Abstract
Turbulence is a significant issue at every site being considered for instream tidal energy development. The turbulence in strong tidal flow creates fluctuating forces that can degrade turbine performance and shortening turbine lifespan. Thus, properly characterizing turbulence is a critical step to designing durable devices and accelerating the pace of technology development. The three Work Packages of this project address the challenge of characterizing turbulence by: validating mobile methods of measuring the turbulence around an operating turbine, improving and validating a numerical models of turbulence and turbine operation in turbulent flow, and developing the data analysis methods to include turbulence analysis in resource and site assessment. The project examined sites in Grand Passage and Minas Passage, two locations where tidal turbines have or will be deployed in flows that display a variety of turbulent features.
Work Package 1 (WP1) used innovative mobile measurement devices to characterize the spatial variation of turbulence in Grand Passage. Two field experiments were conducted in the vicinity of Sustainable Marine Energy Canada (SME) PLAT-I turbine platform. For both field experiments, measurements of turbulent velocities were collected using stream-following surface drifters with a down-looking turbulence-resolving acoustic Doppler current profiler (ADCP). Data were collected over a range of flow conditions and when the turbines were and were not operating. Analysis of the data generated maps to describe the spatial and temporal variations of meanflow velocities and turbulence parameters in the studied area. When turbines are operational, the wake is mapped out, identifying the maximum velocity deficit and how farther downstream the wake expands vertically, the velocity deficit decreases, and the wake begins to recover. WP1 demonstrated that surface drifters are an efficient and economical methods for mapping both mean-flow velocities and resolved turbulence in the vicinity of an operating tidal turbine.
Work package 2 (WP2) developed and validated a high-resolution numerical model of the Grand Passage. The numerical simulations were carried out using a GPU based CFD solver developed at the University of New Brunswick. The research team produced fine grid mesh for Grand Passage with 24 million control-volumes, an average horizontal resolution of 2 m × 2 m, and an innovative smooth transition between the main channel and the shoreline. The numerical simulations run on the mesh clearly illustrates the multi-dimensional, complicated flow structures of the strong tidal flow. The simulations were used to illustrate several important aspects of the PLAT-I project. Although the PLAT-I platform lies outside the wake of Peter’s Island, locations near the deployment site experience periodic, low-velocity, unsteady flow structures. And, a location only 300 m north of the PLAT-I, where and ADCP was previously deployed, has significantly different velocity profiles and larger flow variations than those from locations near the PLAT-I
In the second part of the WP2, numerical models of turbines were embedded into these flow simulations. The researchers developed an innovative methodology based on the actuator line (AL) method and takes advantage of both CPU and GPU processing power to increase computational efficiency. In the initial results, even though a very coarse grid has been used, the simulation successfully produces the wake of the turbine, which can be affected by unsteady tidal flows. The development of a fine resolution simulation with PLAT-I properly located in Grand Passage has been completed and initial testing has indicated a successful implementation of the many-to many technique. The final step of activate the AL method in the rotor block and simulated the turbine for sufficient time to obtain statistical convergence of the flow field variables and turbine performance is under way.
Finally, the third work package further developed data analysis methods and a regional numerical model. The Acadia regional numerical models, which were developed in previous projects, were used to generate years-long data set of 3D simulated tidal flow for all tidal passages in the Bay of Fundy. These data sets were used in both WP1 and WP2, but were also shared with other stakeholders. On the data analysis side, the research team worked with Luna Ocean to develop a data analysis tool, LunaTide, that can better predict tidal flow, using a wide range of measurements (ADCPs, drifters, X-band radar, etc.) and numerical simulation data. Working with FORCE, the numerical simulations and LunaTide were used to show how X-Band radar can be used to predict surface velocities and analyze wakes across the FORCE site. As well, the research team participated in pilot projects to images and videos from drones to collect bathymetry data and analyze tidal flow. When all the results are combined together, the project has produced data sets and data tools that can make maps and forecasts of tidal conditions in all the Bay of Fundy passages, at a unprecedented level of details and accuracy.
Given the short length of the project, the original project proposal was extremely ambitious, a complete mapping of the turbulence characteristics in tidal passage. As well, the project faced with the challenge of having to shift the entire project’s focus from the OpenHydro turbine in Minas Passage to the SME PLAT-I in Grand Passage. Despite the challenges, the project has been successful in developing all the tools necessary to meet its goal: an efficient method to measure the spatial variation of flow around an operating turbine; a high-resolution model capable modelling in-situ tidal turbines in a realistic, fully turbulent flow; and the data analysis tools to combine these results (and others) with the long-term, regional, tidal flow data sets to produce the complete mapping of the flow characteristics. As we see new projects preparing for turbine deployments in the Bay of Fundy, we will start using these tools to provide project developers the information that they require.