effects of modelling detail on airflow prediction and to reduce the computational cost of mesh generation for CFD simulations. Representing the same furniture at different fLOD can result in differences in airflow velocity up to 100% of the ventilation inlet velocity. Additionally, the ventilation regime, especially the positions of the inlet and outlet, can significantly amplify those differences, at least doubling velocity magnitude differences, tripling temperature differences, and more than quadrupling scalar concentration differences for the same furniture across different fLODs. The choice of fLOD should be guided by the intended application, the variable of interest, and the ventilation regime under consideration.

In conclusion, this thesis has developed low-cost assessment methods for indoor airflow to enable ventilation-based performance-based building design. It has shown that low cost is not necessarily tied to reduced accuracy. This thesis is a first step toward more feasible, iterative design processes that include characterising airflow and ventilation to improve indoor air quality.