The Experts below are selected from a list of 261 Experts worldwide ranked by ideXlab platform
Muhammad R Hajj - One of the best experts on this subject based on the ideXlab platform.
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geometrically exact extension of Theodorsen s frequency response model
53rd AIAA Aerospace Sciences Meeting, 2015Co-Authors: Haitham E Taha, Zhimiao Yan, Muhammad R HajjAbstract:The classical unsteady theory of Theodorsen is revisited relaxing some of the major assumptions such as (1) flat wake, (2) small angle of attack, (3) small disturbances to the mean flow components, and (4) time-invariant free-stream. A semi-analytical, geometricallyexact, unsteady potential flow model is developed for airfoils undergoing large amplitude maneuvers. The numerical implementation of the developed model is presented. The unsteady predictions of the developed model are validated against experimental and computational results. Then, the frequency response of the flow dynamics is determined at different angles of attack. While good agreement is shown with Theodorsen’s function at small angle of attack, considerable qualitative and quantitative discrepancies between the obtained frequency response at large angles of attack and Theodorsen’s function are observed.
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Geometrically-Exact Extension of Theodorsen’s Frequency Response Model
53rd AIAA Aerospace Sciences Meeting, 2015Co-Authors: Haitham E Taha, Zhimiao Yan, Muhammad R HajjAbstract:The classical unsteady theory of Theodorsen is revisited relaxing some of the major assumptions such as (1) flat wake, (2) small angle of attack, (3) small disturbances to the mean flow components, and (4) time-invariant free-stream. A semi-analytical, geometricallyexact, unsteady potential flow model is developed for airfoils undergoing large amplitude maneuvers. The numerical implementation of the developed model is presented. The unsteady predictions of the developed model are validated against experimental and computational results. Then, the frequency response of the flow dynamics is determined at different angles of attack. While good agreement is shown with Theodorsen’s function at small angle of attack, considerable qualitative and quantitative discrepancies between the obtained frequency response at large angles of attack and Theodorsen’s function are observed.
James D. Baeder - One of the best experts on this subject based on the ideXlab platform.
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Unsteady response of airfoils due to small-scale pitching motion with considerations for foil thickness and wake motion
Journal of Fluids and Structures, 2020Co-Authors: M. Ryan Catlett, Jason M. Anderson, Camli Badrya, James D. BaederAbstract:Abstract Unsteady pressures, forces, and pitching moments generated by foils experiencing vibratory motion in an incompressible, attached flow configuration are studied within this work. Specifically, two-dimensional, unsteady potential flow and unsteady Reynolds-Averaged Navier–Stokes calculations are performed on various Joukowski foils undergoing sinusoidal, variable amplitude, small-scale pitching motion at a chord-based Reynolds number of 10 6 over a range of reduced frequencies between 0.01–100. These calculated results from both approaches are compared directly to predictions from implementing the Theodorsen model, which treats foils as infinitely thin, flat plates that shed a planar sheet of vorticity. The effects of relaxing these seemingly strict conditions are explored, and the particular terms which control the unsteady responses are identified and discussed. For increasing pitch amplitudes and reduced frequencies the shed wake is seen to become quite non-planar and to form coherent vortex structures. Despite this wake behavior, the normalized airfoil responses at the disturbance reduced frequency are seen to be largely unaffected. However, non-negligible responses are generated across a wide range of other frequencies. Potential flow calculations for symmetric Joukowski foils show that there is marginal effect of foil thickness at reduced frequencies less than one. For higher reduced frequency conditions however, the unsteady lift response is seen to experience both an amplification of level and a phase shift relative to the Theodorsen model. A specific augmenting expression is developed for this behavior through analysis within the potential flow framework.
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Investigation of the unsteady responses of airfoils due to small-scale motion
The Journal of the Acoustical Society of America, 2019Co-Authors: M. Ryan Catlett, Jason M. Anderson, James D. BaederAbstract:Unsteady pressures, forces, and pitching moments generated by foils experiencing vibratory motion in an incompressible, attached flow configuration are studied within this work. Specifically, two-dimensional, unsteady potential flow calculations are performed on Joukowski foils of varying thickness undergoing variable amplitude, small-scale, heaving or pitching motion over a range of reduced frequencies between 0.01 and 100. These calculated results are compared directly to predictions from implementing the Theodorsen model, which treats foils as infinitely thin flat plates that shed a planar sheet of vorticity. The effects of relaxing these seemingly strict assumptions of the Theodorsen model are explored, and focus is placed on results which show deviations from the Theodorsen model. These include altered unsteady responses for finitely thick foils and non-zero mean angle of attack conditions, non-linearity of the wake of shed vorticity, and analysis of the unsteady streamwise force. Further, within the potential flow framework the particular terms which control the unsteady responses are identified.Unsteady pressures, forces, and pitching moments generated by foils experiencing vibratory motion in an incompressible, attached flow configuration are studied within this work. Specifically, two-dimensional, unsteady potential flow calculations are performed on Joukowski foils of varying thickness undergoing variable amplitude, small-scale, heaving or pitching motion over a range of reduced frequencies between 0.01 and 100. These calculated results are compared directly to predictions from implementing the Theodorsen model, which treats foils as infinitely thin flat plates that shed a planar sheet of vorticity. The effects of relaxing these seemingly strict assumptions of the Theodorsen model are explored, and focus is placed on results which show deviations from the Theodorsen model. These include altered unsteady responses for finitely thick foils and non-zero mean angle of attack conditions, non-linearity of the wake of shed vorticity, and analysis of the unsteady streamwise force. Further, within the...
Giuseppe Quaranta - One of the best experts on this subject based on the ideXlab platform.
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Influence of airfoil thickness on unsteady aerodynamic loads on pitching airfoils
Journal of Fluid Mechanics, 2015Co-Authors: Valentina Motta, Alberto Guardone, Giuseppe QuarantaAbstract:The influence of the airfoil thickness on aerodynamic loads is investigated numerically for harmonically pitching airfoils at low incidence, under the incompressible and inviscid flow approximation. Force coefficients obtained from finite-volume unsteady simulations of symmetrical 4-digit NACA airfoils are found to depart from the linear Theodorsen model of an oscillating flat plate. In particular, the value of the reduced frequency resulting in the inversion – from clockwise to counter-clockwise – of the lift/angle-of-attack hysteresis curve is found to increase with the airfoil thickness. Both the magnitude and direction of the velocity vector due to pitching over the airfoil surface differ from their flat-plate values. During the upstroke, namely nose-up rotation, phase, this results in a decrease (increase) of the normal velocity magnitude over the upper (lower) surface of the airfoil. The opposite occurs during the downstroke phase. This is confirmed by comparing the computed pressure distribution to the flat-plate linear Kussner model. Therefore, beyond the inversion frequency, the lift coefficient of a finite-thickness airfoil is higher during upstroke and lower during downstroke than its flat-plate counterpart. A similar dependence is also found for the quarter-chord moment coefficient. Accordingly, a modification to the classical Theodorsen model is proposed to take into account the effects of the airfoil thickness on unsteady loads. The new model is found to accurately predict the unsteady aerodynamics of a thick symmetric and a slightly cambered airfoil with a maximum thickness in the range 4–24 %. The limits of the present inviscid flow analysis are assessed by means of numerical simulation of high Reynolds number ( $\mathit{Re}=10^{6}$ ) flows.
Haitham E Taha - One of the best experts on this subject based on the ideXlab platform.
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geometrically exact extension of Theodorsen s frequency response model
53rd AIAA Aerospace Sciences Meeting, 2015Co-Authors: Haitham E Taha, Zhimiao Yan, Muhammad R HajjAbstract:The classical unsteady theory of Theodorsen is revisited relaxing some of the major assumptions such as (1) flat wake, (2) small angle of attack, (3) small disturbances to the mean flow components, and (4) time-invariant free-stream. A semi-analytical, geometricallyexact, unsteady potential flow model is developed for airfoils undergoing large amplitude maneuvers. The numerical implementation of the developed model is presented. The unsteady predictions of the developed model are validated against experimental and computational results. Then, the frequency response of the flow dynamics is determined at different angles of attack. While good agreement is shown with Theodorsen’s function at small angle of attack, considerable qualitative and quantitative discrepancies between the obtained frequency response at large angles of attack and Theodorsen’s function are observed.
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Geometrically-Exact Extension of Theodorsen’s Frequency Response Model
53rd AIAA Aerospace Sciences Meeting, 2015Co-Authors: Haitham E Taha, Zhimiao Yan, Muhammad R HajjAbstract:The classical unsteady theory of Theodorsen is revisited relaxing some of the major assumptions such as (1) flat wake, (2) small angle of attack, (3) small disturbances to the mean flow components, and (4) time-invariant free-stream. A semi-analytical, geometricallyexact, unsteady potential flow model is developed for airfoils undergoing large amplitude maneuvers. The numerical implementation of the developed model is presented. The unsteady predictions of the developed model are validated against experimental and computational results. Then, the frequency response of the flow dynamics is determined at different angles of attack. While good agreement is shown with Theodorsen’s function at small angle of attack, considerable qualitative and quantitative discrepancies between the obtained frequency response at large angles of attack and Theodorsen’s function are observed.
Amir S Rezaei - One of the best experts on this subject based on the ideXlab platform.
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viscous extension of potential flow unsteady aerodynamics the lift frequency response problem
Journal of Fluid Mechanics, 2019Co-Authors: Haithem E Taha, Amir S RezaeiAbstract:The application of the Kutta condition to unsteady flows has been controversial over the years, with increased research activities over the 1970s and 1980s. This dissatisfaction with the Kutta condition has been recently rejuvenated with the increased interest in low-Reynolds-number, high-frequency bio-inspired flight. However, there is no convincing alternative to the Kutta condition, even though it is not mathematically derived. Realizing that the lift generation and vorticity production are essentially viscous processes, we provide a viscous extension of the classical theory of unsteady aerodynamics by relaxing the Kutta condition. We introduce a trailing-edge singularity term in the pressure distribution and determine its strength by using the triple-deck viscous boundary layer theory. Based on the extended theory, we develop (for the first time) a theoretical viscous (Reynolds-number-dependent) extension of the Theodorsen lift frequency response function. It is found that viscosity induces more phase lag to the Theodorsen function particularly at high frequencies and low Reynolds numbers. The obtained theoretical results are validated against numerical laminar simulations of Navier–Stokes equations over a sinusoidally pitching NACA 0012 at low Reynolds numbers and using Reynolds-averaged Navier–Stokes equations at relatively high Reynolds numbers. The physics behind the observed viscosity-induced lag is discussed in relation to wake viscous damping, circulation development and the Kutta condition. Also, the viscous contribution to the lift is shown to significantly decrease the virtual mass, particularly at high frequencies and Reynolds numbers.
