Session: 10-06-01: Fluid Measurements and Instrumentation

Paper Number: 144771

144771 - Comparative Analysis of Vortex Field Measurement Around Mhk Turbine Blades Using Piv and Cfd Techniques 

Cross-flow turbines have been recently re-discovered in the fields of wind and tidal energy thanks to the high power density they can provide when arranged in arrays. Among them, marine hydrokinetic (MHK) turbines are a viable candidate in harnessing a variety of water resources, as they are able to operate in extremely low hydraulic head. The major advantage of these systems comes from the closer spacing allowed between devices of the same array, and the following establishment of constructive hydrodynamic interactions that significantly enhance the energy output of the farm. Therefore, understanding the synergy behind the operation of these devices is essential in order to effectively address their optimization.

In this paper, we investigate the mechanism of torque generation and maintenance across various configurations of a pair of miniature MHK turbines. The velocity field around the blades was measured for two different tip speed ratios (0.90, 1.20) over the full rotation of the devices, providing insights on the influence of vortex shedding onto the lift generation process. The velocity field was analyzed by performing a momentum balance over moving control volumes (CV), allowing to estimate the force acting on the blades and correlate it to the flow field. The results were then compared with existing two-dimensional CFD studies.

The experiments were carried out in a 2 m long and 0.30 m wide circulating water tunnel facility, at a water depth of 0.105 m. Here, the flow structures of a three-bladed turbine with a diameter of 62.8 mm were compared to the ones of a twin-turbine set-up, for a total of four configurations: one for the single-turbine, and three different relative flow angles for the twin-turbines (0°, 30°, 60°). The flow field analysis was carried out via time-resolved mono-scopic particle image velocimetry (2D PIV) technique around NACA0012 airfoil blades, at a mainstream velocity of 0.316 m/s and a Reynolds number in the order of 10³, based on the chord length of 25.4 mm. These measurements were made possible by adopting blades made of plexiglass, which thanks to their transparency allowed laser light to pass through, thereby drastically reducing shadow disturbance. In order to remove the noise of the random velocity components, flow data coming from three laser positions was combined and then phase-averaged.

The  basic mechanism behind torque production and maintenance was confirmed to be subdividable in two phases: in line with the airfoil theory, a lift force was first generated by the pressure difference due to the flow acceleration on the inner surface of the blade; the same lift was maintained after the onset of dynamic stall by the low pressure associated with a leading-edge vortex, which traveled along the airfoil surface gradually separating from it. Across all the configurations, increasing the tip-speed ratio resulted in a later occurrence of the dynamic stall, together with higher vorticity values. Vortex strength was also enhanced by the hydrodynamic interaction between the two devices in the twin-turbine configuration, with particular attention to relative flow angles of 30° and 60°. To complete the picture, these results are further compared with 2D CFD simulations for the purpose of validating the control volume method for torque estimation, as well as discussing and quantifying the 3D effect, and experimental error.

Presenting Author: Amin Hafdaoui KEIO UNIVERSITY

Presenting Author Biography: Amin Hafdaoui is a Master's student pursuing a double degree program between the School of Open and Environmental Systems at Keio University and the Energy Department at Politecnico di Milano. After completing a Bachelor's degree in Energy Engineering in Milan, focusing on the management and design of energy systems, Hafdaoui is currently researching the optimization of Marine Hydrokinetic Turbines, which represent a promising avenue for the expansion of offshore clean-power generation.

Authors:

Amin Hafdaoui KEIO UNIVERSITY
Shin Tanaka KEIO UNIVERSITY
Minh Doan Fulbright University Vietnam
Shinnosuke Obi KEIO UNIVERSITY