Multi-body Separation Research Articles

Characteristics and optimization of multi-body separation in supersonic ground effects

The electromagnetic launch of aerospace vehicles is an option for reusable earth/orbit space transportation in the future. In contrast to open space, complex ground effects emerge in supersonic near-ground multi-body separations. Here, the separation processes between a supersonic aerospace vehicle (traveling at Mach 1.5) and an electromagnetic sled were studied with a focus on the characteristics of the flow evolution and aerodynamic interference. The results show that the evolution of the supersonic near-ground separation can be divided into three substages: choked flows in the narrow gap, multi-body shock interactions and independent ground effect interference. The aerodynamic loads of the vehicle strongly correspond to the separation stages. The high-pressure region at the leading edge of the electromagnetic sled continuously sweeps the vehicle abdomen and causes significant fluctuations in the aerodynamic lift and moment, which play a dominant role in the separation trajectory. The altitude of the vehicle that flies away from the electromagnetic sled is relatively stable when the preset attack angle is 0° Following this separation, the vehicle has a pitch angle of -0.25° and tends toward a negative angle of attack. A vertical distance of 4.2 m is insufficient for optimal separation. Thus, the separation characteristics were optimized by combining the preset rudder deflection angle of 0° and inclined track with a slope angle of 1.5° Following this separation, the pitch angle of the vehicle is 3.4°, and the vertical distance increases to 14.8 m, which significantly improves the separation safety.

ADRC-based compound control strategy for spacecraft multi-body separation

This paper proposes a compound control method for spacecraft multi-body separation which is based on active disturbance rejection control (ADRC), aiming to address the problems of decreased control accuracy and collisions caused by model uncertainty and vacuum plume disturbance. By comprehensively analyzing the positional relationship and attitude between each separator, the compound flight controller reconstructs the desired attitude of each separator based on artificial potential field method and realizes the tracking of the desired attitudes through ADRC. The closed-loop stability of the system is analyzed, and it is proved that the algorithm is stable in the sense of Lyapunov. The simulation results showed that the multi-body separation compound control strategy based ADRC has more advantages in robustness and security. This method is simple to implement, and it has value of reference and application in spacecraft multi-body separation.

Assessment of turbulence models and mesh types for the simulation of multi-body separation by coupling the CFD and rigid body dynamics

Turbulence models are considered one of the most important sources of numerical errors in the CFD studies on different aeronautical problems. The mesh type can also affect the numerical accuracy and computational efficiency. In this paper, various simulations on the store separation from an aircraft model in the transonic flow have been carried out by coupling the CFD and the rigid body dynamics on the overset mesh. The influence of S-A model, Standard k-epsilon model, SST k-omega model and inviscid model on the simulation results is examined. Different mesh types of both unstructured mesh and structured mesh as component part in overset are also considered. Grid-independence study are first carried out. The effects of different turbulence models and mesh types are then examined by comparing the trajectories, velocities and Eulerian angles. Recommendations are given to get better results for the simulation of multi-body separation problem.

Numerical Investigation on Unsteady Shock Wave/Vortex/Turbulent Boundary Layer Interactions of a Hypersonic Vehicle during Its Shroud Separation

Hypersonic vehicles are drawing more and more attention now and for the near future, especially in the low-altitudes near space, from 20 km to 45 km. The reliable separation of the protecting shroud from the hypersonic vehicle is a prerequisite and critical issue for the success of the entire flight mission. The unsteady multi-body separation characteristics and flow characteristics of hypersonic shroud separation at Mach 7.0 are investigated based on numerical simulation in this paper. The improved delayed detached eddy simulation (IDDES) method, dynamic hybrid overset mesh method, and HLLE++ numerical scheme are used to ensure numerical accuracy. Numerical results show that there are four types of vortexes and three types of shock waves inside the shrouds during the separation process, which generate complex shock wave/vortex/boundary layer interactions. Further, an unsteady process of the expansion-transfer-dissipation of an A-type vortex is found, which is the result of strong shock/vortex/boundary layer interactions. The adverse pressure gradient is the root cause driving the generation and transfer of the A-type vortex during the shroud separation. Furthermore, the transfer process of the A-type vortex only lasts for 5.52 ms but causes a large disturbance to the aerodynamic force of the shroud. The results of this paper could provide a reference for the design of near-space hypersonic vehicles.

A numerical method for multi-body separation with collisions

Multi-body separation with collisions is ubiquitous in aerospace engineering. In this paper, a numerical method for this problem is developed. Coupling with the equations for six-degree-of-freedom rigid-body motion, the unsteady compressible Reynolds-Averaged Navier-Stokes equations are solved. The dynamic overset unstructured grid based on the criteria of wall distance is developed to handle the relative motion of multiple bodies. The collision among rigid bodies is modeled by using an impulse-based method. Efficient algorithms for collision detection and contact determination utilizing hole cutting information of overset unstructured grid are proposed. In the simulation of six-degree-of-freedom rigid-body motion, a collision event can be robustly detected and the collision time can be accurately determined, without needing additional data structures. Test cases of store separation and ball collision are used to validate the accuracy of the dynamic solvers and the collision algorithms. An application to the release of a large number of projectiles from a carrier assesses the capability of the present method in simulating the problems of complex multi-body separation with collisions.

Prediction and evaluation of the stage-separation compatibility of an internally carried air-launch vehicle

This work mainly predicts and evaluates the stage-separation compatibility of an internally carried air-launch vehicle by a multi-body separation wind-tunnel free-flight (WTFF) test without restraining the motional freedom. The Mach number is Ma=1.5, and the initial-launch angle of attack is considered to be α0=0° and α0=3°. The result shows that WTFF test is a feasible prediction and evaluation method for the stage-separation compatibility of an ICAL vehicle when the combined vehicle exhibits static stability. The launch-vehicle can be separated safely from the carrier-aircraft and has a good pitch-angle for ignition when α0=0°. However, the launch-vehicle exhibits a large pitch-angle when it separates completely from the carrier-aircraft when α0=3°, which is unacceptable. Different separation velocities have little influence on the stage-separation compatibility of the ICAL vehicle when α0=3°. The stage-separation compatibility of the ICAL vehicle is first investigated by a free-flight technique in a wind-tunnel, which provides a new research idea for interdisciplinary studies of the kinematics and aerodynamics of multi-body systems.

Numerical simulation of dynamic multi-body separation flowfields

In this paper, three-dimensional unsteady Navier-Stokes equations coupled with six-degree-of-freedom equations are solved numerically to simulate the process of multi-body separation in the condition of high speed and high altitude. Overset grids are adopted to deal with the relative motion between two different parts. In order to study the security of separating process, the trajectories and attitudes of different parts are calculated and the characteristics of flow field during separation are analyzed carefully. Different initial angles of attack are considered during the simulation and its influence on the separation process is analyzed. The result shows that the initial angle of attack has significant impact on the flow field. Large angle of attack would cause shock/shock interactions between the two parts, threatening the safety of the separating process.

Parallel multibody separation simulation using MPI and OpenMP with communication optimization

In this paper we investigate parallel implementations of multibody separation simulation using a hybrid of message passing interface and OpenMP. We propose a mesh block-based overset communication optimization algorithm. After presenting details of local data structures, we present our strategy for parallelizing both the overset mesh assembler and the flow solver by employing message passing interface and OpenMP. Experimental results show that the mesh block-based overset communication optimization algorithm has an advantage in real elapsed time when compared to a process-based implementation. The hybrid version shows that it is suitable for improving the load balance if a large number of CPU cores are used. We report results for a standard multibody separation case.

Multi-body separation simulation with an improved general mesh deformation method

This paper presents an improved general mesh deformation method and its application in multi-body separation simulation. Intelligent Laplacian smoothing algorithm is combined with previously developed radial basis function (RBF) mesh deform technology to avoid mesh quality decreasing and topology damaging in successive large mesh deformation. In the intelligent Laplacian smoothing algorithm, mesh qualities of tetrahedron, triangular prism, hexahedron and pyramid are evaluated by a unified standard which is used to setup the criterion of the smoothing onset. This improved mesh deformation methodology is integrated into a solver of Navier–Stokes equations coupled with six degrees of freedom (6-DOF) moving equations. The store separation trajectory of a generic wing, pylon, and store configuration is successfully simulated to validate the ability of improved mesh deformation methodology in dealing with large magnitude of relative motion among multiple moving boundaries. Numerical simulation results agree well with experimental data, which states that the accuracy and robustness of the above numerical methods can reach the requirement of describing the traces of multi-body separation.

Structure overset grid method and its applications to simulation of multi-body separation

This paper proposes an automatic structure overset grid method, which utilizes the hole-surface optimization with one-step searching, wall-surface grid oversetting, and dynamic overset grid approaches to achieve the high adaptability of overset grids for complex multi-body aircrafts. Specifically, based on the automatic structure overset grids, the method first solves the coupling of Navier-Stokes (N-S) unsteady flow equation and 6DOF motion equation, and establishes the multi-body collision model. Then, the numerical simulation of unsteady flow for complex aircrafts’ multi-body separation, the simulation of multi-body separating trajectory and the separation safety analysis are accomplished. Thus, the method can properly handle practical engineering problems including the wing/drop tank separation, aircraft/mount separation, and cluster bomb projection. Experiments show that our numerical results match well with experimental results, which demonstrates the effectiveness of our methods in solving the multi-body separation problem for aircrafts with complex shapes.

Effect of elastic deformation on the trajectory of aerial separation

Uncertain serious threats to fight vehicle exist during the process of releasing objects or launching weapons. Reynolds-averaged Navier–Stokes equations are solved through CFD technique in this paper. Based on the frame of unstructured mesh, techniques of dynamic chimera mesh and inverse-distance-weighted morphing mesh are adopted to treat the multi-body separation and flexible structure deformation caused by aeroelasticity respectively. Moreover, the six-degree freedom dynamic equations are solved with static aeroelastic equations by means of the modal approach. Effect of elastic deformation on the trajectory of aerial separation is intensively researched. Numerical results of store-separation case and static aeroelastic case calculated using the in-house code both agree well with the experimental data respectively, which validates of the numerical method. An air-to-air missile model of X–X configuration is constructed to research the effect of elastic deformation on the trajectory of the releasing body. Comparison results of flexible and rigid models show that the longitudinal and course trajectory of centroid is affected by the elastic deformation, and the oscillatory cycle of the orientation angle increases. Furthermore, the trajectories of rigid models with various centroid locations are computed, illustrating that the elastic deformation could move the aerodynamic center forward and weaken the margin of the stability. This study demonstrates that more attention should be paid to the effect of elastic deformation during the fining design of the trajectory of aerial separation.

CFD Transonic Trajectory Predictions of Three-Store Ripple Release Using Chimera Method

The complicated unsteady flows with moving boundary were simulated numerically by coupling solving unsteady compressible Navier-Stokes equations and 6DOF rigid-body dynamics equations. The Chimera grid technology was used to handle the relative motion. The three-store ripple release of the wing-store configuration was simulated using this method. The computational results are in good agreement with data from other literature, showing that the method used has a strong applicability to complex multi-body separation problem.

A Meshless Solution Method for Unsteady Flow with Moving Boundary

Using the concept of overlapping mesh method for reference, a new method called as Overlapping Clouds of Points Method (OCPM) is firstly proposed to simulate unsteady flow with moving boundary problems based on meshless method. Firstly, a set of static background discrete points is generated in the whole calculation zone. Secondly, moving discrete points are created around moving body. According to the initial position of moving object in the flow field, the two sets of discrete points can be overlapped. With the motion of moving objects in the calculation field, moving discrete points around the moving body will inherently move. The exchange of flow field information between static points and moving points is realized by the solution of the clouds of points made up of static and moving discrete points using weighted meshless method nearby overlapping boundary. Four cases including piston problem, NACA0012 airfoil vibration flow around a moving sphere in supersonic and multibody separation are given to verify accuracy and practicability of OCPM. The numerical results agree well with exact solution and experimental results, which shows that the proposed OCPM can be applied to the simulation of unsteady flow problem.

Time-Accurate Simulation of the Aircraft External Store Separation

Based on the original unsteady simulation program, the transonic aircraft external store separation was simulated by structured chimera grid approach coupled with a six degree of freedom trajectory code. The chimera grid utilized the hole-map cutting method; the searching efficiency was compared between the stencil walk and inverse map when building interpolation during the procedure. The space format utilized flux difference splitting format FDS based on Roe, moreover adding the min-mod limiter to achieve second order accuracy. The time format utilized the implicit integration and discrete scheme of the Back-Euler method. The three-dimensional trajectory of the store was captured and better fit for the experimental data. The results show that the method is correct and provides a reference for the simulation of the unsteady multi-body separation.

Time-Accurate Simulation of the Aircraft External Store Separation

Based on the original unsteady simulation program, the transonic aircraft external store separation was simulated by structured chimera grid approach coupled with a six degree of freedom trajectory code. The chimera grid utilized the hole-map cutting method; the searching efficiency was compared between the stencil walk and inverse map when building interpolation during the procedure. The space format utilized flux difference splitting format FDS based on Roe, moreover adding the min-mod limiter to achieve second order accuracy. The time format utilized the implicit integration and discrete scheme of the Back-Euler method. The three-dimensional trajectory of the store was captured and better fit for the experimental data. The results show that the method is correct and provides a reference for the simulation of the unsteady multi-body separation.

Applications of dynamic hybrid grid method for three-dimensional moving/deforming boundary problems

In the previous work, the authors had developed a dynamic hybrid grid generation method and an unsteady flow solver for two-dimensional incompressible and compressible unsteady flows with moving or morphing boundary. In this paper, the dynamic hybrid grid generation method and the unsteady flow solver are extended to three-dimensional complex geometries with moving and/or deforming boundaries, and coupled with force and moment calculation, and the integration of the rigid body, six degrees-of-freedom (6DOF) equations of motion. In order to enhance the flexibility and efficiency of moving grid generation, the dynamic hybrid grid method combines the ‘Delaunay graph’ mapping approach, node relaxation based on ‘spring’ analogy and local re-meshing strategy. Firstly, the prism/tetrahedral/Cartesian hybrid grids are adopted to discrete the initial computational domain over complex configurations. Once the bodies move or deform, the grid points in the boundary layer of the moving/morphing bodies are moved firstly with a modified advancing-layer method, the grid points in the outer far-field keep stationary, while the grid points between the last layer of body-fitted grids and the internal boundary of the specified far-field are mapped by the ‘Delaunay graph’ mapping method. But the background grids (the Delaunay graph) themselves are deformed by the simple node relaxation based on ‘spring’ analogy to improve the efficiency. Then the quality of the deformed grids is checked with some criteria. If the deformed grids do not pass the checking step, a local re-meshing procedure is carried out. Based on the dynamic hybrid grids, a parallel implicit finite-volume flow solver for 3D unsteady Navier–Stokes equations is developed also. In order to deal with the problems of multi-body separation, the integration of the rigid body, 6DOF equations of motion is coupled in the same framework of the flow solver. The applications for complex 3D morphing configurations demonstrate the robustness and efficiency of present method.

날개 전개가 가능한 무장의 분리 특성해석

외부장착물의 효율적인 분리해석을 위해 6-DOF 시뮬레이션 프로그램을 개발하였다. 이 S/W는 장착물에 작용하는 공기력을 미리 작성된 공력 데이터베이스를 이용하여 간단한 보간으로 얻기에 전산유체역학에 기초한 방법에 비해 매우 빠른 계산시간을 가지며, 고정된 형상의 무장 뿐 아니라 분리 중 날개를 전개하는 무장에 대해서도 해석이 가능하다. 본 논문에서는 날개전개 무장의 공력 데이터베이스를 MSAP(Multi-body Separation Analysis Program)을 이용하여 작성하고, 비정상 감쇠계수를 Missile Datcom을 이용하여 예측하였다. 이 데이터베이스와 6-DOF 시뮬레이션 프로그램으로 무장의 고정된 날개 뿐 아니라, 날개전개에 따른 무장의 분리 안전성을 판단하였다 6-DOF simulation program is developed in order to increase the efficiency of the store separation analysis. This S/W is much faster than a method based on CFD(Computational Fluid Dynamics) technology, and allows the simulation of stores with fixed shape as well as with extensible wings, because it uses aerodynamic databases which are prepared beforehand. In this paper, aerodynamic databases of stores are obtained with MSAP(Multi-body Separation Analysis Program), and unsteady damping coefficients are modeled with Missile Datcom. These databases and the 6-DOF simulation program are used to predict the trajectory of an external store, while its wings are being deployed. The analysis results indicate that the safe separations of the store can be achieved not only with the wing fixed but with the wings being deployed.

저속통제기 외부장착물 분리해석 및 비행시험

본 논문에서는 저속통제기로부터 비상 시 분리되는 외부연료탱크 및 LAU-131 로켓발사기의 분리안전성을 보이기 위하여 수행한 자유낙하 풍동시험, MSAP을 이용한 해석과 비행 시험의 연구결과를 종합하였다. 자유낙하형의 bomb rack을 사용하는 비행시험 중의 분리안 전성의 확인과 MSAP의 보정용 자료 생성을 위하여 수행한 아음속풍동을 이용한 자유낙하 시험의 결과에 대하여 기술하였다. 보정된 MSAP으로 자유낙하형과 사출형의 bomb rack 을 사용하였을 때의 분리안전성 해석을 수행하였다. MSAP 해석 결과를 이용한 상관관계 해석을 통하여 자유낙하형의 bomb rack을 사용할 경우의 투하 안전성도 보였다. 또한, 실기 비행시험을 통하여 투하시험을 수행하였으며 이를 해석결과와 비교하였다. 마지막으로, 풍동 시험, 해석결과 및 비행시험의 결과를 종합하여 저속통제기의 안전투하영역을 확정하였다. In this paper, we summarize the results of free drop wind tunnel test, separation analysis and flight test in order to verify the safety during the separations of an external fuel tank and the LAU-131 rocket launcher from XKO-l. The wind tunnel test was conducted to show the safety in free drop of the stores and to gather the trajectory data for fine tune of MSAP(Multi-body Separation Analysis Program). The enhanced MSAP was then used to predict the trajectories of the stores with and without the ejector forces. A correlation of MSAP results for free drop case was also made to show the safety of jettison with the free drop type bomb rack. Moreover, the flight test was conducted. and its results were compared to analysis results. Finally, the safe jettison boundary was determined from the flight test.