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Title: Turbulence modeling and simulation and related effects on helicopter response with wake dynamics using finite elements and parallelism.
Creator: Dang, Ying Yi., Florida Atlantic University, Gaonkar, Gopal H.
Abstract/Description: Future helicopters will require all-weather capability for stabilized flight through severe atmospheric turbulence. This requirement has brought into focus the effect of turbulence on handling qualities. Accordingly, there is renewed interest in modeling and simulating turbulence and predicting turbulence-induced rotor oscillations. This thesis addresses three fundamental aspects of the problem: (1) modeling and simulation of turbulence including cross-correlation; (2) three-dimensional...
Show moreFuture helicopters will require all-weather capability for stabilized flight through severe atmospheric turbulence. This requirement has brought into focus the effect of turbulence on handling qualities. Accordingly, there is renewed interest in modeling and simulating turbulence and predicting turbulence-induced rotor oscillations. This thesis addresses three fundamental aspects of the problem: (1) modeling and simulation of turbulence including cross-correlation; (2) three-dimensional dynamic-wake effects on rotor response to turbulence and (3) prediction of turbulence and response statistics. The analysis is based on the theory of isotropic and homogeneous turbulence and Taylor's frozen-field approximation. Quasisteady airfoil aerodynamics and a three-dimensional wake are used. Both the isolated blades and isolated rotors are treated. The parallelization is carried out on a massively parallel MasPar SIMD computer. Major conclusions include: (i) The effects of cross-correlation are negligible when two stations lie on the same blade and appreciable when two stations lie on different blades. (ii) In modeling the three-dimensional wake, 3 harmonics are required and dynamic wake has dominant influence on response statistics. (iii) With increasing comprehensiveness of helicopter-turbulence modeling, the sequential execution times increase dramatically; by comparison, the parallel execution times are far lower and, more significantly, remain nearly constant.
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Date Issued: 1995
PURL: http://purl.flvc.org/fcla/dt/15117
Subject Headings: Helicopters, Turbulence, Rotors (Helicopters), Boundary layer noise
Format: Document (PDF)

Title: Developing interpretive turbulence models from a database with applications to wind farms and shipboard operations.
Creator: Schau, Kyle A., Gaonkar, Gopal H., College of Engineering and Computer Science, Department of Ocean and Mechanical Engineering
Abstract/Description: This thesis presents a complete method of modeling the autospectra of turbulence in closed form via an expansion series using the von Kármán model as a basis function. It is capable of modeling turbulence in all three directions of fluid flow: longitudinal, lateral, and vertical, separately, thus eliminating the assumption of homogeneous, isotropic flow. A thorough investigation into the expansion series is presented, with the strengths and weaknesses highlighted. Furthermore, numerical...
Show moreThis thesis presents a complete method of modeling the autospectra of turbulence in closed form via an expansion series using the von Kármán model as a basis function. It is capable of modeling turbulence in all three directions of fluid flow: longitudinal, lateral, and vertical, separately, thus eliminating the assumption of homogeneous, isotropic flow. A thorough investigation into the expansion series is presented, with the strengths and weaknesses highlighted. Furthermore, numerical aspects and theoretical derivations are provided. This method is then tested against three highly complex flow fields: wake turbulence inside wind farms, helicopter downwash, and helicopter downwash coupled with turbulence shed from a ship superstructure. These applications demonstrate that this method is remarkably robust, that the developed autospectral models are virtually tailored to the design of white noise driven shaping filters, and that these models in closed form facilitate a greater understanding of complex flow fields in wind engineering.
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Date Issued: 2013
PURL: http://purl.flvc.org/fau/fd/FA0004058
Subject Headings: Fluid mechanics, Renewable energy sources, Von Kármán, Theodore -- 1881-1963, Wind energy conservation systems, Wind power, Wind turbines -- Aerodynamics
Format: Document (PDF)

Title: Effects of rotating frame turbulence and dynamic stall on gust response of helicopter blades.
Creator: Madhavan, R., Florida Atlantic University, Gaonkar, Gopal H., College of Engineering and Computer Science, Department of Ocean and Mechanical Engineering
Abstract/Description: Rotating frame turbulence, or RFT, refers to the actual turbulence experienced by the helicopter blades and requires noneulerian description and rotational sampling of measurements. In the stationary case of axial flight, as investigated earlier, its spectra has peaks centered at integer multiples of rotational speed P, as in wind turbines. In forward flight, as investigated here, its instantaneous or frequency-time spectra has split peaks centered at P/2, P, 3P/2, 2P etc. Though...
Show moreRotating frame turbulence, or RFT, refers to the actual turbulence experienced by the helicopter blades and requires noneulerian description and rotational sampling of measurements. In the stationary case of axial flight, as investigated earlier, its spectra has peaks centered at integer multiples of rotational speed P, as in wind turbines. In forward flight, as investigated here, its instantaneous or frequency-time spectra has split peaks centered at P/2, P, 3P/2, 2P etc. Though nonstationary, it is wide sense cyclostationary in that its autocorrelation function R(t1,t2) = R(t1 + 2m pi, t2 + 2n pi) for integers m = n only. The major RFT characteristics--spectral peaks, the consequent transfer of energy essentially from the low-frequency region (<1P) to the high-frequency region (>1P) and cyclostationarity--cannot be predicted by conventional space-fixed description. However, these characteristics are simultaneously predicted by the instantaneous spectra, and for their qualitative and parametric investigation, a closed-form solution of an instantaneous spectrum is presented for a space-fixed turbulence model. The RFT effects on the blade response statistics of rms values and average threshold crossing rates are presented as well. The blade model includes flap bending degrees of freedom and dynamic stall effects. The blade response statistics demonstrate that RFT effects are appreciable for low-advance ratio and low-altitude flight conditions and that dynamic stall increases gust sensitivity.
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Date Issued: 1990
PURL: http://purl.flvc.org/fcla/dt/14613
Subject Headings: Helicopters--Aerodynamics
Format: Document (PDF)

Title: Dynamic stall effects on hingeless rotor stability with experimental correlation.
Creator: Barwey, Dinesh., Florida Atlantic University, Gaonkar, Gopal H., College of Engineering and Computer Science, Department of Ocean and Mechanical Engineering
Abstract/Description: The effects of dynamic stall, lift, drag and pitching moment on the aeroelastic stability of hingeless rotors are predicted. The emphasis is on correlating the predictions with the measured lag-damping levels of a three-bladed model rotor operated in an untrimmed mode. The correlation covers a wide range of test conditions for several values of rotor speed, collective pitch angle, shaft-tilt angle and advance ratio. The database includes cases that vary from near zero-thrust conditions in...
Show moreThe effects of dynamic stall, lift, drag and pitching moment on the aeroelastic stability of hingeless rotors are predicted. The emphasis is on correlating the predictions with the measured lag-damping levels of a three-bladed model rotor operated in an untrimmed mode. The correlation covers a wide range of test conditions for several values of rotor speed, collective pitch angle, shaft-tilt angle and advance ratio. The database includes cases that vary from near zero-thrust conditions in hover to highly stalled forward-flight conditions with dimensionless speed or advance ratios as high as 0.55 and shaft angles as high as 20$\sp\circ$. The aerodynamic representation is based on the ONERA dynamic stall models comprising virtually independent unified lift, drag and pitching-moment models. The nonlinear equations of blade motion and stall dynamics are perturbed about a periodic forced response, and the damping is evaluated by Floquet analysis. The extensive correlation study, based on a rigid-blade flap-lag analysis, demonstrates the viability of Floquet analysis in predicting lag-mode damping under dynamically stalled forward-flight conditions. It also demonstrates the limitations of the linear and quasisteady stall aerodynamic theories. In comparison to these theories, the theory with dynamic stall lift and quasisteady stall drag qualitatively improves the correlation and is viable over the entire range of the database. Addition of dynamic stall drag provides further quantitative improvement. Also presented is a comparative study of two dynamic stall drag models: circulation-like drag variables incorporating unsteady air-velocity variations and conventional drag-coefficient variables. While the formulation with drag coefficients exhibits computational sensitivity to convergence with respect to blade discretization for some isolated cases, the formulation with circulation-like drag variables removes this sensitivity and is computationally robust. The investigation concludes with an elastic blade analysis that includes blade flexibility in lag bending, flap bending and torsion as well as root-flexure elasticity. This analysis shows increasing sensitivity to structural refinements in blade and root-flexure modeling, and this sensitivity increases with increasing pitch setting. Correlation and parametric studies show that the root-flexure elasticity introduces significant bending-torsion couplings that have considerable impact on the stability of a rotor for which the root-flexure is soft, and the blade is stiffer in comparison with the root. This research is expected to serve as a reference comparison with other correlations based on different approaches of modeling dynamic stall and the elasticity of hingeless rotor blades and root-flexure.
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Date Issued: 1992
PURL: http://purl.flvc.org/fcla/dt/12288
Subject Headings: Rotors--Dynamics, Stalling (Aerodynamics), Lift (Aerodynamics)
Format: Document (PDF)

Title: Dynamic stall and three-dimensional wake effects on isolated rotor trim and stability with experimental correlation and parallel fast-Floquet analysis.
Creator: Subramanian, Shanmugasundaram., Florida Atlantic University, Gaonkar, Gopal H., College of Engineering and Computer Science, Department of Ocean and Mechanical Engineering
Abstract/Description: Hingeless rotors are susceptible to instabilities of the lead-lag or lag modes, which are at best weakly damped. The lag mode derives its damping primarily from the complex rotor flow field that is driven by interdependent dynamics of airfoil stall and rotor downwash or wake. Therefore, lag-damping prediction requires an aerodynamic representation that adequately accounts for quasisteady stall, dynamic stall and three-dimensional dynamic wake. Accordingly, this dissertation investigates these...
Show moreHingeless rotors are susceptible to instabilities of the lead-lag or lag modes, which are at best weakly damped. The lag mode derives its damping primarily from the complex rotor flow field that is driven by interdependent dynamics of airfoil stall and rotor downwash or wake. Therefore, lag-damping prediction requires an aerodynamic representation that adequately accounts for quasisteady stall, dynamic stall and three-dimensional dynamic wake. Accordingly, this dissertation investigates these stall and wake effects on lag damping and demonstrates the strengths and weaknesses of the aerodynamic representation with a comprehensive experimental correlation. The database refers to a three-bladed rotor operated untrimmed and to a fourbladed rotor operated trimmed; for both rotors, the blade collective pitch and shaft tilt angles are set prior to each test run. The untrimmed rotor is tested with advance-ratios as high as 0.55 and shaft angles as high as 20°, and it has intentionally builtin structural simplicity: torsionally stiff blades and no swash plate. The trimmed rotor has torsionally soft blades; it is trimmed in the sense that the longitudinal and lateral cyclic pitch controls are adjusted through a swash plate to minimize l/rev root flap moment. Therefore, for the untrimmed rotor, the database refers to lagdamping levels, and for the trimmed rotor, it refers to lag-damping levels as well as to trim results of lateral and longitudinal cyclic pitch controls and steady root flap moments. The dynamic stall representation is based on the ONERA models of lift, drag and pitching moment, and the unsteady wake is described by a finite-state three-dimensional wake model. The root-flexure-blade assembly of the untrimmed rotor is represented by a root-restrained rigid flap-lag model as well as by an elastic flap-lag-torsion model. Similarly, the trimmed rotor is represented by an elastic flaplag- torsion model. The predictions are from three aerodynamic theories ranging from a quasisteady stall theory to a fairly comprehensive dynamic stall and wake theory. This dissertation also addresses the computational aspects of lag-damping predictions by parallel F!oquct analysis based on classical and fast Floquet theories. In a typical trimmed flight, the Floquet analysis comprises (i) trim or equilibrium analysis, (ii) generation of the Floquet transition matrix (FTM) about the trim position, and (iii) eigenanalysis of the FTM. The trim analysis involves the computations of the unknown control inputs that satisfy flight conditions of required thrust and force-moment equilibrium as well as the initial conditions that guarantee periodic forced response. The shooting method is increasingly used for the trim analysis since it generates the FTM as a byproduct and is not sensitive to damping levels. The QR method is used almost exclusively for the FTM eigenanalysis. Presently, the Floquet analysis with shooting and QR methods is widely used for small-order systems (number of states or order M < 100). However, it has been found to be practical neither for design nor for comprehensive-analysis models that lead to large systems (A11 > 100); the run time on a conventional sequential computer is simply prohibitive. Nevertheless, all three parts of Floquet analysis can be algorithmically structured such that they lend themselves well to parallelism or concurrent computations. Furthermore, the conventional Floquet analysis requires integrations of equations of motion through one complete period T; and the bulk of the run time is for repeated integrations over one period. However, for rotors with Q identical blades, it is computationally advantageous to use the fast Floquet analysis, which requires integration through a period T/Q. Accordingly, this dissertation develops parallel algorithms for classical Floquet analysis with classical shooting and for the fast Floquet analysis with fast shooting; in each case the FTM eigenanalysis is baseJ on a parallel QR tibrary routine. The computational reliability· of the sequential anJ parallel Floquet analyses is quantified by (i) the condition number of the converged Jacobian matrix in Newton iteration of trim analysis, (ii) the condition numbers of the FTM eigenvalues of interest, and (iii) the corresponding residual errors of the eigenpairs (eigenvalue and the corresponding eigenYector). These algorithms are applied to study (i) linear flap stability with dynamic wake, (ii) nonlinear flaplag stability with dynamic wake under propulsive- or flight-trim conditions. and (iii) noniinear fiap-iag stabiiity with dynamic staii and wake under flight trim conditions. The parallel and sequential algorithms are compared with respect to computational reliability, saving in run time and growth of run time with increasing system order. Other parallel performance metrics such as speedup, efficiency, and sequential and parallel fractions are included as well. The computational reliability figures of the four algorithms - classical and fast-Floquet analyses each in sequential and parallel modes - are comparable. The fast-Floquet analysis brings in nearly Q-fold reduction in run time in both the sequential and parallel modes; that is, its advantages apply equally to both the modes. 'While the run times for the classical- and fast-Floquet analyses in sequential mode grow in between quadratically and cubically with the system order, the corresponding run times in parallel mode are far shorter and more importantly remain nearly constant. These results offer considerable promise in making large-scale Floquet analysis practical for rotorcrafts with identical as well as with dissimilar blades.
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Date Issued: 1996
PURL: http://purl.flvc.org/fcla/dt/12462
Subject Headings: Rotors (Helicopters), Stalling (Aerodynamics), Drag (Aerodynamics), Wakes (Aerodynamics), Floquet theory
Format: Document (PDF)

Title: Dynamic stall and three-dimensional wake effects on trim, stability and loads of hingeless rotors with fast Floquet theory.
Creator: Chunduru, Srinivas Jaya., Florida Atlantic University, Gaonkar, Gopal H., College of Engineering and Computer Science, Department of Ocean and Mechanical Engineering
Abstract/Description: This dissertation investigates the effects of dynamic stall and three-dimensional wake on isolated-rotor trim, stability and loads. Trim analysis of predicting the pilot's control inputs and the corresponding periodic responses is based on periodic shooting with the fast Floquet theory and damped Newton iteration. Stability analysis, also based on the fast Floquet theory, predicts damping levels and frequencies. Loads analysis uses a force-integration approach to predict the rotating-blade...
Show moreThis dissertation investigates the effects of dynamic stall and three-dimensional wake on isolated-rotor trim, stability and loads. Trim analysis of predicting the pilot's control inputs and the corresponding periodic responses is based on periodic shooting with the fast Floquet theory and damped Newton iteration. Stability analysis, also based on the fast Floquet theory, predicts damping levels and frequencies. Loads analysis uses a force-integration approach to predict the rotating-blade root shears and moments as well as the hub forces and moments. The blades have flap bending, lag bending and torsion degrees of freedom. Dynamic stall is represented by the ONERA stall models of lift, drag and pitching moment, and the unsteady, nonuniform downwash is represented by a three-dimensional, finite-state wake model. Throughout, full blade-stall-wake dynamics is used in that all states are included from trim to stability to loads predictions. Moreover, these predictions are based on four aerodynamic theories--quasisteady linear theory, quasisteady stall theory, dynamic stall theory and dynamic stall and wake theory--and cover a broad range of system parameters such as thrust level, advance ratio, number of blades and blade torsional frequency. The investigation is conducted in three phases. In phase one, the elastic flap-lag-torsion equations are coupled with a finite-state wake model and with linear quasisteady airfoil aerodynamics. The investigation presents convergence characteristics of trim and stability with respect to the number of spatial azimuthal harmonics and radial shape functions in the wake representation. It includes a comprehensive parametric study over a broad range of system parameters. The investigation also includes correlation with the measured lag-damping data of a three-bladed isolated rotor operated untrimmed. In the correlation, three structural models of the root-flexure-blade assembly are used to demonstrate the strengths and the weaknesses of lag-damping predictions. Phase two includes dynamic stall in addition to three-dimensional wake to generate trim and stability results over a comprehensive range of system parameters. It addresses the degree of sophistication necessary in blade discretization and wake representation under dynamically stalled conditions. The convergence and parametric studies isolate the effects of wake, quasisteady stall and dynamic stall on trim and stability. Finally, phase three predicts the rotating blade loads and nonrotating hub loads; the predictions are based on the blade, wake and stall models used in the preceding trim and stability investigations. Although an accurate evaluation of loads requires a more refined blade description, the results isolate and demonstrate the principal dynamic stall and wake effects on the loads.
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Date Issued: 1995
PURL: http://purl.flvc.org/fcla/dt/12426
Subject Headings: Floquet theory, Helicopters, Rotors (Helicopters), Vibration (Aeronautics)
Format: Document (PDF)

Title: Fast-Floquet method of hingeless-rotor trim and stability including comparisons with experiments and approximate methods.
Creator: Ma, Guifa., Florida Atlantic University, Gaonkar, Gopal H., College of Engineering and Computer Science, Department of Ocean and Mechanical Engineering
Abstract/Description: The trim and stability of an isolated hingeless rotor in forward flight are predicted for two coning angles with advance ratio, shaft angle and collective pitch variations. These predictions are correlated with measurements from a test model with four soft-inplane, soft-torsion blades. The test was conducted by the U.S. Army Aeroflightdynamics Directorate at Ames. The collective pitch and shaft angle are set prior to each test point, and the rotor is trimmed as follows: the longitudinal and...
Show moreThe trim and stability of an isolated hingeless rotor in forward flight are predicted for two coning angles with advance ratio, shaft angle and collective pitch variations. These predictions are correlated with measurements from a test model with four soft-inplane, soft-torsion blades. The test was conducted by the U.S. Army Aeroflightdynamics Directorate at Ames. The collective pitch and shaft angle are set prior to each test point, and the rotor is trimmed as follows: the longitudinal and lateral cyclic pitch controls are adjusted through a swashplate to minimize the 1/rev flapping moment at the 12% radial station. The database includes the cyclic pitch controls, steady root-flap moment and lag regressive-mode damping. The predictions are based on a modal approach with both nonrotating and rotating modes, the ONERA dynamic stall models of lift, drag and pitching moment, and a three-dimensional state-space wake model. The periodic shooting method, with damped Newton iteration and the fast-Floquet theory, is used to predict the cyclic pitch controls and the corresponding periodic responses; the equivalent Floquet transition matrix (EFTM) comes out as a byproduct. The eigenvalues and eigenvectors of the EFTM lead to the frequencies and damping levels. The steady root-flap moment is calculated by both the force integration and mode-deflection methods. Although exact, the fast-Floquet theory requires a finite-state representation of all states and is not applicable to numerically and experimentally generated data of response histories. Therefore, the stability is also predicted by three related approximations: generalized Floquet (fast-Floquet) theory and Sparse Time Domain (STD) technique. These approximations can be applied with a finite-state representation of an arbitrary number of states and to response histories; their convergence characteristics and accuracy are examined as well. Two major findings are: (1) The dynamic wake dramatically improves the correlation of the lateral cyclic pitch controls, and (2) all three approximations have excellent convergence characteristics and the converged values agree well with the exact values.
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Date Issued: 2000
PURL: http://purl.flvc.org/fcla/dt/12648
Subject Headings: Floquet theory, Rotors (Helicopters), Stability of helicopters
Format: Document (PDF)

Title: Parallel-computing concepts and methods toward large-scale floquet analysis of helicopter trim and stability.
Creator: Nakadi, Rajesh Mohan., Florida Atlantic University, Gaonkar, Gopal H., College of Engineering and Computer Science, Department of Ocean and Mechanical Engineering
Abstract/Description: The rotorcraft trim solution involves a search for control inputs for required flight conditions as well as for corresponding initial conditions for periodic response or orbit. The control inputs are specified indirectly to satisfy flight conditions of prescribed thrust levels, rolling and pitching moments etc. In addition to the nonlinearity of the equations of motion and control inputs, the control inputs appear not only in damping and stiffness matrices but also in the forcing-function or...
Show moreThe rotorcraft trim solution involves a search for control inputs for required flight conditions as well as for corresponding initial conditions for periodic response or orbit. The control inputs are specified indirectly to satisfy flight conditions of prescribed thrust levels, rolling and pitching moments etc. In addition to the nonlinearity of the equations of motion and control inputs, the control inputs appear not only in damping and stiffness matrices but also in the forcing-function or input matrix; they must be found concomitantly with the periodic response from external constraints on the flight conditions. The Floquet Transition Matrix (FTM) is generated for perturbations about that periodic response; usually, a byproduct of the trim analysis. The damping levels or stability margins are computed from an eigenanalysis of the FTM. The Floquet analysis comprises the trim analysis and eigenanalysis and is routinely used for small order systems (order N < 100). However, it is practical for neither design applications nor comprehensive analysis models that lead to large systems (N > 100); the execution time on a sequential computer is prohibitive. The trim analysis takes the bulk of this execution time. Accordingly, this thesis develops concepts and methods of parallelism toward Floquet analysis of large systems with computational reliability comparable to that of sequential computations. A parallel shooting scheme with damped Newton iteration is developed for the trim analysis. The scheme uses parallel algorithms of Runge-Kutta integration and linear equations solution. A parallel QR algorithm is used for the eigenanalysis of the FTM. Additional parallelism in each iteration cycle is achieved by concurrent operations such as perturbations of initial conditions and control inputs, follow-up integrations and formations of the columns of the Jacobian matrix. These parallel shooting and eigenanalysis schemes are applied to the nonlinear flap-lag stability with a three-dimensional dynamic wake (N ~ 150). The stability also is investigated by widely used sequential schemes of shooting with damped Newton iteration and QR eigenanalysis. The computational reliability is quantified by the maximum condition number of the Jacobian matrices in the Newton iteration, the eigenvalue condition numbers and the residual errors of the eigenpairs. The saving in computer time is quantified by the speedup, which is the ratio of the execution times of Floquet analysis by sequential and parallel schemes. The work is carried out on massively parallel MasPar MP-1, a distributed-memory, single-instruction multiple-data or SIMD computer. A major finding is that with increasing system order, while the parallel execution time remains nearly constant, the sequential execution time increases nearly cubically with N. Thus, parallelism promises to make large-scale Floquet analysis practical.
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Date Issued: 1994
PURL: http://purl.flvc.org/fcla/dt/15085
Subject Headings: Floquet theory, Helicopters--Control systems, Rotors (Helicopters), Parallel processing (Electronic computers)
Format: Document (PDF)

Title: A comparison of a special purpose processor with a general purpose processor and a numerical approach in generating helicopter dynamics equations.
Creator: Ravichandran, S., Florida Atlantic University, Gaonkar, Gopal H., College of Engineering and Computer Science, Department of Ocean and Mechanical Engineering
Abstract/Description: Presently three schemes are used to generate the governing equations of motion. These schemes are: (1) general purpose processors such as REDUCE, MACSYMA and MAPLE, (2) a special purpose symbolic processor DEHIM--Dynamic Equations for Helicopter Interpretive Models and (3) completely numerical approaches such as AGEM--Automatic Generation of Equations of Motion. With REDUCE as a representative multipurpose processor in scheme 1, comparative aspects of these three schemes have been studied by...
Show morePresently three schemes are used to generate the governing equations of motion. These schemes are: (1) general purpose processors such as REDUCE, MACSYMA and MAPLE, (2) a special purpose symbolic processor DEHIM--Dynamic Equations for Helicopter Interpretive Models and (3) completely numerical approaches such as AGEM--Automatic Generation of Equations of Motion. With REDUCE as a representative multipurpose processor in scheme 1, comparative aspects of these three schemes have been studied by applying them to the same set of problems. These problems range from a linear model of a single blade with one degree of freedom to a mildly nonlinear three-bladed rotor model with several degrees of freedom. The derivation process includes the nonlinear equations and the perturbed linear equations about a user-supplied equilibrium state in a rotating frame and then the multiblade equations, which represent transformation into a nonrotating frame using multiblade coordinates. (Abstract shortened with permission of author.)
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Date Issued: 1989
PURL: http://purl.flvc.org/fcla/dt/14517
Subject Headings: Algebra--Computer programs, Helicopters--Dynamics
Format: Document (PDF)

Title: Frequency-time spectral analysis of helicopter turbulence and response in forward flight.
Creator: Vellathottam, George V., Florida Atlantic University, Gaonkar, Gopal H., College of Engineering and Computer Science, Department of Ocean and Mechanical Engineering
Abstract/Description: The atmospheric turbulence that a blade station experiences is called blade-fixed turbulence. It can qualitatively differ from the conventional body-fixed turbulence such as experienced by an element of the body or fuselage. This difference is due to the rotational ,-elocity, which causes fore-and-aft motions of the blade station through the turbulence waves. A closed-form solution of a frequency-time spectrum for the dominant vertical turbulence velocity at an arbitrary blade station is dc,...
Show moreThe atmospheric turbulence that a blade station experiences is called blade-fixed turbulence. It can qualitatively differ from the conventional body-fixed turbulence such as experienced by an element of the body or fuselage. This difference is due to the rotational ,-elocity, which causes fore-and-aft motions of the blade station through the turbulence waves. A closed-form solution of a frequency-time spectrum for the dominant vertical turbulence velocity at an arbitrary blade station is dc,·eloped. This solution makes it possible to explain qualitatively the turbulence cllcrgy transfer due to rotational velocity from the low-frequency region (< 1P or 1/ rcv.) to the high-frequency(> 1P) region with the occurrence of spectral peaks and split peaks at 1P /2, 1P, 3P /2, 2P etc. Comparison of blade responses to bladeand body-fixed turbulence is also presented over a comprehensive range of turbulcuce scale length and advance ratio; the comparison covers frequency-time spectra, correlations including standard deviations, and average threshold-crossing rates of a flapping blade. A major contribution is to formulate both the cyclostationary turbulence and blade response by the frequency-time spectra, which predict simultaneously the time- ancl frequency-dependent characteristics such as the energy culltained in the frequency and time intervals. For low-altitude and low-advanceratio flights, such as nap-of-the earth or NOE flights, rotational velocity effects on turbulence modeling qualitatively affect the prediction of turbulence ancl response statistics.
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Date Issued: 1991
PURL: http://purl.flvc.org/fcla/dt/14683
Subject Headings: Turbulence, Rotors (Helicopters)
Format: Document (PDF)

Title: Trim analysis by shooting and finite elements and Floquet eigenanalysis by QR and subspace iterations in helicopter dynamics.
Creator: Achar, Nagari Shriranga., Florida Atlantic University, Gaonkar, Gopal H., College of Engineering and Computer Science, Department of Ocean and Mechanical Engineering
Abstract/Description: The trim analysis for the initial state and control inputs that satisfy response periodicity and flight conditions, and the Floquet eigenanalysis for a few largest eigenvalues of the Floquet transition matrix (FTM) are investigated. In the trim analysis, the convergence of Newton iteration is investigated in computing the periodic initial state and control inputs sequentially and in parallel. The trim analysis uses the shooting method and two h-versions of temporal finite element methods, one...
Show moreThe trim analysis for the initial state and control inputs that satisfy response periodicity and flight conditions, and the Floquet eigenanalysis for a few largest eigenvalues of the Floquet transition matrix (FTM) are investigated. In the trim analysis, the convergence of Newton iteration is investigated in computing the periodic initial state and control inputs sequentially and in parallel. The trim analysis uses the shooting method and two h-versions of temporal finite element methods, one based on displacement formulation and the other on mixed formulation of displacements and momenta. In each method, both the sequential and in-parallel schemes are used, and the resulting nonlinear equations are solved by damped Newton iteration with an optimally selected damping parameter. The reliability of damped Newton iteration, including earlier-observed divergence problems, is quantified by the maximum condition number of the Jacobian matrices of the iterative scheme. For illustrative purposes, rigid flap-lag and flap-lag-torsion models based on quasisteady aerodynamics are selected. Demanding trim analysis conditions are included by considering advance ratios or dimensionless flight speeds twice as high as those of current helicopters. Concerning the Floquet eigenanalysis, the feasibility of using the Arnoldi-Saad method, one of the emerging subspace iteration methods, is explored as an alternative to the currently used QR method, which is not economical for partial eigenanalysis. The reliability of the Arnoldi-Saad method is quantified by the eigenvalue condition numbers and the residual errors of the eigenpairs. In the three trim analysis methods, while the optimally selected damping parameter provides almost global convergence, the in-parallel scheme requires much less machine time than the conventional sequential scheme; both schemes have comparable reliability of the Newton iteration without and with damping. The Arnoldi-Saad method takes much less machine time than the QR method with comparable reliability.
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Date Issued: 1992
PURL: http://purl.flvc.org/fcla/dt/12297
Subject Headings: Helicopters--Dynamics, Helicopters--Handling characteristics, Stability of helicopters--Mathematical models
Format: Document (PDF)

Rotors (Helicopters) (6)	+ -
Floquet theory (4)	+ -
Helicopters (2)	+ -
Helicopters--Dynamics (2)	+ -
Stalling (Aerodynamics) (2)	+ -

Turbulence (2)	+ -
Algebra--Computer programs (1)	+ -
Boundary layer noise (1)	+ -
Drag (Aerodynamics) (1)	+ -
Fluid mechanics (1)	+ -
Helicopters--Aerodynamics (1)	+ -
Helicopters--Control systems (1)	+ -
Helicopters--Handling characteristics (1)	+ -
Lift (Aerodynamics) (1)	+ -
Parallel processing (Electronic computers) (1)	+ -
Renewable energy sources (1)	+ -
Rotors--Dynamics (1)	+ -
Stability of helicopters (1)	+ -
Stability of helicopters--Mathematical models (1)	+ -
Vibration (Aeronautics) (1)	+ -
Von Kármán, Theodore -- 1881-1963 (1)	+ -
Wakes (Aerodynamics) (1)	+ -
Wind energy conservation systems (1)	+ -
Wind power (1)	+ -
Wind turbines -- Aerodynamics (1)	+ -

Gaonkar, Gopal H. (11)	+ -
College of Engineering and Computer Science (10)	+ -
Department of Ocean and Mechanical Engineering (10)	+ -
Florida Atlantic University (10)	+ -
Achar, Nagari Shriranga. (1)	+ -

Barwey, Dinesh. (1)	+ -
Chunduru, Srinivas Jaya. (1)	+ -
Dang, Ying Yi. (1)	+ -
Ma, Guifa. (1)	+ -
Madhavan, R. (1)	+ -
Nakadi, Rajesh Mohan. (1)	+ -
Ravichandran, S. (1)	+ -
Schau, Kyle A. (1)	+ -
Subramanian, Shanmugasundaram. (1)	+ -
Vellathottam, George V. (1)	+ -

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