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At least 1,495 records · Page 83
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

Limit cycle analysis of large space telescope with CMG nonlinearity

The existence and characteristics of self-sustained oscillations were studied in the Large Space Telescope (LST) system due to the presence of nonlinear gimbal friction in the control moment gyroscopes (CMG's). A continuous data single-axis model of the LST is considered. A solid friction model is used to represent CMG gimbal friction. A rigorous mathematical model is derived for use in a continuous describing function analysis. Conditions for self-sustained oscillations are then determined.

Seltzer, S. M.↗
NASA NTRS1977

On aerodynamic coupling between lateral and longitudinal degrees of freedom

An oscillatory technique for routine measurement of all the direct, cross and cross-coupling moment derivatives due to pitching and yawing has been developed and a series of comprehensive results at Mach numbers 0.7 and 0.25 and at angles of attack up to 40 deg has been obtained in a large wind tunnel. It was found that some of the dynamic cross-coupling derivatives, that at low angles of attack were all zero, at higher angles of attack could reach values that rendered the pertinent cross-coupling terms in the equations of motion comparable in magnitude to the well-established damping terms. Large nonlinear variations with angle of attack were observed in many of the measured derivatives including the important damping-in-pitch and damping-in-yaw derivatives. It was concluded that it may be desirable to include some dynamic cross-coupling derivatives, and the functional variation with angle of attack of many derivatives, in the equations of motion, and to consider all these equations simultaneously, rather than in separate longitudinal and lateral groups.

Orlik-Rueckemann, K. J.↗
NASA NTRS1979

On stability theory

It is found that under mild assumptions, feedback system stability can be concluded if one can 'topologically separate' the infinite-dimensional function space containing the system's dynamical input-output relations into two regions, one region containing the dynamical input-output relation of the 'feedforward' element of the system and the other region containing the dynamical output-input relation of the 'feedback' element. Nonlinear system stability criteria of both the input-output type and the state-space (Liapunov) type are interpreted in this context. The abstract generality and conceptual simplicity afforded by the topological separation perspective clarifies some of the basic issues underlying stability theory and serves to suggest improvements in existing stability criteria. A generalization of Zames' (1966) conic-relation stability criterion is proved, laying the foundation for improved multivariable generalizations of the frequency-domain circle stability criterion for nonlinear systems.

Safonov, M. G.↗
NASA NTRS1980

Diagnostics and control of wavenumber stability and purity of tunable diode lasers relevant to their use as local oscillators in heterodyne systems

Initial operation of the tunable diode lasers (TDL) showed that it was not possible to adjust the wavenumber to one selected a priori in the TDL tuning range. During operation, the operating point would change by 0.1/cm over the longer term with even larger changes occurring during some thermal cycles. Most changes during thermal cycling required using lower temperatures and higher currents to reach the former wavenumber (when it could be reached). In many cases, an operating point could be selected by changing TDL current and temperature to give both the desired wavenumber and most of the power in a single mode. The selection procedure had to be used after each thermal cycling. Wavenumber nonlinearities of about 10% over a 0.5 cm tuning range were observed. Diagnostics of the single mode selected by a grating monochromator showed wavenumber fine structure under certain operating conditions. The characteristics due to the TDL environment included short term wavenumber stability, the instrument lineshape function, and intermediate term wavenumber stability.

Poultney, S.↗
NASA NTRS1982

Planetary-scale waves in the Venus atmosphere

Observed wave-like cloud features on Venus, which at times form a Y-like structure which encircles the planet, were modelled numerically. Linearized primitive equations for a shallow, hydrostatic atmosphere are defined, along with upper boundary layer conditions for the 4-6 day periods detected in the atmospheric response to forcing. The basic state of the Venus atmosphere was considered to display variations in static stability and the mean zonal wind as a function of altitude. Forcing was introduced over a wide range of frequencies in order to produce the target oscillation modes. Combination of a midlatitude Rossby wave and an equatorial Kelvin wave was found to yield the observed Y-shape, which could be preserved with nonlinear coupling.

Covey, C.↗
NASA NTRS1990

Optimization of linear, controlled structures

An approach to the integrated design of linear controlled structures, which is being investigated as part of NASA's controlled structures interaction (CSI) methodology development program, is presented. The approach is integrated in the sense that the structure and its controller are simultaneously designed. The design methodology uses constrained nonlinear optimization procedures based on analytically obtained gradients of the structural responses. Design of the controller is based on the so-called Q-parameterization theory, which parameterizes all closed-loop input/output maps achievable with stabiizing linear controllers. Very general objective and constraint functions are possible, and structural shape can be included in the design variables. This method has been partially implemented and demonstrated; early findings are reported.

Hunziker, K. Scott↗
NASA NTRS1996

Toward a Turbulence Constitutive Relation for Rotating Flows

In rapidly rotating turbulent flows the largest scales of the motion are in approximate geostrophic balance. Single-point turbulence closures, in general, cannot attain a geostrophic balance. This article addresses and resolves the possibility of constitutive relation procedures for single-point second order closures for a specific class of rotating or stratified flows. Physical situations in which the geostrophic balance is attained are described. Closely related issues of frame-indifference, horizontal nondivergence, Taylor-Proudman theorem and two-dimensionality are, in the context of both the instantaneous and averaged equations, discussed. It is shown, in the absence of vortex stretching along the axis of rotation, that turbulence is frame-indifferent. A derivation and discussion of a geostrophic constraint which the prognostic equations for second-order statistics must satisfy for turbulence approaching a frame-indifferent limit is given. These flow situations, which include rotating and nonrotating stratified flows, are slowly evolving flows in which the constitutive relation procedures are useful. A nonlinear non-constant coefficient representation for the rapid-pressure strain covariance appearing in the Reynolds stress and heat flux equations consistent with the geostrophic balance is described. The rapid-pressure strain model coefficients are not constants determined by numerical optimization but are functions of the state of the turbulence as parameterized by the Reynolds stresses and the turbulent heat fluxes. The functions are valid for all states of the turbulence attaining their limiting values only when a limit state is achieved. These issues are relevant to strongly vortical flows as well as flows such as the planetary boundary layers, in which there is a transition from a three-dimensional shear driven turbulence to a geostrophic or horizontal turbulence.

Ristorcelli, J. R.↗
NASA NTRS2002

Very Large Scale Optimization

The purpose of this research under the NASA Small Business Innovative Research program was to develop algorithms and associated software to solve very large nonlinear, constrained optimization tasks. Key issues included efficiency, reliability, memory, and gradient calculation requirements. This report describes the general optimization problem, ten candidate methods, and detailed evaluations of four candidates. The algorithm chosen for final development is a modern recreation of a 1960s external penalty function method that uses very limited computer memory and computational time. Although of lower efficiency, the new method can solve problems orders of magnitude larger than current methods. The resulting BIGDOT software has been demonstrated on problems with 50,000 variables and about 50,000 active constraints. For unconstrained optimization, it has solved a problem in excess of 135,000 variables. The method includes a technique for solving discrete variable problems that finds a "good" design, although a theoretical optimum cannot be guaranteed. It is very scalable in that the number of function and gradient evaluations does not change significantly with increased problem size. Test cases are provided to demonstrate the efficiency and reliability of the methods and software.

Vanderplaats, Garrett↗
NASA NTRS2004

Intelligent, Robust Control of Deteriorated Turbofan Engines via Linear Parameter Varying Quadratic Lyapunov Function Design

A method for accommodating engine deterioration via a scheduled Linear Parameter Varying Quadratic Lyapunov Function (LPVQLF)-Based controller is presented. The LPVQLF design methodology provides a means for developing unconditionally stable, robust control of Linear Parameter Varying (LPV) systems. The controller is scheduled on the Engine Deterioration Index, a function of estimated parameters that relate to engine health, and is computed using a multilayer feedforward neural network. Acceptable thrust response and tight control of exhaust gas temperature (EGT) is accomplished by adjusting the performance weights on these parameters for different levels of engine degradation. Nonlinear simulations demonstrate that the controller achieves specified performance objectives while being robust to engine deterioration as well as engine-to-engine variations.

Turso, James A.↗
NASA NTRS2011

Surface Instability of Liquid Propellant under Vertical Oscillatory Forcing

Fluid motion in a fuel tank produced during thrust oscillations can circulate sub-cooled hydrogen near the liquid-vapor interface resulting in increased condensation and ullage pressure collapse. The first objective of this study is to validate the capabilities of a Computational Fluid Dynamics (CFD) tool, CFD-ACE+, in modeling the fundamental interface transition physics occurring at the propellant surface. The second objective is to use the tool to assess the effects of thrust oscillations on surface dynamics. Our technical approach is to first verify the CFD code against known theoretical solutions, and then validate against existing experiments for small scale tanks and a range of transition regimes. A 2D axisymmetric, multi-phase model of gases, liquids, and solids is used to verify that CFD-ACE+ is capable of modeling fluid-structure interaction and system resonance in a typical thrust oscillation environment. Then, the 3D mode is studied with an assumed oscillatory body force to simulate the thrust oscillating effect. The study showed that CFD modeling can capture all of the transition physics from solid body motion to standing surface wave and to droplet ejection from liquid-gas interface. Unlike the analytical solutions established during the 1960 s, CFD modeling is not limited to the small amplitude regime. It can extend solutions to the nonlinear regime to determine the amplitude of surface waves after the onset of instability. The present simulation also demonstrated consistent trends from numerical experiments through variation of physical properties from low viscous fluid to high viscous fluids, and through variation of geometry and input forcing functions. A comparison of surface wave patterns under various forcing frequencies and amplitudes showed good agreement with experimental observations. It is concluded that thrust oscillations can cause droplet formation at the interface, which results in increased surface area and enhanced heat transfer between the liquid and gas phases as the ejected droplets travel well into the warmer gas region.

Yang, H. Q.↗
NASA NTRSDate not supplied

A Model Predictive Control Approach for In-Flight Acoustic Constraint Compliance

Vehicle noise remains one of the major barriers to public acceptance of Urban Air Mobility-class aircraft. This work focuses on motion planning for aircraft in noise-sensitive areas. A nonlinear Model Predictive Path Integral (MPPI) control law is used to generate a finite-horizon trajectory that satisfies acoustic level constraints at a set of (three-dimensional) observer locations. The MPPI framework places no restrictions on the class of state-dependent cost functionals that can be employed, making it well-suited for use with sophisticated acoustic models and metrics, in addition to dynamic and mission-relevant constraints. The model predictive control architecture is also suitable for implementation in a real-time application. A simulation example demonstrates the ability of the controller to modify the flight trajectory in order to satisfy acoustic constraints at multiple measurement locations.

acoustically-aware vehicle↗
NASA NTRS1998

Study of the effects of MeV Ag, Cu, Au, and Sn implantation on the optical properties of LiNbO3

The authors present the results of characterization of linear absorption and nonlinear refractive index of Au, Ag, Cu and Sn ion implantation into LiNbO3. Ag was implanted at 1.5 MeV to fluences of 2 to 17 x 17(exp 16)/sq cm at room temperature. Au and Cu were implanted to fluences of 5 to 20 x 10(exp 16)/sq cm at an energy of 2.0 MeV. Sn was implanted to a fluence of 1.6 x 10(exp 17)/sq cm at 160 kV. Optical absorption spectrometry indicated an absorption peak for the Au implanted samples after heat treatment at 1,000 C at approx. 620 nm. The Ag implanted samples absorption peaks shifted from approx. 450 nm before heat treatment to 550 nm after 500 C for 1h. Heat treatment at 800 C returned the Ag implanted crystals to a clear state. Cu nanocluster absorption peaks disappears at 500 C. No Sn clusters were observed by optical absorption or XRD. The size of the Ag and Au clusters as a function of heat treatment were determined from the absorption peaks. The Ag clusters did not change appreciably in size with heat treatment. The Au clusters increased from 3 to 9 nm diameter upon heat treatment at 1000 C. TEM analysis performed on a Au implanted crystal indicated the formation of Au nanocrystals with facets normal to the c-axis. Measurements of the nonlinear refractive indices were carried out using the Z-scan method with a tunable dye laser pumped by a frequency doubled mode-locked Nd:YAG laser. The dye laser had a 4.5 ps pulse duration time and 76 MHz pulse repetition rate (575 nm).

Williams, E. K.↗
NASA NTRS1980

The linear and non-linear aerodynamics of three-surface aircraft concepts

It is noted that most modern fighter aircraft rely on vortex interaction to provide lift enhancement at maneuvering angles of attack. It is shown that the close-coupled horizontal canard in a three-surface configuration provides a control surface which in addition to its other control functions, can be used to optimize this vortex interaction. Attention is given to a study intended to provide a detailed understanding of the aerodynamics of this type of configuration. The discussion examines the results of this investigation and hypotheses are presented to explain the linear and nonlinear aerodynamic phenomena observed.

Agnew, J. W.↗
NASA NTRS1983

A refined finite element for vibration analysis of twisted blades based on beam theory

A finite element method of discretizing beam segments of pretwisted rotating blades is presented. Employing the matrix displacement method, stiffness and mass properties are developed from basic mechanics of a pretwisted beam theory. By introducing the proper displacement functions, the effect of rotor blade rotational motion on the stiffness matrix is obtained systematically from the kinetic energy expression. Comparing with other beam elements the derivation of this element is more fundamental. This allows one to apply the same approach to more complicated problems including nonlinear effects or complex dynamic motions. Illustrative examples are given comparing numerical results with available data and other numerical solutions from rotating and nonrotating force fields. These examples show that accurate prediction of vibration frequencies for pretwisted blades can be obtained by employing a quite modest number of degrees of freedom.

Sisto, F.↗
NASA NTRS1984

Nonlinear systems approach to control system design

Consider some of the control system design methods for plants with nonlinear dynamics. If the nonlinearity is weak relative to the size of the operating region, then the linear methods apply directly. Fixed-gain design may be feasible even for significant nonlinearities. It may be possible to find a single gain which provides adequate control of the linear models at several perturbation points. If the nonlinearity is restricted to a sector, that fact may be used to obtain a fixed-gain controller. Otherwise, a gain may have to be associated with each perturbation point Pi. A gain schedule K(p(v)) is obtained by connecting the perturbation points by a function, say p(v), of the scheduling parameter v (i.e., speed). When the scheduling parameter must be multidimensional, this approach is difficult; the objective is to develop an easier procedure.

Meyer, G.↗
NASA NTRS1985

On the nonlinear characteristics of the axisymmetric flow regime: Cylindrical and spherical systems

The physical relationship between steady axisymmetric flows that might be observed in the atmosphere and in laboratory vessels is investigated theoretically. This is accomplished by comparing both the nonlinear structure and the thermal forcing mechanisms in two truncated spectral models of flow in the atmosphere and the rotating laboratory cylinder, respectively. Under statically stable conditions, the response of the internally forced spherical model (which is developed here from a set of new orthonormal basis functions) exhibits steady behavior different from that in the externally forced cylindrical model. Two regions of multiple steady solutions occur in the cylindrical model, under stable conditions, that are not found in the spherical one. The possible physical relevance of these multiple solutions is investigated by determining their location in parameter space with respect to the classical Hadley-Rossby transition curve. The results suggest that the wave flow regime, in an annulus, might develop catastrophically when an upper symmetric flow ceases to exist.

Higgins, R. W.↗
NASA NTRS1986

Nonlinear filtering for spacecraft attitude estimation

Nonlinear filtering techniques are applied to spacecraft attitude estimation using quaternion parameterization for the attitude kinematics. By replacing the angular velocity vector by the gyro output vector, a state dependent noise vector is introduced in the seven-dimensional system equations. The resulting conditional probability density function from the Ito differential rule is governed by the Fokker Planck partial differential equation which is approximated by the second order mean and covariance differential equations. In order to minimize computer loading, the covariance propagation is carried out in six-dimensional state space using a matrix transformation. The star tracker data is used to update the covariance matrix in the seven-dimensional space. The algorithm is simulated for an earth pointing spacecraft mission, using Monte Carlo samples of gyro and star measurements. The performance of the second order filter is compared with the extended Kalman Filter through several simulation runs and drift rates have been identified.

Vathsal, S.↗
NASA NTRS1989

An iterative method for systems of nonlinear hyperbolic equations

An iterative algorithm for the efficient solution of systems of nonlinear hyperbolic equations is presented. Parallelism is evident at several levels. In the formation of the iteration, the equations are decoupled, thereby providing large grain parallelism. Parallelism may also be exploited within the solves for each equation. Convergence of the interation is established via a bounding function argument. Experimental results in two-dimensions are presented.

Scroggs, Jeffrey S.↗