Wind machines for fruit frost mitigation: a quantitative 3D investigation

Wind machines have been increasingly used for fruit frost mitigation in the agricultural community. The basic idea of using wind machines is to bring the warm air from above to the surface and create mixing. However, the efficiency and physical mechanism of air mixing by wind machines are not fully understood from previous studies. The conclusions from previous studies are usually based on a few point measurements only and therefore limited as to abstract general guidelines. Here, a unique field experiment is presented, with high-resolution (0.25m) temperature probing at the scale of a full orchard. In combination with high-resolution numerical simulation, this allows to better understand the effects of turbulent mixing and to quantify wind machine efficiency.

In the field we employed a 9km optical fiber in two horizontal planes at heights of 1 and 2m above the surface. Through this Distributed Temperature Sensing a meshed grid with resolution of ~0.25m over ~ 6 ha is obtained. This allows to quantify the spatial and temporal variation of temperature dynamics at orchard scale. Some findings can be drawn from experimental observations. Wind machines are proved to be an effective method for frost mitigation. In our experiment, the wind machine reduced 50% and 70% of the inversion strength (7K) in an area of 3.39ha and 0.61ha respectively. The warming area strongly depended on the radial distance to the wind machine, inversion strength and advection intensity. In general, the closer distance to the wind machine, the warmer the air is. However, advection plays an important role in the shape and direction of the warming plume. With only 0.2-0.3m/s weak wind at 3-meter height, the center of the warming plume at 1m height drifted 50m in the downwind direction: The background wind in combination with the wind machine changed the warming plume to an asymmetrical ‘pear shape’.

Numerical simulations were used to investigate the sensitivity of the various settings of the wind machine. Operation modes include full 360 degrees rotation (FR), upwind (UHR) and downwind (DHR) rotation (both 180 degrees). The analysis shows that typically the upwind mode results in better mixing efficiencies than the FR and DHR cases. This may be attributed to the enhanced turbulence that is caused by the shear interaction between the machine jet and the upward wind. The evaluation of different settings of the wind machine showed that all levels of warming are generally insensitive to the rotation period (1.6 to 10min). The setting of tilting angle (9°± 3°) gives optimal warming efficiency for all operation modes. With the finding of the current study, we recommend that farmers and the agricultural community test the effectiveness of the upwind operation.