Test results and empirical correlations to account for air permeability of agricultural nets

Fifteen HDPE agricultural nets were tested inside a micro wind tunnel (0.1345 m diameter) to establish their characteristic air flow rate vs pressure drop curves with velocities >4 m s−1. The air pressure drop through the net was accounted for, with reference to the Bernoulli scheme, by means of the loss coefficient. Experimental results confirmed those available in the literature, in terms of the dependence of the pressure drop on the velocity squared and the net porosity, ε, by means of the function h(ε)=(1−ε2/ε2). The influence of the orifice geometry was investigated and an effect equivalent to the increase in net porosity was identified in textile pores with elongated shapes. As with previous studies, the loss coefficient trend was found to fit the product of two functions, one depending on the porosity, and the other on the Reynolds number defined using the pore equivalent diameter. The calculated values of the loss coefficient show deviations from experimental results in the range of 19.9–41.1%. In addition, a new formulation for the loss coefficient, dependent only on the porosity and wet perimeter was proposed. Except for higher porosity nets the simplified formulation, showed the best match with the experimental data. The two formulations of the loss coefficient proposed here were compared with those found in the literature.