Showing papers by "National Technical University of Athens published in 1988"
TL;DR: It is shown that determining whether s(G) ≤ K, for a given integer K, is NP-hard for general graphs but can be solved in linear time for trees.
Abstract: T. Parsons originally proposed and studied the following pursuit-evasion problem on graphs: Members of a team of searchers traverse the edges of a graph G in pursuit of a fugitive, who moves along the edges of the graph with complete knowledge of the locations of the pursuers. What is the smallest number s(G) of searchers that will suffice for guaranteeing capture of the fugitive? It is shown that determining whether s(G) ≤ K, for a given integer K, is NP-complete for general graphs but can be solved in linear time for trees. We also provide a structural characterization of those graphs G with s(G) ≤ K for K = 1, 2, 3.
412 citations
01 Mar 1988
178 citations
TL;DR: In this paper, the authors have studied the Raman spectra of single crystals of LiTaO in two phases in the temperature range between 290 and 1200 K. The number of normal Raman modes observed in each phase (13 optical branches in the ferroelectric phase and 5 in the paraelectric phase) indicates a transition from a ${C}{3v}$ to a ${D}_{3d}$ symmetry, a result that is in agreement with diffraction data.
Abstract: The Raman spectra of single crystals of ${\mathrm{LiTaO}}_{3}$ have been studied over two phases in the temperature range 290--1200 K. Assignments of normal Raman modes in both the ferroelectric and paraelectric phases are given using data from several scattering configurations and taking into account the directional properties (in the ferroelectric phase) and temperature dependence of observed phonons. The number of normal Raman modes observed in each phase (13 optical branches in the ferroelectric phase and 5 in the paraelectric phase) indicates a transition from a ${C}_{3v}$ to a ${D}_{3d}$ symmetry, a result that is in agreement with diffraction data. Most modes in the ferroelectric phase soften with increasing temperature, the only exceptions being three E symmetry modes which are temperature independent. In the paraelectric phase three modes soften with increasing temperature, while the other two appear to be T independent. The manner in which the Raman spectra vary with temperature supports the theory of the order-disorder mechanism for the transition and also suggests that an ``intermediate phase'' exists (mixing of the ${C}_{3v}$ and ${D}_{3d}$ symmetries) within a temperature range of about 100 K below the critical point.
55 citations
TL;DR: In this article, a two-dimensional surface-mounted prism of varying length is mounted on the floor of an open circuit blow down wind tunnel and flow parameters for the developing boundary layer are deduced from the measurements; longitudinal integral time and length scales are estimated through autocorrelations.
Abstract: Velocity and turbulence measurements are presented for the region after reattachment behind a two dimensional surface-mounted prism of varying length. The prism is mounted on the floor of an open circuit blow down wind tunnel and flow parameters for the developing boundary layer are deduced from the measurements; longitudinal integral time and length scales are estimated through autocorrelations. Reattchment on top of the prism, due to its increased length, affects the characteristics of the developing boundary layer; in this case the shear layer originating from the up-stream edge of the prism splits twice at reattachment points on top and behind the prism and the integral length scales of the turbulent eddies are found to be smaller due to the splitting.
53 citations
TL;DR: In this paper, a model solvent technique is presented according to which polystyrene (PS) foam scrap is recovered in the form of small grains, and the process mainly comprises dissolution of the waste into benzene or toluene, filtering, dispersion of the solution into water, and subsequent distillation.
Abstract: A model solvent technique is presented according to which polystyrene (PS) foam scrap is recovered in the form of small grains. The process mainly comprises dissolution of the waste into benzene or toluene, filtering, dispersion of the solution into water, and subsequent distillation. The alternative solvent/non-solvent systems have been studied on the basis of solution rheology, operating conditions during the recycling procedure, and extents of recovery of PS and solvent. Eventually, the toluene/water system has been chosen for a larger scale experiment (LSE), the product of which was used for characterization work. No influence on critical properties was detected due to the solvent technique followed.
49 citations
TL;DR: Both 3-lead and 1-lead ECG signals are used and QRS complexes are considered as events to be detected and a detection accuracy approximating 99% can be reported.
44 citations
TL;DR: In this paper, the surface integrity of high-strength steel plates, namely dual phase, micro-alloyed and high alloyed chromium steels, is investigated.
43 citations
TL;DR: In this paper, the linearized hydrodynamic forces acting on a bottomless cylindrical body having vertical symmetry axis and oscillating in water of finite depth were dealt with, where the flow field around the structure was subdivided into ring-shaped fluid regions, in each of which an axisymmetric eigenfunction expansion for the velocity potential was made.
42 citations
TL;DR: In this paper, an attempt is made to correlate the phenomenon with structural characteristics of the starting material of the solid-state polyamidation (SSP) process, which may proceed through intermediates of lower melting point, resulting in a distinct solid-melt transition.
34 citations
TL;DR: In this article, an efficient boundary differential integral equation (BDIE) method is presented for the analysis of thin elastic plates with free boundaries of any shape resting on biparametric elastic foundation.
Abstract: An efficient boundary differential integral equation (BDIE) method is presented for the analysis of thin elastic plates with free boundaries of any shape resting on biparametric elastic foundation. The plate, which may have holes, is subjected to concentrated loads, line loads, or distributed surface loads. The solution is achieved by converting the governing boundary value problem to an equivalent problem consisting of five coupled boundary equations, two of which are differential and three of which are integral. The boundary differential equations are derived from the boundary conditions, while the boundary integral equations are derived from the integral representations for the deflections of the plate and of the foundation region. A numerical technique based on the discretization of the boundary is developed for the solution of the boundary equations. The computational efficiency of the method is increased by converting the domain integrals attributable to loading into boundary line integrals. Numeric...
32 citations
TL;DR: In this paper, a theoretical model describing the progressive extensible plastic collapse of thin-wall conical and cylindrical shells is presented, which enables the load-deflection curves during axial compression following the deformation history of the shell to be evaluated.
28 Jun 1988
TL;DR: The complexity of a generalization of the unit-execution-time multiprocessor scheduling problem under precedence constraints, in which the number of communication arcs is also minimized, is studied.
Abstract: We study the complexity of a generalization of the unit-execution-time multiprocessor scheduling problem under precedence constraints, in which the number of communication arcs is also minimized. Most versions of the problem are shown NP-complete, and two polynomial algorithms are presented for specialized cases.
TL;DR: In this article, a general expression for the operational matrix of integration, P, in the case of Hermite polynomials is derived, and the optimal control with quadratic index problem and the singularly perturbed optimal control problem are studied.
Abstract: A general expression for the operational matrix of integration, P, in the case of Hermite polynomials is derived. The structure of P is simple and it is therefore computationally attractive. Using this P, the optimal control with quadratic index problem and the singularly perturbed optimal control problem are studied. Some numerical examples are also included.
TL;DR: In this paper, the effect of heavy metal concentration to the anaerobic sludge digestion process is studied by means of simple batch bioassay tests, and it is shown that gas production rate was directly proportional to the concentration of metals in sludge expressed as milliequivalent per litre.
Abstract: The effect of heavy metal concentration to the anaerobic sludge digestion process is studied by means of simple batch bioassay tests. Daily gas production rate and percentage volatile solid reduction decreased with increasing solids concentration. Analysis of the results by least square showed that gas production rate was directly proportional to the concentration of metals in sludge expressed as milliequivalent per litre. Threshold values for inhibition and toxicity limits, corresponding to 20% and 50% decrease in gas production respectively, are derived from the experimental results and can be used as a tool for diagnosis of potential toxicity.
TL;DR: In this paper, a new method is established for the numerical solution of the general type of the finite-part integrals and integral equations of the first and second kind, which are frequently encountered in problems of applied mechanics and especially in plane and antiplane elasticity.
01 Jan 1988
TL;DR: In this paper, a new technique is proposed for the numerical solution of the Cauchy-type singular integral equations, by using the well known Gegenbauer polynomials.
Abstract: A new technique is proposed for the numerical solution of the Cauchy-type singular integral equations, by using the well known Gegenbauer polynomials. A large class of problems of mathematical physics, and especially several plane and antiplane elasticity problems, not possessing closed-form solutions, can be reduced to the solution of certain systems of such a type of singular integral equations. Also by using a certain method the new formula which is used for the numerical solution of the Cauchy-type integral equations can be extended for the general type of the finite-part singular integrals, too.
TL;DR: In this paper, the thermal expansion coefficient of particle-reinforced polymers was evaluated using a theoretical model which takes into account the adhesion efficiency between the inclusions and the matrix.
Abstract: The thermal expansion coefficient of particle-reinforced polymers was evaluated using a theoretical model which takes into account the adhesion efficiency between the inclusions and the matrix — an important factor affecting the thermomechanical properties of a composite. To measure the adhesion efficiency a boundary interphase, i.e. a layer between the matrix and the fillers having a structure and properties different from those of the constituent phases, was considered. This layer is assumed to have varying properties. To obtain information concerning the properties and extent of the interphase, an experimental study of the thermal behaviour of aluminium-epoxy composites was undertaken. Differential Scanning Calorimetry (DSC) measurements were performed to evaluate heat capacity with respect to temperature. In addition, the effects of different factors, such as heating rate and filler concentration on the glass transition temperature of the composite, were examined. The sudden changes in heat capacity values in the glass transition region were used to estimate the extent of the boundary interphase according to an existing theory. Finally, the values of the thermal expansion coefficient, predicted by this model, were compared with theoretical results obtained by other authors and with experimental results.
TL;DR: In this paper, the authors studied the crack initiation from blunt notches under biaxial loading and determined the crack extension angle and the critical fracture stress for notches with finite notch tip radii.
TL;DR: In this article, the authors studied the crack initiation under biaxial loading and determined the crack extension angle and critical stress of fracture for various values of the crack inclination angle and for different values of BIAxial factor k.
TL;DR: In this paper, the main thermodynamic and transport properties of the real NH3/He binary gas mixture are presented in the form of diagrams and analytical expressions, with the temperature and the mass fraction of the mixture as input variables.
Abstract: The behaviour of helium in place of hydrogen as a pressure equalizing gas in triple-fluid absorption refrigeration units is examined. The main thermodynamic and transport properties of the real NH3/He binary gas mixture are presented in the form of diagrams and analytical expressions, with the temperature and the mass fraction of the mixture as input variables. Among others, the mass or mole fraction of the NH3/He gas mixture as a function of the dry and wet bulb temperatures has been plotted. A computer simulation has been developed to calculate the evaporation of NH3 in a NH3/He atmosphere within an insulated tube, allowing measurement of the composition of the mixture via temperature measurement. The results are compared with the corresponding values of evaporation of NH3 in a NH3/H2 atmosphere.
TL;DR: In this article, the effect of atmospheric temperature and humidity changes on the specific power output, specific fuel consumption and nitric oxide emissions from a typical spark-ignited petrol automotive engine is investigated.
TL;DR: In this paper, the exact nonlinear dynamic buckling response of discrete systems under step loading of infinite duration is thoroughly discussed by using one-degree-of-freedom models and the analysis is based on the exact NDE of motions and refers to those systems which when subjected to the same loading applied statically exhibit a limit-point instability.
Abstract: The nonlinear dynamic buckling response of discrete systems under step loading of infinite duration is thoroughly discussed by using one-degree-of-freedom models. The analysis is based on the exact nonlinear differential equations of motions and refers to those systems which when subjected to the same loading applied statically exhibit a limit-point instability. It is found that an unbounded motion may start for the smallest step load which forces the system to pass through an unstable equilibrium state of the postbuckling path with zero total potential energy. This leads to dynamic buckling criteria which allow the determination of exact dynamic buckling loads without solving the corresponding nonlinear differential equations of motion. A comparison of dynamic buckling estimates of previous works with those obtained herein shows that the latter are exact regardless of the magnitude of the initial imperfection. Moreover, some additional results provide a better insight into the actual mechanism of nonline...
TL;DR: In this article, a grid-free Langrangian free vortex method was used for the calculation of the wake characteristics of a HAWT whose geometry was chosen in correspondence to existing experimental results so that comparisons could be possible.
TL;DR: In this paper, the authors presented a gauge theory in ten dimensions based on the gauge group E 8 which is dimensionally reduced, according to the coset space dimensional reduction (CSDR) scheme, to the standard model SU 3c ×SU 2L ×U 1, which breaks further to SU3c ×U1em, which gives similar predictions for sin 2 θ w and proton decay as the minimal SU 5 GUT.
TL;DR: In this article, the efficiency, accuracy and ease in use of linearized stability analyses for establishing the postbuckling response of rigid jointed frames are discussed using as a model a simple rectangular frame.
TL;DR: In this paper, the singular integral equation is applied to a number of appropriately selected collocation points on the integration interval, and then a numerical integration rule is used for the approximation of the integrals in this equation.
Abstract: A new method is proposed, by using some special quadrature rules, for the numerical evaluation of the general type of finite-part singular integrals and integral equations with logarithmic singularities. In this way the system of such equations can be numerically solved by reduction to a system of linear equations. For this reduction, the singular integral equation is applied to a number of appropriately selected collocation points on the integration interval, and then a numerical integration rule is used for the approximation of the integrals in this equation. An application is given, to the determination of the intensity of the logarithmic singularity in a simple crack inside an infinite, isotropic solid.
TL;DR: In this paper, a pseudo-bond graph-modeling approach for gas turbines is described, where the working medium is a compressible fluid, and there is mass flow as well as mechanical, thermal, and fluid power exchanges.
Abstract: A pseudo-bond graph-modelling approach for gas turbines is described. Generally in gas turbines the working medium is a compressible fluid, and there is mass flow as well as mechanical, thermal, and fluid power exchanges. The graph model and the ensuing mathematical model are derived by physical laws. The method presented here offers flexibility and adjustability in dynamic simulations. The technique is applied to the case of a particular gas turbine.
TL;DR: In this paper, the optimum tooth profile of spur gear pumps is calculated and a method of reducing delivery fluctuation is also proposed, which may be achieved through a delivery penalty or through a pump size penalty.
TL;DR: In this article, the stress intensity at the tip of a semi-infinite crack in an orthotropic infinite strip was determined and the clamped strip boundaries were considered, and the stress was shown to be independent of the length of the strip.
Abstract: The stress intensity factor at the tip of a semi-infinite crack in an orthotropic infinite strip was determined. Clamped strip boundaries were considered.
TL;DR: The surface of an Al-4wt%Cu alloy was irradiated with an excimer laser and X-ray measurements showed that this treatment changed the residual stress in the alloy surface layers from tensile to compressive in the first 10 μm below the surface of the laser-treated alloy, the microhardness was found to be higher than that of the as-received alloy as discussed by the authors.
Abstract: The surface of an Al-4wt%Cu alloy was irradiated with an excimer laser Very precise X-ray measurements showed that this treatment changes the residual stress in the alloy surface layers from tensile to compressive In the first 10 μm below the surface of the laser-treated alloy, the microhardness was found to be higher than that of the as-received alloy