Store Separation from a Generic Wing- Wing-Sting-Pylon Configuration

The EDGE-system for trajectory computations is validated against a test case carried out at Arnold Engineering Development Center (AEDC) 4T Transonic Aerodynamic Wind Tunnel, described in Ref. [13]. This case concerns the release of a finned-store from a pylon attached to a clipped delta wing. The wind tunnel experiments were carried out using the AEDC Captive Trajectory Support (CTS).

Apart from the release trajectories and the store attitudes, surface pressure data on the wing, on the pylon and on the store at several positions during the release are available for validation.

The computed test case is transonic at Mach number 0.95, Reynolds number of 3 million based on the root chord of the wing and angle-of-attack of zero degrees.

The full-scale store model length of the store is 3.018 m. The store is mounted on the right side of the wing. Since the flow case is asymmetric a full model is used for the computations.

In Fig. 3, the ERU device is depicted. The physical appearance is not modeled in the computational domain i.e., its effect on the flow in the cavity is neglected. The ERU consists of two pistons with the ejector stroke length 0.10 m which are released simultaneously. The forward ejection location is 1.24 m and the aft ejector location is positioned 1.75 m aft of the store nose. The forward ejector force is 10675.7 N and the aft ejector force is 42702.9 N, both constant in time. This implies an initial pitch up maneuver.

Fig. 3 ERU device

Approximately 55 milliseconds after it has left the original position, the pistons and the store cease to have contact with each other. The computations are started at the subsequent temporal measurement station, 60 milliseconds after the store has left the original position, see Fig. 4b. The effect of the ERU is taken into account through the initial values of the flight mechanics state variables.

Quasi-steady computations mimic the experimental CTS-technique; both rely on the assumption of quasi-steady flow, omitting unsteady effects. Both also adjust for dynamic effects using the same aerodynamic damping coefficients. For the quasi-steady computations, the following roll, yaw and pitch damping coefficients are used:

Clp = -4.0/rad, Cmq = -40.0/rad, Cnr= -40.0/rad.

a. Mounted position

b. Initial position for computations Fig.4 The store in different positions.

The computational grids are generated by a merge of the grid around the wing-sting-pylon configuration and the store grid using the local remeshing routine.

Fig. 5 Cross sectional cut through the Navier-Stokes grid containing 5.8 million nodes.

The Navier-Stokes grid contains 36 layers of prismatic elements, with roughly 6.8 ^.10⁶ nodes in total, depicted in Fig. 5.

Numerical Simulation of Weapons Bay Store Separation

CEAS 2013 The International Conference of the European Aerospace Societies Figure 6 shows cross comparison of trajectory

and attitude angles for quasi-steady and time accurate Navier-Stokes computations versus experiments for the wing-pylon-sting store separation.

a. Trajectory coordinates

b. Euler angles Fig. 6 Computational results for quasi-steady computations versus time accurate Navier-Stokes computations and experiments.

Comparisons between quasi-steady and time accurate computations show small differences for the trajectories but significant differences for the attitude angles. The computed trajectories for the wing-pylon-sting store separation agree reasonably well among the participants and they

also agree well with experiments, especially for the first 0.4 sec. For the latter part of the simulation, the values of the attitude angles differ, especially for the pitch and roll angle.

a. Trajectory coordinates

b. Euler angles

Fig. 7 Computational results for time accurate Euler and Navier-Stokes computations versus experiments.

Figure 7 show comparison between time accurate Euler and Navier-Stokes computations.

The computed trajectory coordinates show small deviations from experiments. The attitude angles deviate to some extent from the experimental ones after t=0.4 sec, especially the roll angle. The Navier-Stokes results show consistently better agreement than the Euler results. The agreement of the computational results with experiments is comparable or better

than results in previous publications for this test however been beyond the scope of this paper.

Weapons bay flow is unsteady by nature, due to the mixing layer that impacts on the aft wall of the bay cavity and entailing extensive pressure oscillations inside the weapons bay.

The aerodynamic forces and moments acting on the store can be unsteady depending on the strength of the mixing layer versus the inertial and gravity forces. This implies that the store movement will depend on at which moment the store is released. If the flow field is truly unsteady, then the standard tool for flight clearance, the wind tunnel CTS technique will be of limited use. The lack of repeatability is an obstacle for certification for flight clearance. It also complicates comparisons between various computational results.

Acknowledgements

The development of the store separation system has been carried out with the support of the Swedish Defense Materiel Administration, FMV. The authors also wish to acknowledge colleagues at FOI, Peter Eliasson for parallelizing routines for trajectory computations and for reviewing the implementation plan, and Henrik Edefur for generating the initial grid files.

References with Comparisons to Quasi-Steady Engineering Methods”,AIAA-2008-0186.

[3] Baum J. D., Lou H. and Löhner R.,”A New ALE Adaptive Unstructured Methology for

the Simulation of Moving Bodies”, AIAA-94-0414(1994).

[4] Baum J. D., Lou H., Löhner R., Goldberg E.

and Feldhun A., ”Application of Unstructured Adaptive Moving Body Methology to the Simulation of Fuel Tank Separation From an F-16C/D Fighter”, AIAA-97-0166 (1997).

[5] Murman S.M., Aftomis, M.J., and Berger, M.J.., ”Simulations of Store Separation from a F/A-18 with a Chartesian Method”, Journal of Aircraft, Vol.41(2004), pp.870.

[6] Eliasson P., “Edge, a Navier-Stokes Solver for Unstructured Grids”, Proceedings of Finite Volumes applications III, ISBN 1-9039-9634-1, pp. 527-534, 2002.

[7] Peng, S.-H., “Hybrid RANS-LES modelling based on zero- and one-equation models for turbulent flow simulation“, Proceedings of 4th Int. Symp. Turb. And Shear Flow Phenomena, Vol. 3, pp. 1159-1164, 2005.

[8] Tang L., Yang J. and Lee, J., ”Hybrid Cartesian Grid/Gridless Algorithm for Store Separation Prediction”, AIAA-2010-508.

[9] Lars Tysell, “An Advancing Front Grid Generation System for 3D Unstructured Grids”, ICAS-94-2.5.1, pp 1552-1564.

Proceedings of the 19th ICAS Congress, Anaheim, California, USA, 1994.

[10] Tysell L., “Implementation of Local Remeshing Routines in Edge”, FOI-R-2550-SE.

[11] Berglind, T. and Tysell, L., “Numerical Investigation of the Impact of Maneuver on Store Separation Trajectories”, AIAA-2010-4241, 2010.

[12] Berglind, T. and Tysell, L., “Time-Accurate CFD Approach to Numerical Simulation of Store Separation Trajectory Prediction”, AIAA 2011-3958, 2011.

[13] Fox J.H., ”Generic wing pylon, and moving finned store”, Arnold Engineering Development Center (AEDC ), Arnold AFB, TN 37389-6001, USA.

Numerical Simulation of Weapons Bay Store Separation

CEAS 2013 The International Conference of the European Aerospace Societies

Figure 8. Views of the computational grid

Figure 9. Pressure distribution on the Wing-Sting-Pylon configuration after 0.00, 0.15 , 0.30, 0.45, 0.60 and 0.75 sec

t = 0.0 sec t = 0.15 sec

t = 0.30 sec t = 0.45 sec

t = 0.60 sec t = 0.75 sec

CEAS 2013 The International Conference of the European Aerospace Societies Abstract

In the present study, numerical investigation about the mutual aerodynamic interaction of the rotors of a multi-rotor UAV (Unmanned Aerial Vehicle) configuration was conducted. For this purpose, time-accurate unsteady flow calculations were performed using a three-dimensional unstructured mesh CFD flow solver.

The fluid motion was assumed to be governed by the three-dimensional, incompressible, inviscid, Euler equations. To handle the relative motion of the rotors, an overset mesh technique was adopted. To reduce the large computational time, the flow solver was parallelized based on a domain decomposition technique. As an application of the present method, simulations were made for a quad-rotor UAV in hover and in forward flight. It was observed that in the case of hovering flight, the mutual aerodynamic interaction of the rotors induces slightly higher inflow than an isolated rotor, and invokes unsteady fluctuating thrust variation. In forward flight, the tip vortices from the upstream rotors affect those at further downstream by reducing the effective angle of attack at the rotor blades and form a complex interactional wake structure. It was found that the mutual aerodynamic interaction leads to a deterioration of the attitude stability of the UAV in forward flight, and this aerodynamic

interaction should be considered seriously in designing accurate attitude control algorithms for multi-rotor UAV configurations.

1 Introduction

For the past few decades, active researches have been conducted for Unmanned Aerial Vehicles (UAVs) as demanded by the practical usefulness for both civilian and military application purposes. Among the several UAV configurations, rotary-wing type vehicles have received quite an attention due to the unique capability of vertical take-off and landing (VTOL) [1]. The research and development have been performed particularly for multi-rotor UAVs, because of the simplicity of the flight mechanism which does not require any anti-torque system and swash plate for flight control [2-5].

Recently, quad-rotor UAVs became one of the standard platforms in the development and also in the practical field application of multi-rotor UAVs. With the four multi-rotors, the two pairs are designed to rotate clockwise and counter-clockwise, respectively, to negate the production of torque on the vehicle. Because the rotors are set at a fixed pitch, the attitude of the vehicle is controlled by the difference of the individual rotor thrust attained by the rotational speed control [1].

Numerical Investigation of Aerodynamic Interaction for a