Stephen L. Woodruff

Discontinuous Spectral Element Approximation of Maxwell’s Equations

Two discontinuous spectral element methods for the solution of Maxwell’s equations are compared. The first method is a staggered-grid Chebyshev approximation. The second is a spectral element (collocation) form of the discontinuous Galerkin method. In both methods, the approximations are discontinuous at element boundaries, making them suitable for propagating waves through multiple materials. Solutions are presented for propagation of a plane wave through a plane dielectric interface, and for scattering off a coated perfectly conducting cylinder.

Prony analysis for power system transient harmonics

Proliferation of nonlinear loads in power systems has increased harmonic pollution and deteriorated power quality. Not required to have prior knowledge of existing harmonics, Prony analysis detects frequencies, magnitudes, phases, and especially damping factors of exponential decaying or growing transient harmonics. In this paper, Prony analysis is implemented to supervise power system transient harmonics, or time-varying harmonics. Further, to improve power quality when transient harmonics appear, the dominant harmonics identified from Prony analysis are used as the harmonic reference for harmonic selective active filters. Simulation results of two test systems during transformer energizing and induction motor starting confirm the effectiveness of the Prony analysis in supervising and canceling power system transient harmonics.

Multiple-scale perturbation analysis of slowly evolving turbulence

A multiple-scale perturbation expansion is applied to extract a closed system of two equations governing the scalar descriptors of the turbulence energy spectrum from a spectral closure model. The result applies when the length scale and total energy input of a force that maintains a steady state of homogeneous isotropic turbulence are perturbed slowly and the energy spectrum consequently evolves slowly compared to the time scales of the turbulence itself.

Topology of the generator bus in a warship integrated power system

Future naval platforms will feature an integrated power system (IPS) that provides power for all ship systems, including propulsion, combat systems, ship service loads and, ultimately, weapons. It is naturally a requirement that the power system be highly reliable and one of the benefits of the all-electric concept is that reliability in general and, specifically, survivability in battle are enhanced by the ability to reassign power components and network paths to dynamically reconfigure the system in response to events. Much research is being directed at the question of how best to reconfigure a system when portions are damaged: this includes choosing the optimal reconfiguration possible in the system, evaluating the effect of transients introduced by the damage and the switching in and out of parts of the network, etc. Little attention has been focused, however, on the how much reconfiguration is possible in a given network, particularly in the event of multiple faults. After all, if there is no alternative path available between source and load, there can be no reconfiguration. Alternatively, little attention has been given to the question of how to design the network so as to give maximum survivability. These questions are vital, regardless of what reconfiguration strategy is chosen.

Development of a Continuous Model for Simulation of Turbulent Flows

The development of a continuous turbulence model that is suitable for representing both the subgrid scale stresses in large eddy simulation and the Reynolds stresses in the Reynolds averaged Navier-Stokes formulation is described. A recursion approach is used to bridge the length scale disparity from the cutoff wave number to those in the energy-containing range. The proposed model is analyzed in conjunction with direct numerical simulations of Kolmogorov flows.

Optimizing the transient response of voltage source converters used for mitigating voltage collapse problems by means of real time digital simulation

Shunt-connected voltage controllers such as STATCOMs have the potential to mitigate voltage collapse problems in distribution systems with heavy induction motor loadings by injecting reactive current into the AC system. In networks with very low X/R ratios, the addition of active (real) power by the STATCOM is required and may be provided by superconducting energy storage systems (SMES). This paper presents a real time network simulation model of a STATCOM-SMES system and shows how control parameters can be optimized for fast transient response by combining simulations with sophisticated optimization strategies. It is concluded that real-time simulation, possibly also in conjunction with hardware-in-the-loop experiments, can aid optimal use of future static compensators in advanced electric power systems.

Study of Power Loss of Small Time-Step VSC Model in RTDS

In this paper, the power loss of power electronic switches modeled as RTDS small time-step voltage source converter (VSC) is studied. The reduction of time step size and resistance at OFF state can decrease the artificial switching loss in the RTDS small time-step VSC models. Higher noise levels in the RTDS simulations indicate more power loss in the RTDS simulations than in equivalent PSIM simulations. The sensitivity of the power loss and the total harmonic distortion (THD) to parameters, including switch model parameters and interface transformer model parameters are studied. With appropriately selected component model parameters, the power loss of the modeled switches can be reduced.

Hardware-in-the-Loop Experiments on the Use of Propulsion Motors to Reduce Pulse-Load System Disturbances

Pulse power loads are assuming increased importance in the development of all-electric naval surface combatants. One possible means for countering the potentially destabilizing effect of pulse loads is by manipulating the power of the propulsion motors to smooth out the variations in system power caused by the pulse loads. Hardware-in-the-loop experiments were conducted with the 2.5 MW motor-dynamometer system at the Center for Advanced Power Systems at Florida State University to test this possibility. RMS variations in system power were successfully reduced and several avenues for controller development were suggested by the results.

Grid-Size Dependence in the Large-Eddy Simulation of Kolmogorov Flow

Through the use of both an a priori analysis of direct numerical simulation data and experiments with large-eddy simulations, a non-Smagorinsky grid-size dependence is established for the Smagorinsky subgrid scale model for low numbers of resolved scales. It is shown that an increase in the Smagorinsky constant as the grid size is increased permits successful large-eddy simulations for cutoffs approaching the energy-containing range of length scales. A detailed comparative analysis is made of the second-order turbulence quantities as determined by the differently resolutioned large-eddy simulations

Evaluation of Turbulence-Model Performance as Applied to Jet-Noise Prediction

The accurate prediction of jet noise is possible only if the jet flow field can be predicted accurately. Predictions for the mean velocity and turbulence quantities in the jet flowfield are typically the product of a Reynolds-averaged Navier-Stokes solver coupled with a turbulence model. To evaluate the effectiveness of solvers and turbulence models in predicting those quantities most important to jet noise prediction, two CFD codes and several turbulence models were applied to a jet configuration over a range of jet temperatures for which experimental data is available.