A COMPARISON OF WIND TUNNEL AND CFD METHODS APPLIED TO NATURAL VENTILATION DESIGN

The design of a naturally ventilated atrium was assessed using both wind tunnel and CFD methods to appraise and modify the response of the system to wind forces. The initial design was expected to be susceptible to flow reversal due to wind forces opposing and ultimately defeating buoyancy forces. Several design options were assessed by both methods. Both the methods were able to provide good information to guide the design development. Crucially, the information and guidance from both methods was consistent; that is either method could have led the design development to a similar final result. Each method has, of course, advantages and limitations, and to some extent these are complementary. INTRODUCTION When designing a naturally ventilated building a knowledge of wind pressures on the external openings are often required to allow the prediction of ventilation performance. In many cases ventilation due to buoyancy alone (i.e. a low or no wind condition) will not be the “worst case” scenario, and the effect of wind must be considered during the natural ventilation design. Wind pressure data can be generated either through wind tunnel measurement or computational fluid dynamics (CFD) predictions. While advanced CFD codes, and wind tunnels, will remain in the hands of design consultants, there have been improvements in the availability of high-powered computing and CFD software targeted at the design field. The question arises; do the low-end CFD methods now becoming available provide the same design advice that may be produced through design consultancy using more sophisticated tools? In the course of a recent investig ation [Jones, Alexander], the design of a naturally ventilated atrium was developed using wind tunnel methods to appraise and modify the response of the system to wind forces. The opportunity was taken to compare the wind tunnel results, and in particular the direction the design improvements were being lead, with those that could be obtained by a limited use of CFD, such as may be feasible within a design practice. The CFD calculations were carried out using the commercially available code FLOVENT . BUILDING DESIGN The investigation was carried out on the design of the Saga Group Headquarters. This is to be a prestige commercial office building, designed by Michael Hopkins and Partner, with Ove Arup and Partners (Figure 1). The natural ventilation strategy is intended to control the thermal conditions in the atrium space, in order to prevent summertime overheating. The office spaces are mechanically ventilated. The designers’ preferred solution was to have inlet and outlet openings on the front facade of the atrium at low and high locations respectively, leaving the top of the atrium structure free to be used as a terrace for the director's suite. The building site is located on the south coast of the UK, with the atrium facing southwards to the sea. Therefore on-shore winds are prevalent and concern was raised over the effect of wind pressures on the use of the atrium facade openings as air inlet/outlet devices. Initially wind tunnel measurements were undertaken to investigate this issue. WIND TUNNEL MEASUREMENT The wind tunnel at the Welsh School of Architecture is an adiabatic atmospheric boundary layer wind tunnel with a working area of 4m x 2m x 1m high. It can be configured for a number of different atmospheric boundary layer profiles. The site to be modelled in this investigation is on a rising slope, facing the sea. There is low level building to the south and east and woodland to the west. The vertical wind velocity profile chosen for these tests simulated that found in open areas, terrain category 1, figure 2. This was felt to be the best approximation of the site conditions for prevailing winds. The boundary layer simulation is adequate for models occupying the lower 1/3 of the windtunnel. A physical scale model of the proposed building and its surroundings were made to a scale of 1:250. The model office building was approximately 30x30x15 cm in size. The office block model is shown in figure 3. Pressure tapping points were placed at 46 locations on one office block model, mainly on the atrium facade. Pressure measurements were made using a tunnel air speed of approximately 8m/s. This speed would allow sufficient flow around the model to provide adequate modelling of Reynolds scale effects. Averages of the surface pressures were taken at each point over 10 seconds. Due to the time scaling of the modelling process, this is equivalent to an average over approximately 1 hour at full scale. Thus short term turbulence effects are not considered in these results. The primary data produced from these tests are mean wind pressure coefficients (Cp) for each tapping point. Cp is a dimensionless coefficient denoting the ratio of the wind pressure at a point on the surface to the potential free air wind pressure for the site at some reference height Z. For these tests the reference height was the buildings roof height. Reference pressures were measured upstream of the model building. The pressure coefficient data can be used to estimate the mean wind pressure at a point on the facade for any wind speed from the relation; P = Cp * (1/2ρUz2), where ρ is the air density, and Uz is wind speed at height Z, in m/s, P is pressure in Pascal (Pa). Cp values are of course dependant on wind direction. Initial testing of the design indicated that for the prevailing wind, the proposed vertically sloping outlet areas would have a relatively high positive pressure relative to the bottom of the atrium facade (the proposed inlet area). This wind induced pressure difference would oppose the buoyancy pressures set up inside the atrium due to solar gains, as illustrated in figure 4. This could lead to • flow reversal at high wind speeds, a continuous air entry at the top of the atrium, or, more importantly, to • the wind and buoyancy forces balancing and negating each other. This latter case would leave minimal ventilation caused only by wind turbulence. The wind tunnel measurements suggested this could occur at moderate wind speeds (i.e. 3m/s). This was clearly undesirable and thus several design options were assessed. An obvious solution to the problem would be to move the outlet to the terrace area at the top of the atrium, where there were strong negative pressures. However, this was not suitable for architectural reasons. Therefore, a number of fixtures and devices were tested, to attempt to produce a relative negative pressure at the sloping vertical outlet. The most promising of these was a 'wing' type of wind deflector which shielded the outlet from direct wind pressure, and promoted a smooth air flow past the outlet. Wind tunnel testing indicated that such a device would reduce positive pressures at the top surface of the atrium and so increase suction pressure at the atrium outlets. With such a device, wind and buoyancy forces worked together, through a wide range of wind directions. A range of variants on the wing design were tested, these are shown in figure 5. The effectiveness of the device was found to be sensitive to the size and placement of the wing, and the effect could be disrupted or negated by the addition of solid shading devices or balustrades on the terrace area. The alterations suggested through this work have been incorporated in the final design, as shown in figure 6. This correspond to case e in figure 5, with both the solar shading and balustrade in the final design being open or porous. CFD As part of the research exercise, the design investigation was repeated using CFD methods. The intent was to compare the design advice generated by the two methods. Flomerics FLOVENT version 1.401/33 was used for this exercise. FLOVENT ® had been in use in the research group for some time, but previously had not been used as a “numerical wind tunnel”. This exploratory use of the tool would arguably emulate the application CFD by members of the design team rather than by external consultants. The simulations were carried out on a standard 90mHz Pentium PC computer. Due to restrictions in available computer power and memory, and in access time to facilities, the CFD simulations were carried out in 2D only. That is, only a vertical section through the centre of the building was considered. This is less than ideal, as only face-on winds could be considered, but it represents a not uncommon option to the CFD user. The CFD simulations were set up only to consider only external air flow, no internal air flow was specified, and thermal calculations were disabled. Thus the calculations simulated the adiabatic wind tunnel measurements. The CFD domain extended considerably beyond the extent of the building. An example geometry for the building and surroundings are shown in figure 7. The feature to the right of the building represents rising ground behind the offices. The grid was set at 0.5m spacing around the building, decreasing upstream (to the left), downstream and upwards. The CFD grid contained approximately 29000 cells, well within the memory constraints of the system. The pressure data produced by FLOVENT were absolute pressures within each cell. Surface pressures were determined from those of adjacent cells. In calculating ventilation, it is the pressure difference between openings that is important, and this was the parameter used for testing. In the data presented, the CFD surface pressures were normalised to acheive the same pressure value at the bottom of the atrium between cases. In the simulations for each case, 2000-3000 iterations were allowed (requiring approximately 5 computer hours each case). Most cases did not achieve full convergence (as defined by FLOVENT, a continuous reduction in field residuals to below 0.5% of the total flux) but rather settled to an oscillating residual that could not be reduced through further calculation, variation of relaxation factors or other computational o