Airfoil In Incompressible Flow Research Articles

Low-Order Modeling of Dynamic Stall on Airfoils in Incompressible Flow

Airfoil dynamic stall in incompressible flow is characterized by two interacting viscous flow phenomena: time-varying trailing-edge separation and the shedding of intermittent leading-edge-vortex structures. In the current work, a physics based low-order method capable of modeling the interactions between the two flow phenomena is developed with the aim of predicting dynamic stall with only a few empirical tuning parameters. Large computational datasets are used to understand the flow physics of unsteady airfoils so as to augment an inviscid, unsteady airfoil theory to model the time-dependent viscous effects. The resulting model requires only three empirical coefficients for a given airfoil and Reynolds number, which could be obtained from a single moderate-pitch-rate unsteady motion for that airfoil/Reynolds number combination. Results from the low-order model are shown to compare excellently with computational and experimental solutions, in terms of both aerodynamic loads and flow-pattern predictions. In addition to formulating a method with limited empirical dependencies, the current research provides valuable insights into the flow physics of unsteady airfoils and their connection to rapidly predictable theoretical parameters.

Simple adaptive V-stack piezoelectric based airfoil flutter suppression system

The suppression of aeroelastic vibrations due to flutter using a trailing edge flap is analyzed to increase the speed operability range of wings. The aeroelastic model of a 3DOF airfoil in incompressible flow is presented and an augmented state-space representation is used for the time domain analysis. A finite element model of a V-stack piezoelectric actuator, used to move the trailing edge flap, is introduced to model the actuator dynamics. The use of the simple adaptive controller is investigated to realize an adaptive flutter suppression system. A parallel feedforward compensator is tuned to make the plant almost strictly passive and a metaheuristic algorithm is used to determine the invariant parameters. Numerical simulations are carried out to verify the performance of the simple adaptive flutter suppression system in presence of speed variations and disturbances. Numerical results proved that the SAC flutter suppression system improves the closed loop system flutter boundary with respect to a gain scheduled PID controller.

Passing-over leading-edge vortex: The thrust booster in heaving airfoil

The thrust generated by heaving airfoil in incompressible flow is studied both numerically and theoretically. It is found that the behaviors of leading-edge vortex (LEV) under different airfoil’s transverse velocities have an important influence on the thrust. When this velocity is small, the LEV is advected downstream regularly; however, as transverse velocity increases, the LEV can be pushed back to the leading edge again and pass over it to the opposite side of the airfoil. In cases with the LEV passing over the leading edge, the maximum transient thrust can be enhanced by several times compared with that in the single-stroke motion. The reason and critical condition for the occurrence of passing-over leading-edge vortex (PO-LEV) are found. Based on the source of the pressure Poisson equation, a near-field force theory is developed and used together with the boundary vorticity flux (BVF) theory to clarify the thrust enhancement mechanisms.

FLEXURAL OSCILLATIONS OF FLEXIBLE AIRFOILS IN SUBSONIC COMPRESSIBLE FLOWS

This paper presents simple and efficient analytical solutions for unsteady subsonic compressible flows past flexible airfoils executing low frequency oscillations. These analytical solutions are obtained with a method using velocity singularities related to the airfoil leading edge and ridges. The method has been validated for the pitching and plunging oscillations of the rigid airfoils by comparison with results based on Jordan’s data for compressible flows and by comparison with the solutions obtained by Theodorsen, Postel & Leppert and Mateescu & Abdo for incompressible flows. The method has been applied to obtain efficient analytical solutions for the flexural oscillations of airfoils in compressible flows, which can be efficiently used in solving aeroelastic problems. Professor Carafoli’s contributions to the aerodynamics of airfoils and wings Starting from 1924, in Paris, Elie Carafoli made significant contributions to the aerodynamic studies of wings and airfoils, included in Aerodynamics of airplane wings [5], Theory of lifting airfoils [6], in which he presented the aerodynamics of airfoils with rounded trailing edge, named later Carafoli airfoils, Influence of ailerons on the aerodynamic characteristics of lifting surfaces [7], and in Experimental studies on monoplane wings [8]. After 1928, he continued his research and academic activity in Romania, bringing new contributions to the aerodynamics of wings and airfoils in incompressible flows, which were mostly included in Aerodynamics [9], book also translated from Romanian into German and Russian. After 1950, Elie Carafoli developed a special interest for compressible flows and published High Speed Aerodynamics [10], initially in Romanian and then translated in English. He made, together with his collaborators, important contributions on the aerodynamics of wings and airfoils in supersonic flows, most of them included in the monograph Wing Theory in Supersonic Flow [11] published by Pergamon Press, to which the present author also collaborated. INCAS BULLETIN No. 2/ 2009 51 DOI: 10.13111/2066-8201.2009.1.2.11 For his outstanding achievements in aerodynamics, Elie Carafoli received very numerous international distinctions and awards, such as Diploma “Paul Tisandier” of the International Aeronautical Federation, Gauss Medal attributed yearly to the best scientist in the world by the Scientific Society Braunschweig, Germany, Apollo 11 Medal, Tsiolkovski Medal and many others distinctions which can not be listed here due to the space limitation. Elie Carafoli was one of the few Honorary Fellows of the Royal Aeronautical Society, Great Britain, and one of the first members of the International Academy of Astronautics (proposed by Theodor von Karman), and he was elected first Vice-President and then President of the International Astronautic Federation. In Romania, Carafoli was elected member of the Romanian Academy in 1948, and later named President of the Technical Sciences Section of the Academy, and he received the greatest Romanian medals, prizes and awards. One of the important realizations of Academician Carafoli was the foundation in 1949 of a research institute of the Romanian Academy, known as the Institute of Fluid Mechanics, which is presently named the National Institute for Aerospace Research “Elie Carafoli”. This institute contributed to the scientific formation of numerous former students and collaborators of Professor Carafoli, who distinguished themselves by contributions in aerodynamics and other related aerospace domains. Many of the former students and collaborators of Professor Carafoli became professors at their turn in Romania, as well as in other countries, such as Canada, France, Germany and United States of America, to name only a few. The present author had the privilege to study Aerodynamics with Professor Carafoli, who was also the Ph. D. supervisor, and to be his close collaborator for more than two decades. He is indebted to Professor Carafoli for many research advices, which materialized not only in many publications resulted from their direct scientific collaboration, but also in his further research and academic activity, first in Romania, and since 1982 in Canada. This author, together with his Masters’ and Ph. D. students, aims to continue the best scientific tradition of Professor Carafoli, with new contributions to the aerodynamics of wings and airfoils in steady and unsteady flows in incompressible, compressible subsonic and supersonic regimes and at low Reynolds numbers, in addition to other contributions in related aerospace domains (a few selected examples from the last ten years are included in references [20–37, 43].

On the Influence of Time‐Varying Flow Velocity on Unsteady Aerodynamics

The effects of a periodic free-stream velocity on the unsteady aerodynamics of an airfoil in incompressible flow are examined. Existing theories are reviewed, and their simplifications and limitations are properly identified. A new general aerodynamic theory for an airfoil undergoing a combination of harmonic pitching, plunging and fore-aft motion is presented. An extension to arbitrary free-stream velocity variations and arbitrary airfoil motion is also given. The theoretical results are validated against numerical predictions made by a modern Euler code.

Bifurcation analysis of airfoils in incompressible flow

Bifurcations of airfoils with non-linear pitching stiffness in incompressible flow are investigated. The harmonic balance method, which is more practical, is adopted for the analysis, and numerical integration results are used as standards of checking. Analytical solutions calculated by asymptotic expansion method and averaging method with the aid of computer algebra have confirmed the feasibility of the harmonic balance method.

Wing-store flutter analysis of an airfoil in incompressible flow

The flutter of two-dimension al airfoil with external store is analyzed to investigate the effects of pylon stiffness on flutter speed. Among the 40 configurations studied, five were tested in the wind tunnel to verify the analytical results. The variations of wing-store flutter speed with the pylon stiffness can basically be divided into three types. The curves of the normal and flutter frequencies vs pylon stiffness have the same pattern. They can be sketched approximately by the aid of the normal frequencies of the two degenerated two-degree-of-freedom systems, i.e., the freely hinged and rigidly connected store eases. A limiting flutter speed for very small pylon stiffness is deduced, which is useful to identify which type of flutter the configuration studied belongs to.

Analysis of limit cycle flutter of an airfoil in incompressible flow

Both experimental and theoretical analyses are perfomed to investigate several types of self-excited oscillations of a two-dimensional wing model with non-linear pitching stiffiness. Free play non-linearity receives especial attention. In the low speed wind tunnel test of the wing model with free play in pitch, an interesting case of double limit cycle flutter is found. Subsequent harmonic balance analyses ascertain not only these two sustained oscillations, but also two other unstable limit cycles. A digital simulation method is also used for flutter analysis. The theoretical results agree well with the experimental data.

Analysis of circulation controlled airfoils

Abstract : A self-contained analysis for arbitrary circulation controlled airfoils in incompressible flow is developed. The analysis predicts the blowing slot conditions required to produce a specified lift coefficient on a given airfoil with given free stream conditions. An iterative procedure is used to find the blowing slot conditions that allow the Thwaites condition of constant pressure in the separated region to be satisfied. With the input given, a potential flow analysis is performed using the Theodorsen method. Boundary layer analyses for the lower and upper surfaces then yield the separation pressure on the lower surface and the boundary layer properties at the slot on the upper surface. The flow is initially laminar and usually becomes turbulent. The Cebeci, Smith finite difference method is used and an eddy viscosity model is used for turbulent flow. Blowing slot values are assumed and a turbulent wall jet analysis is performed to determine the wall pressure at separation on the upper surface.

A note on a solution of Possio’s integral equation for an oscillating airfoil in subsonic flow

Following a procedure analogous to that used by Schwarz for solving the integral equation for an oscillating airfoil in incompressible flow, a method of attack on the same problem for subsonic, compressible flow is outlined. The circulation and kernel to the circulation integral equations are expanded in series with the coefficients M 2 n {M^{2n}} and M 2 n {M^{2n}} ln M M , and a corresponding expansion for the Green’s function is thereby implicitly determined. The solution up to (but not including) terms of O ( M 4 , M 4 ln ⁡ M ) O\left ( {{M^4},{M^4}\ln M} \right ) is stated. A method of iteration for improving a given solution is also indicated.