16:00
Flutter 1
Chair: alexis laporte
16:00
30 mins

A general solver for the prediction of flutter and buffet onset
David Quero, Christoph Kaiser, Jens Nitzsche
Abstract: A versatile solver capable of predicting both flutter and buffet onset (also under the influence of a flexible structure) values is introduced. The methodology involves computing the (generalized) aerodynamic forces using a linearfrequencydomain (LFD) solver based on linearized unsteady Reynoldsaveraged NavierStokes equations (URANS) with an appropriate turbulence closure model. A statespace model of the aerodynamic forces is generated through interpolation of the frequencydomain samples, which is the basis of the pL method [1]. Eigenvalues corresponding to fluid modes may be directly determined, eliminating the need for a priori pole selection as required by traditional rational function approximation techniques. Consequently, the (pre)buffet frequency is accurately represented by the imaginary part of the fluid mode’s eigenvalue.
For automated stability analysis encompassing flutter and (elastic) buffet phenomena, a twostep procedure is employed. An algorithm extracts the dominant fluid mode first. Aerodynamic and structural models are coupled then, and a scalar parameter q_m multiplying the (generalized) mass matrix is varied from infinity to unity, while the dynamic pressure is set to a minimal value. A following sweep involves increasing the dynamic pressure, with both structural and dominant fluid modes included. Classical flutter is encountered if aeroelastic eigenvalues corresponding to structural modes cross the imaginary axis, while (elastic) buffet is predicted if the dominant fluid mode’s eigenvalue does.
Figure 1 illustrates an application for the OAT15A airfoil at Mach number 0.73 and angle of attack 4° [2]. In the first sweep (left), q_m decreases as indicated by the arrow. Buffet onset is caused by an increasing density value, represented by the point at which the dominant fluid mode’s eigenvalue crosses the imaginary axis (right). This solver also accurately predicts classical flutter onset under different conditions, as detailed in the full paper.

16:30
30 mins

Body freedom flutter of scaled vehicles: From blended wing body to conventional configuration
Yingsong Gu, Pengtao Shi, Xinhai Tian, Zhichun Yang
Abstract: In this presentation, the state of the art of BFF study in the last decade is briefly reviewed at first, with an emphasis on the development of flight test vehicles. Next, as an effort of the authors during recent years, the development and BFF investigation of scaled vehicles is introduced. The design, build and test campaigns are detailed, first for a blended wing body (BWB) vehicle, and then for a conventional configuration. Theoretical and experimental correlation study is conducted for the aeroelastic stability of both vehicles. Finally, it is concluded with the pros and cons of the present work.

17:00
30 mins

Unsteady RANSbased Computational Aeroelastic Simulations of X56A Flutter for the Third Aeroelastic Prediction Workshop
Iren Ani Mkhoyan, Peter Blom, Jos Aalbers, Huub Timmermans
Abstract: This paper presents a highfidelity aeroelastic study of the the MultiUtility Technology Testbed (MUTT) X56A, designed to exhibit aeroelastic instabilities such as body free flutter (BFF). The primary objective of this work is to assess the use of highfidelity CFDbased aeroelastic simulations for flutter prediction. This research was originally conducted as part of the Third Aeroelastic Prediction Workshop (AePW3) aiming to enhance the knowledge in aeroelastic predictions using mid to highfidelity computational aerodynamics. This particular study details the contribution from the Flight Physics & Loads group at the Netherlands Aerospace Centre (NLR), exploring two computation methods for generating the Generalised Aerodynamic Forces (GAFs), namely, ZAERO solver (ZONA Technology) using higherorder panel code ZONA6, and an unsteady RANSbased Computational Fluid Dynamics (CFD) and Computational AeroElastic (CAE) simulations implemented within NLR's ENFLOW simulation system for multiblock flow domains. The highfidelity CFD and CAE analyses were performed using the flow solver ENSOLV with unsteady ReynoldsAveraged NavierStokes (RANS) flow formulation implemented with Explicit Algebraic Reynolds Stress Model (EARSM) turbulence modelling based on the TNT $k\omega$. The CAE computational procedure consists of four tool chains, involving structural dynamics (modal) analyses; grid interpolation procedure; steady CFD computations on the undeformed shape; unsteady CFD computations on a deforming grid under prescribed, smallamplitude sinusoidal excitations based on the structural mode shapes; and the transformation of the timedomain unsteady solutions to frequency domain in order to construct the GAF matrices. The X56A configuration used for this study is the 10lb fuel state model released within the AePW3 group. The resulting GAFs were compared to the ZAERO results, showing good agreement for both the rigid body and elastic modes. Earlier work on X56A within AePW3 conveyed the need for further refinement of the highfidelity aeroelastic methodology. Improvement efforts in this regard, included alternative structural dynamics methods for modal model computations, CFD grid refinements, and adjustments to the (un)steady CFD/CAE simulation procedures and methods.

17:30
30 mins

Application of the unsteady compressible source and doublet panel method to flutter calculations
Grigorios Dimitriadis, Spyridon Kilimtzidis, Vassilis Kostopoulos, Vito Laraspata, Leonardo Soria
Abstract: The Source and Doublet Panel Method (SDPM) developed by Morino in the 1970s can model unsteady compressible ideal flow around wings and bodies. In this work, the SDPM is adapted to the calculation of aeroelastic solutions for wings. A second order nonlinear version of Bernoulli's equation is transformed to the frequency domain and written in terms of the generalized mode shapes and displacements. It is shown that the unsteady pressure component at the oscillating frequency is a linear function of the generalized displacements and can therefore be used to formulate a linear flutter problem. The proposed approach has several advantages: the exact geometry is modelled, including thickness, camber, twist and dihedral effects, the motion of the surface can be represented using all six degrees of freedom, the pressure calculation is of higher order and the generalised aerodynamic mass, damping and stiffness load terms are calculated separately. The complete procedure is validated using the experimental data from the weakened AGARD 445.6 wing and three rectangular wings with pitch and plunge degrees of freedom.

