Towards a physics-based understanding of fruit frost protection using wind machines

Towards a physics-based understanding of fruit frost protection using wind machines

Heusinkveld, Vincent (TU Delft Applied Sciences)

van de Wiel, Bas (mentor)
van Hooft, Antoon (graduation committee)

Delft University of Technology

2019-05-17

Wind machines are used to prevent or mitigate the adverse effects of night frost in spring. These frost events occur during clear-sky, low-wind nights in which a thermal inversion builds up from the surface. The machines work by mixing and transporting warm air from aloft downward which consequently erodes the thermal inversion. Various studies have been conducted regarding this frost protection method, which have resulted in empirical regression models that relate affected area and temperature enhancement (i.e. the performance) with inversion strength. In this study, we assessed heightdependent temperature responses and the sensitivity of wind machine performance to various physical processes. Both of which have not been studied thoroughly in earlier works. In this regard, a large field experiment was conducted and experimental analysis was augmented with sensitivity studies using turbulent resolving Large Eddy Simulations (LES). Experimental observations showed that the temperature response strongly depends on the radial distance to the fan and the height above the surface. In agreement with previous studies, the wind machine was able to achieve rotation-averaged temperature increases of up to 50% of the inversion strength (≈3 K) in an area of 3-5 ha at 1 m height. Furthermore, it was observed that low-speed ambient winds (<1 m/s) can cause strong upwind-downwind asymmetries in the protected area, the downwind area being larger. The LES model, inspired by the field experiment, showed similar spatial temperature responses as compared to observations. Sensitivity studies using a simpli_ed case showed that the a_ected area strongly increased for slower axial rotation times (ranging from 3 to 6 min) while the temperature enhancement stayed relatively constant. Furthermore, variation of the horizontal tilt angle showed that, in our model, temperature enhancement was maximized between 8º and 16º. Presumably, those angles, corresponding to near horizontal ow, are very effective in generating strong shear layers which in turn generate Kelvin-Helmholtz like instabilities. These processes increase mixing and vertical transport of heat. Finally, analysis on the ambient wind showed that, in agreement with observations, strong upwind-downwind asymmetries in the affected area exist.

Frost protection
Fruit forst
Wind machine
Inversion
Agriculture
CFD
large-eddy simulation

http://resolver.tudelft.nl/uuid:43faf68c-7e5f-459c-9d1b-88ec7c4c71b9

2020-07-01

Student theses

master thesis

© 2019 Vincent Heusinkveld