11:00
Gust 4
Chair: Felix Arevalo
11:00
30 mins
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Analysis of Linearized Motion- and Gust-Induced Airloads with a Next-Generation Computational Fluid Dynamics Solver
Christoph Kaiser, David Quero, Jens Nitzsche, Bernd Stickan
Abstract: Time-linearized airloads provide a cost-effective way to incorporate unsteady CFD-based methods into aircraft design for assessing loads and stability. RANS methods are necessary to account for non-linearities in modelling transonic or separated flow as required for industrial applications. By limiting excitations around a steady state to very small amplitudes, linearized airloads can be obtained from aerodynamic responses. In this work, two approaches for determining time-linearized motion- and gust-induced airloads [1] are employed within the next-generation CFD framework CODA: the linear frequency domain (LFD) method and the method of system identification in the time domain using forced-motion simulations.
The CFD framework CODA comes with improved capabilities regarding the implementation and modelling of the discretized flow. In particular, automatic differentiation of the implemented equations directly enables the evaluation of the flux Jacobian as needed for the LFD solver [2]. Employing both considered approaches in a unified setting allows verifying
the consistent implementation of CODA’s LFD solver and the unsteady time-integration scheme with moving meshes. Figure 1 shows the agreement of the two approaches for the NACA64 A010 airfoil in transonic flow undergoing pitching oscillations of varying frequencies as well as for a sinusoidal gust excitation.
This work investigates several configurations including the NASA Common Research Model in terms of unsteady integral forces and pressure distributions resulting from rigid body and elastic motion as well as from gusts in varying flow regimes and for different turbulence models. Moreover, the conducted analysis provides the groundwork for verifying aeroelastic studies based on linearized airloads like flutter onset predictions with aeroelastic timemarching simulations employing CODA.
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11:30
30 mins
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Effect of flow separation on discrete gust loads for a free-flying elastic aircraft
Johan Moritz Feldwisch
Abstract: Shock motion and flow separation are aerodynamic nonlinearities, which have a significant effect on dynamic gust loads but are not accounted for in time-linearized aerodynamic models. Shock motion may lead to higher torsional moments (Kaiser, 2019) and even higher loads in case the shock system is dynamically changed (Friedewald, 2023). Gust disturbances are not small and may cause local regions with flow separation during the gust encounter. The detached flow limits the total lift as shown in (Friedewald, 2023) which is promising for a reduction in aerodynamic loads, yielding lighter load carrying structures which in turn may improve overall aircraft performance.
This work investigates the reduction of distributed gust loads due to detached flow for the elastic, free-flying aircraft with discrete gust spectra defined by CS25. The DLR TAU-Code is utilized to solve the RANS equations. Different turbulence models (Spalart-Allmaras, RSM SSG/LRR-lnω) are applied, as the problem of predicting the flow separation correctly remains. The time-linearized solution is achieved by scaling the time-marching responses to small gust amplitudes at the same gust gradients. The nonlinear simulation results are compared to the linear solution, to assess the potential of the load reduction. The investigated transport aircraft is the NASA Common Research model (Vassberg, 2008) for which the structural FERMAT model (Klimmek, 2014) is available.
The results show that the SA and RSM turbulence model predict similar loads for cases with attached flow. For those gust parameters, the root bending moment of the time-linearized and time-marching calculations deviate by about 2%. For medium to long gust gradients large regions of the outer wing show flow separation during the gust encounter. Here, the RSM turbulence model predicts a root bending moment reduction from 12% to 19% where the SA turbulence model predicts a reduction in the range of 14% to 24%.
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12:00
30 mins
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Analysis of inertial gust load relief characteristics of high aspect ratio wings
Sanuja Jayatilake, Mark Lowenberg, Benjamin King Sutton Woods, Branislav Titurus
Abstract: With its promise of enabling enhanced aerodynamic characteristics, the implementation of High Aspect Ratio Wings (HARWs) continues to be a key driver behind the development of novel aircraft configurations. However, due to their inherent non-classical properties associated with increased structural slenderness, HARWs are susceptible to a multitude of adverse aeroelastic phenomena related to gust load responses and aeroelastic instability.
The analysis the gust responses of elastic aircraft dates to 1932, to the works of Küssner. During discrete gust encounters, an elastic structure exhibits responses comprising of several aeroelastic modes. The complexity of these multi-modal responses is exacerbated by the considerably low natural frequencies of HARWs. This is a result of the increased number of modes being stimulated by the excitation defined within the frequency range determined by a given flight speed and a gust gradient.
Among approaches developed to reduce loads during gust encounters, various reconfigurable and hinged concepts have gained increased prominence recently. Extensive wind tunnel experimentation and numerical work has demonstrated their effectiveness in reducing incremental wing root loads, particularly under wider gusts. Further scope for this development includes the exploitation of the inertial shear load relief from the angular acceleration as originally observed in the context of rigid hinged and bird wings [Stevenson et al., 2023].
Whilst the above principle is strictly limited to hinged rigid bodies, the notable flexibility of HARWs still retains the possibility of exploiting the inertial load relief aspect. This research intends to explore this idea using the numerical model of a flexible cantilevered wing. The inertial shear load alleviation qualities associated with various low frequency aeroelastic modes will be explored. To enable this, classical techniques related to modal decomposition will be used to develop the study. In view of varying excitation frequency bands under discrete gusts, the inertial load relief qualities under an aggregation of the low frequency aeroelastic modes of the HARW will be examined. The insights generated will be illustrated with a focus on passive tailoring of inertial properties of HARWs (e.g., mass distribution, inertial amplification mechanisms) to improve the wing root load responses during gust encounters.
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