LABORATORY SIMULATION OF TURBULENT-LIKE FLOWS

Most turbulence studies up to the present are based on statistical modeling, however, the spatio-temporal flow structure of the turbulence is still largely unexplored. Tur- bulence has been established to have a multi-scale instantaneous streamline structure which influences the energy spectrum and other properties such as dissipation and mixing. In an attempt to further understand the fundamental nature of turbulence and its consequences for efficient mixing, a new class of flows, so called “turbulent-like”, is in- troduced and its spatio-temporal structure of the flows characterised. These flows are generated in the laboratory using a shallow layer of brine and controlled by multi-scale electromagnetic forces resulting from a combination of electric current and a magnetic field created by a fractal permanent magnet distribution. These flows are laminar, yet turbulent-like, in that they have multi-scale streamline topology in the shape of “cat’s eyes” within “cat’s eyes” (or 8’s within 8’s) similar to the known schematic streamline structure of two-dimensional turbulence. Unsteadiness is introduced to the flows by means of time-dependent electrical current. Particle Tracking Velocimetry (PTV) measurements are performed. The technique developed provides highly resolved Eulerian velocity fields in space and time. The analysis focuses on the impact of the forcing frequency, mean intensity and amplitude on various Eulerian and Lagrangian properties of the flows e.g. energy spectrum and fluid element dispersion statistics. Other statistics such as the integral length and time scales are also extracted to characterise the unsteady multi-scale flows. The research outcome provides the analysis of laboratory generated unsteady multi- scale flows which are a tool for the controlled study of complex flow properties related to turbulence and mixing with potential applications as efficient mixers as well as in geophysical, environmental and industrial fields

Virtual incidence effect on rotating airfoils in Darrieus wind turbines

Small Darrieus wind turbines are one of the most interesting emerging technologies in the renewable energies scenario, even if they still are characterized by lower efficiencies than those of conventional horizontal-axis wind turbines due to the more complex aerodynamics involved in their functioning. In case of small rotors, in which the chord-to-radius ratios are generally high not to limit the blade Reynolds number, the performance of turbine blades has been suggested to be moreover influenced by the so-called "flow curvature effects". Recent works have indeed shown that the curved flowpath encountered by the blades makes them work like virtually cambered airfoils in a rectilinear flow. In the present study, focus is instead given to a further effect that is generated in reason of the curved streamline incoming on the blades, i.e. an extra-incidence seen by the airfoil, generally referred to as "virtual incidence". In detail, a novel computational method to define the incidence angle has been applied to unsteady CFD simulations of three airfoils in a Darrieus-like motion and their effective angles of attack have been compared to theoretical expectations. The analysis confirmed the presence of an additional virtual incidence on the airfoils and quantified it for different airfoils, chord-to-radius ratios and tip-speed ratios. A comparative discussion on BEM prediction capabilities is finally reported in the study

Enhancing Operational Flood Detection Solutions through an Integrated Use of Satellite Earth Observations and Numerical Models

Among natural disasters floods are the most common and widespread hazards worldwide (CRED and UNISDR, 2018). Thus, making communities more resilient to flood is a priority, particularly in large flood-prone areas located in emerging countries, because the effects of extreme events severely setback the development process (Wright, 2013). In this context, operational flood preparedness requires novel modeling approaches for a fast delineation of flooding in riverine environments. Starting from a review of advances in the flood modeling domain and a selection of the more suitable open toolsets available in the literature, a new method for the Rapid Estimation of FLood EXtent (REFLEX) at multiple scales (Arcorace et al., 2019) is proposed. The simplified hydraulic modeling adopted in this method consists of a hydro-geomorphological approach based on the Height Above the Nearest Drainage (HAND) model (Nobre et al., 2015). The hydraulic component of this method employs a simplified version of fluid mechanic equations for natural river channels. The input runoff volume is distributed from channel to hillslope cells of the DEM by using an iterative flood volume optimization based on Manning\u2019s equation. The model also includes a GIS-based method to expand HAND contours across neighbor watersheds in flat areas, particularly useful in flood modeling expansion over coastal zones. REFLEX\u2019s flood modeling has been applied in multiple case studies in both surveyed and ungauged river basins. The development and the implementation of the whole modeling chain have enabled a rapid estimation of flood extent over multiple basins at different scales. When possible, flood modeling results are compared with reference flood hazard maps or with detailed flood simulations. Despite the limitations of the method due to the employed simplified hydraulic modeling approach, obtained results are promising in terms of flood extent and water depth. Given the geomorphological nature of the method, it does not require initial and boundary conditions as it is in traditional 1D/2D hydraulic modeling. Therefore, its usage fits better in data-poor environments or large-scale flood modeling. An extensive employment of this slim method has been adopted by CIMA Research Foundation researchers for flood hazard mapping purposes over multiple African countries. As collateral research, multiple types of Earth observation (EO) data have been employed in the REFLEX modeling chain. Remotely sensed data from the satellites, in fact, are not only a source to obtain input digital terrain models but also to map flooded areas. Thus, in this work, different EO data exploitation methods are used for estimating water extent and surface height. Preliminary results by using Copernicus\u2019s Sentinel-1 SAR and Sentinel-3 radar altimetry data highlighted their potential mainly for model calibration and validation. In conclusion, REFLEX combines the advantages of geomorphological models with the ones of traditional hydraulic modeling to ensure a simplified steady flow computation of flooding in open channels. This work highlights the pros and cons of the method and indicates the way forward for future research in the hydro-geomorphological domain

Surface-based flow visualization

This is the author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/computers-and-graphics/.With increasing computing power, it is possible to process more complex fluid simulations. However, a gap between increasing\ud data size and our ability to visualize them still remains. Despite the great amount of progress that has been made in the field of\ud flow visualization over the last two decades, a number of challenges remain. Whilst the visualization of 2D flow has many good\ud solutions, the visualization of 3D flow still poses many problems. Challenges such as domain coverage, speed of computation, and\ud perception remain key directions for further research. Flow visualization with a focus on surface-based techniques forms the basis\ud of this literature survey, including surface construction techniques and visualization methods applied to surfaces. We detail our\ud investigation into these algorithms with discussions of their applicability and their relative strengths and drawbacks. We review the\ud most important challenges when considering such visualizations. The result is an up-to-date overview of the current state-of-the-art\ud that highlights both solved and unsolved problems in this rapidly evolving branch of research

On the influence of virtual camber effect on airfoil polars for use in simulations of Darrieus wind turbines

Darrieus vertical-axis wind turbines are experiencing renewed interest from researchers and manufacturers, though their efficiencies still lag those of horizontal-axis wind turbines. A better understanding of their aerodynamics is required to improve on designs, for example through the development of more accurate low-order (e.g. blade element momentum) models. Many of these models neglect the impact of the curved paths that are followed by blades on their performance. It has been theorized that the curved streamlines of the flow impart a virtual camber and incidence on them, giving a performance analogous to a cambered blade in a rectilinear flow. To test the extent of this effect, wind tunnel experiments have been conducted in a rectilinear flow to obtain lift and drag for three airfoils: a NACA 0018 and two conformal transforms of the profile. The transformed airfoils exhibit the virtual camber that the theory predicts is imparted to a NACA 0018 when used in a Darrieus turbine with blade chord-to-turbine radius ratios, c/R, of 0.114 and 0.25. A parallel computational fluid dynamics campaign has been conducted to study the aerodynamic behavior of the same blades in curvilinear flow in Darrieus-like motion with c/R = 0.114 and 0.25, at tip-speed ratios of 2.1 and 3.1, using novel techniques to obtain blade effective angles of attack. The analysis confirms that the theory holds, with the wind tunnel results for the NACA 0018 being analogous to numerical results for the relevant cambered airfoils. In addition, turbine performance is calculated using computational fluid dynamics and a blade element momentum code, for each of the blades in turn. The computational fluid dynamics results for the NACA 0018 agree closely to blade element momentum results for the equivalent cambered airfoil where c/R = 0.25, for both turbine power and blade tangential forces. Agreement between the two methods using geometrically identical blades is poor at both the blade and turbine level for c/R = 0.25. It is concluded that when modeling a Darrieus rotor using blade element momentum methods, applying experimental data for the profile used in the turbine will yield inaccurate results if the c/R ratio is high, in such cases it is necessary to select a profile based on the virtual shape of the blades

Localized flow, particle tracing, and topological separation analysis for flow visualization

Since the very beginning of the development of computers they have been used to accelerate the knowledge gain in science and research. Today they are a core part of most research facilities. Especially in natural and technical sciences they are used to simulate processes that would be hard to observe in real world experiments. Together with measurements from such experiments, simulations produce huge amounts of data that have to be analyzed by researchers to gain new insights and develop their field of science

Numerical Calculation of Transport Properties of Rock with Geometry Obtained Using Synchrotron X-ray Computed Microtomography

Macroscopic properties of rocks are functions of pore-scale geometry and can be determined from laboratory experiments using rock samples. Macroscopic properties can also be determined from computer simulations using 3D pore geometries derived from various imaging techniques. Using 3D imagery and computer simulations, we can calculate the porosity, permeability, formation resistivity factor and cementation exponent in reservoir drill cores. The objective of this thesis was to develop a workflow using Synchrotron X-ray Computed Microtomography (CMT) images and commercially available software in order to determine the macroscopic properties in reservoir drill cores for Midale Marly (M0) and Vuggy Shoal (V6) rocks. The workflow started by using CMT data that provided three-dimensional images of the reservoir rocks taken from drill cores in the Weyburn oil field. The resulting CMT grey scale images were used to isolate the pore space in the rock image. A three-dimensional mesh, representing the pore space, was then used to obtain the solution of the Navier-Stokes equations for an incompressible fluid and Laplace's equation for electrical current flow. Solutions of the Navier-Stokes equations were computed with different inlet pressures for the same pore geometry in order to confirm a direct proportionality between the mass fluid flux and pressure gradient as Darcy’s Law specifies. Previously measured laboratory transport properties were compared with my calculated transport properties on a smaller sub-volume of the same rock core imaged using 0.78 µm resolution CMT images. For the Midale Marly rock, the calculated permeability ranged from 0.01 to 3.53 mD. The formation resistivity factor ranged from 29.3 to 309.43 and the cementation exponent ranged from 1.99 to 2.10. The sample was verified to be nearly isotropic as the permeability was similar for three orthogonal fluid flow directions. Even though the sub-volume analyzed was smaller than a Representative Elementary Volume (REV), the results are within an order of magnitude of the previously calculated laboratory results as completed by Glemser (2007) and fall on the same power law trend. A Vuggy (V6) sample was investigated after the sample had been exposed to CO2, and dissolution within the rock matrix resulted in large visible pore spaces. Using 7.45 µm resolution CMT images, the permeability for a large isolated pore could not be calculated using the previous workflow due to computer memory limitations. Resampling enabled the data to fit into the available computer memory. The permeability values ranged from 2.66x10^5 to 8.59x10^5 mD for resampling the CMT images from 2x to 10x

CFD analyses and performance comparison of micro-hydropowder plants

The project concerns the hydropower renewable energy technology at its micro scale: thanks to an internship performed with the Belgian startup TurbulentHydro, the purpose of this Master Thesis is to evaluate the energy performance of 2 di erent models currently under investigation: the so-called Flatblades and Streamlines con gurations. These layouts are similar in shape but di erent both in dimension and for the turbine used. After a little introduction on renewable energies and hydropower technology, the selected CFD simulation procedure and all its options have been explained as well as the choice of the turbulence model to apply, computing the meaningful parameters to add in the model. This dissertation highlights the uid dynamics behaviour by means of suitable softwares for this purpose: Autodesk Inventor, the 3D CAD mechanical design software in order to build, and edit when necessary, the geometries of the models considered, MeshMixer, a state-of-art software for locally adjusting the mesh of the starting model and OpenFOAM, a free and open source CFD program in order to run and evaluate any details of the analysis, simulating the operation conditions by means of its components and tools. Eventually, the most important CFD results are presented. Di erent con gurations bring di erent results. Regarding Flatblades model, the scope was re ned the CFD initial setup in order to achieve results as close to the real case validation as possible whereas, for Streamlines model, an additional new component has been added for improving the current design in terms of energy output at the turbine level. At the end, conclusions stated possible re nements and improvements for these simulations as well as uncertainties arose from the results that might be avoided for the next stages