Investigation Of Aerodynamic Performance And Multi-objective Optimization Of Non-rigid Stratospheric Airship

Stratospheric airship is a type of “lighter-than-air(LTA)” aircraft whose net lift force obtains from buoyancy.Its aerodynamic drag force is resisted by the propulsion system and attitude is adjusted by the control system.It is be able to hover in a fixed location for a long duration at the height of stratospheric bottom,approximately 20 km.Stratospheric airship has significant value both in military area and civil area,and has become a research hotspot due to its prominent characteristics including long hovering endurance,great payload capacity,high stealthy performance and low cost,etc.Air density at the stratospheric airship working height is much lower than that of the sea-level(only about 1/14).This indicates that lighter weight and larger volume of the stratospheric airship are required to obtain enough net lift force.Non-rigid airship with lighter weight and the ability to achieve larger volume is the preferred structure for stratospheric airship.Wind load is the main external load of stratospheric airship in its working process.The aerodynamic characteristics is the dominant consideration of airship structural design.However,earlier research in this area were mainly concentrated on the force measurement tests of the airship with a double ellipsoid shape,the research on pressure measurement test is still quite limit,especially lack the aeroelastic model wind tunnel experimental studies of non-rigid airship.At the same time,hovering endurance,load capacity together with structural mechanical properties are the key indicators to evaluate the performance of a stratospheric airship.The reduction of aerodynamic drag force means more energy can be saved,and hence the capacity of hovering endurance can be improved.Compared with other aircrafts,the airship weight shows more direct influence on its load capacity.Therefore,it is necessary to conduct multi-objective optimization considering aerodynamic drag force reduction,structural weight reduction and mechanical properties improvement.Based on the research background mentioned above,aerodynamic characteristics of a non-rigid stratospheric airship were systematically researched using the wind tunnel tests and numerical simulation.On this basis,multi-objective optimization analysis of airship was conducted based on the optimization algorithm proposed in this paper.The main content of this work can summarized as follows:(1)Wind tunnel measuring pressure tests on the rigid airship model with a scale of 1/30 were carried out to better investigate the aerodynamic characteristics from the perspective of pressure distribution.The effects of yaw angle,pitch angle and the combined effects of yaw and pitch angle on the aerodynamic characteristics of the airship were analyzed.According to its shape charateristic,the hull was divided into three portions,namely head,middle and tail.The influences of different portions on the hull aerodynamic characteristics were investigated based on the pressure distribution.(2)In order to accurately evaluate the aeroelastic response of the airship,wind tunnel measuring vibration tests of aeroelastic response of airship were designed and conducted.The similarity deviation that do not meet the similarity requirements of the aeroelastic model design was evaluated,and the corresponding correction was given out.The aeroelastic responses were obtained based on the Digital Image Correlation technology.The influences of wind velocity,differential pressure and attitude on aeroelastic response were investigated.What's more,aeroelastic response of the researched airship was evaluated.It was found that the aeroelastic response of the researched airship at hovering altitude is relative small and can be ignored.(3)The aerodynamic drag force has an important influence on the airship performances such as hovering endurance and load capacity.Therefore,the aerodynamic drag force characteristics of the airship were systematically analyzed based on the CFD simulation technology.Influences of environmental parameters,slenderness ratio and fins on the aerodynamic drag force were investigated.Based on the CFD numerical results,estimation formula of aerodynamic drag force coefficient and hull partitioned average wind pressure coefficient were given out.(4)In order to improve the efficiency of multi-objective optimization,a multi-objective optimization algorithm based on the sensitivity analysis was proposed based on the evaluation and improvement of existing parameters sensitivity analysis method and multi-objective optimization algorithm.What's more,the user interface of the algorithm was developed.In the proposed algorithm,the optimization variables are worked as samples of sensitivity analysis,and the sensitivity result is used to guide the optimization process.Optimal result and parameter sensitivity can be simultaneously achieved in a single evolutionary run.Examples showed that the proposed algorithm can obtain multi-objective optimization solutions with higher accuracy and better uniformity.(5)The multi-objective optimization model of the airship was established based on the nonlinear mechanical analysis.Multi-objective optimization considering aerodynamic drag force reduction,structural weight reduction and mechanical properties improvement was conducted using the proposed optimization algorithm.On this basis,Signal-to-Noise Ratio(SNR)-based decision method was applied to determine the most robust scheme that meets the design requirements from the optimized Pareto set.