Alejandro Marcos Aragón

Delft University of Technology | TU · Department of Precision and Microsystems Engineering (PME)

Heat pumping through thermoelectric devices has many advantages over traditional cooling. However, their current efficiency is a limiting factor in their implementation. In this paper, we approach the non-convex topology optimization of thermoelectrical elements for cooling applications through the method of moving asymptotes (MMA) to improve their...

Inorganic scintillators often use exotic, expensive materials to increase their light yield. Although material chemistry is a valid way to increase the light collection, these methods are expensive and limited to the material properties. As such, alternative methods such as the use of specific reflective coatings and crystal optical shapes are crit...

Neural networks (NNs) hold great promise for advancing inverse design via topology optimization (TO), yet misconceptions about their application persist. This article focuses on neural topology optimization (neural TO), which leverages NNs to reparameterize the decision space and reshape the optimization landscape. While the method is still in its...

Nonlinear dynamic simulations of mechanical resonators have been facilitated by the advent of computational techniques that generate nonlinear reduced order models (ROMs) using the finite element (FE) method. However, designing devices with specific nonlinear characteristics remains inefficient since it requires manual adjustment of the design para...

In many scenarios -- when we bite food or during a crash -- fracture is inevitable. Finding solutions to steer fracture to mitigate its impact or turn it into a purposeful functionality, is therefore crucial. Strategies using composites, changes in chemical composition or crystal orientation, have proven to be very efficient, but the crack path con...

Although strain engineering and soft-clamping techniques for attaining high Q-factors in nanoresonators have received much attention, their impact on nonlinear dynamics is not fully understood. In this study, we show that nonlinearity of high-Q Si3N4 nanomechanical string resonators can be substantially tuned by support design. Through careful engi...

Underwater noise resulting from the monopile driving process can cause severe damage to marine wildlife, such as hearing injury, behavioral disturbance, or even death. Although current noise-attenuation techniques used in this process have shown a significant noise reduction at high frequency ranges, mitigating low-frequency noise is still extremel...

Mechanical nonlinearities dominate the motion of nanoresonators already at relatively small oscillation amplitudes. Although single and coupled two-degree-of-freedom models have been used to account for experimentally observed nonlinear effects, it is shown that these models quickly deviate from experimental findings when multiple modes influence t...

Ultrasonic flowmeters face unique challenges since, in addition to withstanding high fluid pressures, they have to avoid crosstalk, which is the interaction of the signals traveling through the fluid and the solid pipe. To avoid the crosstalk, which leads to poor accuracy or complete loss of the required signal, we develop a mounting mechanism base...

Fundamentals of Enriched Finite Element Methods provides an overview of the different enriched finite element methods, detailed instruction on their use, and their real-world applications, recommending in what situations they are best implemented. It starts with a concise background on the theory required to understand the underlying principles beh...

Although strain engineering and soft-clamping techniques for attaining high Q-factors in nanoresonators have received much attention, their impact on nonlinear dynamics is not fully understood. In this study, we show that nonlinearity of high-Q Si3N4 nanomechanical string resonators can be substantially tuned by support design. Through careful engi...

The band-gap frequencies of elastic metamaterials are ideally determined by a metamaterial architecture; yet, in practical situations, are often dependent on the material damping in their constituent(s). The analysis of viscoelastic metamaterials requires however substantial computational resources and, except for oversimplified cases, is solely do...

Mechanical nonlinearities dominate the motion of nanoresonators already at relatively small oscillation amplitudes. Although single and coupled two-degrees-of-freedom models have been used to account for experimentally observed nonlinear effects, it is shown that these models quickly deviate from experimental findings when multiple modes influence...

Topology optimization is an increasingly popular tool for engineers to obtain lightweight yet stiff designs in an automated way. However, the design resolution is directly linked to the computational time required for the optimization. This is especially true for large-scale applications where a small design resolution is required. For instance, op...

This broad review summarizes recent advances and “hot” research topics in nanophononics and elastic, acoustic, and mechanical metamaterials based on results presented by the authors at the EUROMECH 610 Colloquium held on April 25–27, 2022 in Benicássim, Spain. The key goal of the colloquium was to highlight important developments in these areas, pa...

In recent years, the Q-factor of [Formula: see text] nanomechanical resonators has significantly been increased by soft-clamping techniques using large and complex support structures. To date, however, obtaining similar performance with smaller supports has remained a challenge. Here, we make use of torsion beam supports to tune the Q-factor of [Fo...

Ultrasonic flowmeters that use transit-time ultrasonic transducers face measurement errors due to “crosstalk,” whereby the working signal travels through the pipe wall and couplings, interfering with the signal from the fluid. Although various procedures have been proposed to solve the issue of crosstalk, they’re limited to low-frequency ranges, or...

Phononic crystal band gaps (BGs), which are realized by Bragg scattering, have a central frequency and width related to the unit cell’s size and the impedance mismatch between material phases. BG tuning has generally been performed by either trial and error or by computational tools such as topology optimization. In either case, understanding how t...

Quantifying the nanomechanical properties of soft-matter using multi-frequency atomic force microscopy (AFM) is crucial for studying the performance of polymers, ultra-thin coatings, and biological systems. Such characterization processes often make use of cantilever's spectral components to discern nanomechanical properties within a multi-paramete...

The cover image is based on the Research Article An object‐oriented geometric engine design for discontinuities in unfitted/immersed/enriched finite element methods by Jian Zhang et al., https://doi.org/10.1002/nme.7049.

Accelerating topology optimization using efficient algorithms on GPU hardware.

We propose an enriched finite element formulation to address the computational modeling of contact problems and the coupling of non-conforming discretizations in the small deformation setting. The displacement field is augmented by enriched terms that are associated with generalized degrees of freedom collocated along non-conforming interfaces or c...

Quantifying the nanomechanical properties of soft-matter using multi-frequency atomic force microscopy (AFM) is crucial for studying the performance of polymers, ultra-thin coatings, and biological systems. Such characterization processes often make use of cantilever's spectral components to discern nanomechanical properties within a multi-paramete...

In this work, an object‐oriented geometric engine is proposed to solve problems with discontinuities, for instance, material interfaces and cracks, by means of unfitted, immersed or enriched finite element methods. Both explicit and implicit methods, such as geometric entities and level sets, are introduced to describe configurations of discontinui...

Metamaterials can display unusual and superior properties that come from their carefully designed structure rather than their composition. Metamaterials have permeated large swatches of science, including electromagnetics and mechanics. Although metamaterials hold the promise for realizing technological advances, their potential to enhance interact...

We propose a fully immersed topology optimization procedure to design structures with tailored fracture resistance under linear elastic fracture mechanics assumptions for brittle materials. We use a level set function discretized by radial basis functions to represent the topology and the Interface-enriched Generalized Finite Element Method (IGFEM)...

We propose an enriched finite element formulation to address the computational modeling of contact problems and the coupling of non-conforming discretizations in the small deformation setting. The displacement field is augmented by enriched terms that are associated with generalized degrees of freedom collocated along non-conforming interfaces or c...

Finding global optima in high-dimensional optimization problems is extremely challenging since the number of function evaluations required to sufficiently explore the design space increases exponentially with its dimensionality. Furthermore, non-convex cost functions render local gradient-based search techniques ineffective. To overcome these diffi...

This paper discusses an elastic metamaterial for filtering energy from certain frequencies of an incoming wave. The unit cell, which is composed by a single material, is built to obtain a local resonance band gap. Since Bloch-Floquet’s periodic condition is enforced to the unit cell, the dynamic characteristics of the metamaterial is obtained by on...

Stress analysis is an all‐pervasive practice in engineering design. With displacement‐based finite element analysis, directly‐calculated stress fields are obtained in a post‐processing step by computing the gradient of the displacement field–therefore less accurate. In enriched finite element analysis (EFEA), which provides unprecedented versatilit...

An immersed enriched finite element method is proposed for the analysis of phononic crystals (PnCs) with finite element (FE) meshes that are completely decoupled from geometry. Particularly, a technique is proposed to prescribe Bloch–Floquet periodic boundary conditions strongly on non-matching edges of the periodic unit cell (PUC). The enriched fi...

Metamaterials are artificial structures with unusual and superior properties that come from their carefully designed building blocks -- also called meta-atoms. Metamaterials have permeated large swatches of science, including electromagnetics and mechanics. Although metamaterials hold the promise for realizing technological advances, their potentia...

During design optimization, a smooth description of the geometry is important, especially for problems that are sensitive to the way interfaces are resolved, e.g., wave propagation or fluid-structure interaction. A level set description of the boundary, when combined with an enriched finite element formulation, offers a smoother description of the...

Enriched finite element methods have gained traction in recent years for modeling problems with material interfaces and cracks. By means of enrichment functions that incorporate a priori behavior about the solution, these methods decouple the finite element discretization from the geometric configuration of such discontinuities. Taking advantage of...

The Hierarchical Interface-enriched Finite Element Method (HIFEM) is a technique for solving problems containing discontinuities in the field gradient using finite element meshes that do not conform (match) the domain morphology. The method is suitable for analyzing problems with complex geometries or when such geometry is not known a priori. Contr...

During design optimization, a smooth description of the geometry is important, especially for problems that are sensitive to the way interfaces are resolved, e.g., wave propagation or fluid-structure interaction. A levelset description of the boundary, when combined with an enriched finite element formulation, offers a smoother description of the d...

Cover of issue 120 of the International Journal for Numerical Methods in Engineering.

Generating matching meshes for finite element analysis is not always a convenient choice, for instance in cases where the location of the boundary is not known a priori or when the boundary has a complex shape. In such cases, enriched finite element methods can be used to describe the geometric features independently from the mesh. The Discontinuit...

A new enriched finite element technique, named the Discontinuity-Enriched Finite Element Method (DE-FEM), was introduced recently for solving problems with both weak and strong discontinuities in 2-D. In this mesh-independent procedure, enriched degrees of freedom are added to new nodes collocated at the intersections between discontinuities and th...

In this manuscript we thoroughly study the behavior of the virtual element method (VEM) in the context of two-dimensional linear elasticity problems for an engineering audience versed in standard FEM. Through detailed convergence studies we show the accuracy and the convergence rates recovered by VEM, and we compare them to those obtained by the h-...

In this research, a universal framework for automated calibration of microscopic properties of modeled granular materials is proposed. The proposed framework aims at industrial scale applications, where optimization of the computational time step is important. It can be generally applied to all types of DEM simulation setups. It consists of three p...

In this paper we propose a stress recovery procedure for low-order finite elements in 3-D. For each finite element, the recovered stress field is obtained by satisfying equilibrium in an average sense and by projecting the directly-calculated stress field onto a conveniently chosen space. Compared to existing recovery techniques, the current proced...

We introduce a new methodology for modeling problems with both weak and strong discontinuities independently of the finite element discretization. At variance with the eXtendend/Generalized Finite Element Method (X/GFEM), the new method, named the Discontinuity-Enriched Finite Element Method (DE-FEM), adds enriched degrees of freedom only to nodes...

This research aims at developing a universal methodology for automated calibration of microscopic properties of modelled granular materials. The proposed calibrator can be applied for different experimental set-ups. Two optimization approaches: (1) a genetic algorithm and (2) DIRECT optimization, are used to identify discrete element method input m...

We present a structure preserving numerical algorithm for the collision of elastic bodies. Our integrator is derived from a discrete version of the field-theoretic (multisymplectic) variational description of nonsmooth Lagrangian continuum mechanics, combined with generalized Lagrange multipliers to handle inequality constraints. We test the result...

The numerical modeling of phononic crystals using the finite element method requires a mesh that accurately describes the geometric features. In an optimization setting, involving shape and/or topological changes, this implies that a new matching mesh needs to be generated in every design iteration. In this paper a mesh-independent description for...

Topology optimization formulations using multiple design variables per finite element have been proposed to improve the design resolution. This paper discusses the relation between the number of design variables per element and the order of the elements used for analysis. We derive that beyond a maximum number of design variables, certain sets of m...

Generating matching meshes for problems with complex boundaries is often an intricate process, and the use of non-matching meshes appears as an appealing solution. Yet, enforcing boundary conditions on non-matching meshes is not a straightforward process, especially when prescribing those of Dirichlet type. By combining a type of Generalized Finite...

The design of a microvascular flow network embedded in an actively-cooled polymeric material is pre-sented. A multi-objective Genetic Algorithm (GA) com-bined with the finite element method is first used to determine the quasi-optimized network configurations and provide insight into the behavior of the actively-cooled material. The objective funct...

The detection phase in computational contact mechanics can be subdivided into a global search and a local detection. When potential contact is detected by the former, a rigorous local detection determines which sur-face elements come or may come in contact in the current increment. We first introduce a rigorous definition of the closest point for n...

The last few years have seen the proliferation of measures that quantify the scientific output of researchers. Yet, most of these measures focus on productivity, thus fostering the ‘‘publish or perish’’ paradigm. This article proposes a measure that aims at quantifying the impact of research de-emphasizing productivity, thus providing scientists an...

The effect of in-plane deformations on the failure response of heterogeneous adhesives with a second phase of spherical elastic particles is investigated numerically using a 3D cohesive framework. The methodology includes a new interface-enriched generalized finite element scheme for the solution of structural problems with weak discontinuities, al...

Generating matching meshes for problems with complex boundaries is often an intricate process, and the use of non-matching meshes appears as an appealing solution. Yet, enforcing boundary conditions on non- matching meshes is not a straightforward process, especially when prescribing those of Dirichlet type. By combining a generalized finite elemen...

a b s t r a c t This work discusses the computational design of microvascular polymeric materials, which aim at mimicking the behavior found in some living organisms that contain a vascular sys-tem. The optimization of the topology of the embedded three-dimensional microvascular network is carried out by coupling a multi-objective constrained genet...

This article discusses an efficient implementation of tensors of arbitrary rank by using some of the idioms introduced by the recently published C++ ISO Standard (C++11). With the aims at providing a basic building block for high-performance computing, a single Array class template is carefully crafted, from which vectors, matrices, and even higher...

A new generalized FEM is introduced for solving problems with discontinuous gradient fields. The method relies on enrichment functions associated with generalized degrees of freedom at the nodes generated from the intersection of the phase interface with element edges. The proposed approach has several advantages over conventional generalized FEM f...

This paper describes a framework for the design of microvascular polymeric components for active cooling applications. The design of the embedded networks involves complex and competing objectives that are associated with various physical processes. The optimization tool includes a PDE solver based on advanced finite element techniques coupled to a...

Microvascular networks show promise for applications such as self-healing, self-cooling, and structural damage sensing. Fluid-filled micro-scale channels have been investigated extensively in the field of microfluidics, but three-dimensional networks in polymeric structural materials have been achieved only recently. The purpose of microvascular ne...

Microvascular networks show promise for applications such as self-healing, self-cooling, and structural damage sensing. Fluid-filled micro-scale channels have been investigated extensively in the field of microfluidics, but three-dimensional networks in polymeric structural materials have been achieved only recently. The purpose of microvascular ne...

Microvascular networks show promise for applications such as self-healing, self-cooling, and structural damage sensing. Fluid-filled micro-scale channels have been investigated extensively in the field of microfluidics, but three-dimensional networks in polymeric structural materials have been achieved only recently. The purpose of microvascular ne...

Biomimetic microvascular networks with complex architectures are embedded in epoxy matrices using direct-write assembly. Fluid transport in multi-generation bifurcating channels is systematically investigated and maximum flow efficiency is found to occur when Murray's law is obeyed. Nature is replete with examples of vascular networks, whose primar...

Biomimetic microvascular materials are increasingly considered for a variety of autonomic healing, cooling and sensing applications. The microvascular material of interest in this work consists of a network of hollow microchannels, with diameters as small as 10 μm, embedded in a polymeric matrix. Recent advances in the manufacturing of this new cla...

Biomimetic microvascular networks with complex architectures are embedded in epoxy matrices using direct-write assembly. Fluid transport in multi-generation bifurcating channels is systematically investigated and maximum flow efficiency is found to occur when Murray's law is obeyed. Nature is replete with examples of vascular networks, whose primar...

A multi-objective genetic algorithm is used to design 2D and 3D microvascular networks embedded in bio-mimetic self-healing/self-cooling polymeric materials. Various objective functions and constraints are considered, ranging from flow efficiency and homogeneity to network redundancy and void volume fraction. The design variables include the networ...

A general GFEM/XFEM formulation is presented to solve two-dimensional problems characterized by C 0 continuity with gradient jumps along discrete lines, such as those found in the thermal and structural analysis of heterogeneous materials or in line load problems in homogeneous media. The new enrichment functions presented in this paper allow solvi...

The next generation of self-healing polymeric materials will allow multiple healing events through the integration of complex three-dimensional microvascular networks. Traditional techniques for creating these networks, such as soft lithography, do not readily scale to the third dimension. New approaches must therefore be developed to enable the fa...