1,721,055 research outputs found
Optimizing the casting of HPFRCC structural elements via Computational FLuid Dynamics Modelling of fresh concrete behavior
No full textA Lagrangian finite element method for 3D compressible flow applications
No full textWe discuss a Lagrangian approach for the simulation of 3D compressible flows on non-structured tetrahedral meshes. The formulation is nodal-based, in the sense that kinematic and thermodynamic variables are all interpolated with continuous P1 polynomials. The equations are solved with an explicit time-marching scheme without stabilizing terms and with the inclusion only of a shock-capturing viscosity. Several examples featuring shock propagations and mixing of fluids are addressed, with particular emphasis on the stability of the approach and on possible strategies for mesh update
Green's functions for the evaluation of anchor losses in mems
Get PDFThe issue of dissipation has a peculiar importance in micro-electro-mechanical-structures (MEMS). Among the sources of damping that affect their performance, the most relevant are [1]: thermoelastic coupling, air damping, intrinsic material losses, electrical loading due to electrode routing, anchor losses. Moreover, recent experimental results indicate the presence of additional temperature dependent dissipation mechanisms which are not yet fully understood (see e.g. [2, 12]). In a resonating structure the quality factor Q is defined as: Q = 2πW/ΔW (1) where ΔW and W are the energy lost per cycle and the maximum value of energy stored in the resonator, respectively. According to eq. (1), the magnitude of Q ultimately depends on the level of energy loss (or damping) in a resonator. The focus of the present contribution is set on anchor losses and the impact they have in the presence of axial loads. Anchor losses are due to the scattering of elastic waves from the resonator into the substrate. Since the latter is typically much larger than the resonator itself, it is assumed that all the elastic energy entering the substrate through the anchors is eventually dissipated. The semi-analytical evaluation of anchor losses has been addressed in several papers with different levels of accuracy [3, 6]. These contributions consider a resonator resting on elastic half-spaces and assume a weak coupling, in the sense that the mechanical mode, as well as the mechanical actions transmitted to the substrate, are those of a rigidly clamped resonator. The displacements and rotations induced in the half-space are provided by suitable Green's functions. Photiadis, Judge et al. [7] studied analytically the case of a 3D cantilever beam attached either to a semi-infinite space or to a semi-infinite plate of finite thickness. Their results are based on the semi-exact Green's functions established in [4]. More recently Wilson-Rae et al. [9, 10] generalized all these approaches using the involved framework of radiation tunnelling in photonics. Unfortunately, these contributions provide estimates of quality factors that differ quantitatively. In this paper we revisit the procedure of [7], which rests on simple mechanical principles, but starting from the exact Green's functions for the half space studied by Pak [14]. Through a careful analysis utilizing the theory of residues and inspired by the work of Achenbach [15], we show that the results obtained coincide exactly with those of [9], but for the case of torsion
Semi-analytical and numerical estimates of anchor losses in bistable MEMS
No full textAnchor losses are one of the dominant sources of dissipation in micro-structures and their influence is even increased in the presence of axial loads. MEMS in which they are likely to play a major role include bistable elements which are nowadays commonly employed in shock sensors and energy harvesters. In this contribution we compute anchor losses both with a semi-analytical approach based on the theory of beams and a fully 3D finite element method including PML for wave dissipation. The two techniques provide coherent predictions and confirm the expectations of strong anchor losses. A fully analytical model is also proposed in the range of limited compressive forces and represents a valid tool for the design of new devices
Sul comportamento del calcestruzzo allo stato fresco: dalla identificazione delle proprietà reologiche alla simulazione dei procedimenti di getto
No full textCementitious composites with adapted rheology are becoming increasingly used in a wide variety of civil engineering applications. Assessing the fundamental rheological properties of cement suspensions is a crucial task, prodromal not only to mix-design of SCCs and assessment of its fresh state performance, but also to the design of casting procedures, such as pumping, grouting, underground and underwater injections etc., where the rheology of the fluid may discriminate the successful accomplishment of the application.
The measurement of the fundamental rheological properties of cement suspensions is, as well known, a not simple task and requires dedicated and expensive equipments which are not compatible with field applications and may not even be available in every laboratory. Correlations between fundamental properties and field test measurements have hence been sought and, in some cases, quite well assessed, as, e.g., for the yield stress vs. the spread diameter in slump/minislump flow test. As for the viscosity, different attempts have been made, e.g. with the Marsh cone flow time or with the time to reach a prescribed diameter in the slump flow tests.
This works aims at providing further evidence to the aforementioned correlations, with reference to a broad range of mix compositions for cement pastes and mortars formulated from High Performance Fiber Reinforced Cementitious Composites (HPFRCC). A robust assessment by means of Computational Fluid Dynamics (CFD) Modelling will be performed, employing an approach developed by the second author. The “simulation” of the casting process of a structural element made with a highly flowable concrete will be finally performed as an auspice to address the use of CFD in civil engineering as a tool to optimize material composition and casting process
to the intended structural application
On the rheological characterisation of liquefied sands through the dam-breaking test
Get PDF This paper concerns the rheological characterisation of liquefied sands as non-Newtonian Bingham fluids. For this purpose, dam-breaking laboratory tests are often executed and interpreted, offering a viable option to identify the properties of fluidised water-soil mixtures. However, limited attention has been devoted so far to clarify what variables and measurements would allow unambiguous calibration of Bingham parameters, namely, the viscosity η and the yield stress τ y . The numerical results of parametric studies based on the particle finite element method (PFEM) are critically inspected to gain deeper insight into the problem. First, it is confirmed that multiple η − τ y pairs may reproduce the same experimental evidence when formed by only one measurement—usually, the post–dam-breaking displacement of the bottom toe (tip) of the liquefied mass. Then, two alternative procedures are proposed for unambiguous identification of both η and τ y : one is based on monitoring the evolving aspect ratio of the fluid mass during free, gravity-driven flow; the other relies on a slightly different dam-breaking test, also including impact against a rigid obstacle. In particular, the latter approach reduces the relevant duration of the test, reducing the possible influence of reconsolidation effects on the calibration of rheological parameters. Geo-engineerin
Effects of casting process on toughness properties of Fiber Reinforced-Self Compacting Concrete as from EN 14651
No full textUn approccio Lagrangiano ad elementi finiti per la simulazione dell’interazione frana bacino
No full textComputational fluid dynamics modelling of the concrete fresh state behaviour: from identification of rheological properties to casting flow simulation
No full textCementitious composites with adapted rheology are becoming increasingly used in a wide variety of civil engineering applications. Assessing the fundamental rheological properties of cement suspensions is a crucial task, instrumental not only to mix-design of SCCs and assessment of its fresh state performance, but also to the design of casting procedures, such as pumping, grouting, underground and underwater injections etc., where the rheology of the fluid may discriminate the successful accomplishment of the application. The measurement of the fundamental rheological properties of cement suspensions is a not simple task and requires dedicated and expensive equipments which are not compatible with field applications and may not even be available in every laboratory. Correlations between fundamental properties and field test measurements have hence been sought and, in some cases, quite well assessed, as, e.g., for the yield stress vs. the spread diameter in slump/minislump flow test. As for the viscosity, different attempts have been made, e.g. with the Marsh cone flow time or with the time to reach a prescribed diameter in the slump flow tests. This works aims at providing further evidence to the aforementioned correlations, with reference to a broad range of cement pastes and mortars formulated from High Performance Fiber Reinforced Cementitious Composites (HPFRCC). A robust assessment by means of Computational Fluid Dynamics (CFD) modelling will be performed, employing an approach developed by the second author. The “simulation” of the casting process of a structural element made with a highly flowable concrete will be finally performed as an auspice to address the use of CFD in civil engineering to “tailor” the material composition and the casting process to the intended application
A Lagrangian finite element method for non-Newtonian free-surface fluid flows and fluid-structure interaction problems
No full textA Lagrangian Finite Element method is applied to the discretization of the Navier-Stokes equations to simulate fluid problems with significant evolution of the free-surfaces. The same approach is also used to treat fluid-structure interaction problems. Both Newtonian and non-Newtonian fluid behaviours are considered. Different examples are performed to show the potentialities of the approach
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