Shuichi Nagata

Numerical Investigation of 2D Optimal Profile of Backward-Bent Duct Type Wave Energy Converter

A floating type backward-bent duct buoy (BBDB) is a wave energy conversion device with an oscillating water column (OWC) at the front side. The device captures the wave energy using the heaving, the pitching, the surging motion of BBDB, and the heaving motion of OWC. Investigations are carried out to find more reasonable devices than the traditional OWC type floating device. An eigenfunction expansion method is introduced for analyzing the BBDB with OWC. It is confirmed that these solutions give good agreement with several experimental results in this paper. It is shown in a design method how to make BBDB match the turbine characteristics. This feature is being able to select the optimum profile of the turbine and the BBDB individually from each characteristic before comprehensive evaluation of the BBDB and the turbine in the design. After grasping the element characteristics, which are appropriate for the wave energy conversion system, the synthetic design method is built. The BBDB size and the turbine diameter are determined by considering the cost corresponding to the smallest size under the same output. In this way, we can obtain the optimal profiles considering the construction cost including the turbines.

A Study on the Effects of Blade Profile and Non-Uniform Tip Clearance of the Wells Turbine

Several of wave energy devices being studied under many wave energy programs in the United Kingdom, Japan, Portugal, India and other countries make use of the principle of an oscillating water column (OWC). In such wave energy devices, a water column which oscillates due to wave motion is used to drive an oscillating air column which is converted into mechanical energy. The energy conversion from the oscillating air column can be achieved by using a self-rectifying air turbine. Wells turbine is a self-rectifying air turbine which is expected to be widely used in wave energy devices with OWC. There are many reports which describe the performance of Wells turbine both at starting and running characteristics. According to these results, Wells turbine has inherent disadvantages: lower efficiency, poorer starting and higher noise level in comparison with conventional turbines. In order to enhance the performance of Wells turbine, some rotor blade profiles have been recommended by various researchers. The aim of this study is to investigate the effect of rotor blade profile on the performance of Wells turbine. In the study, four kinds of blade profile were selected and tested by model testing under steady flow condition. The types of blade profile are as follows: NACA0020; NACA0015; modified NACA0015; and modified Eppler472. The experimental investigations have been performed by use of test section with a casing diameter of 300 mm. Further, the effect of non-uniform tip clearance on the turbine performance was tested and the result was compared with that of the case of Wells turbine with uniform tip clearance. As an additional experiment, the effects of blade profile and non-uniform tip clearance on the performance under unsteady flow condition have been investigated numerically by using a quasi-steady analysis.Copyright

Analysis of Mixed Convection in a Lid Driven Trapezoidal Cavity

Convection is the heat transfer mechanism affected by the flow of fluids. The amount of energy and matter are conveyed by the fluid can be predicted through the convective heat transfer. The convective heat transfer splits into two branches; the natural convection and the forced convection. Forced convection regards the heat transport by induced fluid motion which is forced to happen. This induced flow needs consistent mechanical power. But natural convection differs from the forced convection through the fluid flow driving force which happens naturally. The flows are driven by the buoyancy effect due to the presence of density gradient and gravitational field. As the temperature distribution in the natural convection depends on the intensity of the fluid currents which is dependent on the temperature potential itself, the qualitative and quantitative analysis of natural convection heat transfer is very difficult. Numerical investigation instead of theoretical analysis is more needed in this field. Two types of natural convection heat transfer phenomena can be observed in the nature. One is that external free convection that is caused by the heat transfer interaction between a single wall and a very large fluid reservoir adjacent to the wall. Another is that internal free convection which befalls within an enclosure. Mathematically, the tendency of a particular

A Sea Trial of Wave Power Plant With Impulse Turbine

A sea trial of wave power plant using an impulse turbine with coreless generator has been carried out at Niigata-nishi Port, in order to demonstrate usefulness of the turbine for wave energy conversion. Oscillating water column (OWC) based wave power plant has been installed at the side of a breakwater and has an air chamber with a sectional area of 4 m2 (= 2m × 2m). The impulse turbine used in the sea trial has fixed guide vanes both upstream and downstream, and these geometries are symmetrical with respect to the rotor centerline in order to rotate in a single direction in bi-directional airflow generated by OWC. The turbine is operated at lower rotational speed in comparison with conventional turbines. The rotor has a tip diameter of 458 mm, a hub-to-tip ratio of 0.7, a tip clearance of 1 mm, a chord length of 82.8 mm and a solidity of 2.0. The guide vane with chord length of 107.4 mm is symmetrically installed at the distance of 30.7 mm downstream and upstream of the rotor. The guide vane has a solidity of 2.27, a thickness ratio of 0.0279, a guide vane setting angle of 30° and a camber angle of 60°. The generator is coreless type and can generate electricity at lower rotational speed in comparison with conventional generator. The rated and maximum powers of the generator are 450 W and 880 W respectively. The experimental data obtained in the sea trial of wave power plant with the impulse turbine having coreless generator was compared to these of Wells turbine which is the mainstream of the turbine for wave energy conversion. As a result, total efficiency of the plant using the impulse turbine was higher than that of Wells turbine.Copyright

An Experimental Study of Negative Drift Force Acting on a Floating OWC “Backward Bent Duct Buoy”

The utilization of renewable energy is required immediately since emissions of carbon dioxide are being restricted. To this end, we are investigating the ocean wave energy converter, especially the floating OWC “Backward Bent Duct Buoy” (BBDB). The BBDB, proposed by Masuda in 1986, is a wave energy converter of the ‘moored floating oscillating water column’ type that is composed of an air chamber, an L-shaped bent duct, a buoyancy chamber, and a turbine. The BBDB has certain positive characteristics. Firstly, the primary conversion performance of the BBDB is better than other floating OWCs. Secondly, the length of the BBDB is shorter than other floating OWCs. Thirdly, as the BBDB advances in the incident wave direction with slow speed waves because of the negative wave drift force, the mooring cost can be reduced. In this research, experiments under a various wave periods were carried out to clarify the characteristics and cause of the generation of negative drift force acting on a BBDB in regular waves with a two-dimensional wave tank at Saga University. The length of the BBDB model is 85cm. To measure the wave drift force, the model is moored with horizontal wire-springs. The motions of the BBDB, such as surge, heave, and pitch, are measured by remotely using image processing. The fluid velocity around the BBDB is measured by using particle image velocimetry (PIV). Motion tests of the BBDB without mooring are also carried out to measure the horizontal velocity of the BBDB in waves. From the experimental results, the characteristics and causes of the generation of negative drift force acting on the BBDB in regular waves are discussed.Copyright

Numerical Investigation of 2-D Optimal Profile of Backward-Bent Duct Type Wave Energy Converter

A floating type Backward-Bent Duct Buoy (BBDB) is a wave energy conversion device with an Oscillating Water Column (OWC) at the front side. The device captures the wave energy using the heaving, the pitching, the surging motion of BBDB and the heaving motion of OWC, and generates the reverse time-mean drift force reducing the mooring loads over specific frequency ranges. Investigations are carried out to find more reasonable devices than the traditional OWC type floating device. An eigenfunction expansion method is introduced for analyzing the BBDB with OWC. It is confirmed that these solutions give good agreement with several experimental results in this paper. It is shown in a design method how to make BBDB match the turbine characteristics. This feature is being able to select the optimum profile of the turbine and the BBDB individually from each characteristic before comprehensive evaluation of the BBDB and the turbine in the design. After grasping the element characteristics which are appropriate for the wave energy conversion system, the synthetic design method is built. The BBDB size and the turbine diameter are determined by considering the cost corresponding to the smallest size under the same output. In this way we can obtain the optimal profiles considering the construction cost including the turbines.Copyright

Tracking of Mesoscale Eddies using Optical Flow Method

The AVHRR (Advanced Very High Resolution Radiometer) data on NOAA satellite have been mainly used to detect the sea surface temperature. AVHRR have relatively high spatial resolution, while had a fault which cannot observe sea surface temperature under cloudy condition. The AMSR-E (Advanced Microwave Scanning Radiometer for EOS) sensor launched in 2002 solved this fault. Since the sensor detects using the GHz band, the sensor detects the sea surface temperature under cloudy condition. In this paper, a program based on an optical flow method is developed to process the AMSR-E data to detect mesoscale surface flow.

Effect of End Plates on the Performance of an Impulse Turbine for Wave Energy Conversion

The objective of this paper is to present the effect of end plate on the performances of the impulse turbine for wave energy conversion by experimental investigation. The experiments have been performed by model testing under steady flow conditions in the study. And then, the performances of the impulse turbine with end plates have been compared with those of the original impulse turbine, i.e., the impulse turbine without end plate. As a result, it is found that the characteristics of the impulse turbine with end plates are superior to those of the original impulse turbine. Furthermore, the effects of end plate size and penetration on the turbine characteristics have been clarified in the study.Copyright