EXPERIMENTAL-OBSERVATIONS OF THE AERODYNAMIC CHARACTERISTICS OF UBAN TREES

The paper presents some of the experimental results that have been obt ained during an extensive investigation of the behaviour of urban tree s in high winds. In particular, results are presented of tests to dete rmine tree mechanical and aerodynamic parameters. The mechanical prope rties that were measured (stiffness, natural frequency, and damping) w ere obtained from tree winching and quick release experiments. It was found that, for the small number of specimens tested, the stiffness of each specimen did not vary greatly through the year, but the magnitud es of natural frequencies and damping ratios were dependent upon wheth er or not the tree was in leaf For a tree in leaf the natural frequenc y was lower and the damping higher than when not in leaf. Measurements of tree deflection in the wind (for a semi-mature plane tree) enabled further estimates of natural frequency and damping ratio to be made, and also enabled drag coefficient values to be measured. It was shown that these coefficients are substantially greater when the tree is lea f, than when it is not in leaf. These experiments did not lend support to the hypothesis that tree drag is proportional to velocity, rather than (velocity)2, as has been previously suggested.