Showing papers by "Technical University of Denmark published in 1978"
TL;DR: The dispersion of sample zones in Flow Injection systems is described by analogy with the mixing of fluids in large-scale chemical reactors as mentioned in this paper, which has resulted in a definition of the sample zone dispersion, its mathematical description, and the identification of the main parameters which can be used to effect the desired degree of mixing.
303 citations
TL;DR: In this article, the origin of meandering and braiding of alluvial rivers is re-analysed in terms of stability theory, and the flow is described by a two-dimensional model.
Abstract: The origin of meandering and braiding of alluvial rivers is re-analysed in terms of stability theory. The flow is described by a two-dimensional model, and the transportation of sediment is separated into bed-load transport and transport of suspended sediment, by use of the improved knowledge of sediment transport mechanisms achieved in recent years. The paper explains why it is important to distinguish between the sediment transported as bed load and that in suspension.The analysis is able to predict whether a river remains stable or tends to meander or braid.The results of the stability analysis are compared with laboratory experiments and data from natural rivers, and the agreement is satisfactory.
214 citations
TL;DR: In this article, the authors used a stereo-photogrammetric system coupled with a stroboscope to study the particle motion near the bottom of a turbulent open channel flow.
Abstract: This study continues the investigation of particle motions near the bottom in a turbulent open channel flow, reported by Sumer & Oguz (1978; hereafter referred to as part 1). Paths of suspended heavy particles were recorded in three dimensions and in time, employing a stereo-photogrammetric system coupled with a stroboscope. In the case of smooth bottom, the measured kinematical quantities concerning the particle motions were found to be in accord with the available information on the ‘bursting process’. Agreement between the particle motion and the bursting process provided further support for the mechanism of particle suspension near the bottom proposed in part 1. Similar experiments were carried out when the bottom was rough. Comparison between the smooth- and rough-bottom cases could be made on the same basis as the flow Reynolds number as well as the particle properties were kept almost unchanged in both the smooth and rough boundary experiments. The observations showed that particle motions close to the rough bottom are very similar in character to those in the smooth-bottom case. The findings of the present paper suggested that the suspension mechanism given for the smooth-boundary flow could be extended to the rough-boundary case.
211 citations
29 Jan 1978
TL;DR: In this article, the equations describing conservation of mass, momentum and energy in a turbulent free surface flow are derived for a controle volume extending over the whole depth, where the equations are applied to the energy balance in a surf zone wave motion.
Abstract: The equations describing conservation of mass, momentum and energy in a turbulent free surface flow are derived for a controle volume extending over the whole depth. The effect of the turbulent surface oscillations are discussed but neglected in the following analysis, where the equations are applied to the energy balance in a surf zone wave motion. This leads to results for the wave height variation and the velocity of propagation. The results cannot be reconciled completely with measurements and the concluding discussion is aimed at revealing how the model can be improved.
180 citations
TL;DR: In this article, a simplified model with independent, noninteracting atoms is proposed to explain the behavior of Urbach's rule in the whole temperature range in the II-VI compound ZnO.
Abstract: The exponential absorption edge (known as Urbach's rule) observed in most materials is interpreted in terms of thermal fluctuations in the band-gap energy. The main contribution to the temperature shift of the band-gap energy is due to the temperature-dependent self-energies of the electrons and holes interacting with the phonons. Since the phonon number is fluctuating in thermal equilibrium, the band-gap energy is also fluctuating resulting in an exponential absorption tail below the average band-gap energy. These simple considerations are applied to derive Urbach's rule at high temperatures, while a simplified model with independent, noninteracting atoms is proposed to explain the behavior of Urbach's rule in the whole temperature range. The three parameters entering Urbach's rule are expressed in terms of parameters derived from the temperature shift of the band gap and from the exciton absorption. Comparison with experiments is performed for the II-VI compound ZnO. It is shown that a good agreement is found between the temperature shift of the exciton line observed experimentally and the temperature shift computed from the steepness parameter of Urbach's rule. The agreement with experimental values for the two other parameters is also satisfactory. It is shown that the band-gap shift (and absorption tail) in ZnO is caused by interaction with both acoustical and optical phonons. While the temperature-dependent polaron contribution can account for the optical-phonon contribution, the deformation-potential interaction with LA phonons is not sufficient to account for the acoustical-phonon contribution.
160 citations
TL;DR: Of 28 patients with chronic nickel dermatitis 17 experienced aggravation following oral ingestion of 2.5 mg nickel, but not a placebo tablet, the dermatitis of 9 of the 17 patients improved during a period of 6 weeks on a low nickel diet.
Abstract: Of 28 patients with chronic nickel dermatitis 17 experienced aggravation following oral ingestion of 2.5 mg nickel, but not a placebo tablet. The dermatitis of 9 of the 17 patients improved during a period of 6 weeks on a low nickel diet. The dermatitis of 7 of the 9 patients flared again when a normal diet was resumed. Nickel excretion over a 24-h period was measured for 14 of the 17 patients by atomic absorption spectrophotometry before, during, and after the diet. Reduced excretion was seen during the diet with no statistically significant difference between patients whose dermatitis improved during the diet and those whose dermatitis showed no change.
145 citations
DOI•
29 Jan 1978TL;DR: In this article, the authors defined a reasonable definition of a quasi-steady wave as one which changes little during the time a water particle takes to pass through it and excluded water particles which may become trapped in a surface roller and surf along with the wave.
Abstract: On gently sloping beaches, almost all water waves break. After the initial breaking the water motion usually appears quite chaotic. However, for a moderate time, for example two or three times the descent time of the "plunge" in a plunging breaker, the flow can be relatively well organised despite the superficial view which is largely of spray and bubbles. If waves continue to break the breaking motion, or "white water" soon becomes fully turbulent and the mean motions become quasisteady. A reasonable definition of a quasi-steady wave is one which changes little during the time a water particle takes to pass through it. We exclude water particles which may become trapped in a surface roller and surf along with the wave. At this stage in its development a wave on a beach may be described as a spilling breaker or as a bore. In fact, there is a range of these waves from those with a little white water at the crest to examples where the whole front of the wave is fully turbulent. In investigating the properties of such waves it is desirable to start by looking at the whole range of related motions. The most obvious extension is to the hydraulic jump; since, in the simplest view, it is equivalent to a bore but in a frame of reference moving with the wave. It is also an example where the mean flow is steady rather than quasisteady.
133 citations
TL;DR: In this article, the authors show how discrete Fourier transformation can be implemented as a filter bank in a way which reduces the number of filter coefficients, leading to new forms of FFT's, among which is a \cos/sin FFT for a real signal which only employs real coefficients.
Abstract: The paper shows how discrete Fourier transformation can be implemented as a filter bank in a way which reduces the number of filter coefficients. A particular implementation of such a filter bank is directly related to the normal complex FFT algorithm. The principle developed further leads to types of DFT filter banks which utilize a minimum of complex coefficients. These implementations lead to new forms of FFT's, among which is a \cos/\sin FFT for a real signal which only employs real coefficients. The new FFT algorithms use only half as many real multiplications as does the classical FFT.
112 citations
TL;DR: For elastic-plastic sheets under biaxial stretching localized necking is investigated in this article assuming that the material follows a kinematic hardening rule, and it is found that the forming limit curves predicted by kinematicshardening are in far better agreement with experimental results than the similar curve predicted by standard flow theory with isotropic hardening, which may be considered as a simple model of a solid that develops a vertex on the yield surface.
104 citations
TL;DR: In this article, the K-edge photoabsorption spectra of the 4D metals calculated by the linear augmented plane-wave method yield good agreement with the measured data, and the prominent systematic features of the spectra, most easily discussed in terms of the l = 1 projected density of final states, are simply related to the underlying band structure.
Abstract: The K-edge photoabsorption spectra of the 4d metals calculated by the linear augmented-plane-wave method yield good agreement with the measured data. The prominent systematic features of the spectra, most easily discussed in terms of the l = 1 projected density of final states, are simply related to the underlying band structure. The results for Zr, Mo, Pd, and Ag are presented here.
99 citations
TL;DR: In this article, the main results of these investigations and compare the data with those reported previously1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,40
Abstract: SUPERSTRUCTURE in taenite of the iron meteorite, Cape York, has already been reported1,2. We have now studied taenite (f.c.c. iron–nickel alloy) from another iron meteorite, Toluca, using Mossbauer and X-ray techniques. Here we report the main results of these investigations and compare the data with those reported previously1,2.
TL;DR: It is proved that the algorithm will always converge to the set of stationary points of the problem, a stationary point being defined in terms of the generalized gradients of the minimax objective function.
Abstract: We present an algorithm for nonlinear minimax optimization subject to linear equality and inequality constraints which requires first order partial derivatives. The algorithm is based on successive linear approximations to the functions defining the problem. The resulting linear subproblems are solved in the minimax sense subject to the linear constraints. This ensures a feasible-point algorithm. Further, we introduce local bounds on the solutions of the linear subproblems, the bounds being adjusted automatically, depending on the quality of the linear approximations. It is proved that the algorithm will always converge to the set of stationary points of the problem, a stationary point being defined in terms of the generalized gradients of the minimax objective function. It is further proved that, under mild regularity conditions, the algorithm is identical to a quadratically convergent Newton iteration in its final stages. We demonstrate the performance of the algorithm by solving a number of numerical examples with up to 50 variables, 163 functions, and 25 constraints. We have also implemented a version of the algorithm which is particularly suited for the solution of restricted approximation problems.
01 Jan 1978
TL;DR: In this paper, the authors presented measurements of film flow rates, pressure gradients, film thicknesses, wave frequencies and velocities, and burnout heat fluxes in one annular and two tubular geometries.
Abstract: Measurements are presented of film flow rates, pressure gradients, film thicknesses, wave frequencies and velocities, and burnout heat fluxes in one annular and two tubular geometries. The more than 250 experiments were performed with steam-water at 30 to 90 bar under both adiabatic and diabatic conditions. On the basis of these data a film-flow model for the prediction of burnout is set up. General film roughness and entrainment correlations are derived from the adiabatic, equilibrium data. The capability of the model is demonstrated by several comparisons between calculations and measurements. This report is submitted to the Technical University of Denmark in partial fulfilment of the requirements for obtaining the lie.tech. (Ph.D.) degree. ISBN 87-550-0593-4 ISSN 0418-6443
TL;DR: In this paper, the authors presented data on dc conductivity and thermoelectric power for four organic conductors related to tetrathiafulvalene-tetracyano-$p$-quinodimethane (TTF-TCNQ) with metal-insulator transitions at temperatures around 50 K.
Abstract: Data on dc conductivity $\ensuremath{\sigma}(T)$ and thermoelectric power $S(T)$ for four organic conductors related to tetrathiafulvalene-tetracyano-$p$-quinodimethane (TTF-TCNQ) are presented. They all qualitatively behave as TTF-TCNQ with metal-insulator ($M\ensuremath{-}I$) transitions at temperatures around 50 K. Tetramethyltetraselenafulvalene-tetracyano-$p$-quinodimethane (TMTSeF-TCNQ) has $\ensuremath{\sigma}(300 \mathrm{K})=1000$ ${\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}$ ${\mathrm{cm}}^{\ensuremath{-}1}$, $\frac{{\ensuremath{\sigma}}_{max}}{\ensuremath{\sigma}(300 \mathrm{K})}=7$, $S(300 \mathrm{K})=8\ensuremath{\mu}\mathrm{V}/\mathrm{K}$. For tetramethyltetraselenafulvalene-dimethyltetracyano-$p$-quinodimethane (TMTSeF-DMTCNQ) $\ensuremath{\sigma}(300 \mathrm{K})=500$ ${\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}$ ${\mathrm{cm}}^{\ensuremath{-}1}$, $\frac{{\ensuremath{\sigma}}_{max}}{\ensuremath{\sigma}(300 \mathrm{K})}=10$, $S(300 \mathrm{K})=11$ \ensuremath{\mu}V/K. The sulphur analogue tetramethyltetrathiafulvalene-dimethyltetracyano-$p$-quinodimethane (TMTTF-DMTCNQ) has $\ensuremath{\sigma}(300 \mathrm{K})=120$ ${\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}$ ${\mathrm{cm}}^{\ensuremath{-}1}$, $\frac{{\ensuremath{\sigma}}_{max}}{\ensuremath{\sigma}(300 \mathrm{K})}=3$, $S(300 \mathrm{K})=\ensuremath{-}30$ \ensuremath{\mu}V/K, while for diethyldimethyltetraselenafulvalene-tetracyano-$p$-quinodimethane (DEDMTSeF-TCNQ) we find $\ensuremath{\sigma}(300 \mathrm{K})=500$ ${\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}$ ${\mathrm{cm}}^{\ensuremath{-}1}$, $\frac{{\ensuremath{\sigma}}_{max}}{\ensuremath{\sigma}(300 \mathrm{K})}=9$, $S(300 \mathrm{K})=18\ensuremath{\mu}\mathrm{V}/\mathrm{K}$. $S(T)$ for the selenium-containing materials is small and metallic above the $M\ensuremath{-}I$ transitions. $S(T)$ for TMTTF-DMTCNQ is high and negative with only slight temperature dependence above 100 K. TMTTF-DMTCNQ and TMTSeF-DMTCNQ are discussed in terms of a simple model of independent stacks. The transport on the DMTCNQ stacks is found to be diffusive while the TMTSeF stacks are in the band regime. Polarized-reflectance data are given for TMTSeF-TCNQ, TMTSeF-DMTCNQ, and TMTTF-DMTCNQ. A plasma edge in the near infrared is found in all cases. The small shifts observed are interpreted as arising from variations in effective bandwidths and degrees of charge transfer. The extrapolated zero-frequency optical conductivity is similar in all materials and greater than the measured dc conductivities, indicating different relaxation mechanisms at dc and at optical frequencies. In the $M\ensuremath{-}I$ transition region zero crossings of $S(T)$ are generally found and at low temperatures both $\ensuremath{\sigma}(T)$ and $S(T)$ show semiconducting behavior. Exceptions are TMTSeF-DMTCNQ doped with methyl-TCNQ (MeTCNQ) and DEDMTSeF-TCNQ, where the transitions are smeared. This smearing is attributed to static disorder.
TL;DR: The character of the transition has a natural explanation that will be considered in this article, where a supercritical flow in a channel is forced to change into a subcritical flow, the transition will normally occur as a hydraulic jump, and the energy loss in the jump will be large.
Abstract: When a supercritical flow in a channel is forced to change into a subcritical flow, the transition will normally occur as a hydraulic jump, and the energy loss in the jump will be large. With increasing supercritical depth and decreasing difference between the corresponding depths the energy loss will diminish and the transition will at last occur in an oscillating manner. The character of the transition has a natural explanation that will be considered.
TL;DR: In this article, the angular dependence of secondary-electron emission (SEE) spectra from tungsten surfaces has been studied as a function of polar angle 0/sup 0/ < or approx.
Abstract: Angle-resolved energy-distribution measurements of secondary-electron emission (SEE) from metals reveal spectral fine structure that relates directly to the density distribution of the one-electron states throughout E-K space located above the vacuum level E/sub v/. The angular dependence of the SEE spectra from (100), (110), and (111) tungsten surfaces has been studied as a function of polar angle 0/sup 0/ < or approx. = THETA < or approx. = 70/sup 0/ along azimuthal directions phi such that the energy- and angle-resolved SEE current j/sub SEE/ (E, ..cap omega..) effectively scans states throughout the 1/48th irreducible body-centered-cubic zone. Calculations have been carried out in both ''reduced'' and ''extended'' K space in order to assess the relative contribution of elastic umklapp scattering to the density distribution of contributing states profiles. The results indicate that the overall secondary-electron yield may be represented as the sum of basically two contributions J/sup total//sub SEE/ = ..integral../sup ..pi..//sub 0/ d..cap omega.. ..integral../sup E//sup max//sub 0/ j/sub SEE/ (E,..cap omega..)2dE = J/sup bulk//sub SEE/ + J/sup surface/ /sub SEE/. The bulk contribution represents emission due to Bloch waves propagating out of states in the semi-infinite crystal; the surface contribution represents that part of the current due to evanescentmore » waves at the metal-vacuum interface. In addition, transmission-induced spectral features are observed (transmission resonances), which are not related to the density-of-states fine structure, but are due to a quantum-mechanical enhancement in the escape probability arising from wave-function matching at the emitter-vacuum interface. Bulk and surface band-structure effects are concurrently manifest in the SEE spectra via the wave-matching conditions imposed at the solid-vacuum interface. The results are discussed within the general conceptual framework provided by ''the (time-reversed) incoming final-state wave-function'' approach to electron emission phenomenology.« less
TL;DR: In this article, a mathematical model for oxygen fluctuations in a river reach, involving photosynthesis, total community respiration, and reaeration, is presented, and analogy is established between diurnal oxygen fluctuations and diurnal pH-fluctuations.
TL;DR: The Flow Injection technique is shown to provide fast, reliable and sensitive methods for the determination of calcium in various aqueous as well as serum samples; spectrophotometric or potentiometric detection can be used as mentioned in this paper.
TL;DR: In this paper, Opulus iridoid glycoside esters were isolated from foliage of Viburnum opulus (Caprifoliaceae) and the structures were elucidated by 1 H and 13 C-NMR spectroscopy and by cleavage of the glycosidic linkage with boron trifluoride etherate in acetic anhydride.
TL;DR: In this paper, steady, slowly varying water waves propagating over a gently sloping bed on a steady current were considered, and an average La-grangian was obtained by heuristic arguments from Clebsch potentials.
Abstract: This paper considers steady, slowly varying water waves propagating over a gently sloping bed on a steady current. The current varies linearly with depth, and so has constant vorticity ω. The analysis is two-dimensional and dissipation is neglected. Definitions, and expressions correct to second order in the amplitude, are given for the radiation stress, wave energy density E and total energy flux. An average La-grangian [Lscr ], obtained by heuristic arguments from Clebsch potentials, leads to the result that for this particular problem E equals the wave action [Lscr ]ω times the angular frequency ωrm relative to a frame of reference moving with the average-over-depth current velocity Um. This determines the variation of the amplitude with distance explicitly. An analytical expression for the height of the mean water surface is found by a heuristic argument which compares the conservation equations for total energy and wave action. All the results have been checked directly by substitution back into the basic equations. Graphs illustrate the effect of the vorticity ω on the wavelength, amplitude and set-down.
TL;DR: In this article, rotationally symmetric solitary wave solutions to the sine-Gordon equation (ring waves) are shown to reach a maximum extension and then shrink, which is referred to as return effect.
Book•
01 Jan 1978TL;DR: This book introduces the reader to the principles used in a broad selection of modern data communication protocols and basic algorithms for use in distributed applications, including not only simple point-to-point data transfer protocols but also multi-party protocols and protocols for ensuring data security.
Abstract: This book introduces the reader to the principles used in a broad selection of modern data communication protocols and basic algorithms for use in distributed applications, including not only simple point-to-point data transfer protocols but also multi-party protocols and protocols for ensuring data security. The presentation uses CSP notation to give a concise description of the protocols considered and to provide a basis for analysis of their properties. Relating theory to practice, with a strong focus on currently important Internet protocols, the book clearly explains how the principles are exploited in real-life protocol designs. Principles of Protocol Design is aimed at third-year students and graduates who are studying computer networks/distributed systems or data communications, as well as professional system designers who are looking for the broad view of the area.
TL;DR: The steam-volatile constituents of fresh radish of Japanese and Kenyan origin have been studied by GC-MS technique as discussed by the authors, and the over-all pattern of compounds in the two materials was similar.
Abstract: The steam-volatile constituents of fresh radish of Japanese and Kenyan origin have been studied by GC-MS technique. The over-all pattern of compounds in the two materials was similar. Ten mustard oils, of which pentyl, hexyl, and 4-methylpentyl isothiocyanate have not previously been reported as products of natural derivation, two related nitriles, dimethyl disulfide, methyl methanethiolsulfinate and 1-methylthio-3-pentanone, a novel sulfide-ketone, together constitute the major volatile sulfur products in the two radish materials. A few non-sulfur volatiles have also been identified. The diversity in chemical structure of the sulfur constituents becomes less surprising when regarded in terms of their biogenetic origin.
TL;DR: In this paper, rather complete Raman and i.r. spectra of the two isotopic ions in aqueous solutions have been obtained and qualitatively interpreted and some revisions of previous assignments are suggested, based on the new data and the isotopic frequency shifts.
TL;DR: In this paper, the authors considered fixed binary convolutional codes which are simultaneously optimal or near-optimal according to three criteria: distance profile d, free distance d, and minimum number of weight d-paths.
Abstract: Fixed binary convolutional codes are considered which are simultaneously optimal or near-optimal according to three criteria: namely, distance profile d , free distance d_{ \infty} , and minimum number of weight d_{\infty} paths It is shown how the optimum distance profile criterion can be used to limit the search for codes with a large value of d_{\infty} We present extensive lists of such robustly optimal codes containing rate R = l/2 nonsystematic codes, several with d_{\infty} superior to that of any previously known code of the same rate and memory; rate R = 2/3 systematic codes; and rate R = 2/3 nonsystematic codes As a counterpart to quick-look-in (QLI) codes which are not "transparent," we introduce rate R = 1/2 easy-look-in-transparent (ELIT) codes with a feedforward inverse (1 + D,D) In general, ELIT codes have d_{\infty} superior to that of QLI codes
TL;DR: The magnetic susceptibility of two LiHo${\mathrm{F}}_{4}$ single crystals has been measured in the range 1.2-4.2 K as mentioned in this paper.
Abstract: The magnetic susceptibility of two LiHo${\mathrm{F}}_{4}$ single crystals has been measured in the range 1.2-4.2 K. Ferromagnetic order occurs at ${T}_{c}=1.527$ K. Above 2.5 K, the susceptibilities parallel and perpendicular to the fourfold $c$ axis are well interpreted by the molecular-field approximation, taking into account the ground state and the first excited state of ${\mathrm{Ho}}^{3+}$ in the crystal field of ${S}_{4}$ symmetry. The experimental results are consistent with ${g}_{\ensuremath{\parallel}}=13.95$ and ${g}_{\ensuremath{\perp}}=0$ for the ground state. The dipolar contribution to the magnetic interaction is about three times larger than the exchange one. Near ${T}_{c}$, the parallel susceptibility is well described by the classical law with logarithmic corrections theoretically predicted by Larkin and Khmel'mitskii for the uniaxial dipolar ferromagnet or by a power law with a critical-exponent value $\ensuremath{\gamma}=1.05$ rather close to 1. The upper limit of the critical region is $\frac{({T}_{max}\ensuremath{-}{T}_{c})}{{T}_{c}}=1.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$.
TL;DR: In this article, a transmission electron microscope investigation has been performed on the adhesion properties of fine gold particles (typical radius 20-30 nm) and it was found that the particle adhesion process is intimately related to the crystallography of the particles and consists both of an elastic part and a plastic part, where twinning plays an important role.
Abstract: A transmission electron microscope investigation has been performed on the adhesion properties of fine gold particles (typical radius 20–30 nm). The adhesion process is found to be intimately related to the crystallography of the particles and consists both of an elastic part and a plastic part, where twinning plays an important role. The main driving force in adhesion is due to the surface energy, but electrostatic forces might also be of importance.