Study On Large Eddy Simulation Of Wind Loads And Aerodynamic Noise Of Super-tall Buildings With Effects Of Corner Modification

With the development of technology and economy,more and more super-tall buildings have been built each year with increased total number and maximum height.Wind loads have been recognized as the critical load in design of super-tall buildings with the increase of height.While aerodynamic noise produced by the super-tall buildings has become the serious environmental problem.Hence,it is paramount important to investigate the wind loads,wind-induced response of super-tall buildings and aerodynamic noise produced by them.Compa red to wind tunnel test and field measurement,CFD simulation possesses many advantages such as reliable prediction,no model fabrication,no interference from measuring equipment,easy visualization of flow field and so on,especially if the large eddy si mulation(LES)is employed.Wind fields,wind loads,wind-induced response and aerodynamic noise of super-tall buildings with square section,square section with corner recession and corner chamfered are all investigated using the LES technologies,with emp hasis on the effect of corner shapes and its ratios of corner modification.This paper will focus on the following aspects and the conclusions will be listed as follows:(1)Current development and their problems existing on wind resistant study,sectional corner modification,and aerodynamic noise of super-tall buildings are summarized in detail.The governing equations of turbulent flows,the CFD simulation method and its applicability are discussed,with emphasis on the basic theories and advantages of LES.Meanwhile,the key technologies for quality control on CFD simulation,such as arrangement of computational domains,the height of first mesh close to the wall,boundary conditions,time step size,turbulence models selection,are also discussed.(2)Considering the shortcoming of turbulent flow produced at the inlet by the Random Flow Generation(RFG),the method of adjusting the turbulent integral scale is proposed to improve the accuracy of LES.The results show that the simulated wind fields have the same characteristics as the atmospheric boundary layer when the modification factors of the turbulent integral scale are respectively set to value of 0.4 and 0.55 under the conditions of terrain category B and C.The flow field around the CAARC Standard Building model is simulated by LES under the conditions of terrain category B,and the effects of turbulent integral scale on wind loads are also investigated.It is found that the simulated wind field is in agreement with wind tunnel test.It is also found that with the increase of turbulence integral length,the turbulent energy increases from the deformation of mean flow velocity,and the mean wind speed will decrease slightly while the turbulence intensity will increase.As a result,the fluctuating wind pressure decreases and the capability of reattachment of separated flow will increase.However the amplitude at high frequency and peak aerodynamic base moment and torque spectrum,peak vortex shedding frequency,the mean drag coefficients,the RMS coefficients of wind pressure,and the mean coefficients of the wind pressure in the negative pressure zone will all decrease.The investigation further finds that the turbulence integral length presents minor effects on correlations of aerodynamic forces in v ertical direction,but it reveals significant influence on correlations of wind pressure in flow and cross-flow directions.(3)LES has been employed to investigate the wind loads on super-tall buildings with square section or square section with corner m odifications.The influences of corner shape or ratios of corner modification on wind pressure distribution and correlations of wind pressure are also analyzed.The inherent mechanism of corner modifications on wind pressure is investigated by using the CF D visualization.Compared to the square section,the square section with corner modifications can reduce the average wind pressure.The ratios of corner modification presents significant influence on mean wind pressure on the side and back surface of the building,and the higher ratios of corner modification,the more decrease of the mean wind pressure.At the section corner,wind pressure coefficients of section with corner modifications are larger than that of square section.In addition,correlations of wind pressure between corner and main surface are reduced for the models with corner modifications.Corner modifications can reduce the amplitude at high frequency band,but rarely influence on low frequency band,the energy of fluctuating wind pressure spectrum are distributed over a wider frequency band.In other words,the energy of vortex shedding are distributed over a wider frequency range,so it is proved that corner modifications can reduce vortex shedding strength in the windward leading edge.The reattachment of separated flow from the windward leading edge increases in leeward.Under the same ratios of corner modification,the reattachment of separated flow from square corner chamfered are stronger than those of square corner recession.In a word,corner chamfered is better than corner recession in decreasing wind pressure.(4)Mathematical expressions of wind loads are established for super-tall buildings with square section or square section with corner modifications by LES.The influence of corner shape,ratios of corner modification and terrain category on the layer aerodynamic forces,the correlations of aerodynamic forces in vertical direction,the base moment and base torque of super-tall buildings are studied.The result shows that the layer drag coefficients,layer lift coefficients,layer torque coefficients,base moment coefficients in cross-wind direction and the average base moment coefficient in along-wind direction for corner modification models are smaller than those of the square sectional model,and the decrease become obvious with the increase of ratios of corner modification.The RMS coefficients of base moment in along-wind direction increase gradually with the increase of ratios of corner modification.Under the same ratios of corner modification,the average and RMS of base moment for square corner recession are bigger than those of the square corner chamfered.With the increase of ratios of corner modification,the peak power spectrum of base moment decreased as well as the curve of spectrum in high frequency range.The spectrum curve changes from narrow-band to wide-band one.The attenuation of the spectral curve is more obvious in high frequency range with the increase of ratios of corner modification,especially for the square model with chamfered corner.(5)Wind loads on super-tall buildings are obtained by LES,with which the analysis of wind-induced response are carried out.The research shows that both the displacement and acceleration on the top of the building decrease with the increase of wind yaw angle.The influence of corner recession on the decrease of the along-wind displacement and acceleration on the top of the model is similar to that of chamfered corner,and the decrease ratios of corner modification increases w ith the increase of ratios of corner modification.Cross-wind displacement and acceleration on the top of the building with corner chamfered decrease significantly with the increase of ratios of corner modification,the decrease amplitude ratios of which a re little difference for the models with different ratios of corner recession.Hence the building with corner chamfered performs better on the displacement and acceleration on the top of the building than the model with corner recession,especially in cros s-wind direction,the effect is more significant with the increase of the ratios of corner modification.(6)LES prediction of flow around circular cylinder under high Reynolds number is carried out.The mean drag coefficient,fluctuating lift coefficient,particularly the vortex-shedding Strouhal number are in good agreement with available reports.The research shows that the vortex-shedding energy is concentrated on a limited band of frequency,and the fluctuating pressure on circular cylinder surface is driven by its alternate vortex shedding.It is found that the lift is mainly produced by the out-of-phase component of fluctuating pressure on the upper and lower surface near the vortex shedding frequency,and the drag is generated by the in-phase component of fluctuating pressure on the upper and lower surface near the two times vortex shedding frequency.Numerical simulation of flow field and aerodynamic noise are investigated for both the two-dimensional circular cylinder and the ground-mounted three-dimensional cylinder.The research shows the aerodynamic noise induced by flow around a circular cylinder is the type of dipolar noise source,and noise radiation intensity is controlled by fluctuating pressure on its surface.Hence,the most effective way to reduce aerodynamic noise produced by the circular cylinder is to reduce the fluctuating pressure on its surface.(7)Based on the secondary development on the Fluent software,aerodynamic noise around super-tall buildings and its inherent mechanism is investigated numerically.Also,the characteristics of spatial distribution of sound pressure level and the propagation of sound waves are discussed.The influence of sectional corner modification,ground effects and wind directions on sound pressure level are also analyzed.It shows that each face of super-tall buildings can be regarded as a sour ce of dipolar noise and the radiation intensity of sound fields are respectively controlled by the corresponding along-wind and cross-wind fluctuating pressure.So the most effective way to reduce the aerodynamic noise is to reduce the fluctuating pressure on building surface.Corner modification on building section has little effect on total sound pressure level in along-wind direction,but has great effect in cross-wind direction.Corner modification also has little effect on the radiation intensity of so und fields when ratios of corner modification is no more than 5%.When the ratios of corner modification is less than 10%,the total sound pressure level decreases obviously.Although the decrease ratio in along-wind direction is small,the decrease ratio in cross-wind direction is significant.With the increase of ratios of corner modification,the radiation intensity of sound fields will decrease.Ground effect can cause the total sound pressure level to increase first and then decrease in vertical direction,with the minimum and maximum respectively occuring at the height of 0.1H and 0.7H,while the highest in the leeward and the lowest in the windward.When the terrain is rougher,the total sound pressure level becomes higher.