Study On The Flow Field Of Wind-blown Sand And Its Effect On Low-rise Buildings

The wind-blown sand disasters in northwest China are widely distributed and serious.The main hazards to civil engineering and traffic infrastructure are as follows: invading engineering structures such as buildings,bridges,pipelines,photovoltaic systems and thermal power systems;erosion of highway subgrade,sand accumulation on the road surface and overturning of vehicles;sand-buried railway tracks,causing train derailment and window damage.However,the current research on wind-blown sand flow is focused on geomorphology and aeolian physics,and there are few studies on the effects of wind-blown sand on civil engineering and traffic infrastructure.Loads of engineering structures under wind-blown sand environment are the sum of wind load and sand particle impact load,which is quite different from the traditional design with the impurity-free wind pressure as the design wind load.Therefore,it is of great theoretical and engineering significance to study the flow fields of wind and wind-blown sand and their effects on engineering structures in the desert area.Based on the frequent occurrence of wind-blown sand and sandstorm in northwest China,the full-scale experimental building in the typical desert area is taken as the research object.The methods of field measurement,wind tunnel test and theoretical analysis are applied to study the characteristics of near-ground wind,sand concentration distribution and wind pressure on the low-rise building in the desert area.The flow fields of wind-blown sand and falling sand and their effects on low-rise buildings are also analyzed.The research results can be good for the improvement of wind-blown sand test technologies and methods,and also provide the theoretical basis for the wind and wind-blown sand resistance design of low-rise buildings in the desert area.The main research contents and findings are as follows:(1)A monitoring platform for wind and wind-blown sand resistance design of low-rise buildings is established,which lays a foundation for field measurement of wind field,wind-blown sand flow field,wind pressure and wind-blown sand pressure of low-rise buildings in the desert area.(2)The characteristics of near-ground wind and sand concentration distribution in the desert area are obtained by field measurement.The measured wind speed profile is basically in agreement with the European wind code.The measured wind speed spectrum is basically in agreement with the von Karman's spectrum.The measured sand concentration decreases exponentially with height.The formulas are proposed to describe turbulence intensity profile,turbulence integral scales profile,the gust factor profile and the correlation between the gust factor and the turbulence intensity in the desert area,respectively.(3)Through field measurement,the wind pressure coefficient,the probability characteristic of wind pressure,the wind pressure spectrum and the correlation of wind pressure on the experimental building in the desert area are systematically analyzed and compared with those of wind tunnel tests.The variation rules of mean wind pressure,fluctuating wind pressure and midline wind pressure with wind direction angle on the experimental building are summarized.There may be local high-pressure areas at the edges or corners of each surface,which should be considered in the design to avoid local damage.The wind pressure on the leeward wall,the crosswind wall,the leeward roof and the edge of the windward eaves presents non-gaussian characteristics.Compared with the windward wall,the wind pressure spectrum values of other surfaces are lower in the low frequency band and higher in the high frequency band.The coherence function of fluctuating wind pressure decreases exponentially with increasing frequency.The correlation coefficient and coherence function of fluctuating wind pressure decrease with the increase of distance,and the correlation coefficient of the windward wall is greater than that of the leeward wall and crosswind wall.(4)Based on the measured wind characteristics in the desert area,wind-blown sand flow fields with fine,coarse and mixed sand are established in the wind tunnel.The results show that at the same wind speed,the concentration and energy of mixed sand are the largest,followed by the fine sand and coarse sand.The concentration and energy increase as the wind speed increases,and decrease exponentially as the height increases.The wind-blown sand flow field can be divided into the flow field inside and outside the sand particle saltation layer.Within the saltation layer,the sand particle movement significantly reduces the wind speed and enhances the turbulence intensity.The influence degree is directly related to the vertical distribution characteristics of sand concentration.Outside the saltation layer,the particle movement has little effect on the wind profile and turbulence intensity.(5)Based on the measured wind characteristics in the des ert area,the number of sand leakage holes at the top of the wind tunnel(0.5 pipe,1.0 pipe,1.5 pipes)is adjusted to establish falling sand flow fields with multiple sand concentrations.When the wind speed is less than the critical value,sand concentration profile presents an exponential decay form;when the wind speed reaches the critical value,the sand concentration decreases first and then increases with height.The sand particle movement significantly reduces the wind speed and enhances the turbul ence intensity in the whole falling sand flow field.The influence degree is directly related to the vertical distribution characteristics of sand concentration.The energy of sand particles increases with increasing sand concentration and wind speed.It also decreases first and then increases with height,and gets the minimum of sand energy at 0.2 m.(6)Base shear forces acting by wind and sand particles in the flow fields of wind-blown sand and falling sand are analyzed.At the same wind speed,base shear forces acting by wind in two types of wind-sand flow fields are smaller than that in the impurity-free wind flow field and base shear forces acting by wind decrease with increasing sand concentration,but base shear forces acting by sand and the ratio of the total base shear forces to base shear forces acting by wind increase with increasing sand concentration.Base shear forces reduction acting by wind and base shear forces acting by sand increase with increasing wind speed.(7)The effects of wind-blown sand and falling sand increase the average base shear force of buildings.The average base shear force coefficient in the wind-blown sand flow fields with coarse,fine and mixed sand bed increases in turn,which is 3%~5%,7%~10% and 11%~13%,respectively.The average base shear force coefficient in 0.5 pipe,1.0 pipe,1.5 pipes falling sand flow fields increases in turn,which is 4%~5%,8%~10% and 12%~14%,respectively.The elastic modulus of the model material has an effect on the mean base shear force.The larger the elastic modulus is,the larger the mean base shear force is.(8)There are positive and negative influences of sand particle movement on the fluctuating base shear force of buildings in wind-sand flow fields.When the wind speed is low,the irregular movement of sand particles can enhance the fluctuating effect in the flow field,so that the fluctuating shear force coefficient of buildings increases.When the wind speed exceeds the critical value,the structure of the wind-sand flow field tends to be stable,and the regular movement of sand particles weakens the fluctuating effect in the flow field,which decreases the fluctuating shear force coefficient of buildings.The roughness of the model material has an effect on the fluctuating shear force.The rougher the surface,the greater the fluctuating shear force.