Effects of unsteady room ventilation on flow structures and the velocity field in a model room

Typically, ventilation systems are used in a steady-state operation, although studies suggest several advantages of unsteady room ventilation, but the underlying mechanisms of action have neither been fully understood nor described. The influence of periodically varied supply volume flow rates on ventilation-relevant characteristics (e.g. mixing, velocity distribution) in a small-scale model room is investigated by full-field optical flow measurements with a particle image velocimetry (PIV) system in this study. In a series of experiments, the effects of different unsteady ventilation strategies on the flow fields are investigated and compared to steady conditions with the same mean exchange rate. Statistical analyses of the velocity field in the room’s longitudinal axis are performed to understand the effects on mean and instantaneous velocity fields. A vortex identification method is used to evaluate the number, size, and distribution of vortices in this plane, and coherent structures are analysed using spectral proper orthogonal decomposition (SPOD) to understand the influence on mixing and flow structures. The number of vortices depends on the inlet conditions: if the volume flow is temporally varied, the number of vortices also changes. FFT analyses of the number of vortices indicate high magnitudes in the period length of the volume flow variations.