Development of a reference method for airflow rate measurements through rectangular vents towards application in naturally ventilated animal houses: Part 2: Automated 3D approach

An accurate measurement of the airflow rate in a naturally ventilated animal house is still an issue due to the large uncertainties of the available techniques. These uncertainties are mainly related to the variability of the velocity profile in a naturally ventilated opening caused by, among others, fluctuations in wind direction and speed. An experimental set-up was built to develop airflow rate measurement methods which can cope with these different profiles and can be further developed for future use in naturally ventilated openings. The methods were compared to a reference technique for mechanical ventilation (VDI2041) as no such reference exists for natural airflows. A relative measurement error from this reference of max. 10% was deemed acceptable. The methods were based on a fully automated traverse movement of respectively a 2D and a 3D ultrasonic anemometer behind the outlet of two different rectangular ducts (respectively 1m and 3m wide). Several airflow rates were imposed, combined with disturbances of the airflow to obtain different velocity profiles. Measurements performed with a 2D ultrasonic anemometer gave rise to relatively high measurement errors (up to -18%) related to the 3D character of the outflow jet. Two methods were developed with a 3D ultrasonic sensor in order to capture the outflow characteristics more adequately. The Basic method concentrated on the area in front of the outlet, while an Extended method paid more attention to the airflow around the edges of the outlet. Both methods succeeded in keeping the relative measurement error below the 10% limit, and even below 5% error for the 3m wide duct. For future use in naturally ventilated openings, more research is necessary to further adapt the methods to continuously changing conditions of wind speed and direction.

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