Thermal and CFD modelling vs. wind tunnel in natural ventilation studies

A major case study of a high rise, city-centre of ce building in continental Europe was undertaken in a true commercial environment. Three dimensional thermal and Computational Fluid Dynamics (CFD) modelling was carried out for major spaces in parallel with wind tunnel tests and results compared. Three different methods were used to assess the ventilation rate in the wind tunnel tests: surface pressure coefficients, tracer gas and direct velocity measurements. The objective was to obtain boundary conditions for interconnected3-storey gardens and 9-storey atria in order to calibrate and fine-tune mathematical models. External air flow was also modelled in a CFD simulated wind tunnel. For these simulations only a very limited surroundings were entered. This was considered as insufficient for reliable generation of boundary conditions so only qualitative comparisons with wind tunnel smoke tests were made. Results from wind tunnel test showed inconsistencies and scatter well beyond expectations. This was attributed to choice of probes and their positioning. These causes were particularly obvious after the initial CFD results. The paper concentrates on comparison of physical testing methods with mathematical models and exposes some of the practical and theoretical limitations of both techniques.