The significance of time step size in simulating the thermal performance of buildings

The determination of the internal air temperature of a building is pivotal to its design in the determination of heating and cooling loads and the assessment of the thermal comfort for the occupants. Autodesk CFD analysis is one of many application programs which can assist in calculating building internal air temperatures and it is expected that using a smaller time step size can provide more accurate results but long term simulations for complex building analysis is significantly lengthening computing time. To solve these issues a larger time step can be used. This speeds up the computing time and also results in a higher predicted (and more realistic) internal temperature fluctuation range. This is reported in this paper, where the larger time scale technique is used with an average capacity personal computer (PC) machines to perform the simulations. This fast Computational Fluid Dynamics (CFD) simulation method is used to simulate the thermal performance of a series of existing housing test modules constructed using a range of walling systems. The performance of the proposed computing technique is assessed by comparing the internal air temperature of each building at the floor level. To find the time step which gives the most accurate simulation of the measured internal air temperature, CFD simulations were carried out for various time steps (15, 30, 60, 80, 100, 120, 150, 180 minutes); it was found that 80 and 100 minute time steps gave the most accurate representation of the real fluctuation. The fastest simulation with the most accurate results was for a 80/100 time step where more than 87% of the results fell within a 3°C range compared to the real data. This also required only 1% of the computing time compared to a 1 minute time step.