Abstract
One source of localized forces on tidal turbine blades, that can lead to lowered efficiencies and failures, is local turbulent eddies forming on and around the blades. These eddies, surviving in the wake, can also have a great influence on the performance and durability of other turbines downstream when in an array. At this time, very few experimental results of flow over turbines are available, with even less work done specifically on turbulence. In such circumstances, when few experimental tests are available, either because of cost or complications in such testing, numerical modeling is a great tool that enables initial studies and characterization for the flow, including strength of turbulence, size and distribution of flow structures, including eddies.
The work performed in this 1 year research project is the numerical modeling of the turbulent flow on a 3-blade horizontal axis turbine in order to study the size, strength and impact of turbulent eddies on the blades, the body of the turbine, and in the wake behind such turbine. In the wake, the turbulent perturbation in the flow behind a turbine will have a great impact on the efficiency, performance and durability of any turbine placed behind as is expected in an array.
This research project uses commercial CFD software (ANSYS CFX) to simulate flow over a 3-bladed turbine in order to test various numerical turbulence models and determine which one(s) are suitable for uses in tidal turbine flow analysis. Numerical results from the study also include flow field (velocities), pressure field and strength of turbulence. The numerical results will be validated with experimental results provided from the original group who made the experimental investigation of the tested turbine in Southampton. Such a validated study will provide a valuable tool to properly quantify turbulence shape and strength on any new tidal turbine design, leading to more robust, streamlined and safe design.
During the year-long period that this grant supported this project, the following research work has been done: a continuous literature review has been underway to ensure a full understanding of the flow physics involved and to gain further knowledge of similar research completed to date as well as best practices in this field of study. A fluid model was created using a turbine geometry that matched an experimental setup for model validation. Simulations were run for a range of tip speed ratios and compared using power and thrust coefficients. Methodology for building this model and result analysis are discussed later in this report. Overall results have good agreement in trends but both power and thrust are underestimated. Current geometry adjustment and mesh convergence studies are underway to investigate their impact on the model solution. Remaining work is discussed in the conclusion.