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Novel L-Shaped Gurney Flap for Rotorcraft Vibration Reduction
No full textThis work concerns an assessment of the rotor blade vibration reduction capabilities of a novel L-shaped trailing edge Gurney Flap. The primary effect of this L-tab is represented by a modification of the reference airfoil mean line shape, both in terms of camber and chord length, this latter being related to the two counter rotating vortical structures developed past the tab vertical prong. Previously validated computational fluid dynamics results are exploited to develop a physically based thin-line reduced order model, which successfully reproduces the mean line modifications induced by the L-tab, in addition to accurately capture the steady aerodynamic forces and the first harmonic of the unsteady loads generated by fixed configurations of the airfoil L-tab system and by oscillating motions of the movable device. A thin-line linear model is also developed for a blade section equipped with a classical trailing edge flap. Comparisons of the aerodynamic loads generated by these two movable devices for equal input oscillation laws, allow to estimate the ranges of reduced frequency where the L-tab is expected to perform better with respect to the trailing edge flap and vice-versa. These two reduced order models are then exploited to build up two separate three degrees of freedom linear aerostructural models for a blade equipped with a partial span L-tab or a trailing edge flap. A higher harmonic control algorithm is then applied and compared between the two devices to reduce separately the JV/rev harmonics of the blade root rotating frame vertical force, flapping and feathering moments. A significant reduction of the vibratory loads is obtained. Moreover, the attainment of similar results with a well known trailing edge device, such the classical flap taken under consideration, is a further confirmation of the potential feasibility of this novel L-tab as an effective alternative mean for vibration reduction on rotor blades
Physically-based Reduced Order Model for Unsteady Aerodynamic Loads of a L-shaped Gurney Flap
No full textA physically-based linear reduced order model is developed for a NACA 0012 section oscillating in pitch and plunge, equipped with a L-tab Gurney flap in unsteady motion. The model allows for a quick and accurate computation of the first harmonic component of unsteady loads on a three degrees of freedom helicopter blade section model. Moreover, a physically-based identification procedure is carried out for fixed configurations of the airfoil and the L-tab, to complete the reduced order model with the mean steady contribution to aerodynamic loads. Numerical computations carried out with a finite volumes solver for compressible Reynolds Averaged Navier-Stokes equations, are performed and used as reference for the derivation of the reduced order model. Structured multi-block overlapped grids in relative motion are used as computational domain. The reliability of numerical simulations is verified by means of convergence analysis and comparisons with empirical and experimental data. The achieved reduced order model is an equivalent three segments piecewise mean line geometry, which correctly reproduces the effects of the airfoil mean line, including the L-tab, as well as contributions to loads of vortical structures behind the movable device. All these effects are well captured both on the mean value and the first harmonic of aerodynamic loads over the blade section. The strong connection of the parameters of the reduced order model with physical quantities is highlighted, as well as its predictive capability for arbitrary parameters of the imposed motion laws
A comparative assessment of vibration control capabilities of a L-shaped Gurney flap
Get PDFThis work presents the capabilities of a novel L-shaped trailing-edge Gurney flap as a device for vibration reduction. The primary effect of this L-tab is represented by a modification of the reference aerofoil mean line shape through by two counter-rotating vortical structures created at the trailing edge. The comparison of the aerodynamic loads generated by the novel L-tab Gurney flap and a classical trailing-edge flap allows to estimate the ranges of reduced frequency where the L-tab is expected to perform better than a trailing edge flap and vice versa. Linear aerostructural models for a typical section representative of a helicopter blade equipped with a partial-span L-tab or a trailing-edge flap are built, and a higher harmonic control algorithm is applied. Performance are compared between the two devices to reduce separately the N/rev harmonics of the blade root rotating frame vertical force, flapping and feathering moments. The attainment of similar results with classical trailing-edge device is a further confirmation of the potential feasibility of this novel L-tab as an effective alternative means for vibration reduction on rotor blades
Linear Reduced-Order Model for Unsteady Aerodynamics of an L-Shaped Gurney Flap
Full text linkAnalysis of ballistic capture orbits in Sun–planet systems is conducted in this paper. This mechanism utilizes purely gravitational forces, and may occur in non-Keplerian regimes. Ballistic capture orbits are generated by proper manipulation of sets of initial conditions that satisfy a simple definition of stability. Six Sun–planet systems are considered, including the inner planets, Jupiter, and Saturn. The role of planets orbital eccentricity, their true anomaly, and mass ratios is investigated. Moreover, the influence of the post-capture orbit in terms of inclination and orientation is also assessed. Analyses are performed from qualitative and quantitative perspective. The quality of capture orbits is measured by means of the stability index, whereas the capture ratio gives information on their statistical occurrence. Some underlying principles on the selection of the dynamical model, the initial true anomaly, and inclination are obtained. These provide a reference for practical cases
Active Control on Helicopter Blades with a L-Shaped Gurney Flap
No full textThis work concerns an assessment of the rotor blade vibration reduction capabilities of a novel L-shaped trailing edge Gurney Flap. Previous numerical and experimental works highlighted how such device is potentially suitable for performance enhancement and stall alleviation on rotorcraft. Moreover, these studies showed that the primary effect of this L-tab is represented by a modification of the reference airfoil mean line shape, both in terms of camber and chord length, this latter being related to the two counter rotating vertical structures developed past the tab vertical prong. Previously validated computational uid dynamics results are exploited to develop a physically based thin-line reduced order model, which successfully reproduces the mean line modifications induced by the L-tab, in addition to accurately capture the steady aerodynamic forces and the first harmonic of the unsteady loads generated by fixed configurations of the airfoil L-tab system and by oscillating motionsof the movable device, respectively. A similar thin-line linear model is also developed for a blade section equipped with a classical trailing edge ap. Comparisons of the aerodynamic loads generated by these two movable devices for equal input oscillating laws allow to estimate the ranges of reduced frequency where the L-tab is expected to perform better with respect to the trailing edge ap and vice-versa. These two reduced order models are then exploited to build up two separate three degrees of freedom linear aerostructural models for a blade equipped with a partial span L-tab or trailing edge ap. A higher harmonic control algorithm is then applied and compared between the two devices to reduce separately the 2/rev, 3/rev, 4/rev and 5/rev harmonics of the blade root rotating frame vertical force, apping and feathering moments. A significant reduction of the vibratory loads is obtained. Moreover, the attainment of similar results with a well known trailing edge device, such the classical ap taken under consideration, is a further confirmation of the potential feasibility of this novel L-tab as an effective alternative mean for vibration reduction on rotor blades
Three-Dimensional Simulation of a Complete Vertical Axis Wind Turbine Using Overlapping Grids
Full text linkThree-dimensional simulations of the aerodynamic field around a three-blade straight-axis Vertical Axis Wind Turbine (VAWT) are presented for two values of the Tip Speed Ratio λ (TSR), namely λ=1.52 and λ=2.5. Numerical simulations were carried out using the over-set grid solver ROSITA (ROtorcraft Software ITAly). The Reynolds-Averaged Navier-Stokes equations are completed by the Spalart-Allmaras turbulence model. A strong interaction between the blade and the blade wakes is evidenced. Dynamic stall is observed in the case λ=2.5. The computed flow-field presents diverse three-dimensional effects, including the interaction between the blades and the tip vortices and the aerodynamic disturbances from the turbine shaft and the support arms. Three-dimensional effects are more relevant for λ=2.5. The comparison to experimental data confirms the general features of the flow
Influence of airfoil thickness on unsteady aerodynamic loads on pitching airfoils
Get PDFThe 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 Küssner 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 (Re=1000000) flows
Numerical Investigation of an L-Shaped Deployable Gurney Tab for Rotorcraft Vibration Control
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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