Nonlinear Modeling of Unsteady Aerodynamics at High Angle of Attack

Allwine, Daniel A.; Strahler, Jeremy A.; Lawrence, Douglas A.; Jenkins, Jerry; Myatt, James

doi:10.2514/6.2004-5275

Cited by 9 publications

(6 citation statements)

References 4 publications

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“…In this study the precise value of the hinge moment of the microflap was not considered. In order to account for flow nonlinearities encountered at high Mach number, large angle of attack and flap deflection angle, the RFA model is enhanced by using a technique referred to as model scheduling, 36 wherein different sets of RFA coefficients are generated at appropriate combinations of the Mach number, angle of attack, and flap deflection angle. This approach resembles the gain scheduling approach used in control system design, commonly employed for control of nonlinear systems.…”

Section: Iiib Modified Rfa Model For Microflapmentioning

confidence: 99%

Computational Study of Microflaps with Application to Vibration Reduction in Helicopter Rotors

2011

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A comprehensive study on unsteady effects of oscillating Gurney flaps, or microflaps, has been conducted. Two-dimensional unsteady airloads, lift, moment and drag, due to an oscillating microflap were computed using a compressible Reynolds-Averaged Navier-Stokes (RANS) flow solver. The CFD results were generated with an overset mesh approach that captures oscillatory microflap motion. Three microflap configurations were examined so as to determine the type most suitable in terms of actuation efficiency and practical implementation. Furthermore, a reduced order model (ROM) for the unsteady microflaps was developed based on CFD simulations, using the Rational Function Approximation (RFA) approach. The resulting RFA model is a state-space, time-domain aerodynamic model that accounts for unsteadiness, compressibility and time-varying freestream effect, suitable for use with comprehensive rotorcraft simulations. The agreement between the ROM and direct CFD calculations was found to be excellent even in presence of strong nonlinear flow effects. The approximate model is suitable for incorporation in a comprehensive code, from which the potential of microflaps for active control of vibrations in rotor can be determined. Preliminary studies with open loop control showed that the microflap produces substantial vibration reduction (52% reduction in vertical shear) on a hingeless rotor configuration resembling the MBB BO-105, confirming the control authority of this novel technique.

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Section: Iiib Modified Rfa Model For Microflapmentioning

confidence: 99%

Computational Study of Microflaps with Application to Vibration Reduction in Helicopter Rotors

2011

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“…This approach resembles the gain scheduling approach used in control system design, commonly employed for control of nonlinear systems (Refs. 34,35). Specifically, the RFA model has been modified by allowing the coefficient matrices, i.e., R, E, C 0 , C 1 , .…”

Section: The Cfd-based Rommentioning

confidence: 94%

“…To account for flow nonlinearities encountered at high Mach number, large angle of attack, and flap deflection angle, the RFA model is enhanced by using a technique referred to as model scheduling (Ref. 34), wherein different sets of RFA coefficients are generated at appropriate combinations of the Mach number, angle of attack, and flap deflection angle. This approach resembles the gain scheduling approach used in control system design, commonly employed for control of nonlinear systems (Refs.…”

Section: The Cfd-based Rommentioning

confidence: 99%

Unsteady Aerodynamics of an Airfoil/Flap Combination on a Helicopter Rotor Using Computational Fluid Dynamics and Approximate Methods

Liu

Padthe²,

Friedmann³

et al. 2011

J Am Helicopter Soc

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The ability to model accurately and efficiently unsteady aerodynamic effects for actively controlled trailing-edge flaps (ACFs) is crucial for practical application of such systems for vibration and noise reduction as well as performance enhancement. Two-dimensional unsteady airloads due to oscillating flap motion are calculated and compared using various computational fluid dynamics (CFD) codes and a CFD-based reduced-order model (ROM). This ROM is based on the rational function approximations approach, which yields a state-space, time-domain aerodynamic model suitable for incorporation into comprehensive rotorcraft simulation codes. The accuracy of this model is demonstrated across a practical range of unsteady flow conditions encountered by active flaps. TwoReynolds-averaged Navier-Stokes solvers (CFD++ and OVERFLOW) are employed in conjunction with various turbulence models, including large eddy simulation based models, so as to examine code independence. Flow physics associated with three-dimensional effects, flap hinge gap, as well as compressibility effects are also examined. Nomenclature

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“…38. Note that in order to account for flow nonlinearities encountered at high Mach numbers, large angles of attack and flap deflections, the RFA model has been enhanced by using a technique referred to as model scheduling, 43 wherein different sets of RFA coefficients are generated at appropriate combinations of the Mach number, angle of attack, and flap deflection angle. Specifically, the RFA model is modified by allowing the coefficient matrices, i.e., R, E, C 0 , C 1 ,..., to vary with M , α, and δ f , as indicated in Eq.…”

Section: Ia Concise Description Of the Rfa Approachmentioning

confidence: 99%

Numerical Evaluation of Microflaps for On Blade Control of Noise and Vibration

Padthe¹,

Liu

Friedmann³

2011

52nd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference

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Active Gurney flaps, or microflaps, are studied to determine their effectiveness in reducing noise and vibration in rotorcraft, as well as improving rotor performance. The effectiveness of the microflap is examined using a comprehensive rotorcraft simulation code. The aerodynamic properties of the microflap is modeled using a nonlinear CFD based reduced order aerodynamic model that takes into account unsteadiness, compressibility and time varying freestream effects. Active control studies employing microflaps are conducted on a hingeless rotor configuration resembling the MBB BO-105, and various spanwise configurations of the microflaps, including a single, a dual, and a segmented five-microflap configuration are evaluated. Results indicate that the microflap is capable of substantial reductions in blade vortex interaction (BVI) noise ranging from 3-6 dB. Vibration reduction ranging from 70-90% is also demonstrated. The interaction of vibration and noise reduction is also examined, and it was found reduction in one objective is accompanied by an increase in the other, a trend also observed when using other active approaches. Finally, the microflap is considered for combined vibration reduction and performance enhancement at a high speed cruise flight condition. The results clearly indicate that the microflaps are very effective for both noise and vibration reduction in helicopters, and they may also have potential for rotor performance enhancement.

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Nonlinear Modeling of Unsteady Aerodynamics at High Angle of Attack

Cited by 9 publications

References 4 publications

Computational Study of Microflaps with Application to Vibration Reduction in Helicopter Rotors

Computational Study of Microflaps with Application to Vibration Reduction in Helicopter Rotors

Unsteady Aerodynamics of an Airfoil/Flap Combination on a Helicopter Rotor Using Computational Fluid Dynamics and Approximate Methods

Numerical Evaluation of Microflaps for On Blade Control of Noise and Vibration

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