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1. Modulation and Annihilation of Aeroelastic Limit-Cycle Oscillations Using a Variable-Frequency Disturbance Generator

   https://doi.org/10.2514/1.J062295  

   Hughes, Michael T.; Gopalarathnam, Ashok; Bryant, Matthew (April 2023, AIAA Journal)

   Nonlinear aeroelastic limit-cycle oscillations (LCOs) have become an area of interest due to both detrimental effects on flying vehicles and use in renewable energy harvesting. Initial studies on the interaction between aeroelastic systems and incoming flow disturbances have shown that disturbances can have significant effects on LCO amplitude, with some cases resulting in spontaneous annihilation of the LCO. This paper explores this interaction through wind-tunnel experiments using a variable-frequency disturbance generator to produce flow disturbances at frequencies near the inherent LCO frequency of an aeroelastic system with pitching and heaving degrees of freedom. The results show that incoming disturbances produced at frequencies approaching the LCO frequency from below produce a cyclic growth-decay in LCO amplitude that resembles interference between multiple sine waves with slightly varying frequencies. An aeroelastic inverse technique is applied to the results to study the transfer of energy between the pitching and heaving degrees of freedom as well as the aerodynamic power moving into and out of the system. Finally, the growth-decay cycles are shown to both excite LCOs in an initially stationary wing and annihilate preexisting LCOs in the same wing by appropriately timing the initiation and termination of disturbance generator motion. 

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2. Toward on-demand modulation and annihilation of aeroelastic limit cycle oscillations with dynamic upstream disturbance generator

   Hughes, Michael; Gopalarathnam, Ashok; Bryant, Matthew (January 2021, Proceedings of the Online Symposium on Aeroelasticity, Fluid-Structure Interaction, and Vibrations)

   Periodic upstream flow disturbances from a bluff body have recently been shown to be able to modulate and annihilate limit cycle oscillations (LCOs) in a downstream aeroelastic wing section under certain conditions. To further investigate these phenomena, we have implemented a controllable wind tunnel disturbance generator to enable quantification of the parameter ranges under which these nonlinear interactions can occur. This disturbance generator, consisting of a pitch-actuated cylinder with an attached splitter plate, can be oscillated to produce a von Karman type wake with vortex shedding frequency equal to the oscillation frequency over a range of frequencies around the natural shedding frequency of the cylinder alone. At vortex shedding frequencies away from the LCO frequency of the wing, forced oscillations were observed in the wing, but the wing did not enter self-sustaining LCOs. However, when disturbances were introduced near the LCO frequency, the initially static downstream wing entered self-sustaining oscillations in the presence of the incoming vortices, and these LCOs persisted when the disturbance generator was stopped. Annihilation of the wing LCOs was also observed disturbance vortices were introduced upstream of the wing in LCO. 

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3. Generation of Large-Scale Gust Structures in a Large Boundary Layer Wind Tunnel: 3D Flow Measurement Experiments

   https://doi.org/10.17603/ds2-0m8r-my92  

   Mokhtar, Nasreldin O.; Fernández-Cabán, Pedro L.; Catarelli, Ryan A. (July 2023, Designsafe-CI)

   This dataset includes flow velocity time series collected in a large boundary layer wind tunnel (BLWT) to investigate the intensity of large-scale turbulent gust structures generated by a novel flow-control instrument. The work leveraged a multi-stage flow conditioning system consisting of an active multi-fan gust generator, termed flow field modulator (FFM), that operated in conjunction with an automated roughness element grid (called Terraformer). The primary goal of the study is to assess the effectiveness of the coupled flow conditioning system (FFM and Terraformer) for increasing and tuning large-scale (particularly near-surface) turbulent structures that will enable characterization of their impact on building loads at relatively large BLWT scales (1:50). The dataset can be used and compared against previously published velocity measurements collected using traditional BLWT flow conditioning approaches (i.e., no active control of large-scale turbulence). 

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4. Acoustic emission of aeroelastic vortex-gust interactions

   Chen, Huansheng; Jaworski, Justin W. (September 2019, International Congress on Acoustics)

   The coupled interaction between an unsteady vortical flow and dynamics of an aerodynamic structure is a canonical problem for which analytical studies have been typically restricted to either static or prescribed structural motions. The present effort extends beyond these restrictions to include a Joukowski airfoil on elastic supports and its aeroelastic influence on the incident vortex, where it is assumed that all vorticity in the flow field can be represented by a collection of line vortices. An analytical model for the vortex motion and the unsteady fluid forces on the airfoil is derived from inviscid potential flow, and the evolution of the unsteady airfoil wake is governed by the Brown and Michael equation. The aerodynamic sound generated by the aeroelastic interaction of an incident vortex, shed Brown-Michael vortices, and the moving airfoil is estimated for low-Mach-number flows using the Powell-Howe acoustic analogy. 

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5. A fully-coupled computational aeroelasticity model for transonic and supersonic flows

   https://doi.org/10.2514/6.2022-3600  

   Ilie, Marcel; Havenar, John (July 2022, AIAA AVIATION 2022 Forum)

   AIAA AVIATION Forum (Ed.)

   The aeroelastic phenomena of fixed-wing aircraft in transonic and supersonic flight regimes plays a critical role in the design of high-speed aircraft. The present research concerns the development of computationally efficient and accurate methods for the computation of aeroelastic systems containing transonic and supersonic flows. Therefore, we propose a fully- coupled, time-marching aeroelastic approach utilizing an URANS model. The computational studies are carried out to assess the effect of the freestream Mach number and angle of attack on the structural dynamics and stresses developed in the wing structure. The studies are carried out for a range of Mach numbers, 𝑴∞ = 𝟎. 𝟖 – 𝟏. 𝟒, and angles of attack, 𝜶 = {𝟐°, 𝟒°, 𝟔°}. The analysis reveals that the aeroelastic deformation of the wing and induced stress in the wing structure increase with the freestream Mach number. 

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