In situ drag coefficient measurements for rooftop trees

Abstract

To evaluate the drag coefficients for roof-top tree crowns, previously developed field test methods for monitoring wind speed, wind direction, and stem deflection were improved using solar cells as a source of power. A field test was conducted for a Norway spruce and a shirakanba located on a rooftop. The drag coefficients showed large variations during low wind speeds, which were caused by the flexural vibration of the tree trunk due to the fluctuating wind speed. To compensate for this effect, the upper percentage of drag coefficients obtained for the 0.5 m/s wind speed classes were used for the exponential regression for drag coefficient as a function of wind speed. As a result, a conservative evaluation of drag coefficients for a greater range of wind speeds could be established instead of using all data. The drag coefficients at 30 m/s wind speed were estimated to be 0.594 for the Norway spruce, and 0.154 for the shirakanba using the upper 50 % of the data. The exponential function convergence was more gradual for the spruce than for the shirakanba, which may be affected by a difference in the streamlining behavior.