Home > Numerical Simulations of Store SeparationTrajectories Using the EGLIN Test

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Scientific Technical Review, 2013,Vol.63,No.1,pp.10-16 UDK: 620.178:623.463.5 COSATI: 20-04, 16-02, 16-04

Yunus Emre Sunay1) Emrah G��lay1) Ali Akg��l1)

1) ROKETSAN Missile Industries Inc., P.K.30: 06780 Elmadağ-Ankara,TŰRIKYE

AFE separation of operational or newly designed missiles from air vehicles is a critical issue in terms of missile integration process. Detailed engineering analyses, wind tunnel and flight tests are needed to be performed. In the view of recent developments in computational methods, the numerical analysis can replace flight and wind tunnel tests in some cases for certification processes. Even validated computational methods can be used to complete a safe separation process. This will result in a cost and time effective integration study. In this paper, the EGLIN test case is used to validate an engineering approach to simulate store separation at transonic and supersonic speeds [1,4]. The analysis results are compared with the experimental values.

In this part, CFD modeling and the simulation of an EGLIN test case model will be explained. GAMBIT (v2.4.6) and TGRID (v5.0.6) are used to generate computational grids for a CFD analysis. Also, the FLUENT commercial program (v12.0.16) is used as a solver. The sign convention, the test case model, the computational grids and the results of the analysis will be explained in detail in the following sections [8,9].

A generic EGLIN test model is used in the validation of the CFD analysis. The test was conducted at the Arnold Engineering Development Center in the Aerodynamic Wind Tunnel (4T) in 1990. The EGLIN test model has three parts; wing, pylon and finned body. The sketch of the EGLIN test case model and the sting used in the wind tunnel test are represented in Fig.1 [5]. The wing consisted of a clipped delta wing with 45�� sweep and a constant NACA 64A010 airfoil section. The pylon has an ogive-flat plate-ogive cross section shape. The store body was an ogive-cylinder-ogive with an aft cylindrical sting. The fins on the store consisted of a clipped delta wing with a 45�� sweep and a constant NACA 0008 airfoil section. The gap between the pylon and the finned body is 0.1778 cm. The geometric specifications of the body are represented in Fig.2.

S

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The experiments were conducted in transonic (M=0.95) and supersonic (M=1.2) flow regimes. The position and the orientation of the store are obtained during the test for both flow regimes. The surface pressure distribution on the model is only available for the transonic flow regime. The sign convention used for the calculation of the computational fluid dynamics simulations and experiments is given in Fig.3.

In the test, aft and forward ejector forces are applied to the store to provide safe separation. The store inertial/mass parameters and ejector parameters are given in Table 1 [3].

The EGLIN geometry was generated for CFD studies based on a test model used in the Arnold Engineering Development Center in the Aerodynamic Wind Tunnel. The geometry consists of three different parts; wing, pylon and store. The generated solid models are shown in Fig.4.

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SUNAY,Y.E. etc: NUMERICAL SIMULATIONS OF STORE SEPARATIONTRAJECTORIES USING THE EGLIN TEST

The initially generated grid for Euler computations has 2,112,822 cells. Deformed computational grids at different time steps are given in Fig.5.

The initial generated grid for the Navier-Stokes analysis has 3,412,792 cells and is shown in Fig.6.

The deformed computational grid for the Navier-Stokes analysis at the 0.2 seconds of the analysis is given in Fig.7. The boundary layer part of the computational grid for the Navier-Stokes analysis is not deformed during the solution.

The computational domain inlet was located at 17 wing length, outlet was located at 25 wing length, upper boundary was located at 17 wing length, lower boundary was located at 25 wing length and side boundary was located at 17 model length away from the center of the store. The solution domain is shown in Fig.8.

The FLUENT commercial flow solver was used to predict the trajectory of the EGLIN test model by using Euler and Navier-Strokes Equations.

�� �� v

(1) where

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,

(2) The calculations took about 3 seconds of the CPU time per iteration and convergence was achieved in about 1,800 iterations for the steady part of the solution [10].

�� �� v

(3) where

0 , ,

(4) The boundary conditions are represented in Fig.8. Downstream, upstream, and all-side boundaries, except the right-side one, were set as far field (characteristics-based inflow/outflow), with a standard atmosphere model for 26,000 ft altitude temperature and pressure free stream conditions. The right-side boundary was defined as symmetric. Solid surfaces were modeled as no-slip, adiabatic wall boundary conditions for the Navier-Stokes analysis [3, 10]. The calculations took about 9 s of the CPU time per iteration and convergence was achieved in about 1,800 iterations for the steady part of the solution. The time step is 0.001 sec and 20 iterations were done for each time step. In this simulation, Fluent uses an implicit, node-based finite volume scheme. Roe��s flux difference splitting scheme is used to compute inviscid fluxes at the boundary of each control volume for the Navier-Stokes analysis and viscid fluxes at the boundary of each control volume for the Euler analysis. A second-order accurate, upwind extrapolation is used to determine the values of the flow variables at the boundary. The

Convergence was determined by tracking the change in the flow residuals and the aerodynamic coefficients during the solution. The solution was deemed converged when the aerodynamic coefficients with respect to the time step had the same frequency and wave length in the solution time. Fig.10 shows residual changes with respect to iteration for the unsteady solution for the Navier-Stokes analysis.

As the store starts to move due to gravity and aerodynamic forces acting on the body, the position of the store has to be changed on the mesh. If the displacements of nodes are large compared to the local cell sizes, cells can become degenerated. This will invalidate the mesh (e.g., result in negative cell volumes) and, consequently, lead to convergence problems. By checking the mesh quality, degenerated cells have to be smoothed or new cells have to be generated. Up to some quality criteria, skew cells are smoothed by the spring analogy method. If the quality of cells exceeds the predefined limit, the mesh has to be updated. Bad quality cells are replaced by created new cells using the re-meshing method. These two methods are explained briefly in the following sections [6,7].

( )

(5) where

neighbor node

1

(6)

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SUNAY,Y.E. etc: NUMERICAL SIMULATIONS OF STORE SEPARATIONTRAJECTORIES USING THE EGLIN TEST

At equilibrium, the net force on a node due to all the springs connected to the node must be zero. This condition results in an iterative equation such that

1

+

�� �� �� = �� G G

(7) Since displacements are known at the boundaries (after the boundary node positions have been updated), Eq.7 is solved using a Jacobi sweep on all interior nodes. At convergence, the positions are updated such that

, 1

+ =

+ �� G G G

(8) where

The experimental and numerical results are compared and presented in this part of the report.

Time-dependent CFD analyses were performed for Mach number 0.95 during 0.45 seconds. The experimental store position and orientation data are compared with the CFD analyses results in Fig.11 and 12, respectively.

The pressure distribution along the store body at the cross section by ��=5º with vertical axes for different time steps is shown in Figs. 13-15.

The experimental and numerical store position and orientation values are compared in Fig.16.

The time-dependent CFD analyses for the supersonic store separation test case at Mach number 1.2 were performed for 0.8 seconds. The experimental store position

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The experimental and numerical store position and orientation values are compared in Fig.19.

In this study, an engineering approach for store separation is studied by using a well-known generic store separation EGLIN test case. The store trajectory and orientation are predicted by using coupled 6DOF and Euler/Navier-Stokes equations. Grid deformation techniques and re-meshing algorithms are used to obtain a grid for the next time step. The Euler and Navier-Stokes results are compared with the experimental data and a good agreement is obtained. It can be concluded that the viscous effects have negligible influence on the results of this type of problems. Hence, the presented engineering approach can be used in the trajectory prediction of real store separation problems.

The authors thank ROKETSAN from Ankara for their support in the study of the problem of missile separation from the aircraft wing.

[1] PARIKH,P., PIRZADEH,S.:

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SUNAY,Y.E. etc: NUMERICAL SIMULATIONS OF STORE SEPARATIONTRAJECTORIES USING THE EGLIN TEST

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