Biomimetic improvement of hydrodynamic performance of horizontal axis tidal turbines

Sasaki Donation and China Scholarship CouncilThis study explored the potential of further improving the hydrodynamic performance of tidal turbines by applying leading-edge tubercles to the blades inspired by the humpback whales. Within this framework, a wide variety of experimental investigations, supported by numerical studies, has been conducted. The study first focused on the design of the leading edge tubercles for a tidal turbine blade. Numerical simulation has been conducted for various designs and the best candidate was then applied onto a representative tidal turbine blade, a 3D hydrofoil can be fitted with various leading-edge designs. Experimental test was conducted in a cavitation tunnel and demonstrated significant benefits in terms of improving the lift coefficient and lift-to-drag ratio especially after stall. The results were then validated and complemented by numerical simulations for further detailed analysis. This simulation explicitly showed that the contra-rotating vortices generated by the tubercles formed a vortex fence prevented the tip vortex from inducing the spanwise flow, which meanwhile energized the flow and maintained more attached. Following that, a set of tidal turbine models with different leading-edge profiles was manufactured and were tested to evaluate the efficiency, cavitation, underwater noise and detailed flow characteristics in the cavitation tunnel. These experimental investigations confirmed that the leading-edge tubercles could: improve the hydrodynamic performance in the low Tip Speed Ratio (TSR) region without lowering the maximum power coefficient; maintain the power coefficient in the low Reynolds number; constrain the cavitation development to within the troughs of the tubercles; hence mitigate the underwater noise levels

Humpback whale inspired design for tidal turbine blades

This study is to further improve the hydrodynamic performance of tidal turbines by applying leading-edge tubercles to the blades inspired by the humpback whales. The study first focused on the design and optimisation of the leading edge tubercles for a specific tidal turbine blade section by using numerical methods to propose an "optimum" design for the blade section. This optimum design was then applied onto a representative tidal turbine blade. This representative 3D blade demonstrated significant benefits especially aft er stall. The experimental measurements were further validated and complimented by numerical simulations using commercial CFD software for the detailed flow analysis. Following that, a set of tidal turbine models with different leading - edge profiles was manufactured and series of model test campaigns were conducted in the cavitation tunnel to evaluate their efficiency, cavitation, underwater noise, and detailed flow characteristics. Based on these experimental investigations it was confirmed that the leading edge tubercles can improve: the hydrodynamic performance in the low Tip Speed Ratio (TSR) region without lowering the maximum power coefficient; constrain the cavitation development to within the troughs of the tubercles; and hence mitigating the underwater noise levels

Experimental analysis of an air cavity concept applied on a ship hull to improve the hull resistance

At the forefront of ship design is the desire to reduce a ship׳s resistance, thus being the most effective way to reduce operating costs and fulfil the international criteria for reduction in CO2 emissions. Frictional drag is always proportional to the wetted surface of the vessel and typically accounts for more than 60% of the required propulsive power to overcome; hence the desire to reduce the wetted surface area is an active research interest. An initial full-scale sea trail on a vessel by introducing air as a lubricating medium has indicated 5–20% propulsive energy savings (DK-GROUP, 2010). Following the report of the fundamental tests with the air cavity concept applied on a flat plate, which was conducted in the Emerson Cavitation Tunnel of Newcastle University (Slyozkin et al., 2014), this paper explores the same concept only this time applied on an existing container ship model to investigate whether it benefits in frictional drag reduction, whilst producing a net energy saving. The middle section of this 2.2 m ship model was modified to accommodate a 0.43×0.09 m2 air cavity in the bottom of the hull and then various model scale tests have been conducted in the towing tank of Newcastle University. The model experiments produced results ranging from 4% to 16% gross drag reduction. Upon applying scaling factors, it is estimated from the experimental results that around 22% gross energy could be saved in a full-scale application with just a 5% reduction in the wetted surface area. Further complementary model tests were also conducted to explore the effect of the air cavity on the stability of the model and on the vertical motion responses in a regular head and following wave. While the cavity did not affect the vessel stability the motion response behaviour seemed to be affected non-linearly by the effect of the air cavity

Systematic study on propulsive performance of tandem hydrofoils for a wave glider

This paper presents the propulsive performance optimization of tandem hydrofoils equipped in a wave glider in head sea conditions with the aid of computational fluid dynamic (CFD) method by using a commercial CFD code, STAR-CCM+. Firstly, this work performs a systematic study on a single 2D hydrofoil to study the effect of varying the pivot position and the torsional spring stiffness to seek for the optimum propulsive performance within a range of velocity. Secondly, parametric studies on the propulsive performance of 2D and 3D six tandem hydrofoils are conducted by varying the oscillation amplitude and the torsional spring stiffness. The result reports: The results show that the torsional springs play a critical role in propulsive performance, comparing to the pivot position. The propulsive performance of the middle hydrofoil is greater than the others in the fore and after positions. When 2D and 3D six tandem hydrofoils achieve the optimum propulsive performance, the frequency ratio is chosen to be around 25 (torsional spring stiffness (k S =0.8N⋅m/rad)) and 17 (torsional spring stiffness (k S =11.8N⋅m/rad)), respectively. Comparing to previous study, the propulsive performance of each hydrofoil in six tandem hydrofoil configuration is greatly improved and none of hydrofoil produce negative thrust

A prediction program of manoeuvrability for a ship with a gate rudder system

The Gate Rudder is a special twin rudder system with two rudder blades placed aside of a propeller. Main advantage of this system is the energy saving originated from the rudder thrust which is induced by the two cambered rudder blades comparably efficient to ducted propellers. However, as any rudder’s prime task, the performance of manoeuvrability is critical to the Gate Rudder too. With the currently available Manoeuvring Modelling Group (MMG) simulation programs, the simulation is only applicable to the traditional single rudder located behind the propeller. Therefore, how to predict the manoeuvring performance for the gate rudder is the focus of this paper. On the other hand, a recent study of the Gate Rudder reveals that this innovative system has remarkable flap effect which is well known as a manoeuvring interaction between rudder blades and ship stern. This phenomenon has been observed in the case of conventional rudder and introduced into an MMG-based theoretical model as interaction factor aH. However, the average values of aH for conventional rudder is between 0.1-0.2 in general whilst the aH value for the Gate Rudder is more than twice as much, showing a superior course keeping ability of the Gate Rudder. The paper presents the manoeuvrability simulation method of a ship with this Gate Rudder system and introduces some examples of comparisons between the model tests and free running tests which was conducted with 2.5 m ship model in the manoeuvring tank at Kyushu University, Japan

The influence of leading-edge tubercles on the sheet cavitation development of a benchmark marine propeller

Cavitation is an undesirable phenomenon in the maritime industry as it causes damage to the propeller, degrading hydrodynamic performance and increasing the subsequent underwater radiated noise (URN). Therefore, mitigating cavitation on marine propellers is an important area of research in order to reduce carbon emissions emitted from the shipping industry and reduce the rate at which ocean ambient noise levels are increasing. The Humpback whale has provided inspiration to research in the fluid-structure interaction field due to the presence of leading-edge (LE) tubercles on the pectoral fins that allow it to perform acrobatic maneuvers to catch prey. This paper assesses the cavitation containment capability of the LE tubercles on a benchmark marine propeller in both heavy and light cavitating conditions using commercial code STARCCM+, unsteady incompressible Reynolds-averaged Navier Stokes (RANS) solver and the Schnerr-Sauer cavitation model to quantify the sheet cavitation present over a range of operating conditions. In summary, in heavy-cavitating conditions, a reduction in sheet cavitation with the inclusion of LE tubercles was observed to a maximum value of 2.75% in all operating conditions considered. A maximum improvement of 3.51% and 1.07% was predicted in propulsive thrust and hydrodynamic efficiency, respectively. In light cavitating conditions, although in some conditions a reduction in cavity volume was observed, this did not result in an improvement in hydrodynamic performance

The influence of leading-edge tubercles on the hydrodynamic performance and propeller wake flow development of a ducted propeller

This study implements leading-edge (LE) tubercles on a benchmark 19A accelerating duct to investigate the impact on the hydrodynamic performance and propeller wake flow development at multiple operating conditions. The study was conducted using Computational Fluid Dynamics (CFD) where the sliding mesh technique was used to describe the propeller rotation and the hydrodynamic flow-field was solved using Improved Delayed Detached Eddy Simulations (IDDES). In summary, it was found that LE tubercles can enhance the thrust of the duct by a maximum of 7.15% and disrupt the coherent vortex structure of the benchmark ducted propeller which will likely influence the noise signature of the propulsor

Cavitation observations and noise measurements of horizontal axis tidal turbines with biomimetic blade leading-edge designs

This paper focuses on the study of cavitation and underwater noise performance of a biomimetically improved horizontal axis tidal turbine (HATT) with a leading edge design inspired by the tubercles on the pectoral fins of humpback whales. Systematic model tests were recently conducted and details of this test campaign together with the findings are summarised in the paper. Several full-scale tidal turbine application cases were studied to understand the full-scale operating conditions considering the characteristics of varied kinds of tidal energy devices, the varying wave height and the flood/ebb tide. A systematic test regime was then designed and conducted. A set of tidal turbines with different leading-edge profiles was manufactured and tested under different loading and hence cavitation conditions. During the tests, cavitation was observed and underwater noise level was measured in comparison with the cavitation and noise performance of a counterpart HATT without tubercles. The tested turbines displayed two main types of cavitation patterns independent of the tubercles. These were steady tip vortex cavitation and relatively intermittent cloud cavitation with a misty appearance. The leading-edge tubercles triggered the cavitation onset earlier for the tidal turbine but constrained the cavitation region to the trough between tubercles with a lesser extent on the blades. The noise performance was strongly related to the blade cavitation hence it was influenced by the leading-edge tubercles. While the turbine was working under the non-cavitating conditions the total noise level was similar to the background noise level. With the increase of the tip speed ratio the noise level was increased, while increasing blade pitch angle reduced the noise level due to lower blade loading. Cavitation inception and noise diagrams are provided as a database for future studies

Numerical simulation of a tidal turbine based hydrofoil with leading-edge tubercles

The tubercles along the leading edges of the humpback whale flippers can provide these large mammals with an exceptional maneuverability. This is due to the fact that the leading-edge tubercles have largely a 3D benefit for the finite hydrofoils, which can maintain the lift, reduce the drag and delay the stall angle. Newcastle University launched a series study to improve a tidal turbine’s performance with the aid of this concept. This paper presents a numerical simulation of the tested hydrofoil, which is representative of a tidal turbine blade, to investigate the flow around the foil and also to numerically model the experiment. This hydrofoil was designed based on an existing tidal turbine blade with the same chord length distribution but a constant pitch angle. The model tests have been conducted in the Emerson Cavitation Tunnel measuring the lift and drag. The results showed that the leading-edge tubercles can significantly improve the performance of the hydrofoil by improving the lift-to-drag ratio and delaying the stall. By applying Shear Stress Transport (SST), Detached Eddy Simulation (DES) and Large Eddy Simulation (LES) via using the commercial CFD solver, Star-CCM+, the tested hydrofoil models were simulated and more detailed flow information has been achieved to complement the experiment. The numerical results show that the DES model is in close agreement with the experimental results. The flow separation pattern indicates the leading-edge tubercles can energize the flow around the hydrofoil to keep the flow more attached and also separate the flow into different channels through the tubercles