MODELOS DE CALIBRACIÓN Y VALIDACIÓN

Though the test was conducted by an outside consultant (Luther 2008) the data was reduced and analysed as part of this research project. Data provided included SF6 gas concentration from the gas analyser, and wind speed and direction from a separate instrumentation system recorded on a data logger. The ventilation rates were calculated as a function of gas concentration and elapsed time. However, the resulting data points occur at a time that was not synchronized with the measurement interval of the data logger. The wind speed and direction were then found at the desired time points via interpolation.

Firstly the ventilation rates were found. The slope of gas concentration was used to differentiate between each dose-decay cycle where a positive slope indicated dosing and a negative slope indicated decay. Once the decay processes were isolated, the ventilation rates for each decay period were found using the two-point average decay method (Sherman 1990) as follows:

T C C V f i         = ln 4.1

where V is the subfloor ventilation rate in air changes per hour [ACH]; C_iis the initial gas concentration of the time period of interest (PPM); Cfis the final gas concentration of the time

Ideally the slope of the decay (C_i/C_f) remains constant during the decay period, but for these tests it did not, indicating that the ventilation rate was varying with time. It is known that in this situation the variability of tracer gas decay test results can be significant (Roulet & Vandaele 1991). The calculation of ventilation rate was therefore very sensitive to the time in each decay period at which the decay slope was sampled. To accommodate this sensitivity, the decay slope was sampled at a consistent time after the start of each decay period for each dose-decay cycle. In addition, calculations were based upon a moving average of the slope, to reduce the effect of any

fluctuations (Sequeira et al. 2010a). The analysis yielded 28 data points from the first day of testing and 21 points from the second day.

To pair these 49 data points with their corresponding wind speeds, some manipulation of the wind speed data was then required. Wind speed and direction were measured in ten-minute intervals, while each decay process took approximately 30 to 45 minutes. Therefore a 30-minute moving average of wind speed was calculated to best represent the average windspeed during the decay times. This was performed as a vector average instead of a simple scalar average. The measured wind speed was separated into north-south and east-west components. A moving average of each component was performed separately and they were then vectorially summed to produce the average wind speed and direction (Sequeira et al. 2010a).

Next this observed wind speed at the building roof height was projected to the meteorological reference height of 10 m. This standard method of relating wind speed at different heights has been used in similar research (Deru and Burns 2003; Swami and Chandra 1988; Williamson and Delsante 2006a). The formula to relate wind speed at any height to the wind speed at 10 m is:

γ

α

_      × = met met H H v v 4.2

where

v

is the wind speed at the location of interest [m/s]; vmet is the wind speed [m/s] at a height

above ground level of 10 m; His the height above ground level of the location of interest [m];

met

H is the meteorological reference height of 10 m; and both

α

and

γ

are defined in Table 4.3 as a function of the terrain class.

Table 4.3: Terrain classification parameters (Deru and Burns 2003)

Using Class III values from Table 4.3, sometimes called “Open” terrain, a scalar of 1.36 is applied to the measured wind speed at measurement height of 4.9 m to project the value to the reference height of 10 m. Thus the observed data are reduced to a set of 49 data points, with each observed ventilation value paired to a corresponding meteorological wind speed.

The prediction of Equation 4.2 and Table 4.3 show that wind speed in an area of open terrain is generally expected to decrease from its free stream speed at 10 m upon approach to the ground. However, per the CFD results the test cell roof peak lies in an area expected to have a local flow obstruction which would causes a contradiction in this general trend. As shown in Table 4.1 the CFD predicts a west or northwest wind results in a roof air speed higher than the free stream wind speed. The roof air speed measurement location may or may not fall within this area of locally- adjusted air speed. There is no separate, independent local wind speed measurement to determine the relationship between roof wind speed and free stream air speed. Thus there is uncertainty in the outcome when projecting wind speed to different heights, especially as Equation 4.2 and Table 4.3 give only general guidance and do not account for local obstructions.

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