A discrete methodology for controlling the sign of curvature and torsion for NURBS

This paper develops a discrete methodology for approximating the so-called convex domain of a NURBS curve, namely the domain in the ambient space, where a user-specified control point is free to move so that the curvature and torsion retains its sign along the NURBS parametric domain of definition. The methodology provides a monotonic sequence of convex polyhedra, converging from the interior to the convex domain. If the latter is non-empty, a simple algorithm is proposed, that yields a sequence of polytopes converging uniformly to the restriction of the convex domain to any user-specified bounding box. The algorithm is illustrated for a pair of planar and a spatial Bézier configuration

Construction of Smooth Branching Surfaces using T-splines

The request for designing or reconstructing objects from planar cross sections arises in various applications, ranging from CAD to GIS and Medical Imaging. The present work focuses on the " one-to-many " branching problem, where one of the planes can be populated with many, possibly tortuous and densely packed, contours. The proposed method combines the proximity information offered by the Euclidean Voronoi diagram with the concept of surrounding curve, introduced in [1], and T-splines technology [2] for securing a flexible and portable representation. Our algorithm delivers a single T-spline that deviates from the given contours less than a user-specified tolerance, measured via the so-called discrete Fréchet distance [3] and is C 2 everywhere except from a finite set of point-neighborhoods. Subject to minor enrichment, the algorithm is also capable to handle the " many-to-many " configuration as well as the global reconstruction problem involving contours on several planes

Is the construction of a sanitary landfill acceptable in a karstic area? The case of the sanitary landfill site in Fokida, Central Greece

Για τη μελέτη της καταλληλότητας μιας συγκεκριμένης θέσης ως Χ.Υ.Τ.Υ. πραγματοποιήθηκαν οι εξής ερευνητικές εργασίες. Εκπόνηση λεπτομερούς γεωλογικής χαρτογράφησης σε αρχική κλίμακα 1:5.000 και στη συνέχεια κατασκευή γεωλογικής- υδρογεωλογικής τομής που περνάει από τη θέση του Χ.Υ.Τ.Υ. Στη συνέχεια εκτελέσθηκε γεωτρητικό πρόγραμμα τόσο με βαθιά γεώτρηση για τη διαπίστωση ύπαρξης στάθμης υπόγειου υδροφόρου ορίζοντα όσο και δειγματοληπτικές γεωτρήσεις με δοκιμές εισπίεσης. Επιπλέον, εφαρμόσθηκαν μέθοδοι εκτίμησης της τρωτότητας υποκείμενου υδροφόρου στρώματος (DRASTIC και EPIC). Τέλος υπολογίσθηκε ο ετήσιος όγκος απορριμμάτων που θα τοποθετούνται στο Χ.Υ.Τ.Υ., ο οποίος δίνει και το μέγεθος του ρυπαντικού φορτίου.This paper investigates the suitability of a specific site for the construction of a sanitary landfill. The following works were performed: detailed geological mapping at a scale of 1:5,000, a geological-hydrogeological cross-section of the sanitary landfill, drilling exploration including the construction of a deep borehole for the detection of any perched aquifer, core logging and in situ permeability tests, implementation of the DRASTIC and EPIC methods to estimate the aquifer’s vulnerability. Finally estimation of the total annual amount of solid waste that will be deposited into the sanitary landfill and determination of the pollution load

Ship-hull shape optimization with a T-spline based BEM-isogeometric solver

In this work, we present a ship-hull optimization process combining a T-spline based parametric ship-hull model and an Isogeometric Analysis (IGA) hydrodynamic solver for the calculation of ship wave resistance. The surface representation of the ship-hull instances comprise one cubic T-spline with extraordinary points, ensuring C2 continuity everywhere except for the vicinity of extraordinary points where G1 continuity is achieved. The employed solver for ship wave resistance is based on the Neumann-Kelvin formulation of the problem, where the resulting Boundary Integral Equation is numerically solved using a higher order collocated Boundary Element Method which adopts the IGA concept and the T-spline representation for the ship-hull surface. The hydrodynamic solver along with the ship parametric model are subsequently integrated within an appropriate optimization environment for local and global ship-hull optimizations against the criterion of minimum resistance

Using acoustic emissions to enhance fracture toughness calculations for CCNBD marble specimens

Rock fracture mechanics has been widely applied to blasting, hydraulic fracturing, mechanical fragmentation, rock slope analysis, geophysics, earthquake mechanics and many other science and technology fields. Development of failure in brittle materials is associated with microcracks, which release energy in the form of elastic waves called acoustic emissions. In the present study, acoustic emission (AE) measurements were carried out during cracked chevron notched Brazilian disc (CCNBD) tests on Nestos marble specimens. The fracture toughness of different modes of loading (mode-I and –II) is calculated and the results are discussed in conjunction with the AE parameters

Coupling an inviscid IGA – BEM solver with X-Foil's boundary-layer model for 2D flows

In this work we couple an IGA-BEM solver for 2D lifting flows with the viscous model in X-Foil [1], a vintage but still widely used software tool for the design and analysis of subsonic airfoils, towards deeper integration of the isogeometric concept in 2D flow models that incorporate boundary-layer corrections. The formulation of the exterior potential-flow problem reduces to a Boundary Integral Equation (BIE) for the associated velocity potential. Adopting the approach presented in [3], the resulting BIE is handled by an IGA-BEM method, combining: (i) A generic B-splines parametric modeler for generating hydrofoil shapes, using a set of 8 design-oriented parameters; (ii) The very same basis of the geometric representation for representing the velocity potential, and (iii) Collocation at the Greville abscissas of the knot vector of the hydrofoil’s B-splines representation, appropriately enhanced to accommodate the null-pressure jump Kutta condition at the trailing edge. For the viscous part of the solution, the two-equation model of X-Foil [2] is employed. X-Foil’s inviscid solver is “circumvented” and inviscid isogeometric parameters are sent to its viscous component, namely the integral momentum and kinetic energy shape parameter equations presented in [2]. The derived coupled system is tested for NACA4412 and NACA0012 airfoils and the output lift and drag coefficients for different angle of attacks are compared to experimental data, uncoupled X-Foil results and one-way coupling results obtained [4] via the software tool PABLO [5]. The so-resulting coupled system can be used in airfoil/hydrofoil shape optimisation algorithms with a variety of optimisation criteria such as maximum lift coefficient, maximum lift-over-drag-ratio, minimum deviation of the airfoil/hydrofoil area from a reference area, etc. REFERENCES [1] Drela, M. (1989) “XFOIL: An analysis and design system for low Reynolds number airfoils”, MIT, Massachusetts, USA. [2] Drela, M., Giles, M. (1987) “Viscous – inviscid analysis of transonic and low Reynolds number airfoils”, AIAA Journal, vol. 25(10), pp. 1347 – 1355. [3] Kostas, K.V., Ginnis, A.I., Politis, C.G., Kaklis, P.D. (2017) “Shape-optimization of 2D hydrofoils using an Isogeometric BEM solver”, Computer Aided Design, vol. 82, pp. 79-87. [4] Kostas, K.V., Ginnis, A.-A.I, Politis, C.G., Kaklis P.D. (2017) “Shape-optimization of 2D hydrofoils using one-way coupling of an IGA-BEM solver with a boundary-layer model”, Coupled Problems 2017, VII International Conference on Coupled Problems in Science and Engineering, June 12-14, 2017, Rhodes (GR)

Experimental investigation of the mechanical properties of Alfas stone

This paper focuses on the experimental investigation of the mechanical properties of the Alfas natural building stone. Two series of uniaxial compression tests and indirect tensile tests (Brazilian tests) were performed in order to determine the uniaxial compressive strength and the indirect tensile strength respectively. Different sets of cylindrical specimens and circular discs were prepared by varying their geometry in order to examine the size effect on the respective strength values. Also, the size effect was investigated with respect to the calculated intact rock modulus and Poisson’s ratio. All specimens were prepared by following the ISRM suggested methods and the load was applied using a stiff 1600 kN MTS hydraulic testing machine and a 500 kN load cell. Strain was measured using biaxial 0/90 stacked rosettes appropriately attached on each specimen

Shape-optimization of 2D hydrofoils using one-way coupling of an isogeometric BEM solver with the boundary-layer model

This work combines an Isogeometric (IGA) Boundary-Analysis-Method (BEM) solver for potential flows with a boundary-layer correction model for developing and testing a framework for the shape optimization of hydrofoils placed in low-speed viscous flow.The formulation of the exterior potential-flow problem reduces to a Boundary-Integral Equation (BIE) for the associated velocity potential exploiting the null-pressure jump Kutta condition at the trailing edge. Adopting the approach presented in [1], the numerical solution of the resulting BIE is performed by an IGA-BEM method, combining: (i)a generic B-splines parametric modeler for generating hydrofoil shapes, using a set of eight parameters, (ii)the very same basis of the geometric representation for representing the velocity potential, and (iii)collocation at the Greville abscissas of the knot vector of the hydrofoil’s B-splines representation, appropriately enhanced so that the null-pressure jump Kutta condition at the trailing edge.As for the boundary-layer correction model, we adopt the one-way coupled approach proposed in [2], where the effect of the boundary layer thickness is neglected and one single iteration is performed. More accurately, the external tangential velocity is inherited from the inviscid model with the condition of vanishing-normal velocity on the airfoil’s surface and then fed to the boundary-layer model. The drag coefficient can then be obtained using the Squire-Young formula [3], which in effect computes the momentum deficit as a function of the outcome of the boundary-layer calculations at the trailing edge.In fine, the optimization environment is developed based on the geometric parametric modeler for the hydrofoil, the IGA-BEM solver which feeds the above boundary-layer correction and an optimizer employing a controlled elitist genetic algorithm. Its performance is tested via shape optimization examples involving a variety of criteria such as, maximum lift coefficient, maximum lift-over-drag-ratio, minimum deviation of the hydrofoil area from a reference area, etc