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

Aeronautical Engineering. A continuing bibliography, supplement 115

This bibliography lists 273 reports, articles, and other documents introduced into the NASA scientific and technical information system in October 1979

Aeronautical Engineering. A continuing bibliography, supplement 112

This bibliography lists 424 reports, articles, and other documents introduced into the NASA scientific and technical information system in July 1979

Computation and control of flow-induced noise behind a circular cylinder using an acoustic analogy approach

The computational aeroacoustics (CAA) research, which focuses on predicting acoustics by means of advanced numerical techniques, has recently gained a great deal of progress. In most applications, the prediction of both the sound source and its far-field propagation is necessary as required by regulations. Recently, powerful computers and reliable algorithms have allowed the prediction of far-field noise through the use of Computational Fluid Dynamics (CFD) data as near-field sound sources. One of the most useful analytical methods, used for the computation of noise, is Lighthill\u27s acoustic analogy. The latter will be used in the present study. Lighthill\u27s acoustic analogy, combined with the two-dimensional incompressible Navier-Stokes flow computation at low Mach Number (M \u3c 1), is used to predict the noise generated by laminar vortex shedding from a circular cylinder at the Reynolds number values Re = 100 and Re = 160. The computed velocity and pressure in the flow field are used as input data for noise source functions. The noise prediction is determined by using Curle\u27s solution of Lighthill\u27s acoustic analogy. Due to the fact that the magnitude of the quadrupole noise source (O (M3)) for this type of flow is much smaller than that of the dipole source (O(M2 )) at low Mach Number, this study concentrates on investigating only the effect of the dipole source on the flow field. The noise amplitude and frequency obtained by using Curle\u27s solution agree well with published data. For both values of Reynolds numbers Re = 100 and Re = 160, the lift dipole source function, caused by the lift force acting on a circular cylinder, is the dominant source term that affects the total acoustic density fluctuation. The objective of this research is to study the suppression of flow-induced noise behind a circular cylinder using a flow control method. The selected method is the electro-magnetic feedback control method developed by Chen and Aubry (2000). The results show that at Re = 100 and Re = 160 the nondimensional acoustic density fluctuation is decreased by five orders of magnitude

Disturbance rejection for U.A.S. aircraft using bio-inspired strain sensing

A bio inspired gust rejection mechanism based on structural inputs is proposed. Insect wings possess a wealth of sensor systems which typically consist of fast reflexive neuronal paths. Stretch and strain sensors on insect wings are used for flight control and can be found across many species. These are used for monitoring of bending and torsion during flight. The fast reflexive and proprioceptive mechanisms based on strain sensing found in nature are the inspiration for this work. A strain feedback controller allows for anticipation of the onset of rigid body dynamics due to gust perturbations. This anticipation stems from sensing of higher order states and the possibility of reacting before lower order states are reached. High bandwidth inner loop compensation is therefore enabled. Forces and moments are proportional to wing strain patterns and can be used in fast reaction inner loops. Strain sensors are used for providing an indirect estimation of the differential forces applied to the aircraft wing and therefore to the aircraft rigid body. These sensors can be distributed over the surface of the aircraft wing to encode multiple degree of freedom disturbances. Sensor locations for disturbance rejection are determined based on metrics associated to the observability Grammian. The locations are preselected based on modal energy analyses and are chosen according to wide field integration patterns. A model for wide field integrated strain based on mass participation factors is proposed as well as one which is based on the physics of the forces and moments acting on the wing producing strain patterns which can be used for disturbance rejection. Models of the differential forces via strains on the wings are proposed. Strain feedback was implemented in four platforms under different types of disturbances. The platforms consisted of a glider, a quadrotor, a wing section for wind tunnel testing and an RC airplane with a full span wing. The disturbances included discrete gusts as well as turbulence. The results of using strain feedback showed not only to be faster than IMU estimations but also to be better when compared to a classical attitude controller implementation

Numerical and theoretical study of flapping airfoil aerodynamics using a parallelized immersed-boundary method

Flight has fascinated humans for centuries. Human inventions such as missiles, aircraft , unmanned aerial vehicles (UAV), and micro air vehicle (MAV) are inspired by natural flying expertise. As natural flyers usually operate in a vortex-dominated environment, interactions between their wings and the vortices have significant influences on force generation and flying efficiency. Some interesting phenomena induced from such vortex-body interactions have gotten a lot of attention in the past few decades. A good example is that birds and insects are credited with extracting energy from ambient vortices. In a simpler form, bio-inspired airfoils with either passive or active flapping motions are found to have the potential to harvest energy from incoming vortices generated from an upstream object, i.e. a cylinder. The current study identified the interaction modes of the leading edge vortex (LEV) and trailing edge vortex (TEV) between the active flapping airfoil and the incoming vortices. The relation between the interaction modes and the energy extraction capacity of an active harvester is investigated guided by a potential theory. The interaction modes induced by a passive energy harvester always benefit the energy extraction efficiency. However, the dynamic response of the passive harvester was found to vary corresponding to the properties of the incoming vortical wake. A profound appreciation of energy extracting mechanisms can provide a solution for the energy consumption issue of MAV and UAV. However, difficulties are encountered in practical applications of energy harvesting on how to detect the locations of generated vortices and what the trajectory of the vortex downstream of the moving body is. Some observations are realized and the fluid dynamics of the phenomena is beyond the fundamentals described in the textbook. One well-known instance is the asymmetric wake formed downstream of a symmetric sinusoidal heaving airfoil. In this study, factors that influence the formation of the asymmetric wakes on both the near wake and far wake regions are demonstrated. Novel vortex models are developed to explore the vortex dynamic mechanisms of the asymmetric wake and its development from the near wake region to the far wake region. In order to analyze the flow fields for the bio-inspired problems, Computational Fluid Dynamics (CFD) provides powerful and convenient tools. The shape of bio-inspired wings/airfoils and their maneuvers are usually very complicated. In CFD, the immersed-boundary (IB) method is an advantageous approach to simulate such problems. In this study, an immersed-boundary method is implemented in a parallel fashion in order to speed up the computational rate.. A variety of numerical schemes have been applied to the IB method, including different spatial schemes and temporal schemes; their performances are investigated. In addition, the IB method has been successfully implemented with the fluid-structure interaction models for studying passive mobile objectives, i.e. the energy harvester. The possibility of coupling other fluid dynamic models, i.e. species transport model and turbulence models, is also demonstrated

Aeronautical Engineering: A special bibliography with indexes, supplement 55

This bibliography lists 260 reports, articles, and other documents introduced into the NASA scientific and technical information system in February 1975

Attenuation of the unsteady loading on a high-rise building using active control

The present work numerically investigates the 3D flow structures around a canonical high-rise building immersed in an atmospheric boundary layer at different oncoming wind angles, using wall-resolved large eddy simulations. The switching between two vortex shedding modes is explored, and the influence of the atmospheric boundary layer on suppressing symmetric vortex shedding is identified. It is shown that the antisymmetric vortex shedding mode is prevalent in the near wake behind the building, with strong coherence between the periodic fluctuations of the building side force and the antisymmetric vortex shedding mode demonstrated. Exploiting this idea, active flow control strategies are designed to alleviate the aerodynamic side-force fluctuations. Two feedback control strategies are then developed to attenuate the building’s unsteady loading when the oncoming flow is normal to the wider side of the building, using pressure sensing on just a single building wall. The sensor response to synthetic jet actuation along the two ‘leading edges’ of the building is characterised using system identification. Both the designed linear controller and the least mean square adaptive controller attenuate successfully the side-force fluctuations when implemented in simulations. The linear controller exhibits better performance, and its effect on the flow field is to delay the formation of dominant vortices and increase the extent of the recirculation region. Motivated by the effect of the downwash flow on the near wake of the building, an open-loop active control in the form of a synthetic jet located on the top surface is then considered. The aim is again to suppress side force fluctuations, this time exploring whether the simpler control setup can be effective across a wide range of oncoming wind angles. The behaviour of the synthetic jet and its effect on the building’s unsteady side force, time-averaged flow fields and unsteady flow structures are investigated numerically. The synthetic jet actuation is found to reduce the side-force fluctuation of the building, enhance the downwash flow and successfully attenuate the antisymmetric vortex shedding. Finally, the possibility of using a robust feedback control strategy mitigating the unsteady loading of a high-rise building exposed to differing oncoming wind directions is explored. A reference linear open-loop transfer function is identified, and the differences between the reference open-loop system and open-loop transfer functions for different oncoming wind angles are assessed by v-gap. An H loop-shaping feedback controller is then designed and achieves the attenuation of the building’s side-force fluctuations for differing oncoming wind angles. This work can provide a theoretical basis for the practical application of novel active control approaches to attenuating the side-force fluctuation of the high-rise building exposed to differing oncoming wind directions.Open Acces