A Ballistic Separation Effect(BSE) in aircraft flow field is predicted with a direct calculation of incremental coefficients. The two step simulation is proposed for generation of bomb trajectory and target point estimation. In the first step, the released bomb trajectory in aircraft flow-field is computed with a time accurate unsteady Computational Fluid Dynamics(CFD) method. The incremental coefficients are calculated and used as an indicator to switch from an unsteady CFD computation in aircraft flow field to an off-line simulation in the free flight condition for the second step. Final velocities and trajectory of released store presented in this paper are used for initial conditions of the off-line simulation with a simple Trajectory Generation Program(TGP). The full trajectory to target points can be accurately predicted with an enhanced initial positioning from the present method.
A three Dimensional numerical flow solver is developed to predict ejecting dynamics. Moreover, the unsteady flow with moving of 6 Degrees Of Freedom(DOF) is simulated by the multi-stage time integration with time accuracy. The CFD analysis is used to compute aerodynamic forces and moments acting on the separated body by solving the inviscid Euler equation. The parallel grid assembly to establish communication among sub-grid systems is developed with static load balancing based on the decomposed grid system of flow solver. A coarse-grainded communication is optimized with a minimized memory allocation and communication load since the parallel grid assembly accesses the decomposed grid information within other processors only by message passing in the distributed memory system, such as the PC cluster. The parallel performance assessment of moving body overset grid application is achieved on Linux PC cluster with 100Mbps-Ethernet network switch.
A highly robust flow solver, including parallel Chimera grid assembly, is used for making trajectory of released store and calculating incremental coefficients. The incremental aerodynamic coefficients in ballistic separation effect regime are computed directly by considering time accurate dynamics, contrary to the classical procedure. A sub-program is simultaneously executed with another grid system in the free-stream condition, while the unsteady computation is achieved to generate trajectory of the released store. Time accurate grid velocities are calculated with 6 DOF motion equations and applied to unsteady calculation. These are also applied to quasi-steady sub-calculation in the free stream condition, while only rotation is updated for Euler angles represent the attitude of store, because altitude effects are neglected in this simulation. The aerodynamic forces and moments of sub-calculation are subtracted from those of unsteady calculation. The differences are incremental coefficients at this situation and disturbed aerodynamic forces by aircraft flow field. Two computations can be executed simultaneously with parallel computing environment with the Multiple Program Multiple Data(MPMD) paradigm. The elimination of the flow field effect is observed by measuring the magnitude of incremental coefficients.The store is considered out of the aircraft flow field if the incremental coefficients converge to zero. A simple TGP algorithm with free-stream database can be executed to trace target points using dynamics in that time as an initial condition. The whole TGP algorithm has two steps: the BSE region and the free flight region. The former step is described and applied in this paper to count BSE in aircraft flow fields.
The direct calculation of incremental coefficients method is tested in the case of released stores in 2-dimensional subsonic and submunition from the dispenser missile during descent with supersonic terminal velocity. The normal shock wave on the submunition interacted with the shock wave from the mother missile and moved suddenly to the aft of body and the angle of oblique shock become large, causing the incremental coefficients to fade out suddenly. The unsteady flow simulation can capture transonic nonlinearity of aerodynamic forces and moments, contrary to the wind-tunnel tests or quasi-steady CFD methods in supersonic test case. The accuracy of the unsteady trajectory calculation is verified by comparing with captive trajectory system data in the Eglin Wing/Pylon/Store separation problem. The computed coefficients and trajectories show good agreement with the experimental data. It is then applied to a generic bomb released from a full body aircraft in some separation scenarios. The computational results show that the current method is capable of simulating store trajectory and investigating aircraft flow field effects.
기존의 항공기에 대해 새로운 장착물의 안전 분리 또는 무기의 분리로부터 목표지점 명중까지의 예측에서, 특히 조종면이 없는 비유도탄(unguided bomb)의 정확한 예측 모델의 계산을 위해 비행시험을 대체할 수 있는 기법을 제안하였다. 2단계 궤적 산출 알고리즘에서 최초 항공기 유동장에서 궤적오차(mis-distance)의 가장 큰 비중을 차지하는 항공기 유동장의 간섭인 Ballistic Separation Effect를 전산유체역학을 이용하여 산출해내고, 이후 항공기 유동장을 벗어난 영역에서는 분리체의 데이터베이스를 이용하는 단순 궤적산출프로그램을 이용한다. 또한 증가계수의 직접 계산법을 통하여 두 단계의 궤적산출기법의 전환자를 측정하고, 항공기 유동장효과의 영역을 검토할 수 있다.
정확성과 안정성을 함께 고려하여 개발된 유동해석자와 함께, PC 클러스터와 같은 병렬환경에 적합하도록 개발된 중첩격자모듈을 이용하여 빠른 시간내에 정확한 궤적을 산출할 수 있었으며, 증가계수 계산을 위한 병행처리 시스템을 이용하여 추가적인 계산시간이 필요치 않는다. 개발된 기법은 아음속/초음속 상태의 2차원 또는 3차원 분리유동장에 적용되어 검토되었으며, 계산된 궤적 결과는 참고문헌의 실험결과와 비교되었다. 또한 실제 항공기로부터 분리되는 일반폭탄에 대한 계산이 몇 가지 분리시나리오에 대해 적용되어 분석되었다.
이와 같은 궤적 모델의 생성을 위해 비행시험을 대체 또는 계획할 수 있는 보조 기법으로 사용될 수 있다.
