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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Impact of Aerodynamic Modeling Assumptions on Flutter Speeds of Vertical-Axis Wind Turbines

Theodorsen’s unsteady aerodynamic theory has been used extensively in flutter speed prediction of wind turbine blades. In this study, three key assumptions of Theodorsen theory (thin airfoil, flat wake, and small angle of attack) have been revisited, and all three assumptions have been addressed in combination to obtain new lift and moment equations that are subsequently applied in flutter calculations for full-scale vertical-axis wind turbine (VAWT) rotors, not just of an individual airfoil or blade. Furthermore, edgewise aerodynamics terms are added to the lift and moment equations to include their effects on flutter speeds. The newly obtained equations were implemented in the OWENS (Offshore Wind ENergy Simulation) toolkit, which is an FEM (Finite Element Method)-based toolkit for aeroelastic analysis of VAWTs. The effect of modifying each of these assumptions has been studied for the flutter RPM prediction of three primary modes of flutter of VAWTs: propeller, butterfly, and tower modes. For the land-based case, the most change was observed for the flutter RPM of the propeller mode, with a maximum increase of 2.62% for the two-bladed UTD 5 MW VAWT case. For the floating offshore case, the primary flutter modes (tower and platform pitch) were not significantly affected.

Engineering↗
NASA NTRS1983

Nonlinear aerodynamic modeling of flap oscillations in transonic flow - A numerical validation

The regime of validity of a nonlinear aerodynamic force and moment formulation, based on concepts from nonlinear functional analysis and applicable to a transonic airfoil with a deflecting flap, is investigated. A time-dependent finite difference technique is used to evaluate the aerodynamic data of the formulation in terms of specified, characteristic motions. Flap-motion histories are generated from the flap inertial equations of motion, with aerodynamic reactions specified by the moment formulation. The motion histories depicting the cases of decaying and growing flap oscillations are compared with histories generated through simultaneous, coupled solution of the fluid-dynamic equations and flap inertial equations of motion. The range of applicability of the formulation is discussed.

Chyu, W. J.↗
NASA NTRS1990

Identification of aerodynamic models for maneuvering aircraft

Due to the requirement of increased performance and maneuverability, the flight envelope of a modern fighter is frequently extended to the high angle-of-attack regime. Vehicles maneuvering in this regime are subjected to nonlinear aerodynamic loads. The nonlinearities are due mainly to three-dimensional separated flow and concentrated vortex flow that occur at large angles of attack. Accurate prediction of these nonlinear airloads is of great importance in the analysis of a vehicle's flight motion and in the design of its flight control system. A satisfactory evaluation of the performance envelope of the aircraft may require a large number of coupled computations, one for each change in initial conditions. To avoid the disadvantage of solving the coupled flow-field equations and aircraft's motion equations, an alternate approach is to use a mathematical modeling to describe the steady and unsteady aerodynamics for the aircraft equations of motion. Aerodynamic forces and moments acting on a rapidly maneuvering aircraft are, in general, nonlinear functions of motion variables, their time rate of change, and the history of maneuvering. A numerical method was developed to analyze the nonlinear and time-dependent aerodynamic response to establish the generalized indicial function in terms of motion variables and their time rates of change.

Chin, Suei↗
NASA NTRS2005

An Investigation of the Impact of Aerodynamic Model Fidelity on Close-In Combat Effectiveness Prediction in Piloted Simulation

Several aircraft airframe modeling approaches are currently being used in the DoD community for acquisition, threat evaluation, training, and other purposes. To date there has been no clear empirical study of the impact of airframe simulation fidelity on piloted real-time aircraft simulation study results, or when use of a particular level of fidelity is indicated. This paper documents a series of piloted simulation studies using three different levels of airframe model fidelity. This study was conducted using the NASA Langley Differential Maneuvering Simulator. Evaluations were conducted with three pilots for scenarios requiring extensive maneuvering of the airplanes during air combat. In many cases, a low-fidelity modified point-mass model may be sufficient to evaluate the combat effectiveness of the aircraft. However, in cases where high angle-of-attack flying qualities and aerodynamic performance are a factor or when precision tracking ability of the aircraft must be represented, use of high-fidelity models is indicated.

Persing, T. Ray↗
NASA NTRSDate not supplied

Unsteady Aerodynamic Modeling of Atmospheric Entry Vehicles in Subsonic and Incompressible flow: A Frequency Response Approach

The determination of time-varying lift force and pitch moment generated by a purely pitching Earth-entry capsule is investigated. Experiments were conducted in the 12-foot Low-Speed Tunnel at NASA Langley Research Center, testing a range of oscillation frequencies at a zero-mean angle of attack with a pitching amplitude of 10 degrees. In light of these measurements, a closed-form set of analytically derived equations for lift and moment was used to develop a semi-empirical formulation, incorporating empirically determined values from the experimental runs. The equations are grounded in potential flow theory, Theodorsen's classical theory of unsteady aerodynamics, and the Joukowski theorem of conformal mapping. The unsteady aerodynamics generated by the oscillating body are then modeled by constructing frequency response functions, with quasi-steady forces and moments serving as inputs and unsteady forces and moments as outputs. The experimentally determined gain and phase variations characterize the unsteady nature of the flow and the system's response and flow time-lag to input flow parameters for a blunt-body entry vehicle. The final semi-empirical model is validated with a set of parameters beyond the initial test matrix.

Entry Systems Modeling↗
NASA NTRSDate not supplied

Unsteady Aerodynamic Modeling of Atmospheric Entry Vehicles in Subsonic and Incompressible Flow: A Frequency Response Approach

The determination of time-varying lift force and pitch moment generated by a purely pitching Earth-entry capsule is investigated. Experiments were conducted in the 12-foot Low-Speed Tunnel at NASA Langley Research Center, testing a range of oscillation frequencies at a zero-mean angle of attack with a pitching amplitude of 10 degrees. In light of these measurements, a closed-form set of analytically derived equations for lift and moment was used to develop a semi-empirical formulation, incorporating empirically determined values from the experimental runs. The equations are grounded in potential flow theory, Theodorsen's classical theory of unsteady aerodynamics, and the Joukowski theorem of conformal mapping. The unsteady aerodynamics generated by the oscillating body are then modeled by constructing frequency response functions, with quasi-steady forces and moments serving as inputs and unsteady forces and moments as outputs. The experimentally determined gain and phase variations characterize the unsteady nature of the flow and the system's response and flow time-lag to input flow parameters for a blunt-body entry vehicle. The final semi-empirical model is validated with a set of parameters beyond the initial test matrix.

Entry Systems Modeling↗
NASA NTRS1986

Modeling aerodynamic discontinuities and the onset of chaos in flight dynamical systems

Various representations of the aerodynamic contribution to the aircraft's equation of motion are shown to be compatible within the common assumption of their Frechet differentiability. Three forms of invalidating Frechet differentiality are identified, and the mathematical model is amended to accommodate their occurrence. Some of the ways in which chaotic behavior may emerge are discussed, first at the level of the aerodynamic contribution to the equation of motion, and then at the level of the equations of motion themselves.

Tobak, M.↗
NASA NTRS1987

Modeling aerodynamic discontinuities and onset of chaos in flight dynamical systems

Various representations of the aerodynamic contribution to the aircraft's equation of motion are shown to be compatible within the common assumption of their Frechet differentiability. Three forms of invalidating Frechet differentiability are identified, and the mathematical model is amended to accommodate their occurrence. Some of the ways in which chaotic behavior may emerge are discussed, first at the level of the aerodynamic contribution to the equations of motion, and then at the level of the equations of motion themselves.

Tobak, M.↗
NASA NTRS1981

Model aerodynamic test results for two variable cycle engine coannular exhaust systems at simulated takeoff and cruise conditions. Comprehensive data report. Volume 3: Graphical data book 1

A graphical presentation of the aerodynamic data acquired during coannular nozzle performance wind tunnel tests is given. The graphical data consist of plots of nozzle gross thrust coefficient, fan nozzle discharge coefficient, and primary nozzle discharge coefficient. Normalized model component static pressure distributions are presented as a function of primary total pressure, fan total pressure, and ambient static pressure for selected operating conditions. In addition, the supersonic cruise configuration data include plots of nozzle efficiency and secondary-to-fan total pressure pumping characteristics. Supersonic and subsonic cruise data are given.

Nelson, D. P.↗
NASA NTRS2016

Longitudinal Aerodynamic Modeling of the Adaptive Compliant Trailing Edge Flaps on a GIII Airplane and Comparisons to Flight Data

A pair of compliant trailing edge flaps was flown on a modified GIII airplane. Prior to flight test, multiple analysis tools of various levels of complexity were used to predict the aerodynamic effects of the flaps. Vortex lattice, full potential flow, and full Navier-Stokes aerodynamic analysis software programs were used for prediction, in addition to another program that used empirical data. After the flight-test series, lift and pitching moment coefficient increments due to the flaps were estimated from flight data and compared to the results of the predictive tools. The predicted lift increments matched flight data well for all predictive tools for small flap deflections. All tools over-predicted lift increments for large flap deflections. The potential flow and Navier-Stokes programs predicted pitching moment coefficient increments better than the other tools.

aerodynamics↗
NASA NTRS2016

Aerodynamic Modeling of Transonic Aircraft Using Vortex Lattice Coupled with Transonic Small Disturbance for Conceptual Design

The need to rapidly scan large design spaces during conceptual design calls for computationally inexpensive tools such as the vortex lattice method (VLM). Although some VLM tools, such as Vorview have been extended to model fully-supersonic flow, VLM solutions are typically limited to inviscid, subcritical flow regimes. Many transport aircraft operate at transonic speeds, which limits the applicability of VLM for such applications. This paper presents a novel approach to correct three-dimensional VLM through coupling of two-dimensional transonic small disturbance (TSD) solutions along the span of an aircraft wing in order to accurately predict transonic aerodynamic loading and wave drag for transport aircraft. The approach is extended to predict flow separation and capture the attenuation of aerodynamic forces due to boundary layer viscosity by coupling the TSD solver with an integral boundary layer (IBL) model. The modeling framework is applied to the NASA General Transport Model (GTM) integrated with a novel control surface known as the Variable Camber Continuous Trailing Edge Flap (VCCTEF).

Transonic Small Disturbance↗
NASA NTRSDate not supplied

Reduced-Order Aerodynamic Modeling Based on CFD Frequency Responses from Multisine Inputs

A system identification analysis was performed to determine reduced-order models of a computational fluid dynamics (CFD) solver for linear aeroelastic analysis and control design. The application was to the FUN3D code and the flexible half-span wind tunnel test article, in transonic flow conditions, used in the NASA-Boeing collaboration called the Integrated Adaptive Wing Technology Maturation (IAWTM) project. Multiple inputs (structural mode displacements and control surface deflections) were simultaneously excited with orthogonal phase-optimized multisines and multiple outputs (generalized aerodynamic forces) were recorded, from which the matrix of frequency responses were computed using a single CFD run. A state-space model was then fit to the frequency response data using a maximum-likelihood estimator.

System identification↗
NASA NTRSDate not supplied

Reduced-Order Aerodynamic Modeling Based on CFD Frequency Responses from Multisine Inputs

A system identification analysis was performed to determine a reduced-order model (ROM) of a computational fluid dynamics (CFD) solver in support of linear aeroservoelastic model development and feedback control design. The approach was applied to the FUN3D code for the half-span wind tunnel test article used in the NASA-Boeing collaboration called the Integrated Adaptive Wing Technology Maturation (IAWTM) project. In a transonic flow condition, multiple inputs (11 structural mode displacements and 3 control surface deflections) were simultaneously excited with orthogonal phase-optimized multisines while multiple outputs (the corresponding 14 generalized aerodynamic forces) were recorded. From these recorded times series, the matrix of frequency responses was computed and subsequently fit using rational function approximations (RFAs). It was found that the entire (14 x 14) matrix of frequency responses could be determined from a single CFD run and that results generally followed trends predicted using other methods. Differences were attributed to the modeling fidelity and nonlinearities from structural mode and control surface interactions at higher reduced frequencies. More accurate fits of the RFAs to the frequency response data were obtained by making two CFD runs, one with only structural mode excitations and one with only control surface excitations, which reduced the degree of nonlinearity in the modeling data.

Aeroservoelasticity↗
NASA NTRS2018

ASPIRE Aerodynamic Models and Flight Performance

NASA's Advanced Supersonic Parachute Inflation Research Experiments (ASPIRE) project was established to test full-scale supersonic parachutes at Mars-relevant conditions, as a risk-reduction activity for NASA's upcomingMars2020 mission. Deployment and inflation of Disk-Gap-Band (DGB) parachutes were examined at Mach number and dynamic pressure conditions relevant to Mars2020, using a sounding rocket platform. The flight tests examined two parachutes: a build-to-print version of the parachute used by the Mars Science Laboratory and a strengthened version of this parachute that has the same geometry but differs in materials and construction. The first flight test (SR01) of the built-to-print parachute took place on October 4, 2017, followed by the first test of the strengthened parachute during flight SR02 on March 31, 2018. A second test of the strengthened parachute with a higher target load, SR03, took place on September 7, 2018. Over the sequence of the three tests, the parachute was exposed to increasing aerodynamic loads with the peak load during SR03 being significantly larger than is expected during a Martian descent. All three tests were successful: the parachute deployment and inflation occurred at the intended conditions, the measurement systems performed as designed and provided the data, and the parachutes survived the aerodynamic loads they were exposed to. The flight tests yielded valuable data on parachute forces and high-speed imagery of the deployment and inflation process.

Parachute↗