Simulation of the influence of hydrophones used for the characterization of pressure field distribution in low frequency, high power ultrasonic reactor vessels

This paper describes the use of a finite element (FE) modeling approach to investigate the influence of different hydrophone designs in laboratory scale reactor vessels. In addition to conventional PVDF membrane and piezoceramic hydrophone, the performance of a conceptual array hydrophone, comprising a 2D matrix of PVDF array elements, will be simulated. The FE modeling concentrates on two issues: the disturbance to the field through the introduction of each hydrophone configuration; and their suitability and response to measuring non-linear effects. To simplify the model the ultrasonic transducer is not directly represented. Here, a pressure loading function is used as the excitation technique, with a sawtooth waveform applied for the simulation of the non-linear detection capability of each hydrophone configuration. The results from the simulation programme demonstrate that the dynamics of the reactor vessel are critical to optimize the performance of the ultrasonic system. In addition, the introduction of a hydrophone alters the wave propagation, and hence the field distribution beyond a given probe location. Nevertheless, the spatial pressure distribution at the active area remains reasonably accurate if within the useable bandwidth of the device. Accordingly, the broadband nature of the membrane device is suited to operation in both the linear and non-linear regimes, with the PVDF array membrane device offering a fast, convenient measurement of the pressure field distribution for industrial applications

The causal differential scattering approach to calculating the effective properties of random composite materials with a particle size distribution

An implementation of the Causal Differential Method (CDM) for modelling the effective properties of a random two-phase composite material is presented. Such materials are commonly used as ultrasonic transducer matching layersor backing layers. The method is extended to incorporate a particle size distribution in the inclusion phase. Numerical issues regarding the implementation and convergence of the method are discussed. It is found that, for a given frequency of excitation, the calculated velocity for the composite has a distribution whose variance increases as the volume fraction of inclusions increases. The model predictions would suggest that to reliably and repeatedly manufacture these composites, with a desired mechanical impedance, a low volume fraction of inclusions should be used

Ultrasonic wave propagation in cylindrical vessels and implications for ultrasonic reactor design

Reactors in which processes are enhanced by ultrasound are hampered by the lack of a theoretical framework on their design. Simulation results of ultrasonic wave propagation in a cylindrical geometry are presented in this work, which are then used to develop guidelines for the design of ultrasonic reactors. These guidelines are used to design a new type of reactor with a novel geometry, operating at a frequency of 27kHz, 39kHz and 82kHz. This reactor is characterized using Weissler's reaction dosimetr

Do Job Search Rules and Reemployment Services Reduce Insured Unemployment?

This paper summarizes state unemployment insurance job search policies based on a recent survey of states by the National Association of State Workforce Agencies. It then reviews research results on the effects of reemployment services on durations of insured unemployment. The paper documents how state administrative practices have changed and questions whether these changes may have affected monitoring of claimant compliance with work search requirements. Since state policies on job search and service referral can affect insured durations of unemployment, these policies can also affect the measured total unemployment rate. This paper reflects the opinions of the authors and does not necessarily reflect the positions or viewpoints of the W.E. Upjohn Institute for Employment Research or the U.S. Department of Labor.unemployment insurance, work test, job search assistance, reemployment, public employment service

Unemployment Compensation and Older Workers

Unemployment compensation in the United States is provided through a federal-state system of unemployment insurance (UI). UI provides temporary partial wage replacement to active job seekers who are involuntarily out of work. For older workers, UI is an important source of income security and a potential influence on work incentives. For many, the transition from full-time work in a career job to retirement is voluntary and orderly. For others, job displacement greatly disrupts plans. The transition often involves many intermediate steps. The chain of transitions may include full- or part-time work on another job which most often is not in the same industry and occupation (a bridge job). There may also be movement between bridge jobs, perhaps back from a bridge job to a career job, and finally a gradual movement into full retirement while out of the labor force. Many issues at the forefront of current UI policy debate are also issues of prime importance to those in the second half of their working life. Issues occur in all the standard areas of UI policy: coverage, eligibility, benefit adequacy, duration of benefits, work incentives, benefit financing, and interaction with other programs. This paper provides a brief background sketch of the labor market situation of older workers to examine issues of prime concern to older workers in these areas of UI policy. Our survey of policy issues suggests that changes in UI rules concerning, initial eligibility, continuing eligibility, wage replacement, and partial benefits should all be examined to evaluate effects on the likely employment patterns of older workers. Particular attention should be given to UI features affecting the choice of self-employment, part-time work, seasonal work, and agricultural jobs. The financing consequences of possible UI program changes should also be estimated, as should the macroeconomic impact of broadening recipiency. UI program features which would promote flexible and extended labor force participation by older workers should also enrich the employment choice environment for other workers. Therefore, it would be useful to examine the impact of such program changes on UI as a built-in stabilizer of aggregate expenditures. While younger workers are usually committed to long-term participation in the labor force, older citizens are often more flexible in choosing to use their time. Worsening labor shortage conditions in the United States mean that efforts to retain older workers in the labor force will intensify. The current and potential influence of UI on the income security and labor force participation of older workers should be well understood.unemployment, insurance, compensation, older, workers, O'Leary, Wandner

Intergovernmental Relations and Employment Policy: The United States Experience

Policies to regulate and support labor markets in the United States have mainly been an initiative of the federal government. Historically, states and localities were reluctant to act independently to build up worker rights and protections for fear of competitively disadvantaging resident industries with added costs. Federal constitutional authority to raise revenue and control commerce among the states governed development of labor market policy in the United States. Labor market support initiatives usually have been forged in difficult economic times with contributions and compromise from the full political spectrum. This paper examines the development of employment policy in the twentieth century by viewing the interplay of federal, state, and local partners. The programs considered include unemployment insurance, training, youth programs, and the employment service. Some attention is also given to governmental policy that influences the geographic mobility of labor. Intergovernmental relations in labor market policy have resulted in a system that performs a wide variety of functions, varies greatly at the local and state levels, but maintains important federal standards nationwide.unemployment, intergovernmental, relations, employment, policy, O'Leary, Straits

Investigating the influence of the constituent materials on the performance of periodic piezoelectric composite arrays

This paper describes a theoretical investigation into the influence of the constituent materials on periodic composite array transducer performance. A finite element (FE) model, configured in PZFlex, is used to analyze the performance of a wedge coupled array transducer operating into a steel component. Here, the improvements offered by new single crystal piezoelectric materials are compared to standard PZT‐based configurations. In addition, new passive polymer materials, possessing low longitudinal loss and high shear loss, are evaluated for their potential to significantly reduce inter‐element mechanical cross talk. The FE results illustrate the potential for the next generation of array transducers incorporating these new materials and this is highlighted in the A‐scan predictions from simulated defects

Experimental assessment of periodic piezoelectric composite arrays incorporating an anisotropic passive phase

This paper discusses the experimental assessment of a number of piezoelectric composite array structures incorporating a novel passive phase exhibiting anisotropic elastic properties. The passive polymer phase has been designed to limit inter-element crosstalk by attenuating lateral propagation across the array aperture. A selection of water coupled linear array coupons, operating with a nominal 400 kHz fundamental thickness mode frequency, has been prepared comprising the novel anisotropic passive phase. As a control, comparisons are made to similarly configured devices employing isotropic filler materials. Scanning laser vibrometry and measurements of electrical impedance characteristic on the array substrate demonstrate that the fundamental thickness mode of the devices configured with anisotropic polymer fillers is not contaminated by parasitic modes of vibration. The reasons for this are explained by considering the dispersion characteristics of the substrate. Water coupled hydrophone measurements of array element directivity; transmit voltage response and subsequently efficiency calculations illustrate that the observed reduction in mechanical cross talk has not been achieved at the expense of element sensitivity. Finally, comparisons between the experimental data and the PZFlex derived array responses are made, with good corroboration demonstrate

A 2D ultrasonic array design incorporating hexagonal-shaped elements for NDE applications

Contemporary 2D Ultrasonic arrays suffer from low SNR and limited steering capabilities. Yet, there is a great desire in the industry to increase the operational frequency, in order to enhance their volumetric imaging resolution. State-of-the art arrays use an orthogonal matrix of rectangular elements as this is a natural step forward from the conventional 1D array structure. The objective of this work is to evaluate properties of triangular, rather than rectangular ceramic pillars in a 1-3 connectivity piezoelectric composite for application in a hexagonal-element 2D array. A 3MHz prototype device exploiting new hexagonal substructure have been manufactured. Measured mechanical cross-coupling level is -21.9dB between neighbouring hexagonal elements, providing validation of simulation result. Corroboration between measured and FE modelled device behaviour is demonstrated

Performance of periodic piezoelectric composite arrays incorporating a passive phase exhibiting anisotropic properties

This paper explores the minimisation of interelement cross talk in 1-D and 2-D periodic composite array structures through the incorporation of a passive phase exhibiting anisotropic elastic properties. Initially the PZFlex finite element code was used to monitor array aperture response as a function of material properties. It is shown that in array structures comprising passive polymer materials possessing low longitudinal loss and high shear loss, inter-element mechanical cross talk is reduced, without a concomitant reduction in element sensitivity. A number of polymer materials with the desired properties were synthesised and their elastic character confirmed through a program of materials characterisation. Finally, a range of experimental devices exhibiting improved directional response, as a result of a significant reduction in interelement cross talk, are presented and the predicted array characteristics are shown to compare favourably in each case