• Energy Research

In the present study the mechanisms of evolution of propeller tip and hub vortices in the transitional region and the far field are investigated experimentally. The experiments involved detailed time-resolved visualizations and velocimetry measurements and were aimed at examining the effect of the spiral-to-spiral distance on the mechanisms of wake evolution and instability transition. In this regard, three propellers having the same blade geometry but different number of blades were considered. The study outlined dependence of the wake instability on the spiral-to-spiral distance and, in particular, a streamwise displacement of the transition region at the increasing inter-spiral distance. Furthermore, a multi-step grouping mechanism among tip vortices was highlighted and discussed. It is shown that such a phenomenon is driven by the mutual inductance between adjacent spirals whose characteristics change by changing the number of blades.

A numerical simulator of a laser Doppler system has been developed to have a computer aided design tool for system analysis. The simulator uses Mie optics to compute the light scattering intensity of the particles crossing the measuring volume. The simulated Doppler signal can be analyzed by numerical simulators of the Doppler processors. The simulators of Burst Spectrum Analyzer (BSA), Intelligent Flow Analyzer (IFA 550) and Counter have been developed. The processor simulators have been experimentally compared with the real processors using a special device that allows the Doppler signals computed by the simulator to be generated. Under well known conditions some numerical experiments have been performed to investigate the accuracy of the LDA technique. In this paper results about measurement errors due to the ambiguity phase noise of the Doppler signal are presented.

Two reports present the results of a two-stage experimental campaign aimed at the study of hydrokinetic turbines operating in cavitating flow conditions. The activity is of primary importance for floating tidal turbines floating that operate in proximity of the free-surface. The activity has been carried out at the Depressurized circulating water channel of CNR-INM, Rome. Two turbine design by Schottel Hydro (Germany) were considered and tested in different installation layouts. During project phase I, and isolated turbine layout was considered, and in phase II a turbine installed downstream of a supporting strut was considered. In both cases, a wide range of turbine operating conditions was considered, by varying current flow speed, turbine rotational speed and pressure in the test section, in order to simulate design as well as off-design real operating conditions. The results allowed to determine a comprehensive characterization of the cavitating flow around the turbine blades and the effects of cavitation on turbine performance and radiated acoustic noise. The activity has been conducted in the framework of the H2020 MaRINET 2 project, with co-funding under the Trans-National Access program, with grants No. 1534 (Phase I) and grant ID f3152cb7-07ad-4de7-931e-b2e3991313d8 (Phase II).

A scaled-model of an HATCT (Horizontal Axis Tidal Current Turbine) was tested in the CNR-INSEAN Circulating Water Channel. Two different analyses were performed during the experiments: preliminary investigation of the performances of the turbine in different working conditions and LDV (Laser Doppler Velocimetry) measurements in the near wake. Velocity measurements are performed in phase locking to the rotor, in order to resolve the blade wake structure at different working regimes. Indeed, the knowledge of all three velocity components (tangential, axial, radial) allows to evaluate through energy balance concerns, the transfer of inflow kinetic energy into mechanical work of the rotor and into different energy contributions in the wake. The LDV analysis of the near wake also contributes to understand some of the most important fluid dynamics aspects related to the design of the rotor and to the different working conditions, and can be used as a preliminary study for understanding the wake evolution and the interaction between two or more turbines in a farm.

A scaled-model of a horizontal axis tidal current turbine (HATCT) is tested in the CNR-INM Circulating WaterChannel. The experiments are designed to establish in the first place the performances of the turbine atdifferent working settings. The second goal is to investigate the hydrodynamics generated by the turbine in thenear wake using the Particle Image Velocimetry (PIV) technique. For this purpose, velocity measurements areperformed in a longitudinal plane and phase-locked to the rotor angle in order to resolve the wake structure atdifferent working regimes. The analysis of the axial and radial velocity fields reveals the flow features of theslipstream, 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 vortexinstabilities. Furthermore the separate contributions of transport, production and dissipation to the turbulentkinetic energy in the wake field are discussed in detail. The vortex unsteadiness is captured and correlated withthe evolution of the kinetic energy transport and production terms. Understanding these phenomenologies isan important step to develop computational tools able to estimate the radiated noise or the potential impactof turbulence on performances of further rotors placed in the wake, as in an array of tidal turbines.

AbstractThe impact of cavitation and inflow perturbation by an upstream support structure on the performance of a horizontal-axis tidal turbine has been studied by experiments at model scale in a depressurized water channel at CNR-INM. Measurements of turbine generated power and thrust have been carried out at varying channel flow speed, tip-speed ratio, and cavitation number. The visualization of cavitation patterns has been associated with the performance measurements to identify the role played by different cavitation types on turbine behavior. Radiated noise measurement were also performed to assess the impact of cavitation phenomena on turbine induced acoustic signature.