Key Technical Issues In Fluid-structure Numerical Simualtion Of Supersonic Parachute

With the rapid development of aeronautic,astronautic,weapon and other relevant fields,work condition of the parachute has been extended from subsonic speed to supersonic speed.Research on supersonic parachute becomes a hotspot in recovery and landing.But the academic research is not mature enough yet.The design and analysis of the supersonic parachute are based on theoretical method and empirical formula of the subsonic parachute,which makes a great error in supersonic region.Based on the two important characteristics of the canopy,material permeability and flexibility,performance of the supersonic parachute is studied in this paper.It aims at constructing theoretical system and analyzing method for the supersonic parachute gradually.It also provides a reference for the design of supersonic parachute.The detail work of this paper is as follows:(1)Flow field model and aerodynamic performance study of the supersonic parachute with material permeabilityCanopy fabric is a special porous media.Its material permeability shows great influence on aerodynamic performance of the parachute.Accurate material permeability model can improve the accuracy of the parachute aerodynamic performance prediction.However,without considering compressibility of the fluid,current material permeability models can't be applied to the simulation of supersonic parachute directly.In this paper,the compressible Ergun equation is introduced into the source term of momentum equation.A new flow field model is established to simulate compressible flow around porous fabric.The numerical calculation is conducted and the aerodynamic performance of the supersonic parachute with material permeability is studied.The influence of material permeability on flow feature,drag performance and stability of supersonic parachute in terminal descent state is explored.(2)Fluid-structure-interaction method study for supersonic self-contact problem with large deformationCanopy fabric is a kind of flexible material which can only bear tension rather than compression.Its shape is greatly influenced by the flow field.The working process of parachute is a typical fluid-structure-interaction problem with strong nonlinearity.Compared with the subsonic parachute,the supersonic parachute has compressible incoming fluid,more complex flow features and larger canopy deformation.So it's much more difficult to simulate the supersonic parachute than the subsonic parachute by a fluid-structure-interaction method.To solve the above problem,an improved Arbitrary Lagrangian Eulerian(ALE)method with body-fitted mesh is developed.It can be applied to supersonic self-contact problem with large-deformation.Firstly,a new virtual structure contact method is proposed.The coupled contact mesh is projected on the virtual region by this method.It ensures that the space between the coupled mesh equals to the size of a local flow field grid.And then it prevents the body-fitted flow field mesh from being excessively compressed and becoming invalid.On the basis of the above,the global remeshing method is added to the original dynamic mesh method.Flow field data at the breakpoint is interpolated to the new mesh obtained by global remeshing method.At the same time,the structural solver restarts at that point.Therefore,lossless restart of the breakpoint can be achieved.The two plates' collision and canopy contact simulations are used to verify the validity of this method.(3)Application of the improved fluid-structure-interaction method on supersonic parachuteOn the basis of the above coupling model and method,this paper studies the terminal descent and inflation stage of a supersonic disk-gap-band parachute.In the structural simulation,the suspension lines and reinforcements are modeled as one-dimensional linear elastic material.The canopy is modeled as Kirchhoff material.In the flow field simulation,the governing equation based on ALE formulation is conducted.And the dynamic and static mesh is combined to reduce computation consumption.The following conclusions are obtained by analyzing the numerical results deeply:1)By analyzing the flow features,it is found that the interaction of the unsteady payload wake and canopy bow shock makes the canopy in an over-pressurized or depressurized state periodically.It is the main reason for the breathing phenomenon of canopy.The abnormal high pressure outside the canopy skirt results in the asymmetric skirt collapse of canopy in the breathing process.And the unsteady flow feature makes this phenomenon strongly random and inevitable.The above analysis reveals the working mechanism of the canopy breathing and collapse.2)The flow field structure with different trailing distance ratio is researched.The flow feature of the parachute-payload system falls into two categories,open and closed flow.The canopy of the open flow is in the wake vortex area of the payload.Its drag performance is very poor.With the increase of the trailing distance ratio,the closed flow transforms to an open one.Bow shock is formed in front of the canopy.The drag coefficient of the canopy is large.The above research provides theoretical basis for the design of the trailing distance ratio.3)The inflation law of the supersonic disk-gap-band parachute is analyzed.The numerical results show that the inflation of the disk is earlier than the band.During the opening process,a couple of vortexes show up in the disk and band respectively and blend together in the end.The shock wave standoff distance reaches a maximum and the canopy inner pressure reaches a minimum when the canopy is over-inflated and achieves the maximum projected area.Then the canopy rebounds to the working diameter due to the bounds of suspension lines and starts to breathe periodically.Maximum stress of the disk occurs when the opening load of the canopy reaches a maximum.While that of the band occurs when the maximum projected area of the canopy is reached.The stress of the canopy in breathing stage is much less than that in inflation stage.The above results reveal the fluid-structure-interaction working mechanism of the canopy inflation process.