Study On Unstable Morphology Mechanism Of Helium Bag Bedded-Inside Envelop

The helium bag bedded-inside envelop is located in the main airbag of the heavy airship at high altitude,and the outer main bag is an air bag.The helium gas is filled inside to generate static lift to balance the self-weight and payload of the hull.As the core carrier to provide the buoyancy of airship,the change of the shape of airbag under different aeration ratio will cause the change of the buoyancy center and center of gravity,and then affect the flight attitude control of airship.Based on the control volume method and the two-way fluidsolid coupling numerical simulation analysis method,the numerical simulation models are established respectively.The helium bag bedded-inside envelop is simulated and analyzed,and the shape displacement of the helium bag at different filling ratio is obtained.At the same time,we combined with the experiments of inflating and deflating helium bag,the shape and displacement of helium bag at different filling ratio were obtained.Firstly,by using LS-DYNA,a non-linear explicit dynamic analysis software,the degassing model of helium bag was established based on the control volume method.The shape displacement of helium bag during degassing was calculated by ANSYS/LS-DYNA solver.The results show that the helium bag model based on the control volume method could not consider the density difference between air and helium which caused the buoyancy load.Therefore,it could not reflect the influence of buoyancy on venting process.Then,based on ANSYS transient dynamics analysis and CFX multi-physical field coupling analysis,considering the buoyancy effect caused by the density difference between air and helium,a multi-phase coupling analysis of air,helium and flexible membrane bag has been carried out.Firstly,the initial inflated equilibrium shape was commutated from the unstressed geometry of the helium bag by inflating simulation,the buoyancy and gravity equilibrium and deformation validate the numerical simulation method.Then,the helium gas flow control method was employed to simulate the helium bag at elevation angles of 0,10,20,30 degrees and at filling percentage of 100,90,80,70 and 60 respectively.Thus,the degassing process of helium bag was simulated.Finally,through the analysis of helium bag deflation models with different elevation angles postures and different filling degrees,it shows that when the elevation angle is zero,approximately 1/3 of the bottom of the helium bag becomes unstable as the filling percentage decreases.The circular uniform sag modes were observed to extend toward the top and a large sag in the circumferential direction;when there is an elevation angle,the sag first occurs in the lower part and exhibit circumferential asymmetry.With the increase of the elevation angle,the sag in the lower part expands toward the top;through analyzing the shape displacement of helium bag during inflation and deflation,the shape and displacement change law of helium bag under different helium filling ratio was verified.By analyzing the shape displacement of helium bag in the process of inflating and deflating,the results show that the upper region expands first and lower region show small displacement during the inflating and deflating of helium bag.With the increase of air volume,the cyst expands from top to bottom and then expands.In the process of ventilation,the lower part of the cyst shrinks first,and there is no obvious deformation near the upper part.With the increase of ventilation volume,the cyst shrinks and sags from bottom to top along the height direction,resulting in compression between sags and subsequent folding.