Working field theory problems with random walks

Abstract Purpose – The purpose of this paper is to demonstrate how Monte Carlo methods can be applied to the solution of field theory problems. Design – This objective is achieved by building insight from Laplacian field problems. The point solution of a Laplacian field problem can be viewed as the solid angle average of the Dirichlet potentials from that point. Alternatively it can be viewed as the average of the termination potential of a number of random walks. Poisson and Helmholtz equations add the complexity of collecting a number of packets along this walk, and noting the termination of a random walk at a Dirichlet boundary. Findings – When approached as a Monte Carlo problem, Poisson type problems can be interpreted as collecting and summing source packets representative of current or charge. Helmholtz problems involve the multiplication of packets of information modified by a multiplier reflecting the conductivity of the medium. Practical implications – This method naturally lends itself to parallel processing computers. Originality/value – This is the first paper to explore random walk solutions for all classes of eddy current problems, including those involving velocity. In problems involving velocity, the random walk direction enters depending on the walk direction with respect to the local velocity. Keywords Finite difference methods, Monte Carlo methods, Random functions Paper type Technical paperCenter for Electromechanic

Transient performance and loss analysis in solid wound rotor pulsed power generators

For magnetic bearing supported flywheels, synchronous power losses can be reduced by proper use of well-established control approaches. Analysis and test results of amplifier and actuator power consumption are presented for several different control schemes: 1) a baseline gain scheduled compensator, 2) an improved compensator with reduced stiffness in the operating range, and 3) the baseline compensator with adaptive open loop control set up to minimize the synchronous currents. To compare the impact of the control changes, power usage and rotor displacement measurements were made on an operating 2.0 kWh flywheel in 2,000 rpm increments from rest to 40,000 rpm. Power consumption was derived from direct measurement of coil currents, coil voltages, and amplifier supply currents using a high-speed digitizer (2 MHz sample rate).Center for Electromechanic

Reconfiguration of shipboard power systems

The electrical system on a ship is well contained. Among the challenges proffered by naval systems is real time monitoring for the purpose of fault analysis and reconfiguration. The power system can be considered as a grid of interconnected trunk lines each with its own equivalent parallel load impedance and series transmission impedance. These equivalent impedances provide a natural means of reducing a rather large, complex system to a small compact system. The reduced system can be optimized to maximize power flow through the equivalent parallel impedances and minimize the loss in the series components. Binary integer optimization techniques offer the most promise in solving these problems quickly.Center for Electromechanic

A new strategy for representation and control of self-contained power systems

Power systems on naval vessels and airplanes are good examples of self-contained power systems. State of the art LEM modules and voltage sensors provide real time current and voltage data. This paper shows how that information can be used to construct dynamic equivalent impedance representations of the system discretized into key trunk lines. Error analysis indicates that the algorithm performing this representation can be updated in one fifth of a cycle if the signal to noise data ratio is 5% or less. The magnitude of the equivalent impedance is sufficient to determine when fault control on either end of the trunk line is required. These same equivalent impedances can be used to determine the best switch configuration to maximize load power while minimizing transmission loss subject to line load capability. The accuracy of the equivalent impedance approach is assessed for transients with active and passive loads.Center for Electromechanic

A fundamental look at energy storage focusing primarily on flywheels and superconducting energy storage

This paper compares energy storage efficiency of Superconducting Energy Storage devices (SMES) with high speed flywheels employing magnetic bearings. Both solid cylinder and shell cylinder flywheels are examined from fundamental physics. Solid cylinder flywheels have a fixed energy density by weight and volume dependent only on the constitutive properties of the flywheel. For a target energy storage, the flywheel’s radius, length, and rotation speed are determined given the governing limitation on hoop stress and the requirement that operation will occur below the first bending mode. No design parameters are open for engineering judgment except the margin of safety. Thus the volume necessary to reach a target energy storage is well defined. The shell cylinder has only the thickness of the shell as an open design variable. The constraint for a SMES system is that the magnetic field density remain below the quench value for the superconductor. This constraint involves the current density, the magnetic field density, and the temperature. A theoretical upper limit can be reached by considering a volume with a B field just under the quench value. In this theoretical upper limit, given the materials available today, the flywheel stores the same energy in a volume 7.4 times smaller than the SMES system even when assuming a 20 T field for the SMES system. Both systems allow for energy to be added and removed rapidly by comparison to battery and capacitive storage, but the flywheel is by far the more efficient choice when examined on a per volume basis.Center for Electromechanic

Pulsed power electromechanics - permanent magnets versus copper coils

A number of emerging military systems operate using short, repetitive, high-power pulses. Rotating electromechanical machines incorporating inertial storage are natural candidates for supplying these high power pulses. The short duty cycle characteristic of these devices introduces an interesting physics trade off in the choice of field excitation. A quantitative comparison of permanent magnet machines to copper coil systems is performed on an equal weigh basis. The results indicate that copper coil based systems using exciters are superior to permanent magnet counterparts in pulsed applications of 20 s and less. The recommended use of copper coils becomes stronger when the issues of magnet life due to vibration, thermal cycling, and slot harmonic heating are considered.Center for Electromechanic

Birth weight in offspring and cardiovascular mortality in their parents, aunts and uncles:a family-based cohort study of 1.35 million births

Background A link between suboptimal fetal growth and higher risk of cardiovascular disease (CVD) is well documented. It has been difficult to assess the contribution of environmental versus genetic factors to the association, as these factors are closely connected in nuclear families. We investigated the association between offspring birthweight and CVD mortality in parents, aunts and uncles, and examined whether these associations are explained by CVD risk factors. Methods We linked Norwegian data from the Medical Birth Registry, the Cause of Death Registry and cardiovascular surveys. A total of 1 353 956 births (1967–2012) were linked to parents and one maternal and one paternal aunt/uncle. Offspring birthweight and CVD mortality association among all relationships was assessed by hazard ratios (HR) from Cox regressions. The influence of CVD risk factors on the associations was examined in a subgroup. Results Offspring birthweight was inversely associated with CVD mortality among parents and aunts/uncles. HR of CVD mortality for one standard deviation (SD) increase in offspring birthweight was 0.72 (0.69–0.75) in mothers and 0.89 (0.86–0.92) in fathers. In aunts/uncles, the HRs were between 0.90 (0.86–0.95) and 0.93 (0.91–0.95). Adjustment for CVD risk factors in a subgroup attenuated all the associations. Conclusions Birthweight was associated with increased risk of CVD in parents and in aunts/uncles. These associations were largely explained by CVD risk factors. Our findings suggest that associations between offspring birthweight and CVD in adult relatives involve both behavioural variables (especially smoking) and shared genetics relating to established CVD risk factors.publishedVersio

The association between BMI and mortality using early adulthood BMI as an instrumental variable for midlife BMI

The article aims to describe the association between midlife body mass index (BMI) and cardiovascular disease (CVD)- and all-cause mortality, and to use early adulthood BMI as an instrumental variable for midlife BMI, in order to obtain an estimate less distorted by midlife confounders and reverse causality. Data from Norwegian health surveys (1974–2003) (midlife BMI, smoking, blood pressure, total cholesterol, heart rate), Military Conscription Records, National Tuberculosis Screenings (early adulthood BMI), National Educational Registry and Cause of Death Registry were linked. Participants with data on BMI in early adulthood and midlife were included (n = 148.886). Hazard Ratio (HR) for CVD mortality was higher in men with midlife obesity relative to normal weight (HR = 1.46(95% CI 1.25, 1.70). For all-cause mortality, HR was higher in those with obesity or underweight in midlife relative to normal weight (Men:HR = 1.19(95% CI 1.09, 1.29), HR = 2.49(95% CI 1.81, 3.43) Women:HR = 1.33(95% CI 1.13, 1.56), HR = 1.61(95% CI 1.22, 2.13)). In instrumental variable analyses, increased BMI became more strongly associated with CVD and all-cause mortality, and the increased risk of all-cause mortality among the underweight attenuated