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Nonlinear Simulations (nonlinear + simulation)

Distribution by Scientific Domains

Engineering	55%

Selected Abstracts

Nonlinear simulation of mixers for assessing system-level performance

INTERNATIONAL JOURNAL OF RF AND MICROWAVE COMPUTER-AIDED ENGINEERING, Issue 4 2005
Nuno B. Carvalho
Abstract This article deals with nonlinear simulation methods intended to evaluate the impact of mixer nonidealities on the performance of a wireless system. Behavioral models capable of accurately describing the mixer's nonlinear dynamic features at the system level are currently unavailable. The possibility of using alternative circuit analysis techniques to reach this goal is discussed. After a brief review of existing mixer analysis methods, the focus is directed to the techniques amenable to efficiently handling periodic carriers modulated by complex stochastic signals. In particular, it is shown how multi-envelope transient methods coupled with a three-dimensional harmonic-balance engine can model a nonlinear dynamic mixer excited by a modulated RF signal accompanied by a strong adjacent channel interferer and with a local oscillator corrupted by phase noise. © 2005 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2005. [source]

Linear, parameter-varying control and its application to a turbofan engine

INTERNATIONAL JOURNAL OF ROBUST AND NONLINEAR CONTROL, Issue 9 2002
Gary J. BalasArticle first published online: 15 JUL 200
This paper describes application of parameter-dependent control design methods to a turbofan engine. Parameter-dependent systems are linear systems, whose state-space descriptions are known functions of time-varying parameters. The time variation of each of the parameters is not known in advance, but is assumed to be measurable in real-time. Three linear, parameter-varying (LPV) approaches to control design are discussed. The first method is based on linear fractional transformations which relies on the small gain theorem for bounds on performance and robustness. The other methods make use of either a single (SQLF) or parameter-dependent (PDQLF) quadratic Lyapunov function to bound the achievable level of performance. The latter two techniques are used to synthesize controllers for a high-performance turbofan engine. A LPV model of the turbofan engine is constructed from Jacobian linearizations at fixed power codes for control design. The control problem is formulated as a model matching problem in the ,, and LPV framework. The objective is decoupled command response of the closed-loop system to pressure and rotor speed requests. The performance of linear, ,, point designs are compared with the SQLF and PDQLF controllers. Nonlinear simulations indicate that the controller synthesized using the SQLF approach is slightly more conservative than the PDQLF controller. Nonlinear simulations with the SQLF and PDQLF controllers show very robust designs that achieve all desired performance objectives. Copyright © 2002 John Wiley & Sons, Ltd. [source]

Draw ratio enhancement in nonisothermal melt spinning

AICHE JOURNAL, Issue 3 2009
Balram Suman
Abstract Nonisothermal melt spinning of materials having a step-like viscosity variation with temperature is studied in this work. A set of nonlinear equations is used to describe the fiber behavior and to obtain the draw ratio, the square of the ratio of the fiber diameter at the entrance to that at the exit of the fiber-spinning device. The fluid-flow equation is based on a slender-jet approximation, and external heating and cooling have been accounted for with a one-dimensional model in order to obtain the fiber temperature and viscosity along the fiber length. The model is similar to that used by Wylie et al. (J Fluid Mech. 2007;570:1,16) but accounts for inertia, shear stress at the fiber surface, surface tension, gravity, cooling, and larger heating rates. Steady-state analysis reveals that the draw ratio increases with an increase in the pulling force, passes through a maximum, and then starts increasing again, resulting in three possible pulling forces for the same draw ratio. However, linear stability analysis reveals that depending on the strength of heating and/or cooling, at most two of the steady states are stable. The stability analysis also predicts complicated oscillatory and nonoscillatory dynamical behavior as the pulling force varies. Nonlinear simulations reveal that an unstable system always tends to limit-cycle behavior. Systems predicted as stable by the linear stability analysis are also stable for large-amplitude perturbations. External heating is found to dramatically enhance the draw ratio of the melt-spinning process. The addition of a cooling section suppresses the draw ratio, but this can be compensated for with a higher heating strength. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source]

Nonlinear simulations of magnetic instabilities in stellar radiation zones: The role of rotation and shear

ASTRONOMISCHE NACHRICHTEN, Issue 10 2007
A.S. Brun
Abstract Using the 3-dimensional ASH code, we have studied numerically the instabilities that occur in stellar radiation zones in presence of large-scale magnetic fields, rotation and large-scale shear. We confirm that some configurations are linearly unstable, as predicted by Tayler and collaborators, and we determine the saturation level of the instability. We find that rotation modifies the peak of the most unstable wave number of the poloidal instability but not its growth rate as much as in the case of the m = 1 toroidal instability for which it is changed to , = /,. Further in the case with rotation and shear, we found no sign of the dynamo mechanism suggested recently by Spruit even though we possess the essential ingredients (Tayler's m = 1 instability and a large scale shear) supposedly at work. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Nonlinear simulations of magnetic Taylor-Couette flow with currentfree helical magnetic fields

ASTRONOMISCHE NACHRICHTEN, Issue 9 2006
J. Szklarski
Abstract Themagnetorotational instability (MRI) in cylindrical Taylor-Couette flow with external helical magnetic field is simulated for infinite and finite aspect ratios. We solve the MHD equations in their small Prandtl number limit and confirm with timedependent nonlinear simulations that the additional toroidal component of the magnetic field reduces the critical Reynolds number from O (106) (axial field only) to O (103) for liquid metals with their small magnetic Prandtl number. Computing the saturated state we obtain velocity amplitudes which help designing proper experimental setups. Experiments with liquid gallium require axial field ,50 Gauss and axial current ,4 kA for the toroidal field. It is sufficient that the vertical velocity uz of the flow can be measured with a precision of 0.1 mm/s. We also show that the endplates enclosing the cylinders do not destroy the traveling wave instability which can be observed as presented in earlier studies. For TC containers without and with endplates the angular momentum transport of the MRI instability is shown as to be outwards. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Incorporating spatially variable bottom stress and Coriolis force into 2D, a posteriori, unstructured mesh generation for shallow water models

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 3 2009
D. Michael Parrish
Abstract An enhanced version of our localized truncation error analysis with complex derivatives (LTEA,CD ) a posteriori approach to computing target element sizes for tidal, shallow water flow, LTEA+CD , is applied to the Western North Atlantic Tidal model domain. The LTEA + CD method utilizes localized truncation error estimates of the shallow water momentum equations and builds upon LTEA and LTEA,CD-based techniques by including: (1) velocity fields from a nonlinear simulation with complete constituent forcing; (2) spatially variable bottom stress; and (3) Coriolis force. Use of complex derivatives in this case results in a simple truncation error expression, and the ability to compute localized truncation errors using difference equations that employ only seven to eight computational points. The compact difference molecules allow the computation of truncation error estimates and target element sizes throughout the domain, including along the boundary; this fact, along with inclusion of locally variable bottom stress and Coriolis force, constitute significant advancements beyond the capabilities of LTEA. The goal of LTEA + CD is to drive the truncation error to a more uniform, domain-wide value by adjusting element sizes (we apply LTEA + CD by re-meshing the entire domain, not by moving nodes). We find that LTEA + CD can produce a mesh that is comprised of fewer nodes and elements than an initial high-resolution mesh while performing as well as the initial mesh when considering the resynthesized tidal signals (elevations). Copyright © 2008 John Wiley & Sons, Ltd. [source]

Analysis of frequency division in microstrip circuits by using the FDTD method

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 5 2008
Oscar González
Abstract The accurate time-domain simulation of the nonlinear dynamics of circuits containing distributed elements is of great interest. Specially at high frequencies, it is essential to consider phenomena, such as crosstalk, packaging effects, and electromagnetic interaction between active and passive elements. These effects are mainly due to the distribute components of the circuit. Therefore, the accuracy in the instability or oscillation analysis of nonlinear circuits is often limited by the models used for such elements. Commercial simulators based on the equivalent-circuit approach hardly take into account the mentioned effects. In this work, the finite-difference time-domain (FDTD) method is applied to the nonlinear simulation of a diode-based microstrip circuit exhibiting a parametric frequency division by two. The FDTD method rigorously solves Maxwell's curl equations in the time domain providing a full-wave characterization of the distributed elements of the circuit. In the example considered, a relatively low operating frequency has been selected to allow a reliable comparison between the results obtained by the proposed technique and those provided by a circuit-based commercial simulator. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 1300,1302, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23352 [source]

On linear-parameter-varying (LPV) slip-controller design for two-wheeled vehicles

INTERNATIONAL JOURNAL OF ROBUST AND NONLINEAR CONTROL, Issue 12 2009
Matteo Corno
Abstract This paper describes the application of linear-parameter-varying (LPV) control design techniques to the problem of slip control for two-wheeled vehicles. A nonlinear multi-body motorcycle simulator is employed to derive a control-oriented dynamic model. It is shown that, in order to devise a robust controller with good performance, it is necessary to take into account the dependence of the model on the velocity and on the wheel slip. This dependence is modeled via an LPV system constructed from Jacobian linearizations at different velocities and slip values. The control problem is formulated as a model-matching control problem within the LPV framework; a specific modification of the LPV control synthesis algorithm is proposed to alleviate controller interpolation problems. Linear and nonlinear simulations indicate that the synthesized controller achieves the required robustness and performance. Copyright © 2008 John Wiley & Sons, Ltd. [source]