Unsteady Aerodynamic Method Research Articles

Improved Approximations to Wagner Function Using Sparse Identification of Nonlinear Dynamics

The Wagner function represents the response in lift on an airfoil that is subject to a sudden change in conditions. While it plays a fundamental role in the development and application of unsteady aerodynamic methods, explicit expressions for this function are difficult to obtain. This has led to numerous proposed approximations that facilitate convenient and rapid computations. While these approximations can be sufficient for many purposes, their behavior is often noticeably different from the true Wagner function, especially for long-time asymptotic behavior. In particular, while many approximations have small maximum absolute error across all times, the relative error of the asymptotic behavior can be substantial. Here, we propose an alternative approximation methodology that is accurate for all times, for a variety of accuracy measures. This methodology casts the Wagner function as the solution of a nonlinear scalar ordinary differential equation, which is identified using a variant of the sparse identification of nonlinear dynamics algorithm. We show that this approach yields accurate approximations using either first- or second-order differential equations. We additionally demonstrate that this method can be applied to model the lift response for a more realistic aerodynamic system, featuring a finite thickness airfoil and nonplanar wake.

Panel/full-span free-wake coupled method for unsteady aerodynamics of helicopter rotor blade

A full-span free-wake method is coupled with an unsteady panel method to accurately predict the unsteady aerodynamics of helicopter rotor blades in hover and forward flight. The unsteady potential-based panel method is used to consider aerodynamics of finite thickness multi-bladed rotors, and the full-span free-wake method is applied to simulating dynamics of rotor wake. These methods are tightly coupled through trailing-edge Kutta condition and by converting doublet-wake panels to full-span vortex filaments. A velocity-field integration technique is also adopted to overcome singularity problem during the interaction between the rotor wake and blades. Helicopter rotors including Caradonna–Tung, UH-60A, and AH-1G rotors, are simulated in hover and forward flight to validate the accuracy of this approach. The predicted aerodynamic loads of rotor blades agree well with available measured data and computational fluid dynamics (CFD) results, and the unsteady dynamics of rotor wake is also well simulated. Compared to CFD, the present method obtains accurate results more efficiently and is suitable to rotorcraft aeroelastic analysis.

Status of Unsteady Aerodynamic Prediction for Flutter of High-Performance Aircraft

Current production unsteady aerodynamics codes are based on panel methods. The capabilities present in these methods to model complex shapes represents one of the major advancements in this methodology. A second advancement is that the new codes are now formulated based on a unie ed analytical capability throughout the Mach range. Examples of these capabilities will be demonstrated. Finally, based on the discussion of requirements for e utter analyses of high-performance aircraft, a description of the needs for the future is presented.Itishoped thatthese recommendations willprovide guidance to those investigators working in the e eldof unsteady aerodynamics. II. Background A. Unsteady Aerodynamic Methods Strip theory aerodynamics originated in the early 1940s, 1;2 and this method was the primary aerodynamic tool for e utter analyses for many years.In 1966, Yates 3 proposed modifying CL® toaccount for e nite span effects with the result being referred to as modie ed strip theory. This aerodynamic method, coupled with the normal modeapproach,and the V‐g‐! solution technique formedthe basis for production e utter analyses in the late 1960s, and it was to this methodology that the author was e rst exposed to production e utter analyses.

Status of Unsteady Aerodynamic Prediction for Flutter of High-Performance Aircraft

Current production unsteady aerodynamics codes are based on panel methods. The capabilities present in these methods to model complex shapes represents one of the major advancements in this methodology. A second advancement is that the new codes are now formulated based on a unie ed analytical capability throughout the Mach range. Examples of these capabilities will be demonstrated. Finally, based on the discussion of requirements for e utter analyses of high-performance aircraft, a description of the needs for the future is presented.Itishoped thatthese recommendations willprovide guidance to those investigators working in the e eldof unsteady aerodynamics. II. Background A. Unsteady Aerodynamic Methods Strip theory aerodynamics originated in the early 1940s, 1;2 and this method was the primary aerodynamic tool for e utter analyses for many years.In 1966, Yates 3 proposed modifying CL® toaccount for e nite span effects with the result being referred to as modie ed strip theory. This aerodynamic method, coupled with the normal modeapproach,and the V‐g‐! solution technique formedthe basis for production e utter analyses in the late 1960s, and it was to this methodology that the author was e rst exposed to production e utter analyses.

Aeroelastic stability of cascades in turbomachinery

State-of-the-art prediction of the aeroelastic stability of cascades in axial-flow turbomachines is reviewed. The first main chapter of the article presents a comprehensive formulation of the two- and three-dimensional classical (unstalled) flutter problem of tuned and mistuned rotor blade rows and bladed disc assemblies. Within the framework of linearized analysis, a complete and generalized theory in modal form is outlined, comprising the various formulations of the cascade flutter problem distributed in fragments throughout the literature. Brief outlines are also made of recent advances in unsteady aero-dynamic methods for turbomachinery aeroelastic applications. The second main chapter contains a parametric study of the classical flutter stability characteristics of compressor and turbine cascades in subsonic and supersonic flow. Stability boundaries and dominant trends in flutter behaviour are outlined, and the significant effects of blade mistuning on the aeroelastic stability of turbomachine bladings are highlighted.

Review of unsteady aerodynamic methods for turbomachinery aeroelastic and aeroacoustic applications

Introduction T HE unsteady aerodynamic analyses intended for turbomachinery aeroelastic and aeroacoustic predictions must be applicable over wide ranges of blade-row geometries and operating conditions and unsteady excitation modes and frequencies. Also, because of the large number of controlling parameters involved, there is a stringent requirement for computational efficiency. To date these requirements have been met only to a limited extent. As a result, aeroelastic and aeroacoustic design predictions are, for the most part, still based on the classical linearized unsteady aerodynamic analyses developed in the early 1970s. During the past decade, significant advances in unsteady aerodynamic prediction capabilities have been achieved. In particular, researchers have developed efficient linearized analyses that account for the effects of important design features, such as real blade geometry, mean blade loading, and operation at transonic Mach numbers, on the unsteady aerodynamic response of the blading to imposed structural and external aerodynamic excitations. The improvements in physical modeling that such linearizations allow are motivating their current implementation into aeroelastic and aeroacoustic design prediction systems. Also, considerable progress has been made on developing time-accurate Euler and Navier-Stokes simulations of nonlinear unsteady flows through blade rows. Although not yet suitable for design use, such analyses offer opportunities for an improved understanding of the unsteady aerodynamic processes associated with blade vibration and noise generation. These recent advances in the theoretical and computational modeling of turbomachinery unsteady flows are reviewed in the present survey.