Simulation of wave propagation through aberrating layers of biological media

Two iterative methods for the calculation of acoustic reflection and transmission at a rough interface between two media are compared. The methods are based on a continuous version of the conjugate gradient technique. One method is based on plane-wave expansions while the other method is based on boundary integral equations and Green's functions. The methods are compared with regard to computational efficiency, rate of convergence, and residual erro

Ultrasound wave propagation through rough interfaces: Iterative methods

Two iterative methods for the calculation of acoustic transmission through a rough interface\ud between two media are compared. The methods employ a continuous version of the conjugate\ud gradient technique. One method is based on plane-wave expansions and the other on boundary\ud integral equations and Green’s functions. A preconditioner is presented which improves the\ud convergence for spectra that include evanescent modes. The methods are compared with regard to\ud computational efficiency, rate of convergence, and residual error. The sound field differences are\ud determined for a focused ultrasound beam distorted by surfaces having a Gaussian roughness\ud spectrum. The differences are evaluated from the root-mean-square differences on the rough surface\ud and in the focal plane

Iterative calculation of reflected and transmitted acoustic waves at a rough interface

A rigorous iterative technique is described for calculating the acoustic wave reflection and transmission at an irregular interface between two different media. The method is based upon a plane-wave expansion technique in which the acoustic field equations and the radiation condition are satisfied analytically, while the boundary conditions at the interface are satisfied numerically. The latter is accomplished by an iterative minimization of the integrated squared error in the boundary conditions by a conjugate gradient technique, leading to a converging and relatively simple scheme. The plane interface result can be used as starting value. Although in principle the method is rigorous, numerical examples show that in practice there is a lower bound on the error in the boundary conditions which can be achieve

The Effect of Search Frictions on Wages

Labor market theories allowing for search frictions make marked predictions on the effect of the degree of frictions on wages. Often, the effect is predicted to be negative. Despite the popularity of these theories, this has never been tested. We perform tests with matched worker-firm data. The worker data are informative on individual wages and labor market transitions, and this allows for estimation of the degree of search frictions. The firm data are informative on labor productivity. The matched data provide the skill composition in different markets. Together this allows us to investigate how the mean difference between labor productivity and wages in a market depends on the degree of frictions and other determinants. We correct for worker self-selection into high-wage jobs. Using within-market variation, we also investigate the extent of (and explanations for) positive assortative matching

Radial profiles of temperature and viscosity in the Earth's mantle inferred from the geoid and lateral seismic structure

In the framework of dynamical modelling of the geoid, we have estimated basic features of the radial profile of temperature in the mantle. The applied parameterization of the geotherm directly characterizes thermal boundary layers and values of the thermal gradient in the upper and lower mantle. In the inverse modelling scheme these parameters are related to the observables (geoid and seismic structure of the mantle) through the viscosity profile which is parameterized as an exponential function of pressure and temperature. We have tested 104 model geotherms. For each of them we have found proper rheological parameters by fitting the geoid with the aid of a genetic algorithm. The geotherms which best fit the geoid show a significant increase of temperature (600-800ºC) close to the 660-km discontinuity. The value of the thermal gradient in the mid-mantle is found to be sub-adiabatic. Both a narrow thermal core-mantle boundary layer and a broad region with a superadiabatic regime can produce a satisfactory fit of the geoid. The corresponding viscosity profiles show similarities to previously presented models, in particular in the viscosity maximum occurring in the deep lower mantle. The best-fitting model predicts the values of activation volume V and energy E which are in a good agreement with the data from mineral physics, except for V in the lower mantle which is found somewhat lower than the estimate based on melting temperature analysis. An interesting feature of the viscosity profiles is a local decrease of viscosity somewhere between 500 and 1000 km depth which results from the steep increase of temperature in the vicinity of the 660-km discontinuity

Using firm data to assess the performance of equilibrium search models of the labor market

Equilibrium search models are useful tools for the evaluation of labor market policies. Recently developed equilibrium search models of the labor market are able to fit the wage distribution perfectly with longitudinal labor supply data, by estimating an appropriate distribution of labor productivity across firms. This paper formally compares such structural estimates to their directly observed counterparts in firm data. More generally, we investigate the extent to which these models are able to explain the observed distributions of wages, productivities and firm sizes across firms, as well as the extent to which they are able to explain the observed relationships between these variables across firms. The parameters that capture search frictions are estimated with worker data that are matched to the data

Modelling planetary dynamics by using the temperature at the core-mantle boundary as a control variable: effects of rheological layering on mantle heat transport

In planetary convection, there has been a great emphasis laid on the usage of the Rayleigh number as a control parameter for describing the vigor of convection. However, realistic mantle rheology not only depends on temperature, pressure, strain-rate and composition, but also on the nature of the dominant creep mechanism, which varies with pressure and also with temperature. It is difficult to study the effects of varying influences from the convective strength without also changing the mantle flow law in the process. We have adopted the approach of using as the sole control parameter, the temperature at the core mantle boundary, T , in modelling planetary dynamics with a composite non-Newtonian and Newtonian CMB rheology, which is temperature-dependent in the upper mantle and both temperature- and pressure-dependent in the lower mantle. Increasing the T strengthens convective vigor and leads to a non-linear increase of averaged temperature, CMB heat-flow and root-mean-squared velocity. The interior viscosity decreases strongly with T and internal heating due to CMB radioactivity. A viscosity maximum is found in the horizontally averaged viscosity profile at a depth around 2000 km. This viscosity hill moves downward with diminishing amplitude in the face of increasing dissipation number and internal heating. The bottom third of the lower mantle appears to be superadiabatic as a consequence of the stiff lower-mantle rheology. The . scaling relationship between the Nusselt Nu number and T shows a relatively insensitive increase of Nu with T .In CMB CMB . terms of an effective Rayleigh number of the whole system, Ra , the power-law exponent of the Nu Ra relationship is E E very low, around 0.12. Strong pressure-dependence of lower-mantle rheology and its large volume relative to the entire mantle would induce a much lower cooling rate of the planet than previous models based on parameterized convection with a temperature-dependent viscosity. q1998 Elsevier Science B.V. All rights reserved

Cytochrome P-450-Substrate Interaction and Hepatic Drug Metabolism in the Mouse

A drug which has entered the circulation, may be eliminated as such by the kidney and excreted in the urine. This depends on the lipid-solubility (lipophilicity) of the drug. In the kidney, lipid-soluble drugs are reabsorbed by the tubules, whereas water-soluble (hydrophilic) compounds cannot cross the tubular membrane. Upon passing through the liver some lipophilic drugs can be modified by conjugation with polar molecules, in order to be excreted in the bile or the urine, for example in the form of glucuronides. A large number of drugs, however, do not possess reactive groups for conjugation, They might tend to remain in the body, which is, in many cases, undesirable. In the liver, however, an enzyme system is present, which is capable of metabolizing these compounds by introducing polar groups with the aid of molecular oxygen, The metabolites produced may be excreted either directly or after subsequent conjugation