Experimental Simulation And Aerodynamic Characteristics Of Transient Wind On Structures

Current structure wind-resistant design methods are based on observation of normal climate condition (e.g. monsoon), steady inflow and quasi-steady aerodynamics are the basic hypotheses of these methods. Building disasters are actually caused by extreme wind climate, including typhoon, thunderstorm and tornado etc. These extreme wind climates share a common feature that wind velocity and direction could transform rapidly. (It is called transient in this paper.) Obviously, steady inflow and quasi-steady aerodynamics assumption cannot be applied to the study on wind loading with transient characteristics, and current wind-resistant design could not effectively prevent structures from disasters. The research on extreme wind climate and its effect become one of the cutting edge issues in wind engineering field in both domestic and overseas, but its development is slow because of some object factors. The major difficulty is shown as following: observed data are scarcely available because of contingency, regional characteristics and short acting time; moreover, the technology of both wind tunnel experiment and numerical simulation are quite immature so that experimental data are difficult to obtain by researchers; non-stationary and non-Gaussian characteristics of extreme winds also pose great difficulty for data analyzing and processing.Focusing on these problems, firstly, a conceptualized model of step transient flow is proposed, and wind tunnel experiment and its implementation method are studied in this paper; secondly, related mechanism of transient flow is revealed by studying the characteristics of wind load of construction under transient flow; finally, wind-resistant design methods are researched, and some design suggestions are presented.The main content includes the parts as follows:According to transient characteristic of extreme wind climate, a conceptualized model of step transient flow is proposed. Both the acceleration of air flow and flow field form after accelerating could be changed so that down-burst step flow and slipping stream step flow can be obtained. On this basis, by applying the multiple fans active control system in the turbulent wind tunnel in Miyazaki University, the above mentioned flow forms are successfully simulated through repeating debugging of fan rotational speed and phase in different positions to obtain flow field in wind tunnel with same power spectrum density as target flow.A series of wind tunnel experiments are designed in order to investigate the characteristics of wind loading of structure under transient flow. Specific investigated parameters includes: 2-D cylinder section and square section, 3-D high-structure, middle-structure and low-structure models, different acceleration and flow field form of inflow, different wind direction of inflow and etc. The number of experimental scenarios is about 190. Based on the above work, it is intended to accumulate the experimental data, on which the mechanism of transient flow is studied.According to the experimental results, adopting the analysis procedure of " from two-dimension to three-dimension, partial to integral, static to dynamic, stationary flow to step flow", wind loading of structure surface under transient flow is studied in both time domain and frequency domain in detail. The specific parameters include: point wind pressure coefficient, surface wind pressure coefficient, drag coefficient, side force coefficient, spatial correlation and running correlation of fluctuating wind pressure, spectrum characteristics of fluctuating wind pressure and etc. On the base of which, this paper mainly discusses the phenomena of Peak-Value of aerodynamics on structure surface during flow accelerating, and quantitative description of integral and local features of wind load of structure under transient flow.According to its non-stationary characteristics of transient flow, by introducing Hibert-Huang Transform (HHT) spectral analysis, the time-varying characteristic of wind load of transient flow is studied, and the time-frequency scalogram during the whole process of acceleration is obtained, thus the mechanism of wind load of transient flow is revealed. Based on this, trend in the wind load of structure under transient flow is extracted by applying theempirical mode decomposition (EMD), and this extraction is regarded as time-varying average component of wind load; by adopting the concept of skewness and kurtosis to study the non-Gaussian characteristics of fluctuation component of transient flow loading, the model of wind load of structure under transient flow is build.Based on the above studies, preliminary study on structural design methods against transient flow is carried out. In wind load, wind load of transient flow are composed of two parts, which are mean wind load with time varying and fluctuant wind load. Mean wind load with time varying can be expressed by coefficient of non-stationary wind pressure, and non-Gaussian fluctuating wind load is obtained by applying empirical formula of spectrum density of fluctuating winds combined with harmonic component representation. Based on an example, the difference of wind load and wind response under transient flow and normal wind in codes is discussed, and it demonstrates that structural safety under transient flow cannot be guaranteed using the methods in design code. The study could be valuable reference for further research on wind resistant design method of structure against extreme wind climate condition.