Marius Paraschivoiu - Academia.edu (original) (raw)

Papers by Marius Paraschivoiu

This paper presents an analysis based on computational fluid dynamics of vertical axis wind turbi... more This paper presents an analysis based on computational fluid dynamics of vertical axis wind turbines when placed in close proximity in a linear array. It has been noticed that VAWTs placed close to each other with counter rotation motions have a higher coefficient of power than a single turbine. This was termed the “coupled vortex effect”. Two mechanisms have been identified to cause this increase in efficiency: the streamtube contraction effect and the vortex effect. The first is due to the blockage effect from neighboring turbines while the later is related to the neighboring turbine acting as a vortex that induces an increased flow field. This paper analyzes each of these effects and studies the influence of the turbine size and the rotation speed. The change of torque on each blade due to these effects is investigated for two different sizes of wind turbines.

This paper investigates the accuracy of a CFD model to capture the complex flow around a small ve... more This paper investigates the accuracy of a CFD model to capture the complex flow around a small vertical axis wind turbine (VAWT) on 2D and 3D grid. Therefore, an Unsteady ReynoldsAveraged Navier-Stokes analysis is performed with parallel OpenFOAM solver based on the Spalart-Allmaras (SA) turbulence model. A grid convergence study is conducted on 2D grids to examine our CFD model sensitivity to grid resolution. Moreover, a 3-D grid of the VAWT is modeled in order to explore the influence of the 3D effects on the aerodynamic performance of the turbine.

Energy, 2021

Abstract This paper demonstrates the capability of CFD to accurately simulate the effect of the f... more Abstract This paper demonstrates the capability of CFD to accurately simulate the effect of the flow turbulence intensity on the performance of vertical-axis wind turbines (VAWT). This effect is quite important as it increases the performance of small VAWTs. For this study, two and three dimensional CFD analysis has been performed on different straight-bladed Darrieus-type rotors. Both a small and a large H-Darrieus VAWT with diameters of 0.5 m and 35 m respectively are investigated using NACA0018 blades. Computational results based on a commercial CFD code (Star CCM+) are compared with experimental measurements. Several simulations based on full URANS calculations are proposed. Firstly, the sensitivity of time step, the number of iterations by time step, and discretization schemes are studied. Secondly, the effect of turbulence intensity on the VAWT performance is simulated and compared with experimental data. Finally, results reveal that the power coefficient of a small turbine is increasing for higher turbulence intensity, up to 20%, but stops increasing afterward. For a small turbine H-Darrieus turbine, the power coefficient is increased by 22% when the turbulence intensity is changed from 0.7% to 20%, however, there is no increase detected in the case of a large H-Darrieus wind turbine. The impact of the turbulence intensity was assessed and a range of behaviors was identified.

Energy, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Progress in Canadian Mechanical Engineering, May 30, 2018

The ground topography effect on the wind flow is significant. The knowledge of the flow behavior ... more The ground topography effect on the wind flow is significant. The knowledge of the flow behavior near ground is crucial in the development of wind power, especially in the choice of suitable sites and for estimation of energy production. In this paper, the numerical prediction of the flow over a three-dimensional hill model and the analysis of placement of Savonius turbines on top of the hill are presented. The numerical analysis is based on the finite volume method implemented in the ANSYS CFX 15 Software using the Shear-Stress Transport (SST) turbulence model. The numerical results for a conventional Savonius rotor and a vertical-axis spiral wind rotor are both satisfactory compared with experimental data. The performances of these turbines, installed on the hilltop, are studied for different height positions. Furthermore, the influence of the hill size on the extracted power is investigated. At TSR=1, the power coefficient of a conventional rotor is increased from 0.15 to 0.32 when the rotor is installed at a height of 0.25 m above the top of the hill, while it reaches 0.40 when the hill is two to three times higher. The helical Savonius rotor tested gives even higher power coefficient of 0.44.

Journal of Computational Physics, 2020

We propose a multiple level-set model to represent the physics governing the three-phase solidifi... more We propose a multiple level-set model to represent the physics governing the three-phase solidification problem. The model couples thermal characteristics of the freezing front with the dynamics of droplet interface. To deal with liquid, solid, and gas phases, two distinct level-sets are used. The liquid-gas interface which is represented by a level-set moves with an external velocity field that is obtained from the Navier-Stokes equations. The solidliquid interface, on the other hand, evolves according to the freezing rate of the liquid. The solid-liquid level-set is comprised of an active and passive part. The active segment of the level-set evolves based on temperature gradients and the latent heat of fusion. The passive segment, however, is merely utilized to impose an angle at the tri-junction point. We propose a Hamilton-Jacobi type equation to impose constant or variable angles at the tri-junction point. In order to consider the effect of volume expansion we modify the continuity, and the energy equation. Importantly, in the case of expansion during solidification we can capture the pointy shape on top of the freezing droplet. For validation we compare numerical results with the analytical Stefan problem with and without the density expansion. In addition, we use experimental results of water droplet freezing, available in the literature, to examine the accuracy of the freezing rate, and the droplet morphological.

Sustainable Cities and Society, 2019

International Journal of Hydrogen Energy, 2017

Journal of Wind Engineering and Industrial Aerodynamics, 2018

Urban wind energy consists of the utilization of wind energy technology in applications to the ur... more Urban wind energy consists of the utilization of wind energy technology in applications to the urban and suburban built environment. The paper provides some views on the progress made recently in the areas of wind resource assessment in the urban habitat; the utilization of suitable wind turbines for enhancing the exploitation of these resources; and the significant role of knowledge of building and urban aerodynamics for an optimal arrangement of interfacing augmented wind with its extraction mechanisms. The paper is not intended to be exhaustive, rather its purpose is to provide some views on the above-mentioned topics from the viewpoint of wind engineering and industrial aerodynamics in the context of buildings and cities.

Transactions of the Canadian Society for Mechanical Engineering, 2014

Mesh resolution requirements are investigated for 2-D and 3-D simulations of the complex flow aro... more Mesh resolution requirements are investigated for 2-D and 3-D simulations of the complex flow around a straight-blade vertical axis wind turbine (VAWT). The resulting flow, which may include large separation flows over the blades, dynamic stall, and wake-blade interaction, is simulated by an Unsteady Reynolds-Averaged Navier–Stokes analysis, based on the Spalart-Allmaras (S–A) turbulence model. A grid resolution study is conducted on 2-D grids to examine the convergence of the CFD model. Hence, an averaged-grid residual of y+ > 30 is employed, along with a wall treatment, to capture the near-wall region’s flow structures. Furthermore a 3-D simulation on a coarse grid of the VAWT model is performed in order to explore the influence of the 3-D effects on the aerodynamic performance of the turbine. Finally, based on the 2-D grid convergence study and the 3-D results, the required computational time and mesh to simulate 3-D VAWT accurately is proposed.

Transactions of the Canadian Society for Mechanical Engineering, 2018

Advances in wind power and tidal power have matured considerably to offer clean and sustainable e... more Advances in wind power and tidal power have matured considerably to offer clean and sustainable energy alternatives. Nevertheless, distributed small-scale energy production from wind in urban areas has been disappointing because of very low efficiencies of the turbines. A novel wind turbine design — a seven-bladed Savonius vertical-axis wind turbine (VAWT) that is horizontally oriented inside a diffuser shroud and mounted on top of a building — has been shown to overcome the drawback of low efficiency. The objective this study was to analyze the performance of this novel wind turbine design for different wind directions and for different guide vanes placed at the entrance of the diffuser shroud. The flow field over the turbine and guide vanes was analyzed using computational fluid dynamics (CFD) on a 3D grid for multiple tip-speed ratios (TSRs). Four wind directions and three guide-vane angles were analyzed. The wind-direction analysis indicates that the power coefficient decreases ...

Journal of Renewable and Sustainable Energy, 2016

Journal of Physics: Conference Series, 2016

Vertical axis wind turbines such as Savonius rotors have been shown to be suitable for low wind s... more Vertical axis wind turbines such as Savonius rotors have been shown to be suitable for low wind speeds normally associated with wind resources in all corners of the world. However, the efficiency of the rotor is low. This paper presents results of Computational Fluid Dynamics (CFD) simulations for an array of Savonius rotors that show a significant increase in efficiency. It looks at identifying the effect on the energy yield of a number of turbines placed in a linear array. Results from this investigation suggest that an increase in the energy yield could be achieved which can reach almost two times than the conventional Savonius wind turbine in the case of an array of 11turbines with a distance of 1.4R in between them. The effect of different TSR values and different wind inlet speeds on the farm has been studied for both a synchronous and asynchronous wind farm.

International Journal of Hydrogen Energy, 2014

Computational Fluid Dynamics (CFD) is employed to investigate the hydrogen jet exiting through di... more Computational Fluid Dynamics (CFD) is employed to investigate the hydrogen jet exiting through different shapes of orifices. The effect of orifice geometry on the structure, development and dispersion of a highly under-expanded hydrogen jet close to the exit is numerically investigated. Various shapes of orifices are evaluated, including holes with constant areas such as elliptical and circular openings, as well as, enlarging circular orifices. A three-dimensional in-house parallel code is exploited to simulate the flow using an unstructured tetrahedral finite volume Euler solver. The numerical simulations indicate that, for a high pressure reservoir hydrogen release, the area of the orifice is the main geometric parameter influencing the centerline pressure at the hydrogen-air interface and the transient peak temperature, while the elliptical or expanding orifices slightly mitigate the auto-ignition risks associated with the accidental release of hydrogen. Therefore, circular openings represent the most conservative geometry for the study of auto-ignition of hydrogen.

ASME 3rd International Conference on Microchannels and Minichannels, Part B cont’d, 2005

Numerical simulation based a design tool devised by an aposteriori error estimation technique ter... more Numerical simulation based a design tool devised by an aposteriori error estimation technique termed the ‘bound method’ is applied in this paper to examine and analyze fluidic flow and species transport phenomena of the electro-osmotic flow in a heterogeneously charged T-shaped micro-mixer. This novel technique provides fast, inexpensive and reliable bounds to the ‘output’ for the slip velocity model of the electro-osmotic flow. The bound method presented here-in is extended to use of an adaptive refinement to generate the ‘ideal’ mesh for computations, the direct equilibration to evaluate the ‘hybrid-flux’ very efficiently, and a parallel calculation to accelerate the error estimates of the output of interest.

ASME 2nd International Conference on Microchannels and Minichannels, 2004

ABSTRACT The micro pumping technology is one of the major and growing research fields in microflu... more ABSTRACT The micro pumping technology is one of the major and growing research fields in microfluidics. The use an electric voltage to induce electro-kinetic fluid flow is an efficient and reliable mechanism for micro pumping systems. The prediction of quantities such as the mass flow rate or the mean species concentration for this type of flow, termed the Electro-Osmotic flow, plays a crucial role in the design and control process of the entire microfluidic system. To this end, numerical techniques are efficient to evaluate these quantities but accuracy depends on the mesh utilizes. The a posteriori finite element output bound method is used to calculate these quantities while offering information regarding accuracy. The bound method applied here-in is based on the flux-free approach and provides relevant, inexpensive, and asymptotic lower and upper bounds to the mass flow rate of an Electro-Osmotic flow in a cross-intersection of a two-dimensional microchannel. To obtain shaper bounds, the flux-free approach is further enhanced by an adaptive mesh refinement strategy. This work focuses on the development of the numerical procedure for Electro-Osmotic flows and reports performance of the method in terms of numerical accuracy and computational cost.

Mathematics and Computers in Simulation, 1997

We present a Bayesian-validated surrogate framework which permits economical and reliable integra... more We present a Bayesian-validated surrogate framework which permits economical and reliable integration of large-scale simulations into engineering design and optimization. In the surrogate approach, the large-scale simulation is evoked only to construct and ...