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Aerodynamic force

About: Aerodynamic force is a research topic. Over the lifetime, 7491 publications have been published within this topic receiving 102639 citations.


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TL;DR: In this paper, an analytical design technique for an active fluttersuppression and gust-alleviation control system is presented based on a rational approximation of the unsteady aerodynamic loads in the entire Laplace domain, which yields matrix equations of motion with constant coefficients.
Abstract: An analytical design technique for an active flutter-suppression and gust-alleviation control system is presented. It is based on a rational approximation of the unsteady aerodynamic loads in the entire Laplace domain, which yields matrix equations of motion with constant coefficients. Some existing rational approximation schemes are reviewed, and a new technique which yields a minimal number of augmented states for a desired accuracy is presented. The state-space aeroelastic model is used to design a constant gain, partial-feedback control system, which simultaneously assures stability and optimizes any desired combination of gust response parameters throughout the entire flight envelope.

404 citations

Journal ArticleDOI
TL;DR: The wake capture force represents a truly unsteady phenomenon dependent on temporal changes in the distribution and magnitude of vorticity during stroke reversal and is well explained by a quasi-steady model.
Abstract: We used two-dimensional digital particle image velocimetry (DPIV) to visualize flow patterns around the flapping wing of a dynamically scaled robot for a series of reciprocating strokes starting from rest. The base of the wing was equipped with strain gauges so that the pattern of fluid motion could be directly compared with the time history of force production. The results show that the development and shedding of vortices throughout each stroke are highly stereotyped and influence force generation in subsequent strokes. When a wing starts from rest, it generates a transient force as the leading edge vortex (LEV) grows. This early peak, previously attributed to added-mass acceleration, is not amenable to quasi-steady models but corresponds well to calculations based on the time derivative of the first moment of vorticity within a sectional slice of fluid. Forces decay to a stable level as the LEV reaches a constant size and remains attached throughout most of the stroke. The LEV grows as the wing supinates prior to stroke reversal, accompanied by an increase in total force. At stroke reversal, both the LEV and a rotational starting vortex (RSV) are shed into the wake, forming a counter-rotating pair that directs a jet of fluid towards the underside of the wing at the start of the next stroke. We isolated the aerodynamic influence of the wake by subtracting forces and flow fields generated in the first stroke, when the wake is just developing, from those produced during the fourth stroke, when the pattern of both the forces and wake dynamics has reached a limit cycle. This technique identified two effects of the wake on force production by the wing: an early augmentation followed by a small attenuation. The later decrease in force is consistent with the influence of a decreased aerodynamic angle of attack on translational forces caused by downwash within the wake and is well explained by a quasi-steady model. The early effect of the wake is not well approximated by a quasi-steady model, even when the magnitude and orientation of the instantaneous velocity field are taken into account. Thus, the wake capture force represents a truly unsteady phenomenon dependent on temporal changes in the distribution and magnitude of vorticity during stroke reversal.

380 citations

Journal ArticleDOI
TL;DR: Since closed-form, analytic expressions are obtained for the generalized aerodynamic forces, insight can be gained into the effects of parameter variations that is not easily obtained from numerical models.
Abstract: The nonlinear equations of motion for an elastic airplane are developed from first principles. Lagrange's equation and the Principle of Virtual Work are used to generate the equations of motion, and aerodynamic strip theory is then employed to obtain closed-form integral expressions for the generalized forces. The inertial coupling is minimized by appropriate choice of the body-reference axes and by making use of free vibration modes of the body. The mean axes conditions are discussed, a form that is useful for direct application is developed, and the rigid-body degrees of freedom governed by these equations are defined relative to this body-reference axis. In addition, particular attention is paid to the simplifying assumptions used during the development of the equations of motion. Since closed-form, analytic expressions are obtained for the generalized aerodynamic forces, insight can be gained into the effects of parameter variations that is not easily obtained from numerical models. An example is also presented in which the modeling method is applied to a generic elastic aircraft, and the model is used to parametrically address the effects of flexibility. The importance of residualizing elastic modes in forming an equivalent rigid model is illustrated, but as vehicle flexibility is increased, even modal residualization is shown to yield a poor model.

372 citations

Journal ArticleDOI
TL;DR: In this article, a force analysis of a cylindrical liquid element subjected to an aerodynamic drag force was performed and the results indicated that for larger injection velocity conditions liquid jets penetrate relatively far into the crosse fields and exhibit surface breakup processes before the column breaks.
Abstract: The breakup processes of liquid jets injected into subsonic air crosse ows were experimentally studied. Test liquids, injector diameters, and air Mach numbers were varied to provide a wide range of jet operation conditions. Results indicate that for larger injection velocity conditions liquid jets penetrate relatively far into the crosse ows and exhibit surface breakup processes before the column breaks. Liquid column trajectories were correlated by liquid/air momentum e ux ratios based on a force analysis of a cylindrical liquid element subjected to an aerodynamic drag force. Drag coefe cients were inferred from the column trajectories and were found to exhibit a weak dependence on liquid viscosity. The heights of the column fracture points were correlated using the time required for an analogous droplet to complete an aerodynamic secondary breakup process. The success of the resulting correlation justie es the assumption that the aerodynamic forces acting on a droplet and those acting on a liquid column have similar effects. This result, combined with the trajectory correlation, leads to the conclusion that the liquid column always breaks at the same streamwise location, in agreement with the present experimental observation.

371 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023145
2022289
2021262
2020310
2019345
2018315