Ted H. Short

A CFD EVALUATION OF NATURALLY VENTILATED, MULTI-SPAN, SAWTOOTH GREENHOUSES

A computational fluid dynamics (CFD) program, Fluent V4, was used to predict natural ventilation rates and airflow patterns of a multi-span, sawtooth greenhouse for various roof and side vent openings and outside wind speeds. Predicted rates were found to be both well above and well below a standard volumetric air exchange rate of 1.0 A.C. min–1. The maximum and most acceptable ventilation rates were obtained with the combined use of a windward side vent and leeward roof vents on all multi-span sections. Ventilation rates ranged from 1.4 to 4.01 A.C. min–1 for the best two-span case and 0.14 to 2.0 A.C. min–1 for the similar four-span case for outside wind speeds of 0.5 and 2.0 m s–1, respectively. Predicted ventilation rates ranged from 0.17 to 0.7 A.C. min–1 when no side vent was used and the roof vents were fully open.

Transpiration, leaf temperature and stomatal resistance of a greenhouse cucumber crop☆

Abstract Transpiration, leaf temperature and stomatal resistance for a greenhouse cucumber crop were determined by simultaneously measuring solar radiation, air flow, transpiration rate, dew point, and air and leaf temperature profiles. Transpiration rate varied with solar radiation and was not uniformly distributed within the canopy. Leaf temperature was lower than air temperature on clear summer days owing to high transpiration rates. Stomatal resistance was computed and was exponentially related to solar energy. No significant correlations between stomatal resistance and other climatic variables were found.

MACHINE VISION EXTRACTED PLANT MOVEMENT FOR EARLY DETECTION OF PLANT WATER STRESS

A methodology was established for early, non-contact, and quantitative detection of plant water stress with machine vision extracted plant features. Top-projected canopy area (TPCA) of the plants was extracted from plant images using image-processing techniques. Water stress induced plant movement was decoupled from plant diurnal movement and plant growth using coefficient of relative variation of TPCA (CRV[TPCA)] and was found to be an effective marker for water stress detection. Threshold value of CRV(TPCA) as an indicator of water stress was determined by a parametric approach. The effectiveness of the sensing technique was evaluated against the timing of stress detection by an operator. Results of this study suggested that plant water stress detection using projected canopy area based features of the plants was feasible.

ESTABLISHING CROP WATER STRESS INDEX (CWSI) THRESHOLD VALUES FOR EARLY, NON-CONTACT DETECTION OF PLANT WATER STRESS

Early, non–contact, non–destructive, and quantitative detection of plant water stress with the application of infrared thermometry using a crop water stress index (CWSI) was established. A CWSI model for plants grown under controlled environments was developed using thermodynamic principles and energy balance of the plant. CWSI threshold values were established with a parametric approach. The effectiveness of the sensing technique was evaluated using timing of the stress detection by a grower. The CWSI–based technique was able to detect the stress one to two days prior to the time of stress detection by visual observation. Overall results of this study suggested that pre–visual and non–contact detection of plant water stress with infrared thermometry application using CWSI is feasible.

The Microclimate and Transpiration of a Greenhouse Cucumber Crop

ABSTRACT Point source sensors and plant growth variables were used to characterize the microclimate of a greenhouse grown cucumber crop. Diurnal cycles and/or vertical profiles of solar radiation, air movement, leaf and air temperature, humidity, transpiration rate, and stomatal resistance, as well as plant architectural parameters were reported. Canopy leaf temperature with high solar radiation was found considerably lower than greenhouse air temperature due to high transpiration rate.

DYNAMIC MODELING OF THE MICROCLIMATE OF A GREENHOUSE CUCUMBER ROW-CROP PART I. THEORETICAL MODEL

ABSTRACT A theoretical model of greenhouse microclimate was developed for describing heat and mass transport processes in a greenhouse row-crop stand, including radiation transfer, energy balance, transpiration and CO2 exchange. The canopy was described as a series of parallel rows with pseudo-rectangular cross-sections and variable architectural parameters. Radiation transfer, convective transport processes, and plant responses to environmental factors were formulated as functions of plant architecture, greenhouse structure and state variables of bulk air above the canopy. Each of the individual submodels was parameterized from experimental data for a dense row cucumber crop. The general theoretical considerations were assembled into a dynamic simulator by applying energy and mass balances simultaneously over differential strata of plant leaves and greenhouse air. Outputs of the simulator included both diurnal courses and vertical profiles of leaf temperature, air temperature, humidity and CO2 concentration in addition to energy and mass exchange.

DYNAMIC MODELING OF THE MICROCLIMATE OF A GREENHOUSE CUCUMBER ROW-CROP PART II. VALIDATION AND SIMULATION

ABSTRACT The general theoretical considerations, described in Part I, were assembled into a dynamic simulator to predict the diurnal courses and vertical profiles of the microclimatic variables within a greenhouse cucumber row crop. A set of climatic variables as defined above the canopy were supplied as driving functions and/or boundary conditions. The simulated results were then compared with experimental data taken simultaneously with the input variables. Quantitative agreements between model predictions and measurements for each output variable were represented by a group of 11 statistical parameters. The index of agreement for the five major microclimatic variables - solar radiation, transpiration, leaf temperature, air temperature, and relative humidity - varied from 0.92 to 0.99, indicating that the newly developed model of this study did possess a high predictive value. Discussions on some preliminary simulation results were also included..

A THEORETICAL MODEL OF SOLAR RADIATION TRANSFER IN A ROW-CROP CANOPY

ABSTRACT A two-dimensional model was developed to describe solar radiation transfer in a dense, row-crop canopy, assuming that radiation distribution along the row direction was uniform. The crop canopy was represented as a series of parallel rows with pseudo-rectangular cross-sections and described by spatial distributions of foliage elements in the vertical and cross-row directions. Penetration functions of direct, diffuse, and downward scattered radiation components were formulated using a ray-tracing technique. With time, location, plant optical and geometrical properties, and global radiation irradiance above canopy as inputs, the model was used to predict both diumal courses and vertical profiles of various components of global shortwave and photosynthetically active radiation. Radiation data above a greenhouse crop on one mostly sunny and one mostly cloudy day were chosen to evaluate the proposed solar radiation model. Global solar radiation was found to be attenuated rapidly as it penetrated down into the canopy. The attenuation rate, however, was a function of plant architecture, time of day and weather conditions. The contributions of direct and diffuse components to the global solar radiation regime were essential while that of downward scattering was minor. A preliminary comparison with a published data set of global solar radiation within a greenhouse row cucumber crop indicated that the proposed theory was in reasonable agreement with the measurements and would be of value in crop microclimate studies.

Validating the CFD Model for Air Movements and Heat Transfer in Ventilated Greenhouses

Ventilation is necessary for controlling the aerial environment in greenhouses. It affects the transfer of both heat and mass though the aerodynamic boundary layers around plant leaves. The objective of this research was to validate a computational fluid dynamics (CFD) model for greenhouse air velocity profiles, heat transfers, and air exchange rates. Validating the CFD model for air movements and heat transfer indicated that the CFD method could be used to predict greenhouse air movements and temperature patterns with reasonable accuracy.

A Portable Polystyrene-Pellet Insulation System for Greenhouses

ABSTRACT A portable polystyrene-pellet insulation system was developed and shown to reduce night thermal con-ductivity of a greenhouse by over 90 percent. Pellets were pneumatically conveyed daily to and from a double-plastic glazed greenhouse. Static electric attraction bet-ween pellets and plastic sheeting was controlled chemically. Monthly and annual heat use for a double-plastic greenhouse and curtain-insulated double-plastic greenhouse was compared to a pellet-insulated greenhouse using a computer model. The model predicted an annual energy saving of 33 percent for the curtain system, and 70 percent for the pellet system under the selected greenhouse operating conditions.