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A numerical optimization algorithm based on the steepest decent along the variation of the optimization function is implemented for maximizing the thrust and/or propulsive efficiency of a single flapping airfoil. Unsteady, low speed laminar flows are computed using a Navier-Stokes solver on moving overset grids. The flapping motion of the airfoil is described by a combined sinusoidal plunge and pitching motion. Optimization parameters are taken to be the amplitudes of the plunge and pitching motions, and the phase shift between them. Computations are performed in parallel in a work station cluster. The numerical simulations show that high thrust values may be obtained at the expense of reduced efficiency. For high efficiency in thrust generation, the induced angle of attack of the airfoil is reduced and large scale vortex formations at the leading edge are prevented.

A potential flow model is used to study the unsteady flow past two airfoils in configuration, each of which is suddenly set into motion. The airfoil bound vortices are modeled using lumped vortex elements and the wake behind the airfoil is modeled by discrete vortices. This consists of solving a steady state flow problem at each time-step where unsteadiness is incorporated through the "zero normal flow on a solid surface" boundary condition at every time instant. Additionally, along with the "zero normal flow on a solid surface" boundary condition Kelvin-s condition is used to compute the strength of the latest wake vortex shed from the trailing edge of the airfoil. Location of the wake vortices is updated at each time-step to get the wake shape at each time instant. Results are presented to show the effect of airfoil-airfoil interaction and airfoil-wake interaction on the aerodynamic characteristics of each airfoil., {"references":["Seiler, P., Pant, A. and Hedrick, K. \"Analysis of bird formations\",\nProceedings of 41st IEEE Conference on Decision and Control, Las\nVegas, Nevada, USA, December 2002.","Poore, S. O., Sanchez-Heiman, A. and Goslow, G. E. Jr., \"Aircraft\nupstroke and evolution of flapping flight\", Nature, Vol 387, pp 799-802,\nJune 1997.","Steven Hoa, Hany Nassefa, Nick Pornsinsirirakb, Yu-Chong Taib, Chih-\nMing Hoa, \"Unsteady aerodynamics and flow control for flapping wing\nflyers\", Progress in Aerospace Sciences, 39, 635-681, 2003.","Bangash, Z. A., Sanchez, R. P. and Ahmed, A., \"Aerodynamics of\nFormation Flight, Journal of Aircraft\", Vol 43, No. 4, pp 907-912, July-\nAugust 2006.","Vachon M., Ray R., Walsh K., Ennix, K., \"F/A-18 Aircraft Performance\nBenefits Measured During the Autonomous Formation Flight Project\",\nAIAA 2002-4491, Aug 2002.","Bowles R. G. A. and Smith F. T., \"Lifting multi-blade flows with\ninteraction\", J. Fluid Mech., vol. 415, pp. 203-226, 2000.","D. Fanjoy, and D. J. Dorney, \"A study of tandem-airfoil interaction in\ndifferent flight regimes,\" AIAA 97-0515, 1997.","L. Zannetti, F. Gallizio, and G. M. Ottino, \"Vortex motion in doubly\nconnected domains,\" J. Fluid Mech., vol. 612, pp. 143 - 152, 2008.","Z. Husain, M. J. Abdullah, and T. C. Yap, \"Two-dimensional analysis of\ntandem/staggered airfoils using computational fluid dynamics,\"\nInternational Journal of Mechanical Engineering Education 33/3.\n[10] Katz, J. and Plotkin, A., Low-speed Aerodynamics, Cambridge University\nPress, 2001.\n[11] A. J. Chorin, and P. S. Bernard, \"Discretization of a vortex sheet, with an\nexample of roll-up,\" J. of Computational Physics., vol. 13, No. 3, 1973.\n[12] A. Fage and F. C. Johansen, \"On the flow of air behind an inclined flat\nplate of infinite span\" Proc. Roy. Soc. A 116, 170, 1927.\n[13] T. Nakagawa, \"On Unsteady Airfoil-Vortex Interaction\", ACTA\nMECHANICA 75, 1-13, 1988\n[14] H. Wagner, \"Uber die Entstehung des Dynamischen Autriebes von\nTragflugeln\", Z.F.A.M.M., Vol. 5, No. 1, pp 17-35, 1925."]}

This paper is aimed at mechanical and civil engineers dealing with practical problems involving flows around structures representing a significant obstacle to the oncoming air (socalled bluff-body geometries). It shows the applicability range of relatively simple twodimensional numerical simulations, explains their limits and offers a set of guidelines, which should help even non-experts to extract the maximum from the often limited computer resources. The results of numerical simulations of the unsteady flow around three cylinders with rectangular cross-sections are presented. The cylinders are of infinite length and have the cross-sections with a width-to-height ratio of 0.2, 1.0 and 5.0. The results obtained by solving 2-D Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations are in reasonable agreement with the experimentally measured forces and pressures. The analysis of the calculated results shows the physical phenomena responsible for the dramatic drag force reduction, which can be observed when the width-to-height ratio of the cylinder is increased.

AIAA AVIATION 2020 FORUM (2020 : Virtual, Online), Canards are used as control surfaces and they operate with control systems including actuators, drivelines and electronics in missiles. Aeroelastic behavior of the canards must be accurately known to comply with the control systems. However, structural nonlinearities such as plastic deformation, bilinear stiffness and free-play change the aeroelastic characteristics of the structure. This study aims to illustrate nonlinear aeroelastic behavior of a missile canard which has free-play in its shaft-actuator joint. Three initial angle of attack positions and two different free-play sizes were evaluated with fluid-structure interaction (FSI) method. Nonlinear behavior of the canard was compared with the linear, which has no free-play, condition. Consequently, FSI results showed that response frequencies decrease as free-play increases and amplitudes of oscillations are affected by initial conditions. © 2020, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.

Over the years, wings have become lighter and more flexible, making them more prone to aeroelastic effects. Thus, aeroelasticity in design becomes more important. In order to determine the response of an aircraft to, for example, a gust, an unsteady aerodynamic model is required to determine the dynamic loads. The three most-commonly used methods in aeroelastic loads analysis are 2D unsteady-airfoil theory, the doublet lattice method (DLM), and the unsteady vortex lattice method (UVLM). In contrast to these existing methods, the current paper proposes a 3D state-space model for unsteady aerodynamic analysis that is both directly written in time-domain, and is a continuous-time model. The main advantages of this are that no approximation errors are made in the conversion to the time domain, and that the time step is only driven by requirements on accuracy. The model is based on potential flow theory, which is implemented by means of vortex ring elements. The model was first verified, and then applied to a pitch-plunge response problem showing the benefits of the current approach over existing methods.

The Active Aeroelastic Test Bench (AATB) is designed for the study of low subsonic unsteady aerodynamics and aeroelasticity. The AATB is equipped with four linear motors offering separate control of the pitch and plunge position of the wing. The AATB allows to perform forced motion unsteady aerodynamic experiments when operated in open loop. In addition, when operated in closed loop, aeroelastic experiments can be performed, with both linear, or nonlinear variable structural behaviour. For this closed-loop operation the directly measured loads are fed back to the real-time controller. This controller then outputs the user desired position, emulating structural behaviour of the wing.

The paper presents main results obtained in the DLR rotorcraft program within the last years such as computational tools (e.g. CFD development and validation for aerodynamics and aeroacoustic), noise studies, environmental studies such as all weather issues, new technologies for active rotor control and active blades, flight mechanics and handling quality studies, recent developments for tiltrotor applications, crash analysis and modelling and helicopter flight tests including studies for noise abatement procedures. The DLR rotorcraft research program is closely connected to the Onera rotorcraft program and is part of international networks of cooperation with many partner organizations. DLR and Onera have been designing rotorcraft airfoils for many years. Almost all Eurocopter helicopters use these airfoils. One important asset to improve the aerodynamic efficiency (i.e. reduce the fuel consumption and the exhaust) of rotorcraft or rotorcraft components is the access to accurate prediction tools. Therefore a large effort was spent during the last two decades to develop so called Computational Fluid Dynamics (CFD) methods which are able to compute the flow around a complete rotorcraft. In order to validate such methods the GOAHEAD (Generation of Advanced Helicopter Experimental Aerodynamic Database for CFD code Valication) project was carried out. One of the main factors restricting the use of rotorcaft in civil and partly in military missions, is the noise emission of these vehicles. Therefore ongoing research concerns itself with the development and validation of methods for the prediction of external and internal noise and specific solutions for noise reduction. This includes the main and tail rotor/Fenestron noise, as well as the engine noise. The successful application of helicopters in Emergency Medical Services (EMS), in rescue operations (Save and Rescue: SAR) and in specific military missions is limited by bad weather conditions, especially degraded visual environments and icing conditions. Helicopters are frequently operated in close proximity to the ground at short distances from obstacles (trees, power poles, etc.). The workload of the pilot and the accident risk in such difficult missions is considerably higher than that of a typical fixed wing aircraft. Therefore research is for flight control, man-machine-interface, sensor development, information display, pilot assistance and navigation systems. At the same time new operational procedures and redundancy concepts under safety- and certification aspects are developed. The results achieved with the flight test aircraft Flying Helicopter Simulator (FHS) of DLR are of specific importance. A complete test environment for the FHS has been developed (system identification, real time simulation, ground simulator, model following control system (MFCS), integration of an active sidestick including the required software in the aircraft and the ground simulator).