A scaled-model of a horizontal axis tidal current turbine (HATCT) is tested in the CNR-INM Circulating Water Channel. The experiments are designed to establish in the first place the performances of the turbine at different working settings. The second goal is to investigate the hydrodynamics generated by the turbine in the near wake using the Particle Image Velocimetry (PIV) technique. For this purpose, velocity measurements are performed in a longitudinal plane and phase-locked to the rotor angle in order to resolve the wake structure at different working regimes. The analysis of the axial and radial velocity fields reveals the flow features of the slipstream, as well as its expansion and dependence on the turbine operating parameters. Analysis of the non- diagonal terms of the Reynolds stress tensor provides insight into the onset of tip vortex pairing and of vortex instabilities. Furthermore the separate contributions of transport, production and dissipation to the turbulent kinetic energy in the wake field are discussed in detail. The vortex unsteadiness is captured and correlated with the evolution of the kinetic energy transport and production terms. Understanding these phenomenologies is an important step to develop computational tools able to estimate the radiated noise or the potential impact of turbulence on performances of further rotors placed in the wake, as in an array of tidal turbines.

This report describes the experimental work performed by CNR-INM in the frame of the NextProp WE4100 work element, entitled "Foil experiments". The goal is to investigate the fluid-structure interaction of three foils made of different materials.

On-blade pressure and stress measurements on Type-B propeller with and without lowercase

Particle Image Velocimetry is employed to investigate the turbulence modulation induced by dispersed elongated, rod-like particles in a turbulent channel flow. Particles with two different aspect ratios AR=40,80 are tested, at a volume fraction of 10-5. Carrier flow velocimetry data and distribution and orientation data of dispersed particles are obtained by an ad-hoc single-camera phase-discrimination technique. Carrier flow data shows that in the near-wall region turbulence modulation by particle occurrs as well as a decrease of average streamwise velocity. Analysis of conditional probability density function of particles location reveals that particles locations statistically match flow regions with instantaneous low vorticity and high streamwise velocity, in particular in the near-wall region.

Contra-rotating propellers - STEP1

In this preliminary work, the feasibility of the combination of a volumetric velocimetry technique such as Defocusing Particle Image Velocimetry and a particle phase-discrimination methodology based on ridge detection algorithm for the analysis of turbulent multiphase flows with non-spherical fiber-like particles is discussed. Experimental results of a dilute suspension of fibers in an open-channel apparatus are provided.

A turbulent channel flow laden with elongated, fiber-like particles is investigated experimentally by optical techniques. The flow-particle inter-coupling is analyzed in the case of particles with an aspect ratio of 40 and 80, at two volume fractions, 10 and 10 . An image processing technique is presented, which is employed to simultaneously obtain carrier flow velocimetry data and distribution and orientation data of dispersed particles. Turbulence enhancement is reported in the near-wall region, with a higher level of increase associated with higher aspect ratio particles. Comparison to fiber data suggests that this mechanism of turbulence modulation stems from a particles orientational behavior. The preferential particle distribution is reported to be dependent on the aspect ratio in the region close to the wall. The probability density function of the fibers' orientation angle appears to be independent of the particle aspect ratio once it is conditioned to the fibers' characteristic size.

The wake hydrodynamics of counter-rotating propellers (CRP) is studied via phase-locked particle image velocimetry for three operating conditions featuring distinct flow dynamics. We focus our work on the flow field and the associated vortex dynamics that take place in the near wake, using a special on-line/off-line phase-locking technique that allows the velocity measurements to be phase-averaged based on both forward and aft propeller angular positions. The effect of propellers relative position on the flow features is investigated with support from high-speed visualizations of the vortex system, which is observed by inducing cavitation in the cores of the vortices. We report the relevant interactions that take place between the forward and aft propellers' tip vorticity, in particular the regular patterns of vortex reconnection and break-up that depend on the advance coefficient, as well as the unique formation of isolated vortex rings and a highly coherent vortex wake.

The hydrodynamics of the flow between and in the wake of contra-rotating propellers is investigated experimentally via Particle Image Velocimetry (PIV) at different advanced coefficients. Results show that the flow dynamics is deeply influenced by the interplay between the two systems of helical vorticity that develop in the wake. The forward and aft propellers' slipstream structure is analyzed in terms of blade loading and relative phase angle. The onset of tip vortex break-up, reconnection and pairing processes occurring within the wake is discussed with a focus on instability mechanisms. Flow field measurements between propellers highlight that the distribution of the tangential component is balanced more effectively by the aft propeller at specific loading conditions.

Contra-rotating propellers represent a non-conventional approach for marine propulsion whose main advantage lies in the increase of propulsive efficiency. This is achieved by recovering part of the energy loss due to the rotational flow generated by a single propeller by means of a contra-rotating downstream propeller. The hydrodynamics of such configuration is quite complex due to the interaction between upstream and downstream propellers and a deep understanding of their features is critical to driving the design phase. In this work a methodology based on planar (2D-2C) Particle Velocimetry is presented to investigate on the flow in the wake of two contra-rotating propellers. An ad-hoc mixed hardware and software phase-locking technique is developed in order to analyze the contribution of each propeller to the overall hydrodynamics of the system.

A computational study of hydrokinetic turbine hydrodynamics by using a hybrid viscous/inviscid-flow model is presented. The methodology integrates Detached Eddy Simulation for the numerical solution of the Navier-Stokes equations with a Boundary Integral Equation Model for potential flows. The formulation generalizes standard actuator disc models and provides a computationally efficient model to analyse the hydrodynamic performance and flowfield of single turbines and arrays in a viscous turbulent onset flow. An application of the hybrid DES/BIEM model to the analysis of the wake field downstream a horizontalaxis turine in uniform flow is presented. Numerical results for a five-bladed bi-directional turbine with elliptical blade sections are compared with experimental results including performance curves and a detailed characterization of the flowfield downstream the rotor using Particle Image Velocimetry. Results demonstrate the capability of the proposed methodology to correctly describe main flow features characterizing the turbine wake.

Our experiments examine the interaction between two coaxially aligned helical vortex systems induced by two counter-rotating propellers of slightly different diameters operating in a water tunnel. Visualizations show that the two helix structures merge and breakup in the regions of closest distance and strongest vorticity. A reconnection process follows, resulting in the formation of isolated vortex rings advected by the accelerated flow of the propeller system.

The deformation of air bubbles in a liquid flow field is of relevant interest in phenomena such as cavitation, air entrainment, and foaming. In complex situations, this problem cannot be addressed theoretically, while the accuracy of an approach based on Computational Fluid Dynamics (CFD) is often unsatisfactory. In this study, a novel approach to the problem is proposed, based on the combined use of a shadowgraph technique, to obtain experimental data, and some machine learning algorithms to build prediction models. Three models were developed to predict the equivalent diameter and aspect ratio of air bubbles moving near a plunging jet. The models were different in terms of their input variables. Five variants of each model were built, changing the implemented machine learning algorithm: Additive Regression of Decision Stump, Bagging, K-Star, Random Forest and Support Vector Regression. In relation to the prediction of the equivalent diameter, two models provided satisfactory predictions, assessed on the basis of four different evaluation metrics. The third model was slightly less accurate in all its variants. Regarding the forecast of the bubble's aspect ratio, the difference in the input variables of the prediction models shows a greater influence on the accuracy of the results. However, the proposed approach proves to be promising to address complex problems in the study of multi-phase flows.

A vertical plunging jet has been investigated experimentally by means of an innovative volumetric shadowgraphy technique. A space carving algorithm has been used for the measurement of the size and concentration of the air bubbles, to follow the air bubbles path inside the investigated volume, providing both spatial and temporal evolution of the same. Furthermore, the air bubble tracking has been performed by means of an algorithm based on the Lucas-Kanade optical flow algorithm. Results highlighted a distribution of the air bubbles that follows the free jet spread inside the investigated volume with a dependence of speed and size from the action exerted by the vertical jet. The volumetric shadowgraph technique has proven effective in characterizing air bubbles, also in presence of relevant void fraction.

A 3D volumetric technique for measuring the evolution over time of the kinematic and geometric characteristics of the bubble population in multiphase flows at moderate void fraction is here proposed. The method is based on a shadowgraphy approach and requires a set of calibrated and synchronized cameras, each placed in front of a bright screen. Synchronized, 2D images of the bubbly flow are analyzed to extract the outline of the bubbles as seen from every camera. Then, each bubble is separately identified as a 3D volume described by the intersection of the cones having vertices on the optical center of each camera and passing through the contour of the bubble. Details about the implementation of the procedure, including the further refinements of the first rough bubble identification and the optimization of the number and geometric arrangement of the points of view, are reported against the results obtained on a reference set of spheres of known dimensions. Application on isolated bubbles demonstrates the ability of the procedure to extract quantitative and self-consistent information over time. These results are consolidated by a hint at a plunging jet test case with a significant void fraction, showing potential for application to situations of practical interest.

In the field of marine hydrodynamics, propeller operations in off-design conditions represent a challenging topic that has been increasingly attracting interest among industry and research institutions. Over the ship operating life, off-design conditions and the associated modifications of the propeller inflow are connected to a wide range of events such as degradation of the overall efficiency, amplification of unwanted side effects of propeller operations (pressure pulses, vibration and noise) and, ultimately, failure of the ship propulsion system. In this work, the performance of a propeller operating in off-design conditions is investigated by a comprehensive experimental activity aimed at synthesizing the cause-and-effect relation between the propeller loads and its inflow. To this purpose, two novel set-ups are employed: the first one is dedicated to the measurement of single blade loads, whereas the second, a boroscopic-based Stereo-PIV (SPIV) system, is focused on the inflow analysis. The overall measurement apparatus is installed on a twin-screw ship model. Experiments, carried out in the CNR-INM towing tank, consist of straight ahead and steady drift motions at drift angles ±13 and ±27. This test matrix is representative of weak and tight maneuvering conditions.

PIV tests on contra-rotating propellers

We report on the activities regarding the cavitation tests performed on the lowercase unit provided by FluidTechno Co., Ltd. The goal is to determine the cavitating behavior for different predefined materials and/or geometries of the water pick-up grid (or inlet). The lowercase is a full-scale unit, however the flow conditions are scaled according to the velocity limit of the test facility (CEIMM cavitation tunnel).

A marine propeller operating in off-design conditions isa challenging topic in marine hydrodynamics that has at-tracted a strong interest in the last decades from industryand research institutions. During actual ship operations,disturbances in the propeller inflow have a negative impacton a wide range of aspects: from reduced overall efficiencyup to the structural failure of the propulsion system. In thiswork, the relation between propeller loads and its inflowin off-design conditions is investigated by means of twonovel experimental set-ups developed to analyze each ofthese aspects, respectively an onboard system to measuresingle-blade loads, and a boroscope-based stereo-PIV sys-tem for the flow characterization. The latter methodologyallows underwater acquisitions and improves greatly uponthe design introduced by Pereira et al (2003). However, thisconcept introduces strong optical aberrations which have tobe dealt with appropriate algorithms. The whole apparatusis used for towing tank testing of a twin-screw model instraight-ahead and steady-drift conditions. Preliminary re-sults obtained in straight-ahead and steady-drift conditions(drift angle?= 13o) are reported to show the potentialof the developed measurement methodology for drawing acomprehensive picture of the propeller performance.

The hydrodynamics of a ship in prescribed motionis addressed by unsteady simulations using a hybridRANS/BEM solver. The interactive methodology com-bines a viscous-flow model addressing the flow aroundthe ship with a potential-flow model describing the loadsinduced by the vorticity generated at the propeller. Animplicit iterative procedure is provided in the time cy-cle through a body-force/effective inflow approach. Thepresent model is validated through comparisons about thevelocity flow-field and blade loads of numerical predictionsand measurements collected during an experimental cam-paign of a ship model in calm water moving in straightahead and steady drift motion. In particular, the nomi-nal tridimensional flow at the propeller disc is providedby the application of a stereo PIV technique, based on theuse of boroscopic equipment, whereas the transient singleblade loads are collected using a novel set-up adopting a 6-component multi-axial force transducer constrained at theroot blade itself. In spite of the inability of the model to ac-count for the blade boundary layer, the flow-field descrip-tion is detailed enough outside the propeller region whileassuring a limited computational burden related to meshgeneration. The present methodology represents a viablesolution to address the flow of maneouvring ships.