Three-dimensional measurements of tree crown movement using an infrared time-of-flight camera

The study of wind–tree interaction represents a challenge in fluid dynamics partly because the three-dimensional tree crown and its branch structure are dynamically adapting to the mean and turbulent wind components. With the advent of infrared technologies these complex, three-dimensional, real time interactions between wind and the response of a tree can be assessed. The present work makes use of a Microsoft Kinect V2, which incorporates an infrared time-of-flight camera, to capture images containing point cloud data of a single garden tree in turbulent winds. The raw output from the sensor is used to translate three-dimensional information into mean crown positions and projected frontal canopy areas. This novel infrared time-of-flight sensor technique facilitates a better understanding of the behaviour of the tree crown against wind speed. Overall, less movement and larger crown areas are observed at lower wind speeds. As the wind speed increases, a general shrinkage in area, more swaying and a smoothing of the crown edges are noted. Correlations between the observed wind force exerted on the tree and the displacement of the crown centre as well as between the wind force and the projected frontal canopy area are determined. The displacement and the drag force are positively correlated for all wind speeds. The drag force and frontal canopy area are positively correlated at low wind speeds and negatively correlated at higher wind speeds with a bi-stable phase between. The infrared time-of-flight sensor has demonstrated its capacity to provide real-time three-dimensional scanning of trees in a wind testing facility providing new insights into the complex wind–tree interaction problem.