Effect of Crop Residues on Weed Emergence

Weed behaviour in crop fields has been extensively studied; nevertheless, limited knowledge is available for particular cropping systems, such as no-till systems. Improving weed management under no-till conditions requires an understanding of the interaction between crop residues and the seedling emergence process. This study aimed to evaluate the influence of maize and wheat residues, applied in three different quantities (1, the field quantity, 0.5, and 1.5-fold amounts of the field quantity), on the emergence of eight weed species: Abutilon theophrasti, Amaranthus retroflexus, Chenopodium album, Digitaria sanguinalis, Echinochloa crus-galli, Setaria pumila, Sonchus oleraceus, and Sorghum halepense. The experiment was conducted over two consecutive years. The results showed that the quantities 1 and 1.5 could suppress seedling emergence by 20 and 44%, respectively, while the quantity 0.5 seems to promote emergence by 22% compared with the control without residues. Weed species showed different responses to crop residues, from C. album showing 56% less emergence to S. halepense showing a 44% higher emergence than the control without residues. Different meteorological conditions in the two-year experiment also exhibited a significant influence on weed species emergence.

Inverse determination of spatially varying material coefficients in solid objects

Material properties such as thermal conductivity, magnetic permeability, electric permittivity, modulus of elasticity, Poisson's ratio, thermal expansion coefficient, etc. can vary spatially throughout a given solid object as it is the case in functionally graded materials. Finding this spatial variation is an inverse problem that requires boundary values of the field quantity such as temperature, magnetic field potential or electric field potential and its derivatives normal to the boundaries. In this paper, we solve the direct problem of predicting the spatial distribution of the field variable based on its measured boundary values and on the assumed spatial distribution of the diffusion coefficient using radial basis functions, the finite volume method and the finite element method, whose accuracies are verified against analytical solutions. Minimization of the sum of normalized least-squares differences between the calculated and measured values of the field quantity at the boundaries then leads to the correct parameters in the analytic model for the spatial distribution of the spatially varying material property.

Interferometry by deconvolution: Part 1 — Theory for acoustic waves and numerical examples

Interferometry allows for synthesis of data recorded at any two receivers into waves that propagate between these receivers as if one of them behaves as a source. This is accomplished typically by crosscorrelations. Based on perturbation theory and representation theorems, we show that interferometry also can be done by deconvolutions for arbitrary media and multidimensional experiments. This is important for interferometry applications in which (1) excitation is a complicated source-time function and/or (2) when wavefield separation methods are used along with interferometry to retrieve specific arrivals. Unlike using crosscorrelations, this method yields only causal scattered waves that propagate between the receivers. We offer a physical interpretation of deconvolution interferometry based on scattering theory. Here we show that deconvolution interferometry in acoustic media imposes an extra boundary condition, which we refer to as the free-point or clamped-point boundary condition, depending on the measured field quantity. This boundary condition generates so-called free-point scattering interactions, which are described in detail. The extra boundary condition and its associated artifacts can be circumvented by separating the reference waves from scattered wavefields prior to interferometry. Three wavefield-separation methods that can be used in interferometry are direct-wave interferometry, dual-field interferometry, and shot-domain separation. Each has different objectives and requirements.

DETERMINATION OF PARAMETERS IN NONLINEAR HYPERBOLIC PDES VIA A MULTIHARMONIC FORMULATION, USED IN PIEZOELECTRIC MATERIAL CHARACTERIZATION

In this paper we consider the problem of determining material parameter curves that appear as coefficients in nonlinear partial differential equations of hyperbolic type. In order to demonstrate our ideas of an identification method for this class of problems, we consider the model problem of identifying c in the nonlinear wave equation dtt - (c(dx)dx)x = 0 from boundary measurements. Motivated by the fact that in many applications, this inverse problem is naturally posed in frequency domain rather than in time domain, we work in the Fourier transformed setting. Here, nonlinearity can be accounted for by using a multiharmonic Ansatz for the measured field quantity. The searched for material parameter curves are approximated by polynomials of arbitrary order, which enables a reformulation of the parameter identification problem purely in frequency domain, although the parameter curve is a function of time domain values of the field quantity. Based on this formulation, we develop a reconstruction algorithm by means of the above-mentioned model problem. Regularization of the typically unstable identification problem is here achieved by bandlimiting the data and restricting the number of degrees of freedom in the solution. We outline the extension of the proposed method to more general material parameter identification problems, focusing especially on the piezoelectric PDEs, for which we also give numerical results.

Chute Flows of Granular Material: Some Computer Simulations

A computer simulation has been developed to describe unidirectional flows of granular materials. Results are presented for a simulation of the two-dimensional flow of disks or cylinders down an inclined plane or chute. Velocity and solid fraction profiles were measured from the simulated systems and compared with theoretical analyses and are compared with the limited experimental results now available. The behavior is shown to be critically dependent on a third field quantity—the “granular temperature”—a measure of the kinetic energy contained in the random motions of the particles.

Evolutionary isotropic radiation

A directionally dependent source distribution activates within a multi-dimensional isotropic dispersive medium. It then develops into a purely pulsatory state. The radiation problem is approached via a convolution principle. Certain basic postulates are imposed to help secure integral convergence at various stages. The Sommerfeld radiation principle holds through an applied initial condition. One field quantity results from stationary phase approximations and represents a superposition of slowly transient spherical wavemodes over a continuously evolving set of wavenumbers and frequencies that must avoid the source frequency. Their radial group velocities vary coincidentally with a positive reception velocity parameter; the associated dispersion is basically a function of the medium, but admits amplitudes with some dependence on source frequency. Another field quantity accumulates relevant residues which contribute to a superposition of quasisteady spherical wavemodes over an intermittently growing set. This depends not only on the medium, but also on the source frequency, imparted to all such wavemodes, as well as an observation criterion, namely that any specific wavemode is observed after its enclosing energy front crosses the observer, in particular, with an invariant group velocity exceeding that common to all slowly transient wavemodes; amplitudes quickly lose their source-induced time dependence. It is this last quantity that survives in the long run and progresses into a non-trivial purely pulsatory steady state consistent with Lighthill’s radiation principle. Its ultimate survival is accomplished through a permanent flow of source-generated non-transient energy, permanency of the energy supply being guaranteed by an indefinitely sustained source amplitude; moreover, both medium and source never conspire to cancel the supply. On an energy basis, the fading of slowly transient modes may be due to the decreasing group velocity of their energy arrival. Spherically symmetric and axisymmetric cases are briefly examined. Finally, arguments and results are applied, with some modifications, to (i) an unsteadily vibrating elastic plate problem, and (ii) radiation of certain internal gravity waves in a Boussinesq fluid.