Hyungi Kim

Numerical simulation of crash impact test for fuel cell group of rotorcraft

Crashworthy fuel cell has been widely applied to rotorcraft for improving the survivability of crews. Because the aircraft fuel cell has many interfaces connected to the airframe, it has been considered as one of the system-dependent critical components. Since the embryonic stage of military rotorcraft history began, US army has developed the detailed military specification document required to perform the crash impact test to make sure of the crashworthiness of fuel cells. However, the crash impact test of the fuel cell requests high cost and spending a lot of time to manufacture fuel cells. Moreover, there is a big risk of failure due to huge amount of crash impact load. Then, at the early design stage, the numerical simulation was needed to reduce the possibility of failure before performing the crash impact test. The present study reports on numerical simulations that use a smoothed particle hydrodynamics method supported by crash simulation commercial software (LS-DYNA). The test conditions of MIL-DTL-27422 are employed for the numerical analysis and the material information of the fuel cell is obtained from the specimen test. The resulting equivalent stresses of the fuel cell are calculated and the vulnerable areas are also evaluated. Moreover, the improved design is reconsidered to reduce the maximum stress value of the fuel cell and future work is mentioned to validate the result of the numerical analysis.

Iterated Improved Reduced System (IIRS) Method Combined with Sub-Structuring Scheme (II) - Nonclassically Damped Structural Systems -

An iterated improved reduced system (IIRS) procedure combined with sub-structuring scheme for nonclassically damped structural systems is presented. For dynamic analysis of such systems, complex eigenproperties are required to incorporate properly the nonclassical damping effect. In complex structural systems, the equations of motion are written in the state space from. Thus, the number of degrees of freedom of the new equations of motion and the size of the associated eigenvalue problem required to obtain the complex eigenvalues and eigenvectors are doubled. Iterated IRS method is an efficient reduction technique because the eigenproperties obtained in each iteration step improve the condensation matrix in the next iteration step. However, although this reduction technique reduces the size of problem drastically, it is not efficient to apply this technique to a single domain finite element model with degrees of freedom over several thousands. Therefore, for a practical application of the reduction method, accompanying sub-structuring scheme is necessary. In the present study, iterated IRS method combined with sub-structuring scheme for nonclssically damped structures is developed. Numerical examples demonstrate the convergence and the efficiency of a newly developed scheme.

Fast update of elastic body simulation under impact load by efficient reduction method

A large number of eigenmodes are required for the realistic simulation of the object under arbitrary dynamic load such as impact load while only a few lower modes are adequate for the simulation of free vibration with initial deformation. For the accurate dynamic simulation under impact, a large amount of computer resources are required to obtain considerable number of eigenpairs. The present study presents an efficient dynamic simulation algorithm and simulation results of elastically deformable body under impact. We employ a modal analysis technique that pre-computes the eigenvalues and the corresponding eigenvectors in the framework of finite element method. For low latency time of real-time simulation, eigenmodes are computed from a reduced system constructed from the original one. To construct reduced system, energy estimation technique in the element level is proposed in the present study. Through a number of examples, it is demonstrated that the proposed method saves computational cost effectively and provides reliable real time results of the deformable solids under impact.

Optimisation of rotorcraft fuel tank for crashworthiness based on a neural network

ABSTRACT Crashworthy fuel tank has been widely implemented among rotorcraft, and they have served a valuable contribution to improving the survivability of crews and passengers. From the early stages of military rotorcraft history, the US Army has developed and implemented a detailed military specification documenting the unique crashworthiness requirements for rotorcraft fuel tank with the aim of reducing the high incidence of fatalities due to post-crash fires. International manufacturers have followed this information to develop their own fuel tank, and have reflected the results of crash impact tests in trial-and-error design and manufacturing processes. Since the crash impact test itself requires lengthy preparation together with costly fuel cell specimens, a series of numerical simulations of the crash impact test with digital mock-ups is necessary, even at the early design stage, in order to minimise trial-and-error testing with full-scale fuel tank. In this study, a number of numerical simulations on fuel cell crash impact tests are performed with the crash simulation software, ANSYS/Autodyn. The resulting equivalent stresses are further analysed to evaluate a number of appropriate design parameters and the artificial neural network and simulated annealing method are simultaneously implemented to optimise the crashworthy performance of fuel tank.

Transformation of Dynamic Loads into Equivalent Static Load based on the Stress Constraint Conditions

Due to the difficulty in considering dynamic load in the view point of a computer resource and computing time, it is common that external load is assumed as ideal static loads. However, structural analysis under static load cannot guarantee the safety of design of the structures under dynamic loadings. Recently, the systematic method to construct equivalent static load from the given dynamic load has been proposed. Previous study has calculated equivalent static load through the optimization procedure under displacement constraints. However, previously reported works to distribute equivalent static load were based on ad-hoc methods. Improper selection of equivalent static loading positions may results in unreliable prediction of structural design. The present study proposes the selection method of the proper locations of equivalent static loads to dynamically applied loads when we consider transient dynamic structural problems. Moreover, it is appropriate to take into account the stress constraint as well as displacement constraint condition for the safety design. But the previously reported studies of equivalent static load design methods considered only displacement constraint conditions but not stress constraint conditions. In the present study we consider not only displacement constraint but also stress constraint conditions. Through a few numerical examples, the efficiency and reliability of proposed scheme is verified by comparison of the equivalent stress between equivalent static loading and dynamic loading.

Adjoint Variable Method Combined with Complex Variable for Structural Design Sensitivity

Abstract The adjoint variable method can reduce computation time and save computer resources because it can selectively provide the sensitivity information for the positions that designers wish to measure. However, the adjoint variable method commonly employs exact analytical differentiation with respect to the design variables. It can be cumbersome to precisely differentiate every given type of finite element. This trouble can be overcome only if the numerical differentiation scheme can replace this exact manner of differentiation. But, the numerical differentiation scheme causes of severe inaccuracy due to the perturbation size dilemma. For assuring the accurate sensitivity without any dependency of perturbation size, this paper employs a complex variable that has been mainly used for computational fluid dynamics problems. The adjoint variable method combined with complex variables is applied to obtain the shape and size sensitivity for structural optimization. Numerical examples demonstrate that the proposed method can predict stable sensitivity results and that its accuracy is remarkably superior to traditional sensitivity evaluation methods.

Efficient Dynamic Response Analysis Using Substructuring Reduction Method for Discrete Linear System with Proportional and Nonproportional Damping

The dynamic response analysis for large structures using finite element method requires a large amount of computational resources. This paper presents an efficient vibration analysis procedure by combining node-based substructuring reduction method with a response analysis scheme for structures with undamped, proportional or nonproportional damping. The iterative form of substructuring reduction scheme is derived to reduce the full eigenproblem and to calculate the dynamic responses. In calculating the time response, direct integration scheme is used because it can be applied directly to the reduced model. Especially for the non proportional damping matrix, the transformation matrices defined in the displacement space are used to reduce the system. The efficiency and the effectiveness of the present method are demonstrated through the numerical examples.

Structural Dynamic System Condensation with Multi-level Sub-structuring Scheme in Large-scale Problem

Eigenvalue reduction schemes approximate the lower eigenmodes that represent the global behavior of the structures. In the previous study, we proposed a two-level condensation scheme (TLCS) for the construction of a reduced system. And we improve previous TLCS with combination of the iterated improved reduced system method (IIRS) to increase accuracy of the higher modes intermediate range. In this study, we apply previous improved TLCS to multi-level sub-structuring scheme. In first step, the global system is recursively partitioned into a hierarchy of subdomain. And next, each uncoupled subdomains condensates by improved TLCS. After assembly process of each reduced subeigenvalue problem, eigen-solution is calculated by Lanczos method (ARPACK). Finally, Numerical examples demonstrate performance of proposed method.

Modified Korteweg–de Vries theory of non-monotonic double layers in multi-species plasma

The analytic solution for the time stationary non-monotonic double layers in multi-species plasma is presented. This solution is the analytic extension of the monotonic double layer and the solitary hole. We have derived the modified Korteweg-de Vries equation in plasmas, taking account of negative ion effects. The effects of negative ion and density on the properties of the non-monotonie double layer are discussed.

A numerical study on the influence of the amount of internal fuel in a bird strike test for the external auxiliary fuel tank of rotorcraft

ABSTRACT The main function of an aircraft fuel tank is fuel storage, but in an urgent situation, such as a crash, the integrity of the fuel tank is directly related to the survivability of the crew. Particularly, when the external fuel tank of the aircraft leaks due to a bird-strike, it is a serious threat to the survivability of the crew. For this reason, when an auxiliary fuel tank is installed outside the aircraft for the purpose of extending the range, robustness against bird-strikes must be demonstrated. The purpose of this study is to reveal the most vulnerable conditions using bird-strike tests. Numerical analysis is carried out using crash simulation commercial software (LS-DYNA) to analyse the influence of a bird-strike on a composite container used as an external auxiliary fuel tank. For the numerical simulation, the structure is modelled as a shell element and the fluid and bird are modelled using a particle method. Using numerical analysis, we calculate the maximum stress and failure index of the composite container in each analysis scenario. Finally, we propose the most vulnerable conditions from bird-strikes after a comparison of the results of each analysis condition.