Overview
Flow over a submerged cylinder subjected to an energetic free surface wave field has not been studied extensively. This is particularly the case for cylinders whose characteristic length is small in comparison to the incident wavelength, where a developing turbulent wake plays a dominant role on the forces exerted on the structure. With the recent interest in renewable energy, ocean waves have the potential to provide reliable, clean energy using wave energy converters (WECs). This research project will specifically address large Reynolds number regimes ++and large amplitude waves, a particular class of flows which, to the PI’s knowledge have not been studied before and are becoming increasingly relevant to a multitude of practical applications. The aim for this research is to be able to bridge the understanding between nonlinear fluid dynamic of the turbulent near wake flow in proximity to free surface and the force production of the oscillating cylinder, specifically investigating the absorption mechanisms of wave energy at the cylinder. Using the United State Naval Academy towing tank, the experiments examine and elucidate near wake flows at high Reynolds number in the small amplitude, linear range of wave forcing as well as the nonlinear regime characterized by increased wave steepness and wave breaking.
The long-term goals of the proposed effort are to understand the mechanisms of incident wave energy absorption for a flow over an oscillating cylinder in close proximity to the free surface, and to form a physics-based model of forcing that can be used in active flow control approaches to maximize absorption of the incident wave energy. To do so, we will characterize the two-dimensional spacetime variability of dynamical processes in the near wake of a cylindrical body shape and relate these to force production on the cylinder. The fluid dynamics-specific objectives of the project are to:
- Elucidate spatiotemporal evolution of flow structures in the cylinder near wake, primarily the cylinder shed vorticity and its relation to the force production experienced by the cylinder.
- Understand how secondary vorticity originating from the free surface affects the dynamics and structure of the cylinder near wake and, in turn, to forcing on the cylinder.
- Characterize wave-turbulent wake interaction. Incident wave phase speeds are expected to be an order of magnitude larger than cylinder vertical/streamwise (heave/surge) velocities. As a result, passing waves will have a pronounced effect on the vorticity transport across the near wake and therefore may have a significant effect on cylinder forcing and wave production of turbulence.
- Formulate conditions for the onset of incident wave breaking at the cylinder and quantify the effect that injected turbulence has on the near wake dynamics and consequently the drag/lift forces on the cylinder.
In support of the primary fluid dynamic goals of the proposed research, specific objectives for the modeling, dynamical systems analysis, and control aspect of the project are:
- Use data-driven and physics-based modeling techniques to derive high fidelity and reduced order control-theoretic models of the fluid dynamic forcing on the cylinder caused by characteristics of the incident wave-wake interaction that are measurable in realistic WEC installation.
- Compare the novel derived models to existing potential flow and high-fidelity numerical fluid solver[1]based solutions with the aim of identifying the primary sources of discrepancies between the two approaches.
Two different PIV experiments will be performed. The first will provide phase locked measurements with PIV realizations obtained at a specific phase (e.g. crest and/or trough) of the incident wave or cylinder displacement. The second experiment will be time series measurements, or “free runs”, when the camera will operate at the maximum frame rate. For this set of experiments, LIF cinematic measurements of the free surface will be performed as well. We anticipate approximately 40 wave cycles passing the cylinder before reflected waves from the beach contaminate the near wake of the cylinder. Multiple run repeats will be employed to achieve sufficient statistical significance for PIV data averaging. Repeatability and shape of regular waves produced by the USNA towing tank wavemaker have been well documented [26, 29] and resemble second order Stokes wave model to within 2% error.
Linking the forces to the flow structure in the cylinder’s near wake is a crucial aspect of this proposal. Our controlled experiments will highlight the underlying flow physics and relate how feedback control of the oscillator restoring and damping forces can incorporate nonlinear fluid dynamic forces to invoke flow control measures that improve forcing and, in turn, wave power absorption.
YEAR 1- Focusing on designing, building and bench testing the spring/damper actuation system, preparation for tow tank experiments including procurement of proposed equipment and sensor calibration, creating a damping and spring coefficient controller to emulate Evans’ conditions, and performing a set of baseline measurements with the cylinder in regular waves with concurrent LIF tracking of the free surface adjacent to the cylinder.
YEAR 2- Focusing on the analysis of data obtained during year 1 experiments. This will include control volume analysis, and development of high fidelity and reduced order data driven models with modification of spring and damping parameters for nonlinear wave conditions, and validating the modeling assumptions to arrive at better optimum power absorption conditions for nonlinear waves. In summer of year 2 we will complete the second experiment in the tow tank with the underwater PIV system. Detailed measurements will be performed at or close to the resonant system condition obtained from the year 1 results. Year 2 goals also include processing and posting experimental data for public use.
YEAR 3- Focusing on analysis of data during year 2 experiments, synthesis of the data with year 1 experiments, presentations at the APSDFD meeting and publication of results in peer reviewed publications.
The proposed research will have theoretical and practical broader impact on understanding nonequilibrium flows past compliant structures in close proximity to oceanic free surface waves. Furthermore, central to our outreach program will be education program in coordination with the established STEM program at the United States Naval Academy. This STEM program operates a variety of initiatives focused on teaching students about math, science, and engineering. Working with the STEM program we propose developing a coordinated problem-based learning module (approximately 23 hours in length) about oceanic waves, and potential for renewable energy and how engineers and scientists use experiments to obtain measurements that let us understand oceanic processes at the free surface.
