A hollow-fiber enzyme reactor, operating under isothermal and nonisothermal conditions, was built employing a polypropylene hollow fiber onto which â-galactosidase was immobilized. Hexamethylenediamine and glutaraldehyde were used as spacer and coupling agent, respectively. Glucose production was studied as a function of temperature, substrate concentration, and size of the transmembrane temperature gradient. The actual average temperature differences across the polypropylene fiber, to which reference was done to evaluate the effect of the nonisothermal conditions, were calculated by means of a mathematical approach, which made it possible to know, using computer simulation, the radial and axial temperature profiles inside the bioreactor and across the membrane. Percent activity increases, proportional to the size of the temperature gradients, were found when the enzyme activities under nonisothermal conditions were compared to those measured under comparable isothermal conditions. Percent reductions of the production times, proportional to the applied temperature gradients, were also calculated. The advantage of employing nonisothermal bioreactors in biotechnological industrial process was discussed.

The problem of the interplay between normal and anomalous scaling in turbulent systems stirred by a random forcing with a power-law spectrum is addressed. We consider both linear and nonlinear systems. As for the linear case, we study passive scalars advected by a 2d velocity field in the inverse cascade regime. For the nonlinear case, we review a recent investigation of 3d Navier Stokes turbulence, and we present new quantitative results for shell models of turbulence. We show that to get firm statements, it is necessary to reach considerably high resolutions due to the presence of unavoidable subleading terms affecting all correlation functions. All findings support universality of anomalous scaling for the small-scale fluctuations.

The statistical properties of velocity and acceleration fields along the trajectories of fluid particles transported by a fully developed turbulent flow are investigated by means of high resolution direct numerical simulations. We present results for Lagrangian velocity structure functions, the acceleration probability density function, and the acceleration variance conditioned on the instantaneous velocity. These are compared with predictions of the multifractal formalism, and its merits and limitations are discussed.

We present a detailed numerical study of anisotropic statistical fluctuations in stationary, homogeneous turbulent flows. We address both problems of intermittency in anisotropic sectors, and the relative importance of isotropic and anisotropic fluctuations at different scales on a direct numerical simulation of a three-dimensional random Kolmogorov flow. We review a simple argument to predict the dimensional scaling for all velocity moments, in all anisotropic sectors. We extend a previous analysis made on the same data set (Phys. Rev. Lett. 86 (2001) 4831) presenting (i) the statistical behavior of spectra and co-spectra; (ii) high-order longitudinal structure functions; (iii) anisotropic fluctuations of the full tensorial two-points velocity correlations. Among the many issues discussed, we stress the problem of the return-to-isotropy, the universality of anisotropic fluctuations and the foliation mechanism. A new a priori test on sub-grid quantities used in Large-Eddy Simulations is also presented.

In this paper we propose a method for wavelet filtering of noisy signals when prior information about the L2 energy of the signal of interest is available Assuming the independence model according to which the wavelet coecients are treated individually we propose a level dependent shrinkage rule that turns out to be the ?minimax rule for a suitable class say of realistic priors on the wavelet coecients The proposed methodology is particularly well suited for denoising tasks where signal?to?noise ratio is low and it is illustrated on a battery of standard test function tions Performance comparisons with some others methods existing in the literature are provided An example in atomic force microscopy AFM is also discussed Key words and phrases? Atomic force microscopy bounded normal mean ?mini? maxity shrinkage wavelet regression

In this paper we propose a non-linear block shrinkage method in the wavelet domain for estimating an unknown function in the presence of Gaussian noise. This shrinkage utilizes an empirical Bayesian blocking approach that accounts for the sparseness of the representation of the unknown function. The modeling is accomplished by using a mixture of two normal-inverse gamma distributions as a joint prior on wavelet coefficients and noise variance in each block at a particular resolution level. This method results in an explicit and readily implementable weighted sum of shrinkage rules. An automatic, level-dependent choice for the model hyperparameters, that leads to amplitude-scale invariant solutions, is also suggested.

We introduce new versions of lattice Boltzmann methods (LBM) for incompressible binary mixtures where fluctuations of total density are inhibited. As a test for the improved algorithms we consider the problem of phase separation of two-dimensional binary mixtures quenched from a disordered state into the coexistence region. We find that the stability properties of LBM are greatly improved. The control of density fluctuations and the possibility of running longer simulations allow a more precise evaluation of the growth exponent.

In this paper we apply a finite difference lattice Boltzmann model to study the phase separation in a two-dimensional liquid-vapor system. Spurious numerical effects in macroscopic equations are discussed and an appropriate numerical scheme involving flux limiter techniques is proposed to minimize them and guarantee a better numerical stability at very low viscosity. The phase separation kinetics is investigated and we find evidence of two different growth regimes depending on the value of the fluid viscosity as well as on the liquid-vapor ratio.

Using an algorithm for simulating equilibrium configurations, we study a fluctuating helical polymer either (i) contained in a cylindrical pore or (ii) wound around a cylindrical rod. We work in the regime where both the contour length and the persistence length of the helical polymer are much larger than the diameter of the cylinder. In case (i) we calculate the free energy of confinement and interpret it in terms of a wormlike chain in a pore with an effective diameter that depends on the parameters of the helix. In case (ii) we consider the possibility that one end of the helical polymer escapes from the rod and wanders away. The average numbers of turns at which the helix escapes or intersects the rod are measured in the simulations, as a function of the pitch p(o). The behavior for large and small p(o) is explained with simple scaling arguments.

Periodicity of the solutions of whole line difference equations is investigated. The considered equations is linear and of convolution type. The expression of the solution by means of a periodic resolvent kernel is given.

We consider the problem of testing for additivity and joint effects in multivariate nonparametric regression when the data are modelled as observations of an unknown response function observed on a d-dimensional lattice and contaminated with additive Gaussian noise. We propose tests for additivity and joint effects, appropriate for both homogeneous and inhomogeneous response functions, using the particular structure of the data expanded in tensor product Fourier or wavelet bases studied recently by Amato & Antoniadis (2001) and Amato, Antoniadis & De Feis (2002). The corresponding tests are constructed by applying the adaptive Neyman truncation and wavelet thresholding procedures of Fan (1996), for testing a high-dimensional Gaussian mean, to the resulting empirical Fourier and wavelet coefficients. As a consequence, asymptotic normality of the proposed test statistics under the null hypothesis and lower bounds of the corresponding powers under a specific alternative are derived.

Objective: The aim of this study was to develop a computational model of HIV infection able to simulate the natural history of the disease and to test predictive parameters of disease progression. Design: We describe the results of a numerical simulation of the cellular and humoral immune response to the HIV-1 infection as an adaptive pathway in a “bit-string” space. Methods: A total of 650 simulations of the HIV-1 dynamics were performed with a modified version of the Celada-Seiden immune system model . Results: Statistics are in agreement with epidemiological studies showing a lognormal distribution for the time span between the infection and AIDS development. As predictive parameters of disease progression we found that HIV-1 accumulates “bit”-mutations mainly in the peptide sequences recognized by cytotoxic CD8 T cells, indicating that cell-mediated immunity plays a major role in viral control. The viral load set-point was closely correlated with the time from the infection to AIDS development. Viral divergence from the viral quasispecies that was present at the beginning of the infection in long-term non-progressors (LTNP) was found to be similar to that found in rapid progressors at the time CD4 T cells drop below the critical value of 200 cells per ml. In contrast, the diversity indicated by the number of HIV strains present at the same time was higher for rapid and normal progressors compared to LTNP, suggesting that the early immune response can make the difference. Conclusion: This computational model may help to define the predictive parameters of HIV dynamics and disease progression, with potential applications in therapeutic and vaccines simulations.

Using importance sampling we give an asymptotic efficient simulation law for risk processes with delayed claims

We study the transition from the collisionless to the hydrodynamic regime in a two-component spin-polarized mixture of 40K atoms by exciting its dipolar oscillation modes inside harmonic traps. The time evolution of the mixture is described by the Vlasov–Landau equations and numerically solved with a fully three-dimensional concurrent code. We observe a master/slave behaviour of the oscillation frequencies depending on the dipolar mode that is excited. Regardless of the initial conditions, the transition to hydrodynamics is found to shift to lower values of the collision rate as the temperature decreases.