| The Mogao grottoes were excavated on the cliff at the eastern foot of Mingsha mountain,25 kilometers southeast of Dunhuang.From the Pre-Qin period of the Sixteen Kingdoms to the Yuan Dynasty,after more than 1000 years of construction,the Mogao Grottoes have formed a huge scale.There are more than 700 caves,more than 45000 square meters of murals and more than 2400 clay-colored sculptures.The Mogao Grottoes,the Yungang Grottoes in Datong,Shanxi,the Longmen Grottoes in Luoyang,Henan and the Maijishan Grottoes in Tianshui,Gansu are collectively known as the four major Grottoes in China.The Mogao Grottoes are located in the west facing east,with the only opening facing the east,and the rest are connected with the rock mass.Therefore,each independent cave can be approximately regarded as a single-side opening building with a special shape.However,the research on the ventilation of Mogao Grottoes at home and abroad is limited.Therefore,it is imperative to conduct an in-depth and systematic study on the natural ventilation of Mogao Grottoes,whether from the perspective of cave site protection or the stability of the micro environment in the caves.Based on the research status of natural ventilation with single-side opening at home and abroad,this paper points out the difficulties caused by the uncertainty and complexity of the natural ventilation airflow in Mogao Grottoes,as well as the differences in the cave environment,cave architectural form and research purposes.Three methods,including mathematical theory,field test and numerical simulation,are adopted for research.Firstly,the opening forms of the caves are divided,and then from the three aspects of outdoor meteorological parameters,cave shape and cave door opening parameters,the impact of8 factors on the natural ventilation of the caves,including the temperature difference between the inside and outside of the cave,wind speed outside the cave,wind direction outside the cave,cave volume,corridor length,the ratio of the depth of the cave to the opening,opening width,and opening height,is analyzed and explored comprehensively.The calculation model of the natural ventilation volume of Mogao Grottoes is established,the concept and its functional expression based on the cave protection strategy are proposed,and the existing cave doors are optimized according to the heat and mass exchange mechanism of the natural ventilation of Mogao Grottoes,and compared with the natural ventilation effect of traditional wooden doors and aluminum alloy doors used at present,In order to explore a new type of cave door that is conducive to the protection of cave sites and can adapt to changes in the environment outside the cave.Firstly,this paper introduces the definition and mathematical model of the singlesided opening of Mogao Grottoes,analyzes the characteristics of natural ventilation of small opening and large opening,and according to the difference of flow characteristics between small opening and large opening,proposes that the judgment of small opening and large opening is an important prerequisite for the analysis of natural ventilation of Mogao Grottoes,and then uses the corresponding calculation model of single-sided opening of caves.Then it introduces the judgment methods and calculation models of natural ventilation under the existing wind pressure,heat pressure and heat pressure wind pressure together.At the same time,based on the previous relevant research,according to the natural ventilation mechanism of different openings,through reasonable assumptions and model simplification,the mathematical model of natural ventilation of Mogao Grottoes is deduced,and the natural ventilation models of large and small openings suitable for Mogao Grottoes are proposed respectively.Especially for the more complex natural ventilation with large openings,this paper proposes two new models,namely,the "equal volume model with the opening as the boundary" and the "independent pipe flow model with the space outside the cave as the boundary",and points out the differences and applicability of these two types of models.It provides the precondition,research boundary and theoretical basis for the following chapters.Secondly,this paper studies the relevant characteristics of natural ventilation in Mogao Grottoes by using experimental testing and numerical simulation methods.In the experimental test,through two types and three groups of field tests in January and September,the relationship between the natural ventilation of Mogao Grottoes and the environmental parameters under the experimental state was finally deduced.In order to make up for the uncontrollable factors in the field experiment,the numerical simulation was carried out.Through the simulation and calculation of the independent changes of eight influencing factors,the temperature field,pressure field,wind field and wind speed streamline under various working conditions were analyzed,and the flow law and heat and mass transfer mechanism of natural ventilation in Mogao Grottoes were explained.Using the method of combining theoretical calculation and numerical simulation,the natural ventilation characteristics of the cave,that is,the local resistance coefficient of the cave door,are systematically and comprehensively studied,and the influencing factors of the local resistance coefficient and the relationship between them are obtained,and then the calculation formula of discharge coefficient Cd is deduced.At the same time,the natural ventilation volume of the cave under each simulated working condition is calculated,the laws and characteristics of the influence of each factor on the natural ventilation volume are analyzed,and the relationship between them is quantified,which provides a basis for the subsequent proposed ventilation volume calculation model.Thirdly,on the basis of field experiments and numerical simulation,a calculation model of natural ventilation for Mogao Grottoes,a special form of ruins,is proposed.The accuracy of this model is verified by experimental results.The accuracy of this model is verified by comparing the results of this model with those of other seven existing models;The stability and reliability of the model are proved by comparing the temperature difference between the inside and outside of the grottoes and the downward ventilation volume of the wind outside the grottoes.At the same time,using this model to test the experimental data in Mogao Grottoes,it shows that this model is generalizable and universal.The results show that this model can accurately predict the natural ventilation of the caves,and make up for the blank of this study in Mogao Grottoes.Fourthly,in order to combine the research of natural ventilation with the protection of caves,this paper proposes the wind speed threshold in caves that is conducive to the protection of sites and cultural relics,and determines the maximum and minimum wind speed allowed in caves.On this basis,the flow field in the grottoes is divided into "dynamic wind zone","favorable wind zone" and "calm wind zone",a new concept of the percentage of favorable wind zone in the grottoes is proposed,and the expression of "PSVZ(%)" function is derived.At the same time,considering the time and number of visitors,the concept of average air age "MMA" of caves is proposed and its expression is derived.For the short-term heavy rainfall weather in recent years,the safe static pressure length "Lssp" of cavern waterproof steam infiltration under heavy rainfall weather is proposed,and its function expression is obtained through the analysis and research of various influencing factors.Finally,according to the theoretical analysis of the natural ventilation of Mogao Grottoes,the results of experimental tests and numerical simulation studies,the existing cave doors are optimized.The natural ventilation effects of the optimized grottoes and the existing grottoes are compared and analyzed by taking the percentage of the optimal wind area,the average air age and the safe static pressure length as the measurement indexes.The results show that the optimization of grottoes improves the sensitivity of existing grottoes to wind speed and direction,and has stronger adaptability to the air flow outside the grottoes with different wind speeds and directions.At the same time,it also gives relevant suggestions for the problems that still exist,which provides guidance for the follow-up application of Mogao Grottoes protection in the cave door and the research and development of new cave doors. |
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