Showing papers by "Helsinki University of Technology published in 1984"

The complex dielectric constant of snow at microwave frequencies

Abstract: The complex dielectric constant of snow has been measured at microwave frequencies. New and old snow at different stages of metamorphosis have been studied. The results indicate that the complex dielectric constant is practically independent of the structure of snow. For dry snow, the dielectric constant is determined by the density. For wet snow, the imaginary part and the increase of the real part due to liquid water have the same volumetric wetness dependence. The frequency dependence of the complex dielectric constant of wet snow is the same as that of water. A nomograph for determining the density and wetness of wet snow from its dielectric constant is given. A snow sensor for field measurement of the dielectric constant has been developed. It can be used for determining the density and the wetness of snow by a single measurement.

On the Stabilization of Finite Element Approximations of the Stokes Equations

Abstract: Consider finite element approximation of the Stokes equations. We present a systematic way of stabilizing it by adding bubble functions to the discrete velocity field. Another way of stabilization is also presented where the finite element spaces are kept unchanged but the discrete incompressibility condition is modified instead.

Exact image theory for the Sommerfeld half-space problem, part II: Vertical electrical dipole

Abstract: Applying the Laplace transform, the exact distributed image current function is obtained for the classical Sommerfeld half-space problem with vertical magnetic current source. The resulting field integral is well behaved when the image current is situated in complex space. Unlike previous approximate images, the present theory is valid for any distance, height of the source, frequency, and half-space parameters. It is demonstrated that the present image theory reduces to the well-known dipole image at complex depth for large dielectric parameters of the half-space. Also, the reflection-coefficient method is obtained as a farfield approximation. Calculation of fields through exact image integration is seen to be simple and accurate and require modest computer capacity and time. In an appendix, some properties of the multivalued Green's function arising from a dipole source in complex space are also studied.

Exact image theory for the Sommerfeld half-space problem, part III: General formulation

Abstract: The general Sommerfeld problem with both \epsilon and \mu discontinuous and a source consisting of arbitrarily oriented electric and/or magnetic dipoles at the same location is considered in terms of image theory. The problem is formulated with electric and magnetic fields instead of potential quantities resulting in a vector transmission-line interpretation of the Fourier transformed problem. The image sources are seen to be located in complex space expressable in terms of a certain basic image current function, which was encountered in part II of this paper on the vertical electric dipole problem. The horizontal electric/magnetic dipole image is solved and found to consist of both vertical and horizontal current components. The image concept is generalized to the most general three-dimensional sources. As a check, the well-known reflection coefficient method is obtained as the far-field approximation of the present theory.

Elementary-flux-pinning potential in type-II superconductors

Abstract: Flux pinning brought about by quasiparticle scattering off small-spatial-extent defects is studied using the quasiclassical method. The theory as applied to the pinning problem is presented in detail and an efficient method is given for solving the transportlike equation of the theory numerically. The pinning potential of a model impurity with hard-sphere---scattering phase shifts is computed over the entire temperature range in a pure superconductor with the Ginzburg-Landau parameter $\ensuremath{\kappa}=0.9$. The pinning energies arising from quasiparticle scattering turn out to be up to 2 orders of magnitude larger than those predicted by the traditional theory for small-spatial-extent pinning centers.

Visibility of halftone dot textures

Abstract: A method for estimating the visibility of halftone dot textures is presented. The estimates can be used in the design of dither signals. The method is based on the properties of human visual contrast sensitivity. The accuracy of the estimates was tested experimentally with dot textures produced by four different kinds of dither signals. The visibility predictions agreed very well with the results of psychophysical measurements.

Efficient octree conversion by connectivity labeling

Abstract: We present an algorithm for converting from the boundary representation of a solid to the corresponding octree model. The algorithm utilizes an efficient new connected components labeling technique. A novelty of the method is the demonstration that all processing can be performed directly on linear quad and octree encodings. We illustrate the use of the algorithm by an application to geometric mine modeling and verify its performance by analysis and practical experiments.

Repulsive interaction of the helium atom with a metal surface

Abstract: The repulsive part of the helium scattering potential at a surface is approximately proportional to the surface electron density The proportionality coefficient is shown to be a well-defined quantity, which can be related to the electron-helium scattering length The spread in the values of the proportionality constant suggested in the literature is shown to be due to different definitions of the coefficient or due to inadequate calculational methods The value calculated using the local density approximation with a self-interaction correction is in very good agreement with the electron-scattering-length measurements

A note on the modeling space of Euler operators

Abstract: Alternative modeling spaces for physical solid objects are discussed and it is shown that the so-called Euler operators completely describe the family of objects bounded by 2-manifold surfaces.

Constant-curvature loss in monomode fibers: an experimental investigation.

Abstract: A new technique for the measurement of constant-curvature (bend) loss as a function of bend radius is described, and it is shown that the influence of transition (microbend) losses can be essentially eliminated. Based on this technique and via the application of curve-fitting procedures and the Newton-Raphson method for two variables, a sensible equivalent step index can be obtained if stress-induced changes in the bent fiber are taken into account. However, the prediction of bend losses at other wavelengths is coarse, and it is shown that, for our fiber, agreement between theory and experiment can be considerably improved via an empirically deduced stress-induced dependence that is far stronger than predicted by published theory.

Retrieval of snow water equivalent from Nimbus-7 SMMR data: Effect of land-cover categories and weather conditions

Abstract: The effect of the four major surface types in Finland (forests, boglands, farmlands, and water (ice)) on the microwave brightness temperature of snow-covered areas is investigated. Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) data are employed to derive the response of each surface type. An algorithm to retrieve the water equivalent of snow cover from Nimbus-7 SMMR data is tested in different dry snow conditions.

Adaptive algorithms for estimating eigenvectors of correlation type matrices

Abstract: In several applications of signal processing recursive algorithms for estimating a few eigenvectors of correlation or covariance matrices directly from the incoming samples are desirable. In this paper such algorithms are derived by starting from an extension of the classical power method of numerical analysis, instead of the usual gradient approach. This viewpoint leads to useful and relatively simple rules for determining the gain parameters of Owsley's stochastic gradient ascent algorithm for sensor array processing and Thompson's adaptive algorithm for unbiased frequency estimation using the Pisarenko method. A new, promising algorithm for adaptive estimation of eigenvectors corresponding to the smallest eigenvalues is introduced. Preliminary numerical results and comparisons are given, and a generalization of Thompson's algorithm for estimating several eigenvectors is represented.

Comment on Quad- and Octtrees

Abstract: The space efficiency of recently reported methods for representing quadand octtrees by a linear code may be improved upon. Gargantini recently proposed a method for representing quadtrees [1] and octtrees [2] by storing, in a l inear array, a code for each black leaf. She shows that such a structure has several good characteristics, among them, space efficiency. The purpose of this note is to recall known results on somewhat more space-efficient l inear coding than Gargantini's. We also point out an addit ional efficiency of using binary division of pixel and voxel arrays when a method similar to Gargantini 's is used. In general, we claim that studying the case of binary division provides for a more general understanding of the underlying structures and that less attention is needed on the separate results concerning quadand octtrees. It is well known that a quadtree may be efficiently represented by l inearly encoding its preorder traversal. The scheme of Kawaguchi and Endo [3] requires less than two bits per node. Because there are one third as many internal nodes as leaves this means less than three bits per leaf and because there are, in general, approximately as many black and white leaves, this means less than six bits per black leaf. Similar results hold for octtrees. This is better than what can be achieved by the methods of [1] and [2]. The method of © 1983 ACM 0001-0783/84/0300-0248 75¢ Kawaguchi and Endo [3] does not, however provide for logarithmic access to leaves, and thus, does not support all the algorithms described in [1]. We conducted some exper iments by encoding a black unit disk in its enclosing box. Using a 21° x 21° pixel array, we arr ived at the following figures: There were 5,198 black leaves; the total number of leaves was 11,968; the total number of nodes was 15,957; and the number of bits in an efficient l inear representat ion of the quadtree [3] was 17,905. Thus, in this case, less than four bits per black leaf were needed. Minor improvements are also possible within Gargantini 's framework: She uses symbols from the set (0, 1, 2, 3, X) for encoding leaves so that the length of a code word is 3(n 1) +2 bits for a 2 n × 2 n pixel array. Separately coding the position of the first 'X' results in a code word of 2n + log2(n) bits as she has shown elsewhere [4]. The binary equivalent of the quadtree [5] is not as well known as the quadtree. It is formed completely analogously, except that at each node, the pixel array is split into two parts (Figure 1). The splits al ternate in the x and y directions. Gargantini 's results apply to this binary pixel tree as well. Now, the code symbols would come from the set (0, 1, X). Also many algorithms go over in a straightforward way to binary trees. A binary pixel tree has obviously at most as many black leaves as a quadtree. Thus, if Gargantini 's l inearization method is used the 248 Communications of the ACM March 1984 Volume 27 Number 3 Research Contributions

Encoding pixel trees

Abstract: Linear representations of quad-trees and other pixel trees are studied for efficient encoding of both black-and-white and multicolor images. Based on a random-line model it is shown that asymptotically, as resolution is increased with respect to image detail, a multicolor image can be represented with less than 5 bits per intersected pixel. This can be compared to the 2–3 bits required by chain encoding schemes. It is also shown that binary pixel trees support linear encoding with the same or better efficiency than the traditional quad-tree. It is shown how geometric probability can help in the understanding of the behavior of image encoding schemes.

Glucoamylase and α-amylase production by immobilized Aspergillus niger

Abstract: Aspergillus niger mycelia or spores were immobilized in calcium alginate gel beads and employed for production of glucoamylase and α-amylase by repeated batch process. The immobilized mycelium produced lower enzyme activities than immobilized spores germinated in a growth medium and subsequently cultured in an enzyme production medium. In repeated batch experiments, free cells could be used for only 4 4-day batches, whereas with immobilized spores at least 11 4-day batches with a gradual increase in enzyme activities in each successive batch were possible. The activity ratio of glucoamylase and α-amylase produced was altered by immobilization.

Continuous vortices with broken symmetry in rotating superfluid 3He-A

Abstract: New NMR measurements are reported on continuous $^{3}\mathrm{He}$-$A$ vortices in tilted magnetic fields. We introduce a symmetry classification of the continuous vortices with broken axial symmetry. It is found that the discrete internal symmetry may in addition be broken in two inequivalent ways, producing two different continuous vortices. Although NMR may not distinguish between these two vortices, the observed vortex satellite peak is well accounted for by spin waves localized in the soft core of such vortices.

Ray-casting and block model conversion using a spatial index

Abstract: A spatial index is a data structure designed to facilitate spatial search, exemplified by the point-in-polyhedron inclusion problem. The 3D extensible cell (EXCELL) index is presented together with algorithms for spatial search and for converting a complex polyhedron (boundary representation) into an octree-like block model. We illustrate the techniques by an application to geometric mine modelling and demonstrate the efficiency of the approach by practical experiments.

Maximum entropy theory of recoil charge distributions in electron-capture collisions

Abstract: A generalized Fermi-Dirac distribution is derived and applied to charge-state distributions in single collisions between multiply charged ions and rare-gas atoms. It relates multiple electron loss in single-electron capture to multiple ionization in multiphoton absorption and discloses inner-shell vacancy formation in double- and triple-electron capture.

Propagating-beam-method analysis of two-dimensional microlenses and three-dimensional taper structures

Abstract: We employ the propagating-beam method to determine the increase in coupling efficiency that is achievable if the end of a two-dimensional monomode taper is rounded before it is placed in front of a semiconductor laser. We find that the coupling efficiency in some cases can be augmented by more than a factor of 2. Subsequently, we describe some results that relate to three-dimensional taper structures and couplers built of two taper structures placed with their wide ends adjacent.

Space sweep solves intersection of convex polyhedra

Abstract: Plane-sweep algorithms form a fairly general approach to two-dimensional problems of computational geometry. No corresponding general space-sweep algorithms for geometric problems in 3- space are known. We derive concepts for such space-sweep algorithms that yield an efficient solution to the problem of solving any set operation (union, intersection, ...) of two convex polyhedra. Our solution matches the best known time bound of O(n log n), where n is the combined number of vertices of the two polyhedra.

Lattice dipole gas and (▽φ)4 models at long distances: Decay of correlations and scaling limit

Abstract: We prove that the scaling limit for a large class of weakV(▽φ) perturbations of the free massless lattice field φ is Gaussian with the covariancec(V)(− Δ)−1. The correlations as well asc(V) are analytic inV. In particular the Mayer series for the dipole gas is convergent for small activity.