Methodology for the Evaluation of Natural Ventilation in ... - Cham

It is more common to compare similar reduced-scale models with full-scale prototypes, asChandra et al. (1983) did in investigating the natural wind on a 1/25 th scale clear plastic modelwith the full-scale prototype building next to it. Also common are wind tunnel investigations ofairflow due to wind-driven ventilation.Wind tunnel experiments examine the pressure fields on the façades and the internal flows.Ernest et al (1991) compared both internal and external measurements of a reduced-scale modelin a wind tunnel to determine indoor air motion as a function of wind direction. Depending onthe scale and the model construction materials, the determination of flow patterns can be donethrough flow visualization or measurements. When comparing the scaled wind models to theprototype, one common issue is the variation of the wind speed and direction that is experiencedby the prototype building versus the normally constant wind speed controlled in the test chamberor wind tunnel. Depending on the frequency of the data measurements, some wind tunnels areoutfitted with a turntable, so that the direction of approach of the wind speed can be varied, asdone by Dutt et al (1992).4.1.6 Combined Buoyancy-Wind ModelingCloser to the actual building case, is the more complex combined wind and buoyancy drivenflow. The wind direction in this case has a significant impact on the flow direction through theopenings. The wind force can either work with or against the buoyancy force. Li and Delsante(2001) presented the analytical solutions to this complex natural ventilation case, and determinedthe stable solutions based on the strength of the heat source and wind. Salt-water models (Huntand Linden 1996, 1997, 2001) have been used extensively to evaluate combined ventilation toprovide quantitative results, including flow velocities and densities. Their model is for a simplesingle space configuration with multiple upper and lower openings. Additionally, some workhas been completed using CFD simulations to evaluate the combined buoyancy and wind forcesin a naturally ventilated building as a method for validating heated water models (Heiselberg etal. 2004). They furthered the analysis by investigating the effect of wind direction in enhancingor hampering the effectiveness of the flow through a simple space.Combined buoyancy-wind driven natural ventilation is complex, with some concern over howbest to model and analyze this complex case. As described in Chapter 2, it is often not as simpleas adding the equations of each type of ventilation. Nevertheless, combined buoyancy-windventilation occurs most often in real world applications, and therefore is an important scenario tocarefully model and understand for application to full-scale buildings.4.2 Flow VisualizationMeasuring airflow at inlet and outlet openings and temperature distribution is not enough tocomprehend fully the airflow patterns within the space or interactions between spaces within abuilding. Flow visualization has historically been used to understand complex fluid flowpatterns. In buildings, it plays an important role. In naturally ventilated buildings, this isparticularly true in understanding how air will enter, flow through, and exit a space, floor, andbuilding. These different scales of airflow require different characteristics in the media used tovisualize the flow patterns. The application and scale of the space being evaluated and fluid usedin scale modeling have a significant impact on the visualization technique used. Issues such as76