WIND-INDUCED AIR EXCHANGE-RATES IN A GREENHOUSE TUNNEL WITH CONTINUOUS SIDE OPENINGS

To study the influence of wind speed on ventilation rates, experimenta l trials were carried out in a plastic tunnel with continuous side ven tilators. This type of greenhouse is the most representative of Medite rranean and arid regions. The injection peak technique using N2O as tr acer gas was used to measure air exchange rates. Simultaneously air pr essure difference between inside and outside at ground level was recor ded together with outside air temperatures. Several combinations of si de ventilator openings were studied under different wind velocities an d directions. Results from pressure measurements show that the thermal buoyancy effect has to influence on the pressure field when the wind velocity exceeds 15 m s(-1) and that the turbulent wind pressure coeff icient increases when adjacent obstacles perturb the wind. Results fro m gas measurements show that the air exchange rate is strongly depende nt on wind velocity and total ventilator area, as expected, and that i t can be expressed as a function of a global wind coefficient having p roperties similar to the turbulent wind coefficient. This indicates th at the induced turbulent component of the air exchange rate can be sig nificant. The reduced form of the ventilation flux, defined as the ven tilation flux per unit ground area and average wind speed is used to c ompare the present results with those of other authors on different ty pes of greenhouses and ventilation systems. It appears that roof openi ngs are more effective than side openings and that the effectiveness o f the ventilation vents depends on the type of opening. The effectiven ess appears to be less when the vents are equipped with a side-mounted flap on their frame. (C) 1996 Silsoe Research Institute.