Research On The Flow Structure Around Large-span Roofs By CFD And PIV Techniques

Large-span spatial structures are very sensitive to wind load because of their characteristics of light mass,flexibility and small damping.The structures always located at the bottom of the atmospheric boundary layer,which experiences rapidly changed wind speed and high turbulence intensity.Flow separation around the structures is complicated and the variety scales of vortices occur on the roofs.One of the most significant flow characteristics and destructive vortex are the separation bubbles and conical vortices,which cause extremely high suctions at the separation zone and the roof comer.Local damage triggered by separation bubbles and conical vortices are inclined to cause the ultimate destruction of the large-span spatial structure roof.In this study,a large-span flat roof and saddle roofs with different rise-span ratios are taken as research objects.The purpose of this study is to investigate the 3D flow structures of the vortices,pressure distribution on large-span roofs under separation bubbles and conical vortices,and examination of the flow mechanism below the vortices,the main contents are as follows:1.According to the characteristics of large-span roof structure,the key technology of CFD simulation is discussed,and the approximate height formula of the first-layer lattice is deduced by the plate incompressible turbulent boundary layer theory.The LES results of three different turbulent flow generation methods are compared and analyzed,the main conclusions are as follows:The upstream and downstream length should not be less than 8h and 16h(h is the height of the model),the watershed width and height are(16h-18h)and 10h.It is suggested that the grid increment coefficient is 1.15.For the Realizable k-? model,the prediction accuracy of the average wind pressure and vortex flow line is the highest.Based on the boundary layer differential equation and the plate incompressible turbulent boundary layer theory,the first floor height of the wall of RANS and LES is determined according to the different y+values for index and logarithmic rate velocity profiles.For the three pulsation inlet generation methods,the random generation method is in good agreement with the wind tunnel results,which is close to the real turbulent state,the linear filtering method and vortex perturbation method predicts over large on the whole.2.Through the flat and saddle roofs PIV test,the phenomenon of separation bubble under uniform and tuibulent flow condition were observed.The similarities and differences of the orbital motion trajectories were analyzed from the time-averaged and transient characteristics,and the 3D flow structure of the separation bubble was obtained,the main conclusions are as follows:?For flat roofs,the separation bubble are formed when the wind direction is perpendicular to the windward leading edge.Under uniform flow,the vortex is generally flat in the downwind direction and almost across the roof.The vertical bisector cross-sectional area is the largest and also with the vortex height.With developing on both sides,the height of the vortex and the distance from the front edge are decreasing.Under turbulent flow,the volume of the bubble,height of the vortex and the distance from the front edge under turbulence flow are much less than the uniform flow.Increasing turbulence results in shorter reattachment length,but it does not change the peak vorticity.The peak of the turbulent kinetic energy is concentrated in the center of the separation bubble,followed by an elliptical ring outward gradually decreasing.With the flow downstream,the vorticity will gradually dissipate.?The typical separation bubble phenomenon on the saddle roof appears between the high point windward and middle point windward,and the reattached length is almost across the whole roof,but in the midnight to low point area cannot form a vortex.The vertical bisector cross-sectional area and the distance from the front edge under turbulent flow are much less than the uniform flow condition.With the incensement of span ratio,the vertical bisector cross-sectional area and the distance from the front edge under turbulence flow are also increasing.However,the influence of span ratio for the vortex height is smaller.3.Through the flat and saddle roofs PIV test,the phenomenon of conical vortices under unifonn and turbulent flow condition is obsened.The characteristics of the conical vortices spatial evolution under different and the same wind angle are analyzed by CFD technique,3D morphological characteristics of the conical vortices are obtained,the main conclusions are as follows:?For flat roofs,a single vortex structure appeared on the main front wind edge under 15°.In the front of the windward on both sides,a pair of conical vortex structure is appeared under 30° and 45° wind direction.On the whole,the conical ortices are an elliptical cone structure,with the separation point as the vertex,and increasing vortex cross-sectional area,the vortex is close to the roof surface.The cross-sectional area of the vortex area and the height of the vortex in uniform flow are even higher than that of the turbulence,and vortex axis position is closer to the front of the windward.The vorticity reaches the maximum near the leading edge and around the vortex.The peak of the turbulent kinetic energy is concentrated at the center of vortex.?For saddle roofs,when the low point is the wind direction,the flow separation at the low point and does not form a vortex.The vortex taking into account the conical vortex and separation bubble characteristics under 15° when the high point is the wind direction.With the incensement of the wind direction,a pair of conical vortices is formed in the front windward on both sides,and the vortex pattern gradually evolves from the flat elliptical cone into a cone.The vortex axis begins to approach the windward.With the increase of the wind direction angle,the distance between vortex of the front wind edge and the height of the vortex are decreasing for two kinds of vector span ratio(1/12,1/6),while the windward wall height has less effect on the vortex position.Whether the flow is uniform or the turbulence,the large curvature of the saddle roof formed a large deflection angle.With the increase of the wind direction angle,the deflection angle is almost straight down trend,while the roof wind suction is increasing.Compared with the uniform flow conditions,the vortex deflection angle performance is smaller under turbulence flow.4.Wind pressure distribution characteristics of flat and saddle roof surface were investigated through the rigid model wind tunnel pressure tests under uniform,turbulent and B terrain flow conditions,and cross-correlation between the test points and the wind pressure spectra are discussed,the main conclusions are as follows:?For flat roofs,the mean and rms pressure in the area of the front windward edge reached the maximum.As the distance between the point and the front edge is increasing,the roof wind pressure gradually decreases.The cross-correlation between the measuring points is exponential decay under the three wind condition,and wind pressure spectrum from the low frequency pulsation transition to high frequency pulsation.?The maximum mean and rms pressure appears near the lower triangular area,and wind suction is also strongest under 30°.With the increase of wind direction angle,the range of cone vortex gradually decreases.The peak value of the wind pressure spectrum near the windward point is proportional to the corresponding roof wind suction.?For saddle roofs,results show that the maximum wind suction is at the location of the low windward point.The wind pressure spectrum at the leading edge is dominated by low frequency.At the low point,spectrum from low frequency to high frequency with the rising trend,and reach the maximum in the high frequency.?For saddle roof,with the increase of the wind direction,the bell shape distribution is more prominent.When the span-ratio is relatively large,the swing range and frequency of the vortex axis are significantly increased,and the vortex axis is the swing motion of the non-equal frequency.Low frequency pulsation dominant the vortex formation stage,and the peak frequency peak gradually increased downstream.5.Based on the wind tunnel test of flat and saddle large-span roofs,the area time history method is adopted to get the most unfavorable pressure,the calculation results are analyzed according to National load specification.The Davenport crest factor method and the Hermit model theory are used to calculate the Gaussian and non-Gaussian peak factors respectively,and the most unfavorable peak wind pressure on the roof surface is obtained.Finally,the local body size and peak suction of the flat roof and saddle roof are defined and the corresponding suggestions are given,which can provide reference for revising relevant provisions in the China Loading Code for Design of Building Structure.