A Wind Tunnel Study of Airflow through Horticultural Crops: Determination of the Drag Coefficient

Greenhouse crops exert a drag force causing a net loss of momentum in the airflow and a negative effect on ventilation. The drag effect produced by the airflow through crops can be modelled as a function of leaf area density (LAD) and the square of air velocity. A wind tunnel was used to determine the drag coefficient CD of four horticultural crops. Three samples with differing values of LAD were tested for each crop. The values of CD obtained for tomato, sweet pepper, aubergine and bean were 0·26, 0·23, 0·23 and 0·22, respectively. Coefficients obtained for the four crops were similar, which suggests that the effect of leaf shape and size is not significant. The drag coefficients found decreased with velocity and remained nearly constant for Reynolds number greater than 6×104. The drag coefficient allows the aerodynamic behaviour of crops to be characterised, although the use of a constant value involves a significant error (15–30%) for velocities usually registered inside greenhouses (0·1–0·5 m s−1). Maximum values of CD were obtained for velocities between 0·5 and 1·5 m s−1 due to the effects of change in leaf position and variation in airflow regime.