Effects of Fuel Slosh on Flutter Prediction Edmond Kwan-yu Chiu * and Charbel Farhat † Stanford University, Stanford, CA 94305, USA The analysis of the sloshing effect of an internal fluid on a flutter envelope has received little attention, at least in the open literature. This problem can affect many aircraft, especially high performance fighter jets with wing-store configurations. This paper dis- cusses the modeling aspect of this problem and presents some computational results that quantify the effects of fuel slosh on the flutter characteristics of a test wing-store configura- tion. The geometry of the studied configuration is that of the AGARD Wing 445.6 with a blunt-nosed under-wing store. Fuel slosh is accounted for using a hydroelastic added mass model. Alternatively, the trivial representation of an internal fluid by mass lumping is also considered. Flutter calculations are performed using both approaches for accounting for the presence of an internal fluid and a validated CFD-based linearized method for solving fluid-structure interaction problems. It is found that for the considered wing-store system, ignoring the sloshing effect of an internal fluid by representing it as a lumped mass system overestimates the added mass effect, underestimates the critical pressure and flutter speed, and therefore leads to conservative conclusions. I. Introduction The cost of a flight test campaign increases rapidly with the number of configurations requiring certifi- cation. 1, 2 In the presence of stores and of fluid in them, the fill level becomes an additional configuration parameter. Sample studies have shown that an internal fluid can alter the natural frequencies and vibration modes of an elastic structure. 3 Hence, it may seem likely that the presence of fluid in stores could alter the flutter characteristics of an aircraft. Unfortunately, it also seems that there is very little, if any, focus on the effects of fuel slosh during flutter flight tests, possibly because of the already large number of other parameters to consider and the already substantial cost associated with flight testing. Furthermore, to the best of the authors’ knowledge, the numerical study of this problem has received little attention, at least in the open literature. Therefore, the objectives of the work reported here are to: (a) numerically predict the effects on flutter of the sloshing motion of an internal fluid, (b) consider for this purpose a hydroelastic added mass model and contrast it with a simple lumped mass representation of the internal fluid, and (c) analyze the implications of the obtained numerical results. To this effect, the remainder of this paper is organized as follows. In Section II, two different computational models are described for representing an internal fluid. In Section III, the computational framework adopted in this study for performing all flutter analyses is outlined. Section IV reports on the flutter analysis of a wing-store geometry based on the AGARD Wing 445.6 using both internal fluid models and considering various fill levels. Finally, Section V concludes this paper. II. Fluid Slosh and Lumped Mass Models Here, two approaches of different fidelity levels are considered for accounting for the effects of an internal fluid in CFD-based aeroelastic computations. The first approach is based on the theory of hydroelastic vibrations of an elastic structure containing an inviscid fluid. It results in an added mass model that * Ph. D. Candidate, Department of Aeronautics and Astronautics, William F. Durand Building, 496 Lomita Mall, Stanford, CA 94305-4035; AIAA Member † Vivian Church Hoff Professor of Aircraft Structures; Chairman, Department of Aeronautics and Astronautics; Professor, Department of Mechanical Engineering and Institute for Computational and Mathematical Engineering; Director, Army High Performance Computing Research Center; William F. Durand Building, Room 257, 496 Lomita Mall, Stanford, CA 94305-4035; AIAA Fellow 1 of 13 American Institute of Aeronautics and Astronautics 50th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference<br>17th 4 - 7 May 2009, Palm Springs, California AIAA 2009-2682 Copyright © 2009 by Kwan Yu Chiu and Charbel Farhat. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Downloaded by STANFORD UNIVERSITY on December 3, 2015 | http://arc.aiaa.org | DOI: 10.2514/6.2009-2682