Simulation And Experimental Research On The Optimal Design Of The Rain Cap For Sounding Humidity Measurement

At present,most of the commonly used radiosonde humidity measurement systems are equipped with rain caps,which leads to obvious solar radiation dry error(SRDB)in humidity measurement.In order to study and improve the influence of rain caps on humidity measurement,the computational fluid dynamics(CFD)method is used to study the influences of different structures and materials of rain caps on the temperature rise of humidity measurement system.The SRDB of humidity measurement is derived by combining with the saturated vapor pressure formula.(1)In this paper,the computational fluid dynamics(CFD)method is used to study the SRDB of humidity measurement system with different types of rain caps.Firstly,three kinds of basic structure rain caps are constructed by Pro / E software,which are cylindrical rain caps,hemispherical rain cap and prismatic rain cap.Then,the model is meshed by ICEM software.Finally,the change of temperature rise in the rain cap under solar radiation is simulated by FLUENT software,and the SRDB under various conditions is compared.(2)A comparative simulation experiment was designed to study the influence of the rain cap and other parameters on SRDB of the humidity measurement,including the structure,size,material and other parameters of the rain cap,as well as altitude,solar altitude angle and air velocity.In order to find the most suitable structure and material of the radiosonde rain cap,the effects of various variables on the SRDB of the humidity measurement system are studied.The results show that: although the rainproof effect of the three structures is similar,the cylindrical rainproof cap performs better in terms of radiation protection.Using waterproof and breathable membrane instead of traditional plastic rainproof cap can greatly reduce the SRDB of radiosonde humidity measurement.(3)A temperature and humidity measurement system is designed.The hardware circuit includes a main probe installed with a rain cap.A temperature and humidity sensor is installed in the probe to measure the internal temperature and humidity of the rain cap.Another temperature sensor is installed on the motherboard to measure the external environment temperature of the rain cap.Comparing the experimental data with the corresponding CFD simulation data,the results show that the overall change trend of the experimental data and the corresponding simulation data is consistent,but compared with the simulation data,the value of the experimental data is slightly larger due to the laboratory environment,the installation position of the temperature and humidity sensor and the manual pasting of the surface material of the rain cap used in the experiment.The above simulation results and experimental results show that the rain cap can play a better rainproof effect,but there is still solar radiation drying error.The rain cap with new waterproof and breathable membrane material can reduce the SRDB of high altitude humidity measurement to the greatest extent,which provides a theoretical basis for the optimization design of the rain cap.