Research On Snow Drifting On Roofs Using Wind Tunnel Test And Numerical Simulation Based On Lagrangian Approach

In snowy and windy regions,the unbalanced snow loads on building roofs caused by wind-induced snow drifting are recognized as crucial factors leading to the collapse of roof structures.The growing construction of long-span and lightweight structures thus raises higher demands for the accurate prediction of drifting snow loads on roofs.As powerful tools suitable for the mechanism study of snow drifting on building roofs,numerical simulation offers the capability of obtaining details for the two-phase flow in the vicinity of the roof,while the wind tunnel test shows better reliability and accuracy with comparatively mature similarity theories.In this study,the characteristics of snow drifting on roofs as well as detailed influences of several key factors are explored via the combination of numerical simulation and wind tunnel test from the following four parts:1.Experimental investigation of snow drifting on flat roofs during snowfall in a cryogenic wind tunnel.Given the fact that quantitative analysis concerning the features of snow drifting on roofs during snowfall has rarely been reported,a series of tests for snow drifting on 2D flat roofs are thus carried out in a cryogenic wind tunnel with concurrence of snowfall.The similarity requirement for correct scaling of snowfall intensity is derived according to the similarities of snow distribution patterns and the development state of snow transport on the roof.Detailed influences of roof span and snowfall intensity on the characteristics of snow drifting are then analyzed in terms of snow distributions,transport rate and its development.By comparisons with published experimental data from snow redistribution tests without snowfall,the similarities and differences between snow drifting with and without snowfall are uncovered to obtain the typical distribution patterns on roofs.2.A study of snow drifting on monoslope roofs during snowfall: Wind tunnel test and numerical simulation.The research object is further extended to 3D monoslope roofs for an insight into the impact of roof slope on the features and mechanisms of snow drifting under snowfall conditions.High-density silica sand particles are employed in the conventional wind tunnel to create the snow precipitation environment.Based on the results under different wind velocities and snowfall intensities,comparisons of particle distributions,degree of erosion and transport rate are presented for different sloped roofs.Then,the flow patterns around the roof before(without snow)and after snowfall(with snow cover)are predicted using numerical simulation.The correlation between the distributions of surface friction velocity with the typical snow distribution pattern obtained from the experiment is discussed and summarized,to elaborate the varying characteristics of snow drifting with roof slope.3.A numerical model for snow drifting simulations on flat roofs using Lagrangian approach.The Lagrangian description presents superiority in capturing particle behaviors near the roof surface in comparison to the Eulerian two-phase flow approach commonly adopted in snow drifting predictions on building roofs.Several representative numerical methods are thus selected from typical models of blowing snow over flat terrains,of which the feasibility and accuracy are evaluated by the scaled test of snow redistributions on the flat roof conducted in the wind tunnel.The optimal model is determined by comparing the predicted transport rate on the roof with estimations from empirical formulas and by comparing simulated snow distributions with experimental data.Snow particle behaviors above the roof are then provided in detail based on the developed model,including particle speeds,snow transport flux,and spatial distributions.In particular,a preliminary insight is given into the role of splash entrainment and its contribution to unsaturated snow transport on the flat roof.4.Application of snow drifting model considering the effect of slope on gable roofs.Characteristics of snow drifting on gable roofs substantially differ from those on the flat roof,which is mainly reflected in the stress state at the initiation of particle movement,as well as the grain-bed collision process.The existing snow drifting model for flat roofs is thus further improved with modifications on the threshold of snow particles and the splash function correspondingly.The accuracy of the modified numerical model is then verified by comparing the predicted results with data obtained from the wind tunnel test of snow redistributions on gable roofs.On this basis,systematic analysis and discussions are presented for the detailed influences of roof slope on the flow pattern around the gable roof,snow particle behaviors close to the roof surface,as well as the features of snow transport on the roof.