Evaluation of the drag coefficients of tree crowns by numerical modeling of their free fall

The method for evaluation of wind loads and drag coefficients for the trees of different species, size and morphology in natural conditions that does not require special equipment is presented for the first time. Field experiments and numerical simulation were performed for five trees of the Scots pine trees (Pinus sylvestris L.) under free fall. The field experiments were carried out in the forest for symmetrical 23-27 m tall pine trees. The trees were cut by the chainsaw operator and the videos of a tree falling were obtained. Still images were captured from the video every second after the start of movement. Then the tree stem center lines from the images were obtained. After each run, the length and diameter of the stem and the geometrical characteristics of the tree crown were checked. Numerical experiments were carried out using the deformation model of the tree stem that is inspired by the Cosserat theory of elastic rods. Under the assumption that the drag force is distributed along the tree stem according to a triangular load and increases linearly with velocity, the dynamic global behavior of the real trees was reproduced and the tree stem center lines were obtained. By comparing the data of the two experiments, the drag coefficients of the whole pine trees were found.