Wind tunnel investigation of natural ventilation through multiple stacks. Part 2: Instantaneous values

Wind tunnel experiments have been performed on a scale model to study unsteady natural ventilation through multiple stacks. Part 1 of the paper concentrated on the characteristics of the mean flows. Part 2 concentrates on the instantaneous characteristics of the flows, both in terms of the experimental measurements and in terms of an unsteady envelope flow model. Further investigations into the experimental measurement techniques are described. These relate to improving the hot-wire calibration, to the instantaneous flow balance with multiple stacks (and the importance of model rigidity) and to the increased importance of internal air motion with the higher flow rates associated with multiple openings. The experimental measurements of stack reversal percentage are examined in some detail, regarding the effects of wind direction and Reynolds number and how the reversal percentage relates to a simple pressure parameter. Following on from Part 1, the effect of opening configuration on the instantaneous properties of the external wind pressures is investigated by examining their correlations. As with the mean values it is found that the correlations are not entirely independent of opening configuration. The experimental data is used to assess the performance of an unsteady envelope flow model, both in terms of calculating instantaneous values and mean values such as reversal percentage. The model is shown to perform well with multiple stacks. It is then used to estimate the effect of Reynolds number on the reversal percentage at model-scale and full-scale.