Topic
Airfoil
About: Airfoil is a research topic. Over the lifetime, 24696 publications have been published within this topic receiving 337709 citations. The topic is also known as: aerofoil & wing section.
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TL;DR: In this article, the optimal shape of flying wings for subsonic and transonic speeds is examined using a suite of tools developed around a three-dimensional, time-spectral, Euler computational fluid dynamics solver.
Abstract: The optimal shape of flying wings for subsonic and transonic speeds is examined using a suite of tools developed around a three-dimensional, time-spectral, Euler computational fluid dynamics solver. The first result in the study is a lift-constrained drag minimization, performed on an unswept, rectangular wing. When the spanwise twist distribution of the wing is varied, the elliptic optimum predicted by the low-speed inviscid theory can be reproduced. With this result as a reference, three different optimization formulations are explored. These formulations consider the addition of bending moment constraints, static-stability constraints, and dynamic-stability constraints. In each case, the design space of the problem is explored using both planform and shape variables to determine the optimal shape. These techniques are used to show that the addition of stability constraints has a significant impact on the optimal surface shape of the wing. In particular, it is shown that at lower speeds, the airfoil sha...
92 citations
TL;DR: In this paper, a computational method for predicting unsteady viscous flow through two-dimensional cascades accurately and efficiently is presented, which is intended to predict the onset of the aeroelastic phenomenon of stall flutter.
Abstract: A computational method for predicting unsteady viscous flow through two-dimensional cascades accurately and efficiently is presented. The method is intended to predict the onset of the aeroelastic phenomenon of stall flutter. In stall flutter, viscous effects significantly impact the aeroelastic stability of a cascade. In the present effort, the unsteady flow is modeled using a time-linearized Navier-Stokes analysis. Thus, the unsteady flow field is decomposed into a nonlinear spatially varying mean flow plus a small-perturbation harmonically varying unsteady flow. The resulting equations that govern the perturbation flow are linear, variable coefficient partial differential equations. These equations are discretized on a deforming, multiblock, computational mesh and solved using a finite-volume Lax-Wendroff integration scheme. Numerical modeling issues relevant to the development of the unsteady aerodynamic analysis, including turbulence modeling, are discussed. Results from the present method are compared to experimental stall flutter data, and to a nonlinear time-domain Navier-Stokes analysis. The results presented demonstrate the ability of the present time-linearized analysis to model accurately the unsteady aerodynamics associated with turbomachinery stall flutter.
92 citations
29 Jun 1997
TL;DR: In this paper, the authors show that adding oscillatory addition of momentum without the addition of mass flow is very effective in delaying separation, particularly from lifting surfaces, and that the incremental improvements in the airfoil characteristics are insensitive to changes in Reynolds number provided the latter is sufficiently large.
Abstract: Oscillatory addition of momentum without the addition of mass flow is very effective in delaying separation, particularly from lifting surfaces. It is much more robust than the steady blowing traditionally used for this purpose. Experiments carried out on different airfoils revealed that this flow depends on many parameters such as: the location of the blowing slot, the steady and oscillatory momentum coefficients (if oscillatory blowing is used), the frequency of the imposed oscillations and the shape and incidence of the airfoils. The incremental improvements in the airfoil characteristics are insensitive to changes in Reynolds number provided the latter is sufficiently large. Preliminary results suggest that the improvements in the airfoil performance are not hindered by compressibility at subsonic speeds or by sweep. Furthermore the method can be applied to diffusers and thus used for thrust augmentation and vectoring. It can also be applied to helicopter rotors and alleviate the effects of dynamic stall, some preliminary results on pitching airfoil are shown. The oscillatory blowing and suction used most often can be replaced by other means providing the appropriate oscillatory component of the momentum coefficient at the right frequency. The method is therefore independent of the device producing the oscillations. It became clear that a delay of separation is a distinctly different task than the promotion of .reattachment yet both are important in operating an airfoil or a flap near its natural stall. Some examples, showing the instantaneous (or phase locked to the disturbance and ensemble-averaged) pressure distributions and voiticity distributions over a flap will be given. The integration of geometrical design with the imposed forced oscillations providing the maximum pressure recovery over the shortest possible distance is now under consideration.
92 citations
TL;DR: Spectral data are presented for the noise produced due to the turbulent three-dimensional vortex flow existing near the rounded tip of lifting airfoils as discussed by the authors, and the results are obtained by the comparison of sets of two-and threedimensional test data for different airfoil model sizes, angles of attack, and tunnel flow velocities Microphone cross correlation and cross-spectral methods were used to determine the radiated noise.
Abstract: Spectral data are presented for the noise produced due to the turbulent three-dimensional vortex flow existing near the rounded tip of lifting airfoils The results are obtained by the comparison of sets of two- and three-dimensional test data for different airfoil model sizes, angles of attack, and tunnel flow velocities Microphone cross-correlation and cross-spectral methods were used to determine the radiated noise Corrections were made for tunnel shear layer and source directivity effects Interpretation of the results are aided by a three-dimensional flow analysis developed for this study which determines open tunnel and finite aspect ratio corrections heretofore neglected in tip vortex studies Hot wire measurements were made in the tip vortex formation region for the specification of governing flow parameters The spectral data is normalized in a format considered most useful for subsequent quantitative prediction of this noise mechanism for practical systems such as helicopter rotors Comparison is made to the analysis of George and Chou A recommended prediction method is given
92 citations
TL;DR: In this article, the importance of unsteady effects on laminar boundary layers was found to diminish rapidly with increasing longitudinal pressure gradients, whereas turbulent separation on airfoils was significantly affected by oscillatory motion when the incidence approached the stall angle.
Abstract: Incompressible laminar and turbulent flows over flat plates and airfoils have been investigated numerically and experimentally in unsteady flow conditions. Important differences were found between laminar and turbulent flat plate flows over a wide range of oscillation frequencies. Also, the importance of unsteady effects on laminar boundary layers was found to diminish rapidly with increasing longitudinal pressure gradients, whereas turbulent separation on airfoils was significantly affected by oscillatory motion when the incidence approached the stall angle. The calculated hysteresis in turbulent separation followed in a qualitative sense the well-known trends of dynamic stall delay and reattachment. However, the numerical analysis failed to indicate some of the important features of dynamic stall observed in the present experiment and in previous studies.
92 citations