Multi-Scale Synthetic Eddy Method For Large Eddy Simulation Of Structural Wind Effect

With the increase of height,span and softness of structures,the wind sensitivity of structures becames more and more obvious,which makes structural design of resistance to wind face a great challenge.When the wind-sensitive structure is in the strong wind,its vibration response may be quite obvious,which affects the safty and serviceability of the structure.Strong vibration of structure can make residents uncomfortable,result in damages to building enclosures,and even cause damages to structural components.Using large eddy simulation method to analyze the wind effects of structures has advantages of modeling convenience,low cost and abundant data.In order to ensure the accuracy and stability of large eddy simulation for structural wind effect,it is necessary to provide realistic inflow turbulence and adopt reasonable large eddy simulation method.It is difficult for traditional turbulence generation method to generate inflow turbulence which satisfies the predefined mean velocity profile,Reynolds stress(or turbulence intensity),turbulence integral scales and turbulence spectra.With the purpose of using large eddy simulation method more accurately and effectively to study the wind effect of structures,a multi-scale synthetic eddy method was proposed to generate turbulent inflow boundary condition.The synthetic eddy method was modified by extending single length scale eddies to multi-scale eddies and improving the convection method of eddies.The multi-scale synthetic eddy method could satisfy the predefined mean velocity profile,Reynolds stress(or turbulence intensity),turbulence integral scales and turbulence spectra,which was verified and validated systematically in this paper.In order to analyze the structural wind effect by using the large eddy simulation method based on the multi-scale synthetic eddy method proposed in this paper,four aspects of research were carried out as follows:(1)Study on the theory of multi-scale synthetic eddy method.Using large eddy simulation method to accurately calculate boundary layer flow and get wind loads on buildings is based on a realistic turbulent inflow condition.The essential condition for generating realistic inlet turbulence is that the synthetic turbulence has coherent structures and satisfies statistical characteristics of turbulence.In order to generate more realistic inlet turbulence for large eddy simulation,a multi-scale synthetic eddy method was proposed,which defined variable anisotropy multi-scale eddies.The idea of the multi-scale synthetic eddy method is consistent with the fact that turbulence is composed of eddies of different sizes and that turbulence length scale changes with the distance from the wall.Scale and intensity constraint parameters solved by optimization algorithm were introduced to control turbulence spectra and turbulence integral scales.The convection method of eddies in the multi-scale synthetic eddy method was improved,so that multi-scale eddies moved with the mean velocity varied with the distance from the wall.To keep the random and uniform distribution of multi-scale eddies,a new method of regenerating multi-sclae eddies on the inlet of the multi-domain was proposed.The theoretical analysis of statistical characteristics of the synthetic turbulence was carried out to conduct the validation and verification of the multi-scale synthetic eddy method.(2)Research on self-sustaining property of statistical characteristics of turbulent boundary layer considering multi-scale characteristics.The developments of the turbulence in the flow field of flat plate boundary layer and atmospheric boundary layer were systematically analyzed.The statistical characteristics of turbulence such as mean velocity profile,Reynolds stresses,turbulence integral scales and turbulence spectra were studied respectively.In the case of turbulent flat plate boundary layer,the Lund cycle method was used to generate the target inlet turbulence,and then the average wind profile,Reynolds stress,turbulence integral scale and fluctuating wind speed spectrum of the target were calculated according to the target inlet turbulence.Then,the inlet conditions were generated by using the multi-scale synthetic eddy method,synthetic eddy method method and consistent discrete random inflow generation method according to the turbulence characteristics of the target.Large eddy simulation(LES)was used to simulate the turbulent boundary layer of a flat plate,and the retention of the statistical characteristics of turbulence was tested.In the case of turbulent flat plate boundary layer,the target inlet turbulence was generated by the precursor simulation method.According to the mean velocity profile,Reynolds stresses,turbulence integral scales and turbulence spectra calculated by the target inlet turbulence,turbulence inflow conditions were generated by the multi-scale synthetic eddy method,the synthetic eddy method and the consistent discrete random inflow generation method.Large eddy simulations of turbulent flat plate boundary layer were conducted under four inlet conditions.In the case of the atmospheric boundary layer,the target turbulence characteristics were obtained by the wind tunnel test of the atmospheric boundary layer.Large eddy simulations of atmospheric boundary layer were conducted under three inlet conditions generated by the multi-scale synthetic eddy method,the synthetic eddy method and the consistent discrete random inflow generation method.Self-sustaining property of the simulated turbulence in the cases of turbulent flat plate boundary layer and atmospheric boundary layer was analysed to validate and verify the multi-scale synthetic eddy method.(3)Large eddy simulation of wind load on structures based on limited velocity field method.Based on existed researchs of the large eddy simulation of wind loads on buildings,an accurate and systematic scheme of large eddy simulation was proposed to calculate wind loads on building surfaces.In this scheme,reasonable boundary conditions of flow field were adopted to improve the accuracy of simulation results according to the characteristics of large eddy simulation of structural wind loads.The stability of LES calculation of the flow field around the building was enhanced by limiting the velocity magnitude.The influence of various inlet turbulence conditions on wind load on structure surface was compared to verify the effectiveness and accuracy of the scheme.(4)Analysis of wind-induced responses of structures considering multi-scale characteristics of inlet turbulence.Taking into account different levels of reduced velocity,a finite element model of CAARC standard high-rise building with a scale of 1:300 was established.Wind loads transformed from the time history of wind pressure obtained by the wind tunnel test and the large eddy simulation were applied to nodes of the finite element model of CAARC standard high-rise building.Wind-induced responses of the structure were calculated by Ansys structural analysis software.The displacement and acceleration responses were compared to verify the multi-scale synthetic eddy method for calculating wind-induced responses of structures.