Flow through concertina-shape screens

• Folding a fine-mesh screen into a concertina shape may allow an increased airflow.
• Flow resistance of a concertina screen is determined by fold angle and resistance before folding.
• Downstream velocity profile close to a concertina screen is different from a flat one.

An insect-proof screen acts as a mechanical barrier, preventing insects from reaching the plants in a greenhouse and thus reducing pesticide application requirements. However, the exclusion of very small insects requires fine-mesh screens which impede ventilation and thus increase temperature and humidity within the greenhouse. To minimise the effect on ventilation the screen resistance to airflow should be as low as possible. Experiments and computational fluid dynamics (CFD) simulations were used to compare the airflow resistance of flat vs. concertina-shape screens. The CFD results were validated against experimental data and then further simulations were done to explore additional concertina-screen configurations. Folding a fine-mesh screen into a concertina shape reduced airflow resistance: for upstream air velocities of 0.42–0.85 m s−1 a screen with a typical 60° fold allowed, on average, about 23–32% higher airflow than a flat screen with similar through-screen pressure drops. The simulation results allowed a detailed examination of the through-screen and downstream flow patterns, and showed that the downstream velocity profile close to a concertina screen was different to that of a near flat screen. Velocities downstream from the concave parts of the concertina screen were lower than those downstream from the convex parts, resulting in a form of ‘sinusoidal’ velocity distribution rather than the uniform velocity distribution downstream from a flat screen. The CFD results further suggest that for a given pressure drop the airflow may increase with decreasing fold angle; this depends on the pre-folding screen resistance to airflow.