Store Separation Research Articles

Detached Eddy Simulations of Cavity-Store Interactions at Subsonic Turbulent Flow

Weapons bays have gained much attraction in the last decade, mainly in the context of next-generation aircraft. Although internal store carriage provides numerous advantages, aero-mechanical challenges still exist, particularly for safe store separation. Therefore, it is essential to gain fundamental knowledge of the flow field within weapons bays, which can be achieved by studying the flow within a more simplified geometry of a cavity. In this study, detached eddy simulations are performed using the Elastic-Zonal-Navier–Stokes-Solver (EZNSS) to characterize the unsteady turbulent flow within NASA’s benchmark rectangular cavity with a store model located at various positions. Simulations are performed at a Mach number of 0.4 and a Reynolds number of 7 million to form a transitional cavity flow, which is common in jet-fighter weapons bays. The numerical results are validated with experimental data for the empty cavity and cavity-with-store configurations. The effect of the store’s position on the cavity flow characteristics is analyzed and verified, as well as the aerodynamic loads exerted on the store. Results show a complex interaction between the store model and the cavity flow field, manifested by distortion of the wall pressure fluctuations and mean flow structures and large amplitude fluctuations of the loads exerted on the store. The insights reported herein can serve future development efforts of more accurate numerical frameworks for cavity-with-store configurations towards improving their applicability for weapons bays store separation in certification procedures.

Trajectory reconstruction of a wingtip segment separated from an aircraft using CFD simulation

Reconstructing the trajectory of the disintegrated component in an aircraft using numerical methods is anticipated to be a valuable technology for air crash investigation. The motion of separated objects in the air is significantly influenced by aerodynamic loads, making it necessary to employ computational fluid dynamics to estimate the forces and moments acting on the objects. To address the limitations of current methods, a multi-zone dynamic mesh method was utilized and validated via a store separation case, demonstrating good consistency with wind tunnel data. Since the trajectory of the wingtip is highly dependent on the center of gravity and mass properties, a predictive-level finite element model was developed using 3D scanning and reverse engineering techniques. Trajectories of the wingtip for a Tu-154M under various initial conditions were reconstructed, revealing nonlinear variations over time. By comparing the predicted crash position with the actual one, it is possible to ascertain the aircraft's condition at the moment when the wingtip separated.

Numerical investigation of store separation from cavity problems at high speeds

In this work, the ability of open-source CFD tools to conduct store separation simulations from cavities is evaluated and validated using a generic test case from the literature. Firstly, the ability and accuracy of these tools for solving cavity flows at high speeds are evaluated. Secondly, their competence in predicting the trajectory of a generic store from a generic deep cavity is checked. Finally, and in order to reduce the associated computational costs, a release-time dependability factor from the recent literature is studied and evaluated. The obtained results using the selected open-source CFD tools matched quite well with the wind tunnel results. Furthermore, the results show that predicting the release-time dependability using a quantified index/factor can be a potential remedy for reducing the computational cost for this type of CFD simulations.

Comparison of Data-Driven Approaches to Rotorcraft Store Separation Modeling

In this study, two surrogate modeling approaches will be investigated for their ability to predict store surface pressure distributions as a store is launched from a rotorcraft in hover. To generate these surrogate models, three computational fluid dynamics (CFD) simulations were completed while varying the propulsive force assigned to the store. Once the surrogate models were generated, an additional CFD simulation with a new propulsive force was assigned to the store. The results of this validation indicate that although the proper orthogonal decomposition (POD) surrogate model struggled to provide detailed predictions of store-distributed loads, mean load variations could be modeled well. The results further indicated that by leveraging the convolutional neural network (CNN)-based surrogate model, a significant improvement in distributed load modeling could be obtained. It was further identified that once distributed loads were integrated, both POD- and CNN-based surrogate models provided a viable path for the generation of a trajectory prediction-based surrogate model for rotorcraft applications. Both POD- and CNN-based surrogate models provided significantly reduced computational costs. For each rotorcraft store separation CFD simulation, the computational cost required 10 days of simulation time across 880 central processing unit cores. While using the surrogate model, comparable predictions could be produced in under 3 min on a single core.

Static Ejection Test for Separation Analysis of 2,000lb-Class Store

Static ejection tests were conducted using the 2,000lb-Class Store to provide ejector model for the store separation simulation. In this study, static ejection test device for 2,000lb-class store was constructed and reaction force applied to store was measured over time. In addition, the trajectories of the ejected store were obtained using photogrammetry and compared with the simulations using developed ejector model. The results of the static ejection test were analyzed to determine the cartridge-orifice combination to be used for store separation. Flight tests were performed by applying the analysis results and verified that the store was safely separated from the aircraft.

UAV Design and Configuration Aerodynamics - An ADE Experience

UAVs are becoming an essential part of the modern defense forces world over in the recent years. They are widely used for surveillance, reconnaissance, target for missiles and combat. They are available in various weight classes from few kilograms to tons. Apart from design of aircraft, establishing an accurate aero data base for control law design for the aircraft is a main challenge in itself. Aeronautical Development Establishment (ADE) is involved in the design and development of many UAVs for the last 30 plus years and has experienced several challenging design and engineering problems. Over the years, various UAVs like Ulka, Sparrow, Lakshya, and Nishant for different mission requirements have been designed and developed. The evolution of design methodology in ADE and the present configuration design activities of UAVs and aircraft stores separation studies with a focus on CFD are discussed in this paper.

Grid Method Based Store Separation Suite Using Paranam Mesh Free Solver

A store separation suite based on Point Prediction methodology was developed at CSIR-National Aerospace Laboratories (NAL) using PARANAM (Parallel NAL MCIR) mesh free CFD solver along with 6-DOF code and dynamic point cloud pre-processor [7,8]. This store separation suite is well validated and tested and is being used widely for various strategic store separation projects at CSIR-NAL [7,8]. However, in some scenarios point prediction based method is difficult to employ owing to either large number of trajectory studies or time and resource requirements. In such scenarios a quick but sufficiently accurate method is needed. The Grid method which is an approximate method based on aerodynamic database has been used to develop a store separation suite to achieve the same. In this work we have exploited mesh free nature of PARANAM solver to generate the aerodynamic database required for Grid method efficiently. The suite has been applied to standard Eglin test case for carrying out validation. Further we have demonstrated the utility of such a tool in parametric studies for assessing sensitivity.

Cfd Simulations of Store Separation: An Indian Perspective

CFD analysis played a very major role in predicting the safe separation of many stores including missiles, guided bombs, drop tanks from various airborne platforms namely, fighter aircraft, helicopters etc. Number of indigenous CFD based stores separation suites are developed by coupling flow solvers, 6 Degrees-of-freedom (6DOF) trajectory programs and efficient pre and post processors. These packages were validated against reliable experimental results and were applied to many practical applications. These analyses gave very good confidence to the designers, test pilots and certifying agencies to clear the flight trials without much wind tunnel testing. This paper highlights the author’s experience of use of CFD techniques to predict external store separation from fighter aircraft pertaining to Indian system.

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.

Investigation of Aeroelasticity Effect on Missile Separation from the Internal Bay

There is a strong aerodynamic interference when launching the missile in the embedded mode. During the separation process, the carrier aircraft safety may be threatened due to large slenderness ratio, low structural stiffness, and aeroelasticity effects of the missile. The present study simulates missile separation in the presence of the aeroelasticity effects based on the computational fluid dynamics (CFD), rigid body dynamics (RBD), and computational structure dynamics (CSD) coupling method. A hybrid dynamic grid method consisting of the mixed overset unstructured grid and deformation grid is utilized. In order to verify the accuracy of the coupled numerical method, store separation from a wing and AGARD 445.6 wing flutter are first simulated as two standard test cases. The verification results imply that the present coupled numerical method is reliable and capable in simulation of the aeroelastic effect in missile separation. The influence of aeroelasticity on the separation trajectory of a missile from the internal bay is systematically studied at different states. Numerical results show that aeroelasticity substantially affects the missile angular displacement, while it has a slight impact on the linear displacement of the center of mass. Mach number and flight altitude are two important flight parameters that characterize the aeroelasticity effect on missile separation from the internal bay.

Machine learning-based surrogate modeling approaches for fixed-wing store separation

In pursuit of deriving a limited expense store trajectory prediction model, this work investigates the application of two data-driven surrogate modeling approaches for the prediction of surface pressure and shear stress distributions of a store separating under supersonic conditions. Through the use of computational fluid dynamics (CFD), store separation simulations at three supersonic Mach numbers were carried out, M = 1.2, 1.4, and 1.6, for fixed-wing store separation. Pressure and shear stress distributions were recorded and used to construct a proper orthogonal decomposition (POD) and convolutional neural network (CNN) based surrogate model such that predictions of store load distributions could be obtained under new operating conditions. Load predictions are then integrated and coupled with the equations of motion to predict store trajectories at intermediate Mach numbers of M = 1.3 and M = 1.5. Results demonstrated that both the CNN and POD-based surrogate models were proven capable of providing both high fidelity distributed load and trajectory predictions at a greatly reduced computational expense. Computational run times for a single trajectory prediction simulation while using CFD simulation was 6 hours on 220 CPUs. When using the POD-based surrogate model, run time was reduced to one minute on a single CPU while the CNN-based surrogate model required 15 minutes on a single GPU.

Numerical Simulation about the Characteristics of the Store Released from the Internal Bay in Supersonic Flow

To understand the influence of the initial release conditions on the separation characteristics of the store and improve it under high Mach number (Ma = 4) flight conditions, the overset grid method and the Realizable turbulence model coupled with an equation with six degrees of freedom are used to simulate the store released from the internal bay. The motion trajectory and the attitude angle of the store separation under the conditions of different centroid, velocity, height and control measures are given by the calculated result. Through analysis, the position of the centroid will affect the separation of the store, which needs to be considered in the design. Increasing the launching height is conducive to the separation of the store. If the store has an initial velocity, it can leave the internal bay more quickly and reduce the probability of collision with the wall. Cylindrical rod and slanted aft wall control measures can improve the attitude of the store and make the store fall more smoothly.

High Speed Marker Tracking for Flight Tests

Flight testing is a mandatory process to ensure safety during normal operations and to evaluate an aircraft during its certification phase. As a test flight may be a high-risk activity that may result in loss of the aircraft or even loss of life, simulation models and real-time monitoring systems are crucial to access the risk and to increase situational awareness and safety. We propose a new detecting and tracking model based on CNN, that uses fiducial markers, called HSMT4FT. It is one of the main components of the Optical Trajectory System (SisTrO) which is responsible for detecting and tracking fiducial markers in external stores, in pylons, and in the wings of an aircraft during Flight Tests. HSMT4FT is a real-time processing model that is used to measure the trajectory in a store separation test and even to assess vibrations and wing deflections. Despite the fact that there are several libraries providing rule-based approaches for detecting predefined markers, this work contributes by developing and evaluating three convolutional neural network (CNN) models for detecting and localizing fiducial markers. We also compared classical methods for corner detection implemented in the OpenCV library and the neural network model executed in the OpenVINO environment. Both the execution time and the precision/accuracy of those methodologies were evaluated. One of the CNN models achieved the highest throughput, smaller RMSE, and highest F1 score among tested and benchmark models. The best model is fast enough to enable real-time applications in embedded systems and will be used for real detecting and tracking in real Flight Tests in the future.

Improved Delayed Detached-Eddy Investigations on the Flow Control of the Leading-Edge Flat Spoiler of the Cavity in the Low-Aspect-Ratio Aircraft

The internal weapon bay is widely used in modern aircraft; however, because the unsteady flows of the cavity would cause dangerous store separation and intense aerodynamic noise, the leading-edge spoiler is an easy and efficient passive flow-control method. The flow control of the leading-edge flat spoiler before the cavity of a low-aspect-ratio flying-wing aircraft is investigated based on numerical simulation. Numerical results show that the leading-edge flat spoiler completely changes the cavity flow; it obviously lifts up the shear layer and reduces the pressure inside the cavity. For the store separation from the weapon bay, the leading-edge flat spoiler is a very good passive flow-control method that curbs the nose-up trend of the store and produces a safe and stable store separation. Besides, the leading-edge spoiler reduces the noise in the rear of the cavity (max 8.2 dB), but increases the noise in the middle of the cavity (max 11.3 dB). In addition, the leading-edge spoiler brings in a large drag increase to the aircraft (39.41% when the height of spoiler is 0.2 m), which would affect the operational stability of the aircraft. The results of this paper could provide a reference for the flow control of weapon bays and the design of aircraft.

Enhancement on parallel unstructured overset grid method for complex aerospace engineering applications

In the present study, an efficient overset grid method by means of parallel implicit hole-cutting is proposed for the sake of simulating unsteady flows in aerospace engineering involving multiple bodies in relative movement. In view of the degraded computational efficiency and robustness for conventional overset grid assembly, several innovative techniques are developed within the overset grid assembly process, viz., a bookkeeping alternative digital tree method to speed up the donor-cell searching, a fast parallel advancing front algorithm to accelerate the wall-distance calculation and a message-passing strategy with efficient information communication and lower storage expenditure within distributed computational architecture. The contribution of the developed techniques is evidenced by comparison with the existing alternative ways in terms of computing efficiency. Subsequently, the overset grid method is embedded into an in-house programed URANS solver to examine its capability in predicting the flow field of complex applications such as helicopter, store separation and component deploying. Results show that the developed overset grid methodology is, in practice, able to resolve the aerodynamic characteristics of complex aerospace engineering with a high-fidelity flow topology and accuracy.

Aerodynamic characteristics of store during lateral jet assisted separation from cavity using free drop technique

Aimed at the problem of store separation from internal cavity, this paper innovatively puts forward a separation scheme of using lateral jet to assist store safe separation. The jet ensures that the store is continuously subjected to down head moment during separation, so as to ensure safe separation. The wind tunnel free drop test technique with lateral jet is established, which can ensure that there is no support interference in the motion process of the store and more truly simulate the motion of the store. The feasibility of the new separation scheme is proved by wind tunnel test. The test shows that the new scheme can also change the more dangerous state into a safe state. Through the analysis of the test data, the pitching moment coefficients of the store under different pressures in the high-pressure cylinder are obtained, and the effects of aircraft and cavity on the pitching moment of the store are obtained. The 3 stages of store passing through cavity shock wave are found. The results show that the lateral jet provides the store with continuous head down moment, and makes the store overcome the head up moment caused by the shock wave in front of the cavity, so as to ensure the safety of separation.

Recording Store Separation Safe and Sound

Recording Store Separation Safe and Sound

A spacetime formulation for unsteady aerodynamics with geometry and topology changes

AbstractA spacetime formulation is presented to solve unsteady aerodynamic problems involving large deformation or topological change such as store separation, slat and flap deployment or spoiler deflection. This technique avoids complex CFD meshing methods, such as Chimera, by the use of a finite-volume approach both in space and time, and permits a locally varying real timestep. The use of a central-difference scheme in the time direction can yield non-physical transient solutions as a consequence of information travelling backwards in time. Therefore, an upwind formulation is provided and validated against one-dimensional and two-dimensional test cases. A hybrid formulation (central in space, upwind in time) is also given and unsteady cases are computed for a spoiler and spoiler/flap deployment, with all three formulations compared, demonstrating that the use of an upwind time stencil yields more representative physical solutions and improves the rate of convergence.

Unsteady CFD Studies for Gust Modeling in Store Separation


 
 
 Aircraft and different store configurations must be certified before a flight. There is a small but finite probability of aircraft being hit by gust wind at the time of separation. Most store separation analyses from airborne platforms do not consider the gust phenomena because of the complexity and inadequate knowledge of its behavior. A dedicated task group was recently created to understand the gust-related phenomena in aircraft safety. Of the various gust cases, vertical gust is most severe and can cause instability leading to store collision. The situation is compounded in a long and heavy store due to its large projected area. No test procedures exist for simulation or practical tests of gust. A study was conducted to identify a test procedure for gust simulation using MIL standard data and Indian conditions. The current paper studies the emergency release condition where a vertical gust is hitting the aircraft to ascertain safe separation. A discrete gust with a 1-cosine shape and specified length and amplitude is imposed at the inflow boundary. The gust is allowed to sweep the computational domain containing the airborne platform and the store. The computed trajectory of the store, the miss distance, and its angular rates in the presence of gust are analysed in this work to study the safe separation of a store from an airborne platform. Simulations are also carried out to determine the effect of gust at the highest dynamic pressure in the flight envelope.
 
 

A new strategy for solving store separation problems using OpenFOAM

The ability of OpenFOAM to solve the problem of a store separating from an air vehicle (store separation problem) has been evaluated using a dynamic mesh (Overset/Chimera) technique for an industry-class (transonic and generic) benchmark test case. The major limitations of the standard libraries have been determined. To tackle these challenges, a new strategy has been proposed and implemented using only open-source libraries and tools. The strategy combines porting, modifying, and adapting an overset library from the OpenFOAM fork platform (foam-extend) to the standard OpenFOAM platform (ESI). Furthermore, in order to overcome the well-known weakness of the standard OpenFOAM compressible solvers, the newly adapted overset library was integrated with an open-source, density-based, and coupled solver ( HiSA), which uses the OpenFOAM technology. Additionally, a force restrained model was developed to consider the externally applied forces on the store by the store ejectors. The accuracy of the developed strategy has been compared with wind tunnel tests and the solutions of two well-known commercial codes, showing good agreements with them. While the study has focused on simulations with inviscid Euler equations (typical of the test case considered here), the viscosity effect on the solution has also been studied with Navier–Stokes equations and compared with other results in the literature, showing minor differences. To the best of the authors’ knowledge, this is the first work which studies and validates the store separation problem in transonic regime with OpenFOAM.