IFASD2024 Paper Submission & Registration
17 – 21 June 2024, The Hague, The Netherlands

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16:00   Flutter 1
Chair: alexis laporte
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 linear-frequency-domain (LFD) solver based on linearized unsteady Reynolds-averaged Navier-Stokes equations (URANS) with an appropriate turbulence closure model. A state-space model of the aerodynamic forces is generated through interpolation of the frequency-domain samples, which is the basis of the p-L 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 two-step 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.
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.
30 mins
CFD-based computational aeroelastic analysis of body freedom flutter in X-56A
Iren Ani Mkhoyan, Huub Timmermans, Jos Aalbers
Abstract: This study presents a comprehensive high-fidelity analysis of body free flutter phenomenon exhibited by the Multi-Utility Technology Testbed (MUTT) X-56A. The primary objective of this study is to assess the use of CFD-based Computational Aeroelastic Simulation (CAS) for flutter prediction. This research was originally conducted as part of the third Aeroelastic Prediction Workshop (AePW3) aiming to enhance the knowledge in aeroelastic prediction using mid to high-fidelity computational tools. 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 panel code and the flow simulation system ENFLOW (NLR) for multi-block flow domains. The high-fidelity CFD and CAS analyses were performed using the ENSOLV code that solves the unsteady RANS equations on a deforming multi-block structured grid. The four-phase computational chain consisted of pre-processing where the structural modes obtained from NASTRAN are interpolated onto the aerodynamic surface grid; a steady RANS analysis in ENSOLV on the undeformed grid representing the jig shape; and unsteady RANS simulations in ENSOLV using consecutive forced vibrations of the splined structural modes at different values of reduced frequencies using a 3-period sine-excitation with a small amplitude. The total number of runs performed in the forced vibration computational chain equalled the number of structural modes multiplied by the number of reduced frequencies of interest. The post-processing chain entailed the transformation of the time-domain GAFs obtained with unsteady aeroelastic simulations to frequency domain, followed by flutter boundary prediction using the p-k method. The results obtained from the ENSOLV analyses were compared to those obtained from ZAERO analyses as well as the results. Initial series of CAS analyses indicated discrepancies in the predictions of the GAFs, in particular at lower reduced frequencies. This was remedied by remodelling of the aerodynamic surface grid and the far field volume grid, and by implementing a preconditioning scheme to improve convergence of the URANS solver at low Mach numbers. These changes resulted in significant improvements in the results compared to previous analyses, both in accuracy and computational cost.
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.

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