Aerodynamic ground effect in fruitfly sized insect takeoff

Aerodynamic ground effect in flapping-wing insect flight is of importance to comparative morphologies and of interest to the micro-air-vehicle (MAV) community. Recent studies, however, show apparently contradictory results of either some significant extra lift or power savings, or zero ground effect. Here we present a numerical study of fruitfly sized insect takeoff with a specific focus on the significance of leg thrust and wing kinematics. Flapping-wing takeoff is studied using numerical modelling and high performance computing. The aerodynamic forces are calculated using a three-dimensional Navier--Stokes solver based on a pseudo-spectral method with volume penalization. It is coupled with a flight dynamics solver that accounts for the body weight, inertia and the leg thrust, while only having two degrees of freedom: the vertical and the longitudinal horizontal displacement. The natural voluntary takeoff of a fruitfly is considered as reference. The parameters of the model are then varied to explore possible effects of interaction between the flapping-wing model and the ground plane. These modified takeoffs include cases with decreased leg thrust parameter, and/or with periodic wing kinematics, constant body pitch angle. The results show that the ground effect during natural voluntary takeoff is negligible. In the modified takeoffs, when the rate of climb is slow, the difference in the aerodynamic forces due to the interaction with the ground is up to 6%. Surprisingly, depending on the kinematics, the difference is either positive or negative, in contrast to the intuition based on the helicopter theory, which suggests positive excess lift. This effect is attributed to unsteady wing-wake interactions. A similar effect is found during hovering

Rotorcraft aeroelastic stability

Theoretical and experimental developments in the aeroelastic and aeromechanical stability of helicopters and tilt-rotor aircraft are addressed. Included are the underlying nonlinear structural mechanics of slender rotating beams, necessary for accurate modeling of elastic cantilever rotor blades, and the development of dynamic inflow, an unsteady aerodynamic theory for low-frequency aeroelastic stability applications. Analytical treatment of isolated rotor stability in hover and forward flight, coupled rotor-fuselage stability in hover and forward flight, and analysis of tilt-rotor dynamic stability are considered. Results of parametric investigations of system behavior are presented, and correlation between theoretical results and experimental data from small and large scale wind tunnel and flight testing are discussed

Survey of Army/NASA rotorcraft aeroelastic stability research

Theoretical and experimental developments in the aeroelastic and aeromechanical stability of helicopters and tilt-rotor aircraft are addressed. Included are the underlying nonlinear structural mechanics of slender rotating beams, necessary for accurate modeling of elastic cantilever rotor blades, and the development of dynamic inflow, an unsteady aerodynamic theory for low frequency aeroelastic stability applications. Analytical treatment of isolated rotor stability in hover and forward flight, coupled rotor-fuselage stability are considered. Results of parametric investigations of system behavior are presented, and correlations between theoretical results and experimental data from small- and large-scale wind tunnel and flight testing are discussed

Computational Fluid Dynamics Simulations of Oscillating Wings and Comparison to Lifting-Line Theory

Computational fluid dynamics (CFD) analysis was performed in order to compare the solutions of oscillating wings with Prandtl’s lifting-line theory. Quasi-steady and steady-periodic simulations were completed using the CFD software Star-CCM+. The simulations were performed for a number of frequencies in a pure plunging setup. Additional simulations were then completed using a setup of combined pitching and plunging at multiple frequencies. Results from the CFD simulations were compared to the quasi-steady lifting-line solution in the form of the axial-force, normal-force, power, and thrust coefficients, as well as the efficiency obtained for each simulation. The mean values were evaluated for each simulation and compared to the quasi-steady lifting-line solution. It was found that as the frequency of oscillation increased, the quasi-steady lifting-line solution was decreasingly accurate in predicting solutions

Flight of the dragonflies and damselflies

This work is a synthesis of our current understanding of the mechanics, aerodynamics and visually mediated control of dragonfly and damselfly flight, with the addition of new experimental and computational data in several key areas. These are: the diversity of dragonfly wing morphologies, the aerodynamics of gliding flight, force generation in flapping flight, aerodynamic efficiency, comparative flight performance and pursuit strategies during predatory and territorial flights. New data are set in context by brief reviews covering anatomy at several scales, insect aerodynamics, neuromechanics and behaviour. We achieve a new perspective by means of a diverse range of techniques, including laser-line mapping of wing topographies, computational fluid dynamics simulations of finely detailed wing geometries, quantitative imaging using particle image velocimetry of on-wing and wake flow patterns, classical aerodynamic theory, photography in the field, infrared motion capture and multi-camera optical tracking of free flight trajectories in laboratory environments. Our comprehensive approach enables a novel synthesis of datasets and subfields that integrates many aspects of flight from the neurobiology of the compound eye, through the aeromechanical interface with the surrounding fluid, to flight performance under cruising and higher-energy behavioural modes

Aeronautical Engineering: A continuing bibliography, supplement 120

This bibliography contains abstracts for 297 reports, articles, and other documents introduced into the NASA scientific and technical information system in February 1980

FluSI: A novel parallel simulation tool for flapping insect flight using a Fourier method with volume penalization

FluSI, a fully parallel open source software for pseudo-spectral simulations of three-dimensional flapping flight in viscous flows, is presented. It is freely available for non-commercial use under [https://github.com/pseudospectators/FLUSI]. The computational framework runs on high performance computers with distributed memory architectures. The discretization of the three-dimensional incompressible Navier--Stokes equations is based on a Fourier pseudo-spectral method with adaptive time stepping. The complex time varying geometry of insects with rigid flapping wings is handled with the volume penalization method. The modules characterizing the insect geometry, the flight mechanics and the wing kinematics are described. Validation tests for different benchmarks illustrate the efficiency and precision of the approach. Finally, computations of a model insect in the turbulent regime demonstrate the versatility of the software

CFD Analysis of Plunging Flat Plates at Low-Reynolds Number

Unsteady aerodynamics include the study of flows that pass an object subjected to oscillations, and therefore, has a strong link with bioinspired flows and flapping airfoils. The central objective of the dissertation is to study the propulsive characteristics of two flat plates at a Reynolds number, Re, of 3.1 × 103 with straight and sharp trailing edges. The reduced frequency, k, is kept between 1.0 and 5.0 with a nondimensional amplitude, k, ranging between 0.125 and 0.500. The problem is solved numerically using Computational Fluid Dynamics (CFD), and results show that different from what is typically observed in airfoils, the mean thrust coefficient, Ct, does not increase monotonically with the reduced frequency, for h = 0.250, having a dip around k = 3.0. The same is not verified for the mean power coefficient, CP , which increases continuously with the reduced frequency for the entire dimensionless amplitude domain studied. Through analysis of pressure contours around the flat plates, a low-pressure zone was detected near the trailing edge, creating a suction effect in that zone. By analyzing the CP and Ct over one period, the recirculation influence was evidenced, displaying a strong effect on the thrust coefficient parameter. However, results also show the influence of another phenomenon. The LEV’s evolution with k was analyzed, via the normalized distance between its center and the surface of the plate, evidencing a behavior similar to the Ct one previously observed. An attempt to relate the phenomena with the Strouhal number, St, was made, identifying a predictable feature of the performance parameters for St ranging between 0 and 1. The trailing-edge shape revealed to influence the propulsive coefficients, being the overall CP values higher and the Ct lower for the flat plate with a straight trailing edge, when compared to the plate with a sharp one.A análise aerodinâmica em regime transiente passa pelo estudo de objetos sujeitos a oscilações e rotações e, portanto, possui uma forte associação com escoamentos bioinspirados. O principal objetivo desta dissertação é o estudo das características propulsivas de duas placas planas com bordo de fuga reto e afiado para um número de Reynolds, Re de 3, 1 × 103 . A frequência reduzida, k, é mantida entre 1, 0 e 5, 0 com uma amplitude adimensional, h, variando entre 0, 125 e 0, 500. O problema é resolvido numericamente usando Dinâmica de Fluidos Computacional (DFC), e os resultados mostram que, contrariamente ao descrito na literatura para perfis alares, o coeficiente de tração médio, Ct, não aumenta monotonamente com a frequência reduzida, reduzindo em torno de k = 3, 0 para h = 0, 250. O mesmo não se verifica para o coeficiente de potência médio, CP , aumentando continuamente com a frequência reduzida para todo o domínio de amplitude adimensional estudado. Analisando os contornos de pressão em torno das placas planas, uma zona de baixa pressão foi detetada perto do bordo de fuga, criando um efeito de sucção naquela zona. Ao analisar os parâmetros CP e Ct ao longo de um período foi evidenciada a influência da recirculação, apresentando um forte impacto no coeficiente de tração. Contudo, os resultados evidenciam a influência de outro fenómeno. A evolução do vórtice de bordo de ataque com k foi analizada, recorrendo a uma distancia normalizada entre o centro do vórtice e a superfície da placa plana, resultando num comportamento similar ao de Ct previamente observado. É realizada uma tentativa de relacionar os fenómenos observados com o número de Strouhal, St, identificando-se um carácter previsível dos parâmetros de desempenho no intevalo 0 = St = 1. A geometria do bordo de fuga aparenta ter influência nos coeficientes propulsivos obtidos, sendo, na sua generalidade, os valores de CP mais altos e os de Ct mais baixos para a placa de bordo de fuga reto, relativamente à placa de bordo de fuga afiado

Quantitative analysis of take-off forces in birds

The increasing interest on Unmanned Air Vehicles (UAV’s) and their several utilities blended with the need of easy carrying and also the stealth, lead to the need to create the concept of Micro Air Vehicles (MAV’s) and the Nano Air Vehicles (NAV’s). Due to the current interest and the present lack of knowledge on the insect’s and bird’s flight, this study was intended to interpret the forces involved on the moment of the take-off of a bird, recurring to an experiment involving a fast data acquisition force sensor and high speed camera, in addition known facts from earlier studies. In order to do that a bibliographic revision was done, to know what was already studied and to find what could yet be studied. That way could be formed a link on the factors involved on the propulsion of a bird at the moment of take-off. The main conclusions obtained by this work is that the bird can produce movements that will enhance the total moment when the bird stretches its neck forward and moving head down followed by stretching even more its neck and moving head up impelling himself into the air, resulting in a main role on the mechanical forces (against perch) for the bird first moments momentum. Columba livia can generate about 4 times its weight worth mechanic force (against perch) and above 8 times its weight during the 2nd downstroke.O interesse crescente nos Veículos Aéreos não Tripulados “Unmanned Air Vehicles (UAV’s)” e suas diversas utilidades em conjunto com a necessidade de seu fácil transporte e furtividade, levaram à necessidade de criar o conceito dos Micro Veículos Aéreos “Micro Air Vehicles (MAV’s)” e os Nano Veículos Aéreos “Nano Air Vehicles (NAV’s)”. Este tipo de veículos tem como fonte inspiradora os insetos e aves devido à necessária produção simultânea de sustentação e propulsão. Tal como no voo convencional, também no voo animal podem ser identificadas as fases de levantamento (descolagem) e aterragem como diferenciadas do voo longe de uma superfície de apoio. Este trabalho é dedicado ao estudo da fase de levantamento de voo de uma ave columba livia. Foram realizadas experiências para medir a força inicial produzida pela ave para iniciar o voo e a respetiva trajetória na zona próxima do ponto de apoio inicial. Estas medidas foram efetuadas com um sensor de força dotado de elevada velocidade de aquisição de dados e uma camara de alta velocidade. As principais conclusões obtidas com a realização deste trabalho é o facto de que a ave consegue produzir movimentos, que aumentar o momento total quando a ave estica o pescoço para a frente e movendo a cabeça para baixo seguido por continuação de esticamento do pescoço e movimento da cabeça para cima impelindo-se para o ar, resultando num papel principal relativamente às forças mecânicas (contra o poleiro) para o momento linear actuante nos primeiros momentos. Columba livia consegue gerar cerca de 4 vezes o seu peso em força mecânica e acima de 8 vezes o seu peso durante o 2º downstroke