S.R. Turnock

Impact of a free-falling wedge with water: synchronized visualization, pressure and acceleration measurements

A fixed 25deg deadrise angle wedge is allowed to fall from a range of heights into static water. A high-speed (up to 5000 frames s?1) camera is used to visualize the impact and subsequent formation of jet flows and droplets. Unsteady pressure measurements at six locations across the wedge surface are measured at 10 kHz. Two accelerometers (10g, 100g) are mounted above the apex of the wedge and measure the vertical acceleration. A purpose-built position gauge and analysis of the synchronized video allows the wedge motion to be captured. The synchronization of these data with the digital images of the impact makes it particularly suitable for the validation of computational fluid dynamics simulations as well as theoretical studies. A detailed experimental uncertainty analysis is presented. The repeatability of the test process is demonstrated and the measured pressures are comparable to previous studies. A 2.5 ms time delay is identified between the point of impact observed from the video and the onset of actual wedge deceleration. The clear definition of the free surface provides insight into jet formation, its evolution and eventual breakdown, further assisting with the development of numerical predictions

Tribological design constraints of marine renewable energy systems

Against the backdrop of increasing energy demands, the threat of climate change and dwindling fuel reserves, finding reliable, diverse, sustainable/renewable, affordable energy resources has become a priority for many countries. Marine energy conversion systems are at the forefront of providing such a resource. Most marine renewable energy conversion systems require tribological components to convert wind or tidal streams to rotational motion for generating electricity while wave machines typically use oscillating hinge or piston within cylinder geometries to promote reciprocating linear motion. This paper looks at the tribology of three green marine energy systems, offshore wind, tidal and wave machines. Areas covered include lubrication and contamination, bearing and gearbox issues, biofouling, cavitation erosion, tribocorrosion, condition monitoring as well as design trends and loading conditions associated with tribological components. Current research thrusts are highlighted along with areas needing research as well as addressing present-day issues related to the tribology of offshore energy conversion technologies.

The Use of Computational Fluid Dynamics to Assess the Hull Resistance of Concept Autonomous Underwater Vehicles

Autonomous underwater vehicles (AUVs) provide an important tool for collecting detailed scientific information from the oceans depths. The hull resistance of an AUV is an important factor in determining the powering requirements and range of the vehicle. This paper discusses the use of computational fluid dynamics (CFD) to determine the hull resistance of three existing AUVs, of differing shape and size. The predictions are compared with available experimental data and good agreement found. This work has demonstrated that with use of suitable shape parameterisation it is possible to carry out concept design evaluation using a RANS flow solver.

Enhancing Performance of a Horizontal Axis Tidal Turbine using Adaptive Blades

Free stream tidal turbines are a source of growing interest in the marine renewable energy field. Some designs use variable pitch blade control devices in order to maximize the efficiency of the turbines; however these are complex to design, construct and maintain under the severe load conditions sub sea devices experience. Adaptive materials have been used in the wind industry to create bend-twist coupled blades in an effort to bring turbine efficiency ever closer to the Betz limit, and increase annual energy capture. This work encompasses a feasibility study, focusing on hydrodynamic performance calculations, wherein the blade geometry is artificially reconfigured as a function of flow velocity. The pressure distribution over the blade is also analysed. The concept of a passively adaptive tidal turbine blade is shown to increase annual energy capture, reduce blade loading and delay cavitation inception.

The use of computational fluid dynamics to aid cost-effective hydrodynamic design of autonomous underwater vehicles

The missions being proposed for autonomous underwater vehicles (AUVs), by both marine scientists and industry, are becoming increasingly complex and challenging. In order to meet these demands the next generation of AUVs will need to be faster, to operate for longer durations, and to be more manoeuvrable than existing vehicles. It is therefore vital that the hydrodynamic forces and moments acting on a self-propelled manoeuvring AUV can be predicted accurately at the initial design stage. In order to achieve this, the use of a computational-fluid-dynamics-based analysis is suggested. The approaches developed are predominantly steady state and suitable for running on a workstation personal computer using a commercial software licence. It is estimated that the proposed simulations would take a competent user less than 1 month for a new concept design. The total cost of these simulations is significantly lower than the cost of building a model and having it commercially tested to capture the same level of detail for the resistance, propulsion, and manoeuvring performance. Based on the validation studies presented, it is estimated that on a 2×106 element structured mesh a competent user should be able to predict hydrodynamic forces to within at least 10 per cent and moments to within 20 per cent of in-service performance.

Accurate Capture of Propeller-Rudder Interaction using a Coupled Blade Element Momentum-RANS Approach

Abstract This paper compares experimental results with numerical simulations using three different body-force propeller models. The results demonstrate that as long as the radial variation of axial and tangential momentum generated by the propeller are included, the influence of the unsteady propeller flow can be removed and calculations with a steady propeller model can be performed for the evaluation of the influence of the propeller on the rudder.

Internal actuation of underwater robots using Control Moment Gyros

A novel control scheme based on internal actuation using Control Moment Gyros (CMGs) is proposed as the basis of an unrestricted attitude control system suitable for application to Autonomous Underwater Vehicles (AUVs). The coupled dynamic equations of a CMG actuated underwater robot are derived, taking into account interactions with the fluid environment. A nonlinear feedback control law is developed based on energy considerations of the coupled system. Singularities, redundancy and null motion are discussed in the context of CMGs and a mathematical escapability condition is developed based on the differential geometry of null motion. A geometric study of a CMG pyramids singularities forms the basis of a global steering law that guarantees real-time singularity avoidance whilst maintaining exactness in some constrained workspace. The practical design considerations of a CMG pyramid developed for a laboratory scale underwater robot are discussed. Finally the results of practical experiments using the CMG pyramid are presented to demonstrate the steering and control performance of the system.

Model predictive control of a hybrid autonomous underwater vehicle with experimental verification

In this work, model predictive control is used to provide transit and hover capabilities for an autonomous underwater vehicle where the description of the system dynamics used include terms measured experimentally. The resulting controller manoeuvres the vehicle in the presence of constraints on the actuators and results obtained from the deployment of the vehicle in an inland lake for the study of the zebra mussel, an invasive species, are also given.

Design and performance of an electric tip-driven thruster

A 250 mm diameter demonstrator electromagnetic tip-driven propeller (ETDP) unit has been designed, built and tested in a towing tank at zero, advance speed and for a range of advance speeds. The aims have been to assess the performance, efficiency and general practicalities of such a propeller for use on underwater vehicles. An overview is given of the general design philosophy and practical design issues of the demonstrator. The testing procedure is described and performance results are presented.

Computational Evaluation of the Periodic Performance of a NACA 0012 Fitted With a Gurney Flap

A detailed computational investigation into the periodic two-dimensional performance of a NACA 0012 section fitted with 2 and 4 percent h/c Gurney flaps operating at a Reynolds number of 0.85×106 is presented. The aim of the work was to determine the suitability of the incompressible Reynolds-averaged Navier-Stokes (RANS) formulation in modeling the vortex shedding experienced by lifting sections with blunt, sharp edged features. In particular, whether under-converged steady state calculations could be used for section design performance evaluation in place of the computationally intensive time accurate flow simulations. Steady, periodic, and time-averaged two-dimensional lift and drag coefficients, as well as vortex shedding frequency, were predicted and compared with the available experimental data. Reasonable agreement was found, once sufficiently fine grids had been generated, and the correct time step determined for the time accurate simulations.