Estimating Wind Damage in Forested Areas Due to Tornadoes

Research Highlights: Simulations of treefall patterns during tornado events have been conducted, enabling the coupled effects of tornado characteristics, tree properties and soil conditions to be assessed for the first time. Background and Objectives: Treefall patterns and forest damage assessed in post-storm surveys are dependent on the interaction between topography, biology and meteorology, which makes identification of characteristic behavior challenging. Much of our knowledge of tree damage during extreme winds is based on synoptic storms. Better characterization of tree damage will provide more knowledge of tornado impacts on forests, as well as their ecological significance. Materials and Methods: a numerical method based on a Rankine vortex model coupled with two mechanistic tree models for critical wind velocity for stem break and windthrow was used to simulate tornadic tree damage. To calibrate the models, a treefall analysis of the Alonsa tornado was used. Parametric study was conducted to assess induced tornadic tree failure patterns for uprooting on saturated and unsaturated soils and stem break with different knot factors. Results: A power law relationship between failure bending moments and diameter at breast height (DBH) for the hardwood species provided the best correlation. Observed failure distributions of stem break and windthrow along the tornado track were fitted to lognormal distributions and the mean of the critical wind speeds for windthrow were found to be higher than that for stem break. Relationships between critical wind speed and tree size were negatively correlated for windthrow and positively correlated for stem break. Higher soil moisture contents and lower knot factors reduced the critical wind speeds. The simulations show varying tree fall patterns displaying forward and backward convergence, different tornado damage widths and asymmetry of the tracks. These variations were controlled by the relative magnitudes of radial and tangential tornado velocities, the ratio between translational speed and maximum rotational wind speed and the mode of failure of the trees. Conclusions: The results show the complexity of predicting tornadic damage in forests, and it is anticipated that this type of simulation will aid risk assessments for insurance companies, emergency managers and forest authorities.

Study on the Elastoplastic Dynamic Failure of Single-Layer Cylindrical Reticulated Shell Structures under Wind Loads

Critical wind velocity of dynamic failure is an important factor for the wind-resistance design of three-way single-layer cylindrical reticulated shells. This paper investigates the elastoplastic dynamic failure of reticulated shells under three dimensional wind loads, the ultimate capacity, buckling mode and plastic development distribution of reticulated shells under wind loads are investigated. The influence of initial geometrical imperfections, initial static loads, supporting types, rise-span ratio, long-width ratio on the failure performance of reticulated shells are presented. The comparison of the ultimate bearing capacity shows that the critical wind velocities under dynamic wind loads are much lower than that under static wind loads for reticulated shells.

Aeroelastic Modeling and Analysis of a Horizontal Axis Wind Turbine

Dynamics of the horizontal axis wind turbine is investigated in this paper. The wind turbine rotor modelling is based on the nonlinear Eular beam model, the generalized dynamic wake model and the Beddoes-Leishman dynamic stall model are used to predict the aerodynamic loads. The Newton-Rafson iteration is adopted to get the solution in each time step and the second-order backward difference is used in time marching. A horizontal axis wind turbine is analysed in fixed rotor speed and the structural responses in rated conditions and critical wind conditions are obtained. The critical wind velocity is acquired from simulation. The influences of the rotor speed and yaw angle on the critical wind velocity of the rotor are reserched.

Numerical Analysis for Galloping of Iced Quad Bundle Conductors

In order to attain the purpose of anti-galloping, a simplified model for iced quad bundle conductors of three degrees of freedom in vertical, horizontal and torsional directions is established by means of the Hamilton principle, in which the effect of spacers stiffness and damping is considered. Based on the model, the influence of related parameters such as fluid density, damping ratio on conductor galloping amplitude and critical wind velocity is analyzed. Simultaneously, the relation curve between elastic property of spacers and conductor galloping is obtained. The results indicate that the conductor galloping can be weakened to some degree with the proper enlargement of damping ratio, the reasonable setting of spring stiffness on spacers and the avoidance of areas such as the wind outlet and windward as much as possible when choosing the line path and so on.

Research on the Influence of the Aerodynamic Measure on the Flutter Derivative of the Steel Truss Suspension Bridge

Flutter derivative is a significant index of the structure flutter stability. Identifying flutter derivative precisely contributes to the bridge flutter stability analyzing. In this paper, we take a research on the Liujiaxia Bridge in Gansu Province, China. Different flutter derivatives, which were got via segment model vibration tests with different aerodynamic measures, were classified, and made comparison in order to get the law of how different aerodynamic measures effect on the flutter derivative. The results show that, setting central stabilized plate, Build-in deflector, flange plate all affect flutter derivative significantly, which leads to changes in the flutter critical wind velocity of the structure. Setting central stabilized plate above the deck contributes to identify the flutter derivative of the 0° and positive attack angle, while setting central stabilized plate will contribute to flutter derivative identification at negative angles. It will make it difficult to identify the flutter derivative at 0° and -3° if the built-in deflector was set. Wind plate contributes to the identification of the flutter derivative at +3°, however, it will make it harder to identify the flutter derivative at 0° and -3°.

Conductors Vibration Model Improved and Nonlinear Vibration Analysis

Vibration model can reflect conductors nonlinear vibration under wind excitation .The paper improves and expounds the detailed derivation process based on the previous studies, and studies the movement type of conductors. Comparing with the vertical displacement change of conductors in different conditions to prove that the model derived is reasonable. The paper analyses conductors nonlinear vibration in the case of external condition change to study the movement change of conductors. Lyapunov stability theory and Routh-Hurwitz criterion are used to obtain critical wind velocity of conductors, namely the conditions for conductor galloping. The results shows model derived can reflect conductors nonlinear vibration under wind excitation.