Application of Semi-Empirical Ventilation Models in A Mediterranean Greenhouse with Opposing Thermal and Wind Effects. Use of Non-Constant Cd (Pressure Drop Coefficient Through the Vents) and Cw (Wind Effect Coefficient).

The present work analyses the natural ventilation of a multi-span greenhouse with one roof vent and two side vents by means of sonic anemometry. Opening the roof vent to windward, one side vent to leeward, and the other side vents to windward (this last vent obstructed by another greenhouse), causes opposing thermal G_T (m³ s⁻¹) and wind effects G_w (m³ s⁻¹), as outside air entering the greenhouse through the roof vent circulates downward, contrary to natural convection due to the thermal effect. In our case, the ventilation rate R_M (h⁻¹) in a naturally ventilated greenhouse fits a second order polynomial with wind velocity u_o (R_M = 0.37 u_o² + 0.03 u_o + 0.75; R² = 0.99). The opposing wind and thermal effects mean that ventilation models based on Bernoulli's equation must be modified in order to add or subtract their effects accordingly—Model 1, in which the flow is driven by the sum of two independent pressure fields G M 1 = | G T 2 ± G w 2 | , or Model 2, in which the flow is driven by the sum of two independent fluxes G M 2 = | G T ± G w | . A linear relationship has been obtained, which allows us to estimate the discharge coefficient of the side vents (C_dVS) and roof vent (C_dWR) as a function of u_o [C_dVS = 0.028 u_o + 0.028 (R² = 0.92); C_dWR = 0.036 u_o + 0.040 (R² = 0.96)]. The wind effect coefficient C_w was determined by applying models M1 and M2 proved not to remain constant for the different experiments, but varied according to the ratio u_o/∆T_io^0.5 or δ [C_wM1 = exp(−2.693 + 1.160/δ) (R² = 0.94); C_wM2 = exp(−2.128 + 1.264/δ) (R² = 0.98)]. [ABSTRACT FROM AUTHOR]