An experimental assessment of the effect of current on wave buoy measurements

This is the final version. Available on open access from Elsevier via the DOI in this recordWave measurement buoys provide characterisation of wave climates that forms the basis for the design of offshore systems. These buoys are commonly subjected to currents which affect the resulting wave measurements, and if not accounted for will result in errors in the estimated sea state parameters. The present work provides results and observations from experiments aimed at assessing the impact that currents have on wave buoy measurements, thereby informing processing techniques to more accurately include this effect. Through scaled testing (circa 1:15) in a combined wave–current test tank, buoy motions (diameter, D = 0.24m) are recorded in current only, waves only, and combined wave–current including oblique conditions. From these, the wave-induced motions are extracted and compared against three prediction methods based on established transfer function approaches as well as a frequency-domain hydrodynamic coefficient (HC) model based on potential flow. The scaled buoy was observed to have large, complex, irregular oscillatory vortex-induced motions (VIM) exceeding the buoy diameter. Both the magnitude and frequency of these oscillations was found to be significantly altered by the mooring stiffness and configuration whilst the addition of collinear waves was found not to affect the magnitude of VIM. Furthermore, due to the lack of VIM heave response and a large difference between the frequencies of the vortex-induced and wave induced horizontal motions, it was found that the VIM did not significantly alter the interpretation of the wave climate for the tested conditions. The HC model was found to accurately capture the observed modified hydrodynamics for opposing wave–current conditions, where larger horizontal motions than (typically) predicted are observed for all frequencies. This behaviour is concluded to result from increased excitation forces owing to the higher wavenumbers. The experiments highlight the potential effects of VIM on wave measurement performance of wave buoys, along with the complex and mooring-dependent nature of the response. Altered dynamics in the presence of currents are described which must be accounted for to avoid errors and the presented prediction methods provide a mechanism to account for these effects in wave processing methodologies which can subsequently reduce uncertainty in our understanding of the offshore environment.Engineering and Physical Sciences Research Council (EPSRC

Wave buoys in current - experimental results and observations

This is the author accepted manuscript. The final version is available from EWTEC via the link in this recordWave buoys are used extensively for the characterisation of deployment locations for offshore technologies, where they are subjected to currents in addition to the ocean waves. It has been frequently observed that the measurements from wave buoys are affected by these currents, but the majority of deployed buoys cannot measure, or account for, this effect. Presented here are a series of experiments conducted at the FloWave Ocean Energy Research Facility, deploying a spherical wave buoy scale model with a simplified mooring in a series of combined wave-current sea states. The resulting openaccess dataset provides 6 degree of freedom buoy motion and force data (current only) in addition to wave gauge and acoustic doppler velocimeter measurements of the sea states. Experiments were conducted under a range of combined wave-current conditions with variables including velocity, wave period, and relative wave-current angle. It is observed that vortex induced motions (VIM) are significant and highly sensitive to the mooring configuration and current speed. Nevertheless, the wave-induced response and buoy motion amplitude is found to agree with linear wave-current theory predictions in most following wavecurrent conditions. This agreement is poorer in opposing conditions where larger surge motions than predicted are consistently observed. Wave buoy outputs in directional irregular seas were also found to closely match wave gauge outputs. If properly considered, it is suggested that the effect of the current on existing buoy technologies may be accounted for without hardware modification or additional sensing, but through updated analytical tools.Engineering and Physical Sciences Research Council (EPSRC

Experimental data of bottom pressure and free surface elevation including waves and current interaction

This is the final version. Available on open access from MDPI via the DOI in this recordData Availability Statement: The data presented in this study are openly available: https://doi.org/10.7488/ds/3120.Abstract: Force plates are commonly used in tank testing to measure loads acting on the foundation of a structure. These targeted measurements are overlaid by the hydrostatic and dynamic pressure acting on the force plate induced by the waves and currents. This paper presents the dataset of bottom force measurement with a six degree-of-freedom force plate (AMTI OR6-7 1000, surface area 0.464 m×0.508 m) combined with synchronised measurements of surface elevation and current velocity. The data covers a wave frequencies between 0.2 to 0.7 Hz and wave direction between 0◦ and 180◦ . These variations are provided for 0 and 0.2 m/s current speed and a variation of current in the absence of waves covering 0 to 0.45 m/s. The dataset can be utilised as a validation dataset for models predicting bottom pressure based on free surface elevation. Additionally, the dataset provides the wave and current induced load acting on the specific load cell and a fixed water depth of 2 m, which can subsequently be removed to obtain the often-desired measurement of structural loadsEngineering and Physical Sciences Research Council (EPSRC

The dispersion of spherical droplets in source–sink flows and their relevance to the COVID-19 pandemic

In this paper, we investigate the dynamics of spherical droplets in the presence of a source-sink pair flow field. The dynamics of the droplets is governed by the Maxey-Riley equation with Basset-Boussinesq history term neglected. We find that, in the absence of gravity, there are two distinct behaviours for the droplets: small droplets cannot go further than a specific distance, which we determine analytically, from the source before getting pulled into the sink. Larger droplets can travel further from the source before getting pulled into the sink by virtue of their larger inertia, and their maximum travelled distance is determined analytically. We investigate the effects of gravity, and we find that there are three distinct droplet behaviours categorised by their relative sizes: small, intermediate-sized, and large. Counterintuitively, we find that the droplets with minimum horizontal range are neither small nor large, but of intermediate size. Furthermore, we show that in conditions of regular human respiration, these intermediate-sized droplets range in size from a few $\mu$m to a few hundred $\mu$m. The result that such droplets have a very short range could have important implications for the interpretation of existing data on droplet dispersion.Comment: 14 pages, 7 figure

Experimental assessment of head losses through elliptical and sharp-edged orifices

Due to the European electricity market liberalization and the appearance of other renewable electricity producers, an increase of installed peak power capacity is relevant to provide larger amount of electricity in a shorter turbine duration. When the discharge increase is not too large, it is frequently efficient to place an orifice at the entrance of the existing surge tank. Actually, the surge tank modifications have to be designed case-by-case. The introduction of head losses helps to manage maximum and minimum water level following the management of downstream discharge control and transient events. It allows to keep the same safety level. The placed orifice should commonly produce asymmetric losses. It is important to note that target head losses are evaluated with a unidimensional numerical model which performs transient simulation for relevant flow directions in the whole water way system and hydropower plant. A previous study performed by the authors focused on the effects of sharp-edged orifice parameters. This research carries out the analysis of corresponding elliptical orifices to tested sharp-edged orifices. The goal is to evaluate the head loss modification in both directions due to the shape change

Standardising Marine Renewable Energy Testing: Gap Analysis and Recommendations for Development of Standards

Marine renewable energy (MRE) is still an emerging technology. As such, there is still a lack of mature standards and guidance for the development and testing of these devices. The sector covers a wide range of disciplines, so there is a need for more comprehensive guidance to cover these. This paper builds on a study undertaken in the MaRINET2 project to summarise recommendations and guidance for testing MRE devices and components, by reviewing the recently published guidance. Perceived gaps in the guidance are then discussed, expanding on the previous study. Results from an industry survey are also used to help quantify and validate these gaps. The main themes identified can be summarised as: the development progression from concept to commercialisation, including more complex environmental conditions in testing, accurately modelling and quantifying the power generated, including grid integration, plus modelling and testing of novel moorings and foundation solutions. A pathway to a standardised approach to MRE testing is presented, building on recommendations learnt from the MaRINET2 round-robin testing, showing how these recommendations are being incorporated into the guidance and ultimately feeding into the development of international standards for the marine renewable energy sector