Computational fluid dynamics modeling the store separation from a jet trainer aircraft

Abstract

The issue of ensuring the safe and predictable separation of munitions from aircraft has become an area of growing interest in recent years. This highlights the necessity of eliminating potential risks associated with the interaction between munitions and the aircraft, while ensuring the safe and effective execution of operational missions. Solutions to this problem include flight tests, wind tunnel experiments, and Computational Fluid Dynamics (CFD) analyses. The Captive Trajectory System (CTS) and grid methods are wind tunnel techniques designed to solve the store separation problem. In this thesis, both CTS and grid methods were modeled using CFD. Furthermore, a new method called the 'Captive Carry with Flow-Field' was developed as an alternative to the conventional grid method. The primary objective of this research is to validate the munition trajectory calculated through the developed CFD methodology with experimental results, and to compute the trajectory of the munition separating from a jet trainer aircraft using the CTS, grid, and Captive Carry with Flow-Field methods. Furthermore, a comprehensive investigation was conducted to assess the impact of mach number and angle of attack on store separation analyses. The results of simulations performed at various mach numbers and angles of attack clearly illustrated the effect of these parameters on the munition's trajectory behavior. The CFD studies related to store separation were conducted using the commercial software Simcenter Star-CCM+. In the first phase of this thesis, a Computational Fluid Dynamics (CFD) methodology was developed for the Captive Trajectory System (CTS) using the Eglin geometry, a widely employed configuration in store separation studies, along with its corresponding wind tunnel test data. The methodology development included selecting an appropriate turbulence model, conducting a mesh independence study, and determining a suitable time step to ensure the accuracy of unsteady simulations for store separation analyses. Unsteady analyses were carried out using the Spalart Allmaras, Realizable K-ε, and K-ω SST turbulence models, with the munition's trajectory computed for each model. The simulation results were then validated against wind tunnel data. After identifying the optimal turbulence model, a detailed mesh independence study was performed to confirm that the numerical results remained unaffected by variations in mesh density. This study tested five different mesh with varying densities, during which aerodynamic forces along the X, Y, and Z axes acting on the munition were calculated. The evaluation of these results led to the selection of the most accurate and efficient mesh for store separation analyses. Subsequently, the determination of an appropriate time step, a key factor in unsteady simulations, was undertaken. Using four different time steps, unsteady simulations were conducted, and the munition's trajectory was compared to wind tunnel results to identify the optimal time step. These unsteady CFD analyses were performed at a mach number of 0.95 and an altitude of 26,000 feet. For the trajectory calculation, the overset mesh algorithm was employed in unsteady analyses. The trajectory was calculated by solving the RANS equations under the assumption of compressible and viscous flow. The developed CFD methodology was validated by comparing the computed trajectory with wind tunnel results. In the second phase of the study, different methods were modeled using CFD to calculate the trajectory of the munition separating from the jet trainer aircraft. Initially, the CTS methodology developed in the first phase was employed to model the separation process. The K-ω SST turbulence model and a time step of 0.001 seconds, identified during the validation phase, were used in the CFD simulations of the munition separation. A thorough mesh independence study was conducted to enhance the reliability and accuracy of the CFD analyses by assessing the influence of mesh density on numerical results. As part of this study, five different mesh configurations were tested, and the aerodynamic forces acting on the munition along the X, Y, and Z axes were evaluated. The results demonstrated that the numerical outcomes remained consistent regardless of changes in mesh density. Following the identification of the optimal mesh density, the trajectory of the munition separating from the jet trainer aircraft was computed for various flight conditions. Another approach used for trajectory calculation was the grid method. Two aerodynamic databases were generated for this method. The first was the freestream aerodynamic database for the munition, which contained force and moment coefficients derived from CFD analyses under various flight conditions. The second was the aerodynamic database for the aircraft-munition configuration, developed by placing the munition in different positions and attitudes under the aircraft and performing CFD analyses. After developing both the aircraft-munition configuration and freestream databases, a 6-DOF model was created in MATLAB Simulink. The aerodynamic data from the CFD analyses were used as inputs in this model to calculate the munition's displacements and Euler angles under different flight conditions. An alternative approach to the grid method, the 'Captive Carry with Flow-Field' method, was developed to calculate the munition's trajectory. This method enables a rapid assessment of separation analyses for newly designed munitions across various aircraft configurations. The Captive Carry with Flow-Field approach models the non-uniform flow field experienced by the munition due to aerodynamic disturbances caused by the aircraft during separation. For this method, a freestream database was created through CFD analyses of the munition under different flight conditions, and a flow-field database for the jet trainer aircraft (without the munition attached) was also generated using CFD. These databases were utilized in a 6-DOF model in MATLAB Simulink to calculate the munition's displacements and Euler angles under different flight conditions. In the third and final phase of this thesis, the trajectory of the munition separating from the jet trainer aircraft was computed under various flight conditions using the CTS, grid, and Captive Carry with Flow-Field methods, and the results were compared. The Captive Carry with Flow-Field method demonstrated a significant advantage in its ability to rapidly compute the munition's trajectory. The developed method has been proven to serve as an efficient and rapid solution for the design and evaluation processes of new munitions. Furthermore, a detailed investigation was conducted into the effects of mach number on store separation analyses. In this context, the trajectory of the munition separating from the jet trainer aircraft was evaluated under various flight conditions using different Computational Fluid Dynamics (CFD) methods, and the results were systematically compared. Additionally, the influence of the angle of attack on store separation analysis was thoroughly examined. To this end, the munition's trajectory was simulated under a range of flight conditions employing various CFD approaches, and the effects of this parameter on the munition's behavior were meticulously analyzed.In the third and final phase of this thesis, the trajectory of the munition separating from the jet trainer aircraft was computed under various flight conditions using the CTS, grid, and Captive Carry with Flow-Field methods, and the results were compared. The Captive Carry with Flow-Field method demonstrated a significant advantage in its ability to rapidly compute the munition's trajectory. Additionally, the effects of mach number and AoA on store separation analyses were examined in detail.