History of Controlled Environment Horticulture: Greenhouses

Modern greenhouses are intensive farming systems designed to achieve high efficiency and productivity. Plants are produced year-round in greenhouses by maintaining the environment at or near optimum levels regardless of extreme weather conditions. Many scientific discoveries and technological advancements that happened in the past two centuries paved the way for current state-of-the-art greenhouses. These include, but are not limited to, advancements in climate-specific structural designs and glazing materials, and temperature control, artificial lighting, and hydroponic production systems. Greenhouse structures can be broadly grouped into four distinct designs, including tall Venlo greenhouses of the Netherlands, passive solar greenhouses of China, low-cost Parral greenhouses of the Mediterranean region, and gutter-connected polyethylene houses of India and African countries. These designs were developed to suit local climatic conditions and maximize the return on investment. Although glass and rigid plastic options are available for glazing, the development of low-cost and lightweight plastic glazing materials (e.g., polyethylene) enabled widespread growth of the greenhouse industry in the developing world. For temperate regions, supplemental lighting technology is crucial for year-round production. This heavily relies on advancements in electro-lighting during the 19th and 20th centuries. The development of hydroponic production systems for the controlled delivery of nutrients further enhanced crop productivity. This article addresses important historical events, scientific discoveries, and technological improvements related to advancements in these areas.

Analysis of Strain-Hardening Viscoplastic Wide Sheets Subject to Bending under Tension

The present paper provides an accurate solution for finite plane strain bending under tension of a rigid/plastic sheet using a general material model of a strain-hardening viscoplastic material. In particular, no restriction is imposed on the dependence of the yield stress on the equivalent strain and the equivalent strain rate. A special numerical procedure is necessary to solve a non-standard ordinary differential equation resulting from the analytic treatment of the boundary value problem. A numerical example illustrates the general solution assuming that the tensile force vanishes. This numerical solution demonstrates a significant effect of the parameter that controls the loading speed on the bending moment and the through-thickness distribution of stresses.

Approximate methods for calculating the impact of a massive body on a plate lying on the base

Normal impact of a massive body on a uniformly stretched plate lying on the base is investigated. A hinged round or rectangular plate on an elastic base, or an infinite plate on the surface of an ideal incompressible fluid is considered. The solution to the elastoplastic impact is in good agreement with numerical calculations and experimental data. With a small parameter of elastic collapse, that is, with the developed local plastic deformations, a solution to the problem of impact with rigid-plastic local collapse can be used. Approximate formulas for calculating the main characteristics of rigid-plastic impact are set up.

An Analytic Solution for an Expanding/Contracting Strain-Hardening Viscoplastic Hollow Cylinder at Large Strains and Its Application to Tube Hydroforming Design

This paper presents a semi-analytic rigid/plastic solution for the expansion/contraction of a hollow cylinder at large strains. The constitutive equations comprise the yield criterion and its associated flow rule. The yield criterion is pressure-independent. The yield stress depends on the equivalent strain rate and the equivalent strain. No restriction is imposed on this dependence. The solution is facilitated using the equivalent strain rate as an independent variable instead of the polar radius. As a result, it reduces to ordinary integrals. In the course of deriving the solution above, the transformation between Eulerian and Lagrangian coordinates is used. A numerical example illustrates the solution for a material model available in the literature. A practical aspect of the solution is that it readily applies to the preliminary design of tube hydroforming processes.

Solar Radiation Transmittance Characteristics of Textile Woven Fabrics suitable for Greenhouse covering Materials

Nowadays, greenhouse covering materials have a vital role in terms of a protective cultivation process. Many farmers use polyfilms, rigid or semi-rigid plastic panels, and glazing materials as greenhouse covering materials in the present scenario. However, these plastic covering materials are known for their high cost, short service life, and cause of harmful environment. Solar transmittance property is one of the main criteria for choosing any greenhouse covering materials. This study prepares various woven fabrics made of polyester, cotton, and polyester–cotton blend yarns. Their solar transmittance characteristic is analyzed to develop fabric and compare it with a polyethylene film already used as a greenhouse cladding material to substitute for plastic materials. The solar transmission of polyester fabric is achieved as high as 70% in the photosynthesis active radiation, suitable for a commercial greenhouse material. In addition, the polyester fabric has tensile strength and extension much higher than that of commercial plastic greenhouse material.

A Method for Determining the Workability Diagram by Varying Friction Conditions in the Upsetting of a Cylinder between Flat Dies

This paper aims to develop a method for determining the workability diagram by varying frictional conditions in the cylinder upsetting test. The method is based on a known theoretical relationship between the average stress triaxiality ratio and in-surface strains if the initiation of fracture occurs at a traction-free surface. This relationship is valid for any rigid/plastic strain hardening material obeying the Mises-type yield criterion and its associated flow rule, which shows the wide applicability of the method. The experimental input to the method is the strain path at the site of fracture initiation. Neither experimental nor numerical determination of stress components is required at this site, though the general ductile fracture criterion involves the linear and quadratic invariants of the stress tensor. The friction law’s formulation is neither required, though the friction stress is the agent for varying the state of stress and strain at the site of ductile fracture initiation. The upsetting tests are carried out on normalized medium-carbon steel C45E, for which the workability diagram is available from the literature. Comparison of the latter and the diagram found using the new method shows that the new method is reliable for determining a certain portion of the workability diagram.

A Dynamic Method to Predict the Earthquake-Triggered Sliding Displacement of Slopes

The earthquake-induced permanent displacement is an important index of the potential damage to a slope during an earthquake. The Newmark method assumes that a slope is a rigid-plastic body, and the seismic responses of sliding masses or seismic forces along the slide plane are ignored. The decoupled method considers no relative displacement across the sliding plane, so it overpredicts the seismic response of the sliding mass. Both dynamic and sliding analyses are performed in the coupled method, but when Ts/Tm is large, the results are unconservative. In this paper, a method is proposed to predict the earthquake-triggered sliding displacement of slopes. The proposed method is based on the Newmark rigid method, coupled method, and decoupled method considering both the forces at the sliding interface and the system dynamics under critical conditions. For the flexible system, the displacements are calculated with different stiffness values, and the results show that as the stiffness increases and tends to infinity, the critical acceleration and displacements of the proposed method are close to those of the Newmark method. The proposed method is also compared with the Newmark method with the period ratio Ts/Tm. At small values of Ts/Tm, the flexible system analysis results of the displacement are more conservative than those of the rigid block model; at larger values of Ts/Tm, the rigid block model is more conservative than the flexible system.

Totimorphic assemblies from neutrally stable units

Inspired by the quest for shape-shifting structures in a range of applications, we show how to create morphable structural materials using a neutrally stable unit cell as a building block. This unit cell is a self-stressed hinged structure with a one-parameter family of morphing motions that are all energetically equivalent. However, unlike kinematic mechanisms, the unit cell is not infinitely floppy and instead exhibits a tunable mechanical response akin to that of an ideal rigid-plastic material. Theory and simulations allow us to explore the properties of planar and spatial assemblies of neutrally stable elements, and solve the inverse problem of designing assemblies that can morph from one given shape into another. Simple experimental prototypes of these assemblies corroborate our theoretical results and show that the addition of switchable hinges allows us to create load-bearing structures. Altogether, totimorphs pave the way for structural materials whose geometry and deformation response can be controlled independently and at multiple scales.

Simulation Analysis of Daylight Characteristics and Cooling Load Based on Performance Test of Covering Materials Used in Smart Farms

In this study, seven of the most commonly applied covering materials in smart farms are selected as the representative samples for analysis, namely, glass, soft film (polyethylene, PE), soft film (polyolefin, PO), rigid plastic film (ethylene tetra fluoro ethylene, ETFE), rigid plastic sheet (poly methyl methacrylate, PMMA), rigid plastic sheet (polycarbonate, PC double layer), and woven film. For each covering material, visible light transmittance and reflectivity, solar radiation transmittance and reflectivity, thickness, solar heat gain coefficient, and the coefficient of heat transmission are measured according to the test methods in the Korean Industrial Standards (KS) to derive input data for the respective materials. In addition, using the optical and thermal input data as derived above, simulations are performed on the cooling load and daylight characteristics of smart farms to derive basic reference data for the selection of adequate covering materials for the design. Based on the analysis result of the daylight characteristics for each covering material, for a shading rate of 60%, the average values of indoor illuminance were 19,879 lux, 20,012 lux, 19,393 lux, 19,555 lux, 16,560 lux, 16,228 lux, and 11,173 lux for glass, PE film, PO film, ETFE, woven film, PMMA, and PC double layer, respectively, between 6 am and 8 pm, which correspond to the hours when daylight enters indoor spaces. Considering the target light intensity for strawberry growth at 10,000–40,000 lux, the above results confirm that all the sample covering materials had an indoor illuminance level above the lower limit range of the target light intensity. For the cooling load evaluation, the PC double layer had the lowest value of 482.8 W/m2, and PO film had the highest value of 633.8 W/m2. The difference between the cooling loads of the PC double layer and the PO film is 151 W/㎡, which amounts to 23.8%, thus indicating that the selected covering material significantly impacts the cooling load. The cooling load exhibited a pattern similar to that of the coefficient of heat transmission and solar heat gain coefficient, which are key influencing factors for most of the sample materials. However, for PMMA, the cooling load was low because it had a higher coefficient of heat transmission than other materials, but its solar heat gain coefficient was relatively low. Another possible reason for the difference is that the solar heat gain coefficient impacts the cooling load. When the cooling set temperature was controlled from Case 1-1 to Case 1-2, the cooling load decreased by 10.7% on average. In addition, when the cooling set temperature changed from Case 1-1 to Case 1-3, the cooling load decreased by 26.1% on average. For cooling set temperature control, maintaining the temperature around the lower temperature range of the optimal growth temperature of the plants increases the yield, but it also incurs increased cooling load and cost. In terms of cost only, while maintaining the cooling temperature for 24 h at 30 °C, which is the upper limit of the optimal growth temperature, would be advantageous, it will lead to a deterioration of the quality and reductions in yield. Therefore, as follow-up studies for further investigation of the findings of this research, there is the need for an evaluation of the yield and quality with respect to the range of cooling set temperatures. When the internal shading rate was increased to 40% (Case 2-2) with reference to Case 2-1, which was a greenhouse without the application of internal shading, the cooling load decreased by 27.4% on average. Furthermore, when the internal shading rate increased to 50% (Case 2-3) with reference to Case 2-1, the cooling load decreased by 29.3% on average. When the internal shading rate increased to 60% (Case 2-4), the cooling load decreased by 31.5% on average.