
In memory of
Andreas N. AlexandrouUniversity of Cyprus · Department of Mechanical and Manufacturing Engineering
Andreas N. Alexandrou
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Publications (95)
The classification of a concrete mixture as self-compacting (SCC) is performed by a series of empirical characterization tests that have been designed to assess not only the flowability of the mixture but also its segregation resistance and filling abil-ity. The objective of the present work is to correlate the rheological parameters of SCC matrix,...
The start-up flow of a Bingham plastic in a channel is considered and Safronchik’s solution [1] for the initial
evolution of the yield surface and the core velocity is revisited. Stricter time bounds for the validity of the above
solution are derived and the solution is extended to include the velocity profile in the evolving yielded zone.
Comparis...
The purpose of this study is to investigate the potential of a small-scale, combined-cooling-heating-and-power system, consisting of a 1 MWe gas turbine subsystem coupled to a 0.5 MWe photovoltaic (PV) subsystem for application in Cyprus. The proposed system is completely autonomous, without any interconnections to a central power grid. To allow ma...
In this study a completely autonomous, zero-emission photovoltaic (PV)-based system is modeled for residential applications. Apart from the PV subsystem, an electrolyzer-hydrogen storage-fuel cell subsystem is integrated to the system to fully fulfill a varying load profile throughout the year. The fuel cell and electrolyzer components are based on...
We have found two inadvertent errors in our paper [1], and thus would like to make the following corrections: [...]
In this work we revisit the issue of obtaining true material constants for semisolid
slurries. Therefore, we consider the circular Couette flow of Herschel-Bulkley fluids. We first show
how true constants can be obtained using an iterative procedure from experimental data to theory
and vice versa. The validity of the assumption that the rate-of-str...
In this study a small-scale, completely autonomous combined cooling, heating, and power (CCHP) system is coupled to a photovoltaic (PV) subsystem, to investigate the possibility of reducing fuel consumption. The CCHP system generates electrical energy with the use of a simple gas turbine cycle, with a rated nominal power output of 1 MWe. The nomina...
We provide benchmark results for a transient variant of the lid-driven cavity problem, where the lid motion is suddenly stopped and the flow is left to decay under the action of viscosity. Results include Newtonian as well as Bingham flows, the latter having finite cessation times, for Reynolds numbers Re ∈ [1, 1000] and Bingham numbers Bn ∈ [0, 10...
A solar-heating-and-cooling (SHC) system, consisting of a flat-plate solar collector array, a hot water storage tank, and an absorption chiller unit is designed and modeled to satisfy thermal loads (space heating, domestic hot water, and space cooling). The system is applied for Nicosia, Cyprus, a location with prolonged summer-like conditions, whe...
Three-dimensional direct numerical simulations of a Bingham fluid flowing past a confined circular cylinder have been used in order to investigate viscoplastic effects in the wake-transition regime. The case of a cylinder confined in a plane channel with a fixed blockage ratio (ratio of the cylinder diameter to the channel height) of has been studi...
Decentralized, liquefied natural gas-fueled, trigeneration plants are considered as alternatives to centralized, electricity-only generating power plants to improve efficiency and minimize running costs. The proposed system is analyzed in terms of efficiency and cost. Electrical power is generated with a gas turbine, while waste heat is recovered a...
A residential solar-heating-and-cooling (SHC) system is a possible alternative for the reduction of fossil fuel
consumption to cover residential energy loads. These systems can cover all thermal loads, i.e. space heating, space
cooling and domestic hot water preparation. Space cooling is fulfilled through a heat-activated (hot water), single-effect...
Liquefied natural gas (LNG) is considered a safe and transportable fuel and could become a fueling option for distributed generation applications, particularly in locations which are situated far from centralized power plants. Depending on the location and the related needs in energy type, these systems could vary in size, technology (e.g., gas tur...
A small-scale combined-cooling–heating-and-power (CCHP) plant is proposed as a possible alternative to large-scale, centralized, electricity-only power plants. The proposed system is based on a non-ideal gas turbine (Brayton) cycle, integrated with a cooling plant and a district energy network. The study analyzes whether the proposed system could b...
The objective of this presentation is to show and explain why semisolid slurries can fill thin sections at seemingly unlimited flow velocity; the suitability of SSM parts with very thin sections is a characteristic of the process that is often overlooked by the industry. This characteristic provides a unique opportunity for new advanced application...
An essential element for the integration of a semisolid process in the production of complex commercial components is the availability of accurate mathematical and computational tools that could describe both the rheological behavior and the material characteristics of the suspension, which are strongly affected from its internal structure and its...
We study the flow of a thixotropic fluid around a cylinder. The rheology of the fluid is described by means of a structural viscoplastic model based on the Bingham constitutive equation, regularised using the Papanastasiou regularisation. The yield stress is assumed to vary linearly with the structural parameter, which varies from zero (completely...
We extend our recent work on the creeping flow of a Bingham fluid in a lid-driven cavity, to the study of inertial effects, using a finite volume method and the Papanastasiou regularisation of the Bingham constitutive model [J. Rheology 31 (1987) 385-404]. The finite volume method used belongs to a very popular class of methods for solving Newtonia...
A standard method used to determine material properties of semi-solid slurries is the squeeze flow experiment; a fixed amount of material is squeezed under constant force or velocity and the relation between the force and the displacement of the sample provides information about the rheology of the slurry. The objective of this work is to contribut...
Understanding the time-dependent flow behavior of metal alloys in semisolid state is essential for the further development of the process. In the present investigation, the thixotropic behavior of semisolid slurries is modeled using conservation equations and the Herschel-Bulkley fluid model. The rheological parameters are assumed to be functions o...
The breakup of liquid bridges under the action of capillary forces is used for studying the rheology of suspensions under
stretching. The experiments were performed with suspensions of finegrained (3–30 µm) sand in glycerin for sand volume fractions
up to 0.465. The bridge thinning process was registered using an electro-optical measuring device an...
The objective of this work is to study quantitatively the errors introduced by the standard Newtonian and power-law assumptions used in the determination of the material properties of viscoplastic fluids from circular Couette experiments. The steady-state circular Couette flow of a Herschel-Bulkley fluid is solved assuming that the inner cylinder i...
The purpose of this work is to simulate shear rejuvenation and aging effects in shear thinning yield stress fluids in a typical rotational rheometer and to provide a common framework to describe the behavior of yield stress materials in general. This is particularly important in the determination of material constants under both steady and unsteady...
The objective of this work is to study quantitatively the errors introduced by the standard Newtonian and power-law assumptions used in the determination of the material properties of viscoplastic fluids from circular Couette experiments. The steady-state circular Couette flow of a Herschel-Bulkley fluid is solved assuming that the inner cylinder i...
When SSM material is subjected to a sudden transient the rate of buildup (aging) is negligible compared to the rate of breakdown (shear rejuvenation). While this is generally true, due to prolonged processing or the geometry the local shear rate in some regions may become equal or lower than the critical value, where aging becomes as important as s...
Due to the distinct rheology of semisolid slurries the process has well established advantages over competing near-net-shape manufacturing technologies. Despite the obvious advantages of the process its adoption by the casting industry, however, has been slow. This is primarily due to lack of confidence of how these slurries flow in die cavities. T...
Numerical simulations are undertaken for the transient squeeze flow between parallel plates under a constant load. The conservation equations are solved via the finite element method in space and the Crank–Nicolson method in time. The constitutive equation used to describe the semi-concentrated fiber suspension is an integral equation proposed by D...
The time-dependent, compressible extrusion of a Carreau fluid is solved over the reservoir-die-extrudate region using finite elements in space and a fully implicit scheme in time. A nonmonotonic slip law based on experimental data on polyethylene melts is assumed to hold along the die wall and the velocity at the entrance of the reservoir is taken...
The flow and shape evolution during the compression of a finite amount of a Bingham plastic is investigated by means of numerical simulations. The problem relates to the popular compression test used for the rheological characterization of non-Newtonian fluids. The flow is modelled in Lagrangian coordinates using the Papanastasiou regularization fo...
Semisolid materials are suspensions of metal alloys which are processed while in a mushy state. In this state, the suspensions behave as viscoplastic materials with time-dependent material properties. During processing, the material is injected at high speed into a mold cavity with the process lasting only fractions of a second. The short-term tran...
A rotating vane-cup rheometer is used to determine the rheological properties of semi-solid slurries, and a procedure is established for characterizing the rheology with emphasis given to the proper and self-consistent evaluation of the material constants. Introduction Semi-solid slurries behave as visco-plastic materials with time-dependent materi...
The time-dependent, two-dimensional compressible Newtonian flow over the reservoir-die region is solved assuming that slip occurs along the die wall following a nonmonotonic slip law. The combination of compressibility and nonlinear slip leads to self-sustained oscillations of the pressure drop and of the mass flow rate at constant piston speed, wh...
Systematic experimental work and modeling efforts have been conducted to characterize the yield behavior of commercial aluminum
alloys in the semisolid state. In this study, extensive compression experiments were performed to measure the yield stress
of semisolid aluminum slurries at high solid fractions (0.5 to 1.0), and a cone penetration method...
The problem of a bubble rising due to buoyancy in a Hele–Shaw cell filled with a viscous fluid is a classical free-boundary problem first posed and solved by Saffman & Taylor [11]. In fact, due to linearity of the flow equations the problem is reduced to that of a bubble transported by uniform fluid flow. Saffman and Taylor provided explicit expres...
An essential element for the integration of the semisolid metal (SSM) process in the production of complex commercial components is the availability of accurate mathematical and computational tools that could describe both the material characteristics and the dynamics of semisolid slurries during die filling. Computational simulation and modeling i...
Development of numerical models to simulate flow characteristics of semi-solid metals in the die cavity requires a good understanding of the transient and equilibrium flow properties of semi-solids and knowledge of the basic parameters that control the process. This paper presents transient rheology behavior of a MHD A356 aluminum alloy under const...
We investigate numerically the interactions of two-dimensional jets of Bingham plastic and Herschel–Bulkley fluids with a vertical surface at a distance from the die exit. This problem simulates the early stages of filling of a planar cavity. Our main objective is to explain the flow instabilities observed during the processing of semisolid materia...
The overall objective of this research is to investigate the fundamental physics of film condensation in reduced gravity. The condensation of vapor on a cool surface is important in many engineering problems,including spacecraft thermal control and also the behavior of condensate films that may form on the interior surfaces of spacecraft. To examin...
Summary The advantages of semisolid processing derive from the characteristic rheology of the material whose state lies between that of a pure solid and that of a pure liquid. As a solid, the material maintains its structural integrity, and, as a liquid, it flows with relative ease. To successfully produce parts using this process, it is important...
In this paper we study steady flow of Herschel–Bulkley fluids in a canonical three-dimensional expansion. The fluid behavior was modeled using a regularized continuous constitutive relation, and the flow was obtained numerically using a mixed-Galerkin finite element formulation with a Newton–Raphson iteration procedure coupled to an iterative solve...
Understanding the time-dependent flow behavior of semisolid materials (SSM) is essential for the simulation of semisolid processes. During processing, the structure of the material changes with the processing history due to agglomeration and de-agglomeration of particles or particle–particle interactions. Analysis of experimental results show that...
The present study concentrates on Bingham fluid filling of a 2-D cavity and examines the relative importance of inertial, viscous and yield stress effects on the filling profile. The results presented are obtained using PAM-CAST/SIMULOR, which was modified by introducing a regularized Bingham fluid constitutive relation. The results identify five d...
The spreading behavior of initially spherical, stagnant, viscous droplets on a planar interface is studied numerically. The droplet stability, equilibrium shape, and the fluid motion within the drop are examined. For a droplet spreading on a solid surface, either a no-slip or a partial-slip boundary condition is enforced at the contact point. In ad...
The focus of this ground-based program is the study of film condensation phenomena under variable, reduced-gravity conditions. Experimental tests, combined with numerical modeling, are employed to gain an improved understanding of the fundamental fluid physics responsible for condensate film growth, film instability and the resulting interfacial mo...
Processing of metal alloys in their mushy state represents a new trend in metal processing. The process produces components with low porosity, high crack resistance along with fine microstructure and mechanical properties that are better than those produced by liquid casting, and comparable to those of forged alloys. In the present work, a summary...
Crystals grown from specially prepared liquid solutions have important industrial applications. The efficiency of these crystals is usually a function of their size. Therefore, during the synthesis of the crystals, it is important to optimize their size, and to control the defect location and concentration. For this reason, crystals have been grown...
* Presents the principles of control volume early for use throughout the book * Emphasizes the constitutive equation that relates deformation to stress. * Examines in detail the flow of Newtonian fluids-which can often be easily generalized to non-Newtonian fluids mechanics. * Includes many applications on topics like liquid droplets, gas bubbles,...
The effective use of the semisolid process presumes a good theoretical framework to describe it. The understanding of semisolid behavior should be based upon a combination of basic and empirical knowledge in versatile models which, by numerical and physical experimentation and feedback, can be used to predict the actual behavior of these materials....
Semisolid metal processing is gaining interest very rapidly in the casting industry. It offers distinct advantages over other near-net-shape technologies, such as a more homogeneous microstructure, less porosity and thus improved mechanical properties. A perfect control of die filling during processing is however necessary, especially in the case o...
The flow development of HerschelâBulkley fluids in a sudden three-dimensional square expansion is studied numerically. The flow is modeled using the mixed-Galerkin finite element formulation to solve the conservation of mass and momentum equations. The HerschelâBulkley material behavior is described using a regularized model based on the Papanastas...
Herschel-Bulkley fluids are materials that behave as rigid solids when the local stress τ is lower than a finite yield stress τ0, and flow as nonlinearly viscous fluids for τ > τ0. The flow domain then is characterized by two distinct areas, τ < τ0 and τ > τ0. The surface τ = τ0 is known as the yield surface. In this paper, by using analytic soluti...
Crystals grown from liquid solutions have important industrial applications. Zeolites, for instance, a class of crystalline aluminosilicate materials, form the backbone of the chemical process industry worldwide, as they are used as adsorbents and catalysts. Many of the phenomena associated with crystal growth processes are not well understood due...
Non-harmonic forcing of cylinder wakes at frequencies far removed from subharmonics of the natural shedding frequency is studied using a periodic freestream flow. A two-dimensional numerical formulation based on a Galerkin finite element method and a fully implicit segregated solution procedure is used. It is shown that the non-harmonic forcing cau...
The importance of processing of metal and metal matrix composites in semisolid state is increasing quite rapidly due to the distinct advantages it offers over similar methods of near-net-shape making. In this investigation we are using a two-phase model to predict the behavior of SSM in a three-dimensional isothermal flow through a sudden square ex...
Monitoring capillarity-driven necking and breakup of liquid bridges is used to study tensile rheology of suspensions. A specimen of the tested fluid is placed between two parallel disks and kept in the gap by capillary forces. The disks then are rapidly pulled apart and kept at a fixed distance. As a result, the fluid forms a liquid bridge which is...
The problem of steady-state propagation
of a finger or a bubble of inviscid fluid through a
Hele–Shaw cell filled by a viscous non-Newtonian,
including visco-plastic (Bingham) fluid is
addressed. Only flows symmetric relative to the cell
axis are considered. It is shown that, using
a hodograph transform, this non-linear free boundary...
The free surface interface shape between a condensing pure vapor and liquid is investigated numerically. The location and shape of the interface is determined by a front tracking method. A classical mixed Galerkin finite element method is used for the numerical solution of the Navier Stokes, energy, and continuity equations in both the vapor and li...
Crystals grown from special liquid solutions find important industrial applications. Most often the physics and chemistry of the growth processes are not well understood due to complex microscopic chemical and thermo-fluid phenomena. Microgravity could help elucidate these phenomena and allow the control of defect concentration and crystal size. We...
The quasi-conservative stochastic averaging (QCSA) method is applied to a Bouc-Wen-Baber hysteretic system (BWB) under Gaussian white noise excitations. The stationary probability density of the system's response amplitude and energy is obtained for different excitation levels and different damping ratios. These results are compared with the studie...
The interest in the theoretical investigation of the flow behavior of fiber suspensions during manufacturing processes has increased considerably due to the increased interest in the use of composite materials. The unsteady flow of semi-concentrated fiber suspensions in two dimensions is investigated using the Generalized Eulerian-Lagrangian (GEL)...
The advantages of the Generalized Eulerian–Lagrangian (GEL) formulations in modeling and numerical solution of unsteady flow of fiber suspensions are discussed. This formulation offers the flexibility of selectively applying an Eulerian (Spatial), a Lagrangian (Material), and any combination of these two frames of reference in the same computationa...
This paper deals with the flow behavior and the resulting fiber orientation during compression molding of semi-concentrated fiber suspensions. The suspension is characterized using the Dinh and Armstrong integral constitutive relations, expressed in terms of the finite deformation tensors. The equations of motion are expressed in a convenient Gener...
One element of WPI's project-based curriculum is its interdisciplinary Advanced Aeronautics Design Program. Students participating in the program are involved in the design, construction, and flight testing of non-traditional aircraft such as an ultralight solar-powered vehicle, microwave-powered long endurance aircraft, or a flying oblique wing. T...
A mathematical and a numerical approach to model the injection mold filling stage is presented. The approach is based on a Generalized Eulerian Lagrangian (GEL) formulation. In purely Lagrangian formulations the solution is obtained at mesh points which follow the local fluid velocity. On the other hand, in Eulerian formulations the mesh is fixed a...
In this paper we are focusing on a method to solve three-dimensional solidification problems typical of the aforementioned applications. Apart from the three-dimensionality of the problem, additional difficulties with this type of free-boundary problem arise from the need to adequately account for the non-linearities in the a priori unknown geometr...
A generalized Eulerian-Lagrangian approach is used to study the compression molding of viscoelastic polymers. The complete, unsteady, two-dimensional equations of motion are used in the analysis. The White–Metzner model is used to describe the constitutive behavior of the material. The solution is obtained using a mixed Galerkin finite element meth...
Increase in energy demands coupled with rapid depletion of natural energy resources have deemed solar energy as an attractive alternative source of power. The focus was to design and construct a solar powered, remotely piloted vehicle to demonstrate the feasibility of solar energy as an effective, alternate source of power. The final design include...
The present work is a study of the thermo-viscous effects in the extrusion of a “power law” liquid with temperature dependent viscosity by including the effects of solidification. This is achieved by using a novel inverse isotherm finite element method. The method is based on an inversely defined (but not inverted) formulation where the isotherms a...
The finite element method was used to study steady, incompressible, laminar flow in porous tube systems. The Navier-Stokes equations were used to describe the flow phenomena in the tube and shell space while the continuum theory of Brinkman was used to describe flow in the porous tube wall. The solution method, based on the Galerkin finite element...
In free boundary problems a priori unknown geometric parameters need to be evaluated simultaneously with the primary field unknowns. These problems are highly nonlinear and difficult to handle computationally. Here an accurate and efficient method is presented which can appropriately handle the complexities associated with these problems. The metho...
Non-isothermal extrusion, of non-dilute fiber suspensions with solidification is analyzed by means of an integral constitutive equation for melt fiber suspension and a novel, streamlined inverse finite element method (IIFEM), to account for the solidification. Newton iteration is used to solve simultaneously the highly non-linear system of intergro...
In free boundary problems, in addition to the primary field variables, velocity, pressure and temperature, unknown geometric parameters need to be evaluated. A characteristic feature of the problems is a demarcation line which separates two domains with different material properties. The problems are highly non-linear and difficult to handle comput...
The flow and solidification of planar jets are analysed by means of an efficient inverse isotherm finite element method. The method is based on a tessellation that is constructed by isotherms as characteristic co-ordinate lines transverse to the flow direction. Thus opposite sides of finite elements lie on isotherms. The method allows the simultane...
Steady axisymmetric thin film flows on a rotating cone of Newtonian and non‐Newtonian liquids are analyzed by means of the axisymmetric boundary layer with swirl equations, Galerkin finite‐element discretization with free surface parameterization, and Newton iteration, which permits the simultaneous evaluation of the boundary layer thickness with t...
The extrusion of a rod-like fiber suspension is a Newtonian solvent, as a first step to the fast and inexpensive production of composite materials, is investigated. The analysis is carried out by means of an integral constitutive equation for a non-dilute suspension, streamlined finite element for liquid with memory, and Newton iteration of nonline...
The steady two-dimensional flow past bluff bodies is simulated
numerically using a hybrid Eulerian-Lagrangian model. The boundary layer
effects, such as the location of the separation points and the rate of
the generation of vorticity, are determined by a boundary layer solver.
This solver uses Prandtl's boundary layer equations transformed by the...
Summary The purpose of this work is to address lingering concerns about the reliability of the semisolid metal process (SSMP) and its inability to produce consistently high quality parts. We first provide typical examples that lead to questions about the consistency of the process, and then demonstrate that the inconsistent results can be explained...
Contenido: Introducción y definiciones básicas; Leyes de conservación para sistemas cerrados; Leyes de conservación para sistemas abiertos; Vista diferencial del movimiento de fluidos: cinemática de fluidos y deformación; Formas diferenciales de las leyes de conservación; Análisis dimensional de sistemas de fluidos; Soluciones analíticas exactas; C...
The steady two-dimensional flow past bluff bodies is simulated numerically using a hybrid Eulerian-Lagrangian model. The boundary layer effects, such as the location of the separation points and the rate of the generation of vorticity, are determined by a boundary layer solver. This solver uses Prandtl's boundary layer equations transformed by the...