The ventilation measurements described in Chapter Six have provided an estimate of ventilation rate at a point in time, i.e. for a specific wind condition. The ventilation performance of a courtyard house can be studied in more detail using a combination of wind tunnel scale modeling ( to establish pressure coefficients at the triangular openings for different wind directions) and computer simulation modeling ( to provide ventilation rates and patterns indoor air movement). The best existing example of a typical courtyard house was the first case study, Prince Nasser’s palace. The wind tunnel experiment was therefore carried out to measure the pressure coefficients at the openings, and these pressure coefficients were then used to set the boundary conditions at the openings during the computer simulation of ventilation rate and indoor air movement described in Chapter Nine.     

This chapter describes the scale modeling study of the courtyard house in the wind tunnel. A model was built for the chosen house, to perform the pressure coefficient (Cp) tests at points corresponding to the location of the triangular openings. These values will be used in Chapter Nine for the computer simulation tests (CFD) which will give the ventilation rates and air movement for the selected test rooms. Also, smoke visualization tests were performed in the wind tunnel to study the patterns of wind flow over the house and in the courtyard. These tests also involved visualization of air movement inside the courtyard using small polystyrene balls. The movement of these tiny balls indicated  the areas of relative strength of air movement in the courtyard.

In conducting wind tunnel experiments it is intended to simulate two regions of flow near and above the model: the boundary flow near the floor of the tunnel and the model, and the main flow above it. The use of the wind tunnel in environmental experiments  enables the simulation of natural wind on a scaled model in order to measure its effect on the model which can then be translated to the real building.

The boundary layer is a layer of air which covers the earth at a varying thickness that is dependent  on the height of the surface details, and above which the surface friction has no effect on the flow of  wind. Air movement can be divided into two flows; the main flow, where ground friction has no effect, and the boundary layer flow, where friction has an effect. The boundary layer thickness may be typically 166m over large bodies of water or 500 m over large cities. The movement of air in this layer is generally turbulent  with rapid changes in speed and direction. There are three factors which affect this motion, temperature, surface  roughness (which determines the thickness of the boundary layer) and distance from the earth. Figure 7.1 illustrates the effect of ground roughness on the boundary layer thickness and wind flow gradient. As the distance from the earth increases, air speed increases and turbulence decreases (1).

Wind gradients and boundary layer (6)

For wind tunnels to achieve the best wind simulation they should be long enough to allow the boundary layer to grow over their length. For shorter wind tunnels, to achieve this, they should employ a number of roughness devices such as, wooden blocks or Lego, which is used in the wind tunnel at The Welsh School of Architecture, Architectural Science Research and Development (6). Click here t see more about this.

Stage One: Choosing the House

Stage Two: Building the model

Stage Three: Scaling criteria

Tunnel Setting

Test Results