Numerical Computation of Flapping-Wing Propulsion and Power Extraction

AIAA Paper No. 97-0826, 35th AIAA Aerospace Sciences Meeting, Reno, Nevada, Jan. 1997.Numerical procedures are presented for the systematic computation of unsteady flows over moving airfoils or airfoil combinations, and these procedures are applied to the investigation of flapping-wing propulsion and power extraction. Flow solutions about single foils are computed using an unsteady, two-dimensional panel code coupled with a boundary layer algorithm and driven using an interactive graphical user interface. Flow solutions about airfoil combinations are computed using a companion, multi-element version of the panel code. Results for pitching-only and plunging-only motions compare favorably with theory and reasonably well with experimental results. Extensive computations are performed over the broad parameter space for combined pitching and plunging motions using the foil as both a propulsive device and as a wingmill or power-extraction device. Results modeling flight in ground effect are compared with other numerical and experimental results

Conpressibility Effects on Dynamic Stall of Oscillating Airfoils

A research proposal to investigate the "Compressibility Effects on Dynamic Stall of Oscillating Airfoils" was submitted to ARO and the project was funded in April 1986. The aim was to obtain a basic understanding of the effect of compressibility on the phenomenon of dynamic stall under typical flight conditions encountered by a helicopter in forward flight., so that eventually a means for its control can be devised and thus, its flight envelope can be expanded. The initial phase of the study was devoted to building a drive system to produce the necessary unsteady airfoil motion. A novel design was arrived at after reviewing the various possibilities and was built. It uses a four-bar chain mechanism of which the airfoil is one of the links.U.S. Army Research Office ARO 23394.10-EG, MIPR ARO 137-86U.S. Army Research Office ARO 23394.10-EG, MIPR ARO 137-8

An Investigation of the Fluid-Structure Interaction in an Oscillating-Wing Micro-Hydropower Generator

in Fluid Structure Interaction II, Eds. Chakrabarti, S.K., Brebbia, C.A., Almorza, D. and Gonzalez-Palma, R., WIT Press, Southampton, UK, 2003, pp. 73-82.Results of a computational and experimental investigation of the fluid-structure phenomena occurring in an oscillating-wing micro-hydropower generator are presented. The generator consists of tandem wings which oscillate in a combined pitch-plunge mode with approximately 90 degree phase angle between the two motions. Two-dimensional inviscid and viscous flow codes are used to predict the oscillatory flow field and the power transferred from the water flow to the oscillating wings. Experimental results of water tunnel tests of this hydropower generator are also described and comparisons between the measured and predicted power output are given

Jet Characteristics of a Plunging Airfoil

Presented as Paper 98-0101 at the AIAA 36th Aerospace Sciences Meeting, Reno, NV, 12–15 January 1998The article of record as published may be found at https://doi.org/10.2514/2.641Water-tunnel tests of a NACA 0012 airfoil that was oscillated sinusoidally in plunge are described. The flowfield downstream of the airfoil was explored by dye flow visualization and single-component laser Doppler velocimetry (LDV) measurements for a range of freestream speeds, frequencies, and amplitudes of oscillation. The dye visualizations show that the vortex patterns generated by the plunging airfoil change from drag-producing wake flows to thrust-producing jet flows as soon as the ratio of maximum plunge velocity to freestream speed, i.e., the nondimensional plunge velocity, exceeds approximately 0.4. The LDV measurements show that the nondimensional plunge velocity is the appropriate parameter to collapse the maximum streamwise velocity data covering a nondimensional plunge velocity range from 0.18 to 9.3. The maximum streamwise velocity at a given streamwise distance downstream starts to exceed the freestream speed as soon as the nondimensional plunge velocity exceeds 0.25. Furthermore, this maximum jet velocity has been shown to be a linear function of the nondimensional plunge velocity

Transonic blade flutter: A survey of new developments

This paper presents a review of current work in transonic blade flutter research. Aerodynamic analyses for the prediction of attached flow flutter, choke flutter, and stall flutter are described. Also reviewed are unsteady aerodynamic measurement and flutter test programs that have recently been completed or are in progress to investigate transonic blade flutter phenomena

On the Design of Efficient Micro Air Vehicles

Design and Nature - Comparing Design in Nature with Science and Engineering Eds. Brebbia, C.A., Sucharov, L.J. and Pascolo, P., WIT Press, Southampton, UK, 2002, pp. 67-76.In the past few years aeronautical engineers have recognized the possibility of building very small air vehicles, so-called Micro Air Vehicles (MAVs), in response to specific military and commercial needs. Remotely controlled or autonomous MAVs are difficult to detect because of their small size and low noise emission or, if detected, they may be mistaken for small birds or insects. Yet video cameras and other sensors have become so miniaturized in recent years that it is possible to mount them on MAVs for the purpose of transmitting information which may be difficult to obtain in other ways. In this paper the authors first explain various aspects of the physics of thrust generation due to wing flapping and then review the major computational and experimental results which they achieved during the past few years. They conclude with a description of their MAV which is currently under development

Airfoil Geometry and Flow Compressibility Effects on Wing and Blade Flutter

AIAA Paper No. 98-0517, 36th AIAA Aerospace Sciences Meeting, Reno, Nevada, Jan. 1998.An unsteady, two-dimensional, incompressible potential-flow solver and an unsteady, two-dimensional, compressible Euler/Navier-Stokes flow solver are coupled with a two-degree-of-freedom structural model for the time-domain computation of aeroelastic response. Comparisons are made between results from the two flow solvers and with flutter boundary predictions of linear theory. Presented results demonstrate similar destabilizing effects for both increasing airfoil thickness and increasing Mach number. More importantly, it is shown that linear theory yields un-conservative flutter-velocity predictions. While linear theory predicts that single-degree-of-freedom (pitching) flutter cannot occur except with an unrealistically high sectional moment of inertia, it is shown here that thicker airfoils in compressible flows may easily achieve single-degree-of-freedom flutter under realistic conditions

The Unsteady Aerodynamics of Flapping-Foil Propellers

Proceedings of the 9th International Symposium on Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines, Eds. Ferrand, P. and Aubert, S., Presses Universitaires de Grenoble, Grenoble, France, 2001, pp. 123-147.It is the objective of this paper to summarize the authors' recent work on flapping foils. Water tunnel experiments on sinusoidally plunging foils are described which elucidate the change in vortical wake pattern shed from the foil's trailing edge. These experiments were carried out using dye flow visualization and laser-Doppler velocimetry. It is found that the wake pattern is a strong function of the maximum non-dimensional plunge velocity, with the wake topology changing from a typical Kármán vortex street to an inverse Kármán vortex street to an asymmetric wake structure as the non-dimensional plunge velocity increases. These results are partly reproducible with inviscid panel code and Navier-Stokes code predictions. Additional interesting features are obtained if two degrees of freedom are permitted (pitch-plunge motions). Depending on the pitch/plunge amplitudes and the phasing between the two motions, the foil either produces thrust or extracts energy. A water tunnel experiment is described which demonstrates the possibility of power generation from a slowly flowing, shallow river. Additional interesting features are found if two airfoils in close proximity to each other are studied. Experiments with two airfoils arranged in a biplane configuration and oscillating in counter-phase show significant thrust and propulsive benefits in comparison to single flapping foils

Computational Simulation of Dynamic Stall on the NLR 7301 Airfoil

The article of record as published may be found at http://dx.doi.org/10.1006/jfls.2000.0299The dynamic stall behavior of the supercritical NLR 7301 airfoil is analyzed with a 2-D thin-layer Navier-Stokes code. The code solves the compressible Reynolds-averaged Navier- Stokes equations with an upwind biased numerical scheme in combination with the Baldwin}Lomax or the Baldwin}Barth turbulence models. The effect of boundary layer transition is incorporated using the transition length model of Gostelow et al. The transition onset location is determined with Michel's formula or it can be specified as an input parameter. The two turbulence models yield significantly different steady-state lift coefficients at incidences greater than 8 degrees. The use of the one-equation Baldwin}Barth model together with the Gostelow transition model is found to give substantially better agreement with the experimental data of McCroskey et al. than the Baldwin}Lomax model. Also, the unsteady computations are strongly affected by the choice of the turbulence model. The Baldwin}Barth model predicts the lift hysteresis loops consistently better than the algebraic turbulence model. However, the one-equation model improves the prediction of the moment hysteresis loops only for one test case.Deutsche Forschungsgemeinschaft (DFG)Naval Postgraduate SchoolDeutsche Forschungsgemeinschaft (DFG)Naval Postgraduate Schoo

Computation of unsteady flows over airfoils

Two methods are described for calculating unsteady flows over rapidly pitching airfoils. The first method is based on an interactive scheme in which the inviscid flow is obtained by a panel method. The boundary layer flow is computed by an interactive method that makes use of the Hilbert integral to couple the solutions of the inviscid and viscous flow equations. The second method is based on the solution of the compressible Navier-Stokes equations. The solution of these equations is obtained with an approximately factorized numerical algorithm, and with single block or multiple grids which enable grid embedding to enhance the resolution at isolated flow regions. In addition, the attached flow region can be computed by the numerical solution of compressible boundary layer equations. Unsteady pressure distributions obtained with both methods are compared with available experimental data.Approved for public release; distribution is unlimited