*Research Centre CIAIMBITAL. University of Almería. Ctra. de Sacramento s/n. 04120 Almería, Spain.*

*Research Centre CIAIMBITAL. University of Almería. Ctra. de Sacramento s/n. 04120 Almería, Spain.*

*Research Centre CIAIMBITAL. University of Almería. Ctra. de Sacramento s/n. 04120 Almería, Spain.*

*Research Centre CIAIMBITAL. University of Almería. Ctra. de Sacramento s/n. 04120 Almería, Spain.*

*Universidad de Guadalajara. Av. Independencia Nacional 151. Autlán de Navarro, Jalisco 48900 Mexico.*

The present work examines the variations in the aerodynamic characteristics of four insect-proof screens by means of wind tunnel
tests and digital image processing. The tested insect-proof screens were examined in three different conditions: (i) in their new, unused
state; (ii) under conditions of accumulated dust and dirt after a period of 3 to 4 years of use; and (iii) under clean conditions after a
period of 3 to 4 years of use and a cleaning treatment with high-pressure water. The deterioration of the screens caused the mesh to
become less tense, therefore increasing its thickness and improving its aerodynamic behaviour despite a slight increase of the thread
diameter and a subsequent decrease of the 2-dimensional porosity. The pressure drop coefcient, _{φ}_{d,φ}_{φ}_{d,φ}

*b*and

*c*(second-order polynomial regression coefcients);

_{h}

_{hx}

_{hy}

_{i}

_{r}

^{2}]);

_{φ}

_{p}

_{px}

_{py}

^{2}(coefcient of determination);

_{p}(Reynolds number based on the screen’s permeability);

_{p}

^{2}]);

_{t}

^{2}]);

^{-1}m

^{-1}]);

^{-2}m

^{-2}]);

^{-2}m

^{-2}]);

^{-3}]);

Insect-proof screens are a widespread means of preventing the entrance of harmful insects into greenhouses. Insect-proof screens are commonly manufactured with HDPE monofilament-woven fabrics of different densities of warp and weft threads (threads/cm
^{2}). In the Mediterranean Basin, the use of insect-proof screens in greenhouse vents is a standard crop management practice; in the Province of Almería, Spain, insect-proof screens are employed in the side and roof vents in 99.1% and 95.4% of the greenhouses, respectively (

A number of research works have highlighted the negative effect of insect-proof screens on the greenhouse ventilation capacity and, by extension, on the greenhouse microclimate. Screens of low porosity increase both the temperature and the humidity inside the greenhouse (

In addition to determining the geometric and aerodynamic characteristics of the insect-proof screens, it is of great interest to quantify the influence of both the deterioration of the screens and the accumulation of dirt over time on these characteristics and thus on the natural ventilation capacity of the greenhouse. Among the few research works in this field of study, ^{2} greenhouse in Israel, the pressure drop coefficient of the screen can increase by a factor of up to 20 (from 12 to 200).

The effect of material aging and the accumulation of dirt on the geometrical and aerodynamic characteristics of insect-proof screens is of great research interest since their placement in the greenhouse vents negatively affects the ventilation capacity of the greenhouses (

The aim of this work was to analyse the effect of material aging and the accumulation of dirt on the aerodynamic characteristics of the HDPE monofilament-woven fabrics that are employed as insect-proof screens in greenhouses. Variations in the aerodynamic behaviour of screens affect the ventilation capacity of greenhouses and therefore crop production. As a result, knowledge of the effect of material ageing and of the accumulation of dirt on the screen permeability is fundamental for an accurate estimation of ventilation capacity in greenhouse modelling. This work complements the information presented in the previous study by

In this study, wind tunnel experiments were performed on samples from four screens with the objective of determining the effect of material aging and of the accumulation of dirt on the aerodynamic characteristics of insect-proof screens. Insect-proof screens were assessed in three different conditions: (i) newly installed, (ii) after 3 to 4 years of use with no cleaning treatment, and (iii) after 3 to 4 years of use after a high-pressure water cleaning treatment.

The four insect-proof screens manufactured with HDPE monofilament-woven fabrics were installed in the side vents of two multi-span, naturally-ventilated greenhouses located at the agricultural research station of the University of Almería in south-eastern Spain (36° 51' N, 02° 16' W and 87 masl). Both Greenhouse 1 (24×45 m) and Greenhouse 2 (18×45 m), were divided in half by a polyethylene sheet. Natural ventilation consisted of two side vents (18×45 m) and three roof vents in Greenhouse 1, and of two side vents and two roof vents in Greenhouse 2 as described in

Screens 1 and 2 were installed in August 2007 (date of purchase July 2007), and screens 3 and 4 in September 2008 (date of purchase August 2008). The mechanisms used to hold the insect-proof screens in the side vents did not affect the integrity of the screens over time: U-shaped (omega) metal frames were attached to the vent structure, and the screen was then inserted and held in place by polyethylene stoppers, ensuring that the screens were not subjected to movements that may have affected their structure during usage, according to the method described in

Screen 1 had a thread density of 13×30 threads/cm
^{2} and was designed by the Engineering Department of the University of Almería. Screens 2, 3 and 4 are commercial models with a thread density of 10×20 threads/cm
^{2}. The two-dimensional geometric characteristics of the new screens and the cleaned and uncleaned used screens (^{2} were analysed. Twenty-four images were taken of each sample with a microscope that incorporated a Motic DMWB1-223 digital camera (MoticSpain S.L., Barcelona, Spain) with a 4× lens and a resolution of 10.5 μm/pixel. The analysis of each image included the geometric characteristics of 30 pores, 2 wefts and 9 warp threads (mesh 1) and 12 pores, 1 weft and 5 warp threads (meshes 2, 3 and 4). The following steps were performed for each color digital image of the fabric: (i) the images were converted to grayscale; (ii) a gray colour was manually assigned to the areas of the pores to be analysed; (iii) the gray areas corresponding to the pores were converted to white by the software, leaving the rest of the image black; (iv) the vertices of each pore were automatically identified by the software; (v) the correct identification of the vertices was verified by the user, who could then correct any potential errors caused by the software; and, finally, (vi) the relevant geometric parameters were calculated by the software. The following parameters were obtained: thread density
_{r} (weft × warp) [threads/cm
^{2}]; porosity
^{2}/m
^{2}]; weft pore length
_{px} and warp pore length
_{py }[m]; weft diameter
_{hx }and warp diameter
_{hy} [m]; mean thread diameter
_{h} [m]; pore circumference diameter
_{i }[m]; and pore surface area
_{p} [mm
^{2}]. Further details on the methodology used for the digital image processing can be found in

Statistically significant differences were found between all the geometric parameters of the new screens and of the clean used screens, except for the
_{hx} of the 4 mesh (^{2}/m
^{2}) by comparing the calculated surface of the pores
_{p} and a total reference surface
_{t}, maintaining the correct proportion between pores surface area and the solid surface area (threads) (^{*}) was estimated based on black and white digital images of the insect-proof screens. This procedure compares the number of white pixels (corresponding to the pores) with the total number of pixels in each image, resulting in a certain degree of error due to the difficulty of maintaining the correct proportion between pores and threads (

The effect of time on the geometric characteristics on the insect-proof screens as described by

Although the wind tunnel allows testing at speeds of up to 10 m/s, the experiments were carried out at 0 to 3 m/s to ensure that no damage to the sensors and wind tunnel components was caused when testing the unclean used screens. Once the screen is in place in the greenhouse, it is highly unlikely that the air velocity through it reaches the maximum tested speed of 3 m/s. In fact, the maximum air velocity registered at the vents in Greenhouse 1 with natural ventilation and with an insect-proof screen of 0.39 porosity was around 1.0 m/s in the study performed by

The wind tunnel experiments allow the determination of the pressure drop
_{p}, the screen permeability [m
^{2}], a coefficient that is independent of the nature of the fluid and dependent on the geometry of the porous medium (_{φ}, the pressure drop coefficient; and, lastly,
_{d,φ}, the discharge coefficient that results from the presence of insect-proof screens.

The airflow through the porous medium (the insect-proof screen) can be described by modifying Darcy’s equation (

where
^{-1} m
^{-1}] and
_{a} is the air density [kg/m
^{3}]. A second-degree polynomial can be used (

The zero-order term
_{p} and

The thickness

Bernoulli’s equation provides an alternative way of describing the relationship between the pressure drop caused by the insect-proof screens and the air velocity through the screens (

where
_{φ} is the pressure drop coefficient as a result of the presence of an insect-proof screen, and can be obtained from

The coefficient
_{φ} can be used to predict the pressure drop caused by the screens below a certain limit of the Reynolds number (_{p}<10
^{5} (_{p} based on the permeability of the screen. Re
_{p} can be obtained as follows (

In addition, the discharge coefficient due to the presence of insect-proof screens
_{d,f} can be calculated as (

A similar discharge coefficient has been used in the literature for monofilament-woven fabrics (

Regression analyses were carried out to study the relationship between different parameters (statistically significant for

The porosity values
^{*} were obtained for the four analysed screens. The following results show the pressure drop curves produced by the screens under three different conditions: (i) new, unused screens, (ii) unclean used screens, and (iii) clean used screens. The pressure drop coefficient
_{φ}, and the discharge coefficient that results from the presence of the insect-proof screens
_{d,φ} are then presented, in both cases including an analysis of the effect of time and the accumulation of dirt on these values.

^{*} for the four screens analysed under the three testing conditions. Based on these data, it has been found that the two parameters are related as follows:
^{*} + 0.045 (
^{2} = 0.85 and
^{*}) of the new screens and the clean used screens for determining this expression, whereas the latter study only used the porosity data of the clean used screens. The values of the calculated porosity

Compared to the new screens, the unclean used ones caused a reduction of the porosity

The following values of reduction in porosity
^{*} + 0.045] that is used to estimate the porosity values

In summary, deterioration of the screen mesh over time lead to a slight decrease in the two-dimensional porosity of the screen but the accumulation of dust and dirt produced a far greater reduction in this parameter. It could be argued that a reduction of the porosity of the screens should produce a reduction in the air permeability. However, this does not always hold true due to the fact that a variation in porosity produced by material ageing can also be associated with a modification in the tri-dimensional shape of the pore. This reinforces the need to perform aerodynamic analyses when determining the effect of material ageing and dirt on the screen permeability.

_{p},
_{φ} expressed as a function of Re
_{p}, for screens 1, 2, 3 and 4.

The pressure drop caused by the clean used screens was slightly lower to that caused by the new ones (_{d,φ}) was lower for threads with an elliptic cross-section than for those with a circular one. In the present work, the values of thread diameter and thickness were greater for the clean used screens than for the new ones, with significant statistical differences at the 99.0% confidence level (

The values determined for
_{p} and
_{p} increases with porosity, in the present study this was not the case. The permeability
_{p} of the new screens was inferior to that of the clean used ones (_{p} of the unclean used screens was the lowest. As regards inertia

It should be highlighted that the passage of time, together with the exposure to environmental conditions, produces two contrasting effects on the aerodynamic behaviour of the screens. On the one hand, the deterioration of the mesh causes it to become less tense, resulting in a lower pressure drop (

Comparison of the screens by analysis of the parameters
_{p} and
_{d,φ} and on the discharge coefficient due to the presence of insect-proof screens
_{d,φ} as a result of the presence of insect-proof screens.

The pressure drop coefficient
_{φ} was clearly greater for the unclean used screens than for the new ones and the clean used ones (_{φ} was slightly lower for the clean used screens than for the new ones. As commented above, this difference may be due to the deterioration and the tension loss, which increase the thickness of the mesh and the spherical shape of the threads. To compare the
_{φ} values of the different screens, the following air velocity

Comparison of the unclean used screens and the new ones revealed increases in
_{φ} of between 36.0% (screen 2 for
_{φ} to decrease with respect to the new screens, although this decrease is only relevant in the case of screen 4 (8.8% for
_{φ} of between 16.5% (screen 3) and 61.2% (screen 4), for
_{φ} and the inverse of air velocity, whose slope is 2
_{p}
_{φ} values increases as the air velocity drops (and therefore 1/
_{φ} are produced when air velocity thought the greenhouse screens is lower than 0.3 m/s, (1/

Once the values of
_{φ} are known, the values of the discharge coefficient
_{d,φ} for each screen can be derived from _{d,φ} of between 5.3% (screen 3 for
_{d,φ} of between 0.8% (screen 3
_{d,φ} is between 7.5% (screen 3) and 21.3% (screen 4), in both cases for

^{-1}) in a greenhouse with and without insect-proof screens can be considered proportional to the ratio between the discharge coefficients at the vents. In turn, the discharge coefficient at the vents depends on the coefficient
_{d,φ}. The reduction observed in the coefficient
_{d,φ} due to the accumulation of dust and dirt on the insect-proof screens results in a reduction of the natural ventilation capacity of the greenhouse.

The comparison of clean used screens with new ones showed a reduction of the pressure drop coefficient
_{φ }of up to 8.8% (screen 4 for
_{d,φ} was 7.3% (screen 4 for

In addition, the accumulation of dirt on the screens has a major bearing on their aerodynamic characteristics. Comparison between the clean and unclean used screens showed an increase in
_{φ} of up to 61.2% (screen 4 for
_{d,φ} of up to 21.3% (screen 4 for

In view of the results obtained in the present work, it is recommended that insect-proof screens are manufactured with geometrical and aerodynamic characteristics that avoid approaching the limit of 8 Pa established by

Lastly, we can conclude that the mechanical deformation of screens over time produces a reduction of the resistance to airflow throughout the insect-proof screens installed in the greenhouse openings, whereas ventilation is hindered by the dirt accumulation. A regular cleaning treatment of the insect-proof screens is a simple measure that improves ventilation inside the greenhouse and avoids major reductions in the natural ventilation capacity of the greenhouse, as quantified in the present study. Future research will focus on the study of the effect of material degradation and accumulation of dust on the optical properties of the insect-proof screens and on crop productivity.

The authors wish to express their gratitude to the Research Centre CIAIMBITAL of the University of Almería (Spain) and the National Council of Science and Technology (CONACYT) of Mexico, for their support throughout the development of this study.