| Thunderstorm downburst is defined as an intense downdraft impacting the ground and forming a damaging outflow.In the non-typhoon area,downburst is the main reason for the formation of extreme wind loads,which is extremely dangerous to buildings.However,there isn't any provisions about the thunderstorm downburst wind field nor its relative wind load on structures in China Wind Code.It is very urgent to study on thunderstorm downburst and its wind load characteristics on the structure in structural engineering.In view of this specific situation,the mechanism of the impact of steady thunderstorm wind field and its load on the high-rise building is analyzed and studied in this paper,which mainly includes the following aspects:(1)A static impinging jet model was used to simulate the steady thunderstorm downburst wind field.Wind field on flat terrain and embankment topography is measured by using the hot wire anemometer.The test results show that: the initial jet parameters and jet velocity have a significant effect on the thunderstorm downburst wind field,which can be eliminated by the dimensionless of the velocity profile.The maximum radial velocity appears at 1 times the diameter of the jet in flat conditions,the maximum radial velocity is almost 1.05 times jet velocity,and the height of maximum radial velocity comes at 0.037 times the diameter of the jet.The radial velocity profile of the steady state thunderstorm downburst is "nose-shaping",which is extremely different from the atmospheric boundary layer.Topography has a great influence on the downburst wind field.Compared with the relative flat condition,the cornice position has an acceleration effect of radial velocity,which is up to 1.2 times.Behind the cornice,although thunderstorm downburst wind field spreads higher,but the intensity of the wind field decays much faster.According to the average radial velocity of the flat conditions obtained by the hot wire anemometer,a new radial velocity profile,which is varied with radial position,is proposed in solving the shortcomings of the existing downburst radial velocity model not being applicable to all radial positions.(2)Large eddy simulation method is chosen to simulate thunderstorm downburst wind field on both flat and embankment topography,the results of numerical simulation are compared with the experimental results to ensure the reliability of physical test and numerical simulation.According to the simulation results of the transient wind field,this paper analyzed the whole process of thunderstorm downburst from the formation of sink current to hitting the ground,and till it spreads to the surroundings by front vortex way.Analysis shows that the development of front vortex has a decisive effect on the transient wind field of the thunderstorm downbursts.According to the large eddy simulation,the vertical velocity of the thunderstorm in the flat conditions is obtained,and a mathematical model of the vertical wind velocity profile for all radial position is proposed.(3)By the pressure test of the square section of the high-rise building impacted by steady state thunderstorm,the amplitude characteristics,spectrum characteristics and correlation of wind load on high rise buildings was analyzed in detail.Experiments were carried out separately for both flat terrain and embankment topography,and the influence of embankment topography on the wind pressure characteristics of high rise buildings is analyzed.(4)Based on the pressure test of rigid model,the experimental study on the wind load characteristics of high rise building under the influence of the steady state thunderstorm is carried out.By summarizing the changing law of radial,lateral and torsional wind load on each layer with the radial position and height variation,the mathematical model is proposed for the mean,root mean square deviation,power spectrum and coherence function of wind load on each layers at the 0° direction,which provide a basis for the wind resistant design of the steady thunderstorm downbursts. |
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