Performance evaluation of a tidal current turbine with bidirectional symmetrical foils

As one might expect, tidal currents in terms of ebb and flood tides are approximately bidirectional. A Horizontal Axial Tidal Turbine (HATT) with unidirectional foils has to be able to face the current directions in order to maximize current energy harvesting. There are two regular solutions to keep a HATT always facing the direction of the flow, which are transferred from wind turbine applications. One is to yaw the turbine around the supporting structure with a yaw mechanism. The other is to reverse the blade pitch angle through 180° with a pitch-adjusting mechanism. The above solutions are not cost-effective in marine applications due to the harsh marine environment and high cost of installation and maintenance. In order to avoid the above disadvantages, a turbine with bidirectional foils is presented in this paper. A bare turbine with bidirectional foils is characterized in that it has nearly the same energy conversion capability in both tidal current directions without using the yaw or pitch mechanism. Considering the working conditions of the bidirectional turbine in which the turbine is installed on a mono-pile, the effect of the mono-pile on the turbine’s performance is evaluated in this paper, especially when the turbine is downstream of the mono-pile. The paper was focused on the evaluation of the hydrodynamic performance of the bidirectional turbine. The hydrodynamic performance of the bare bidirectional turbine without any supporting structure was evaluated based on a steady-state computational fluid dynamics (CFD) model and model tests. Performance comparison has been made between the turbine with bidirectional foils and the turbine with NACA foils. The effect of the mono-pile on the performance of the bidirectional turbine was studied by using the steady-state and the transient CFD model. The steady-state CFD model was used to evaluate the effect of the mono-pile clearance, which is the distance between the mono-pile and the turbine on the performance of the turbine. The transient CFD model was used to determine the time-dependent characteristics of the turbine, such as time-dependent power and drag coefficients. The results show that the bare bidirectional turbine has nearly the same energy conversion capability in both tidal current directions. The performance of the bidirectional turbine is inferior to the turbine with NACA foils. At the designed tip speed ratio, the power coefficient of the turbine with NACA foils is 0.4498, which increases by 1.6% compared to the 0.4338 of the bidirectional turbine. The turbine’s performance decreases due to the introduction of the mono-pile, and the closer the turbine is to the mono-pile, the greater effect on the turbine’s performance the mono-pile has. At the designed clearance of 1.5 DS, the presence of a mono-pile decreases the peak Cp value by 1.82% and 3.17% to a value of 0.4156 and 0.4004 for the turbine located in the mono-pile upstream and downstream, respectively. The mono-pile can result in the fluctuation of the turbine’s performance. This fluctuation will detrimentally harm the life of the turbine as it will lead to increased wear and fatigue issues

Numerical investigation of a wave glider in head seas

A wave glider comprises a surface boat, which harvests energy from wave and solar power, a submerged glider containing six pairs of tandem hydrofoils and a tether connecting them in between. This paper presents a numerical simulation to predict the wave glider dynamic performance in head seas with the aid of computational fluid dynamic (CFD) method. The simulation involves two commercial CFD software packages, FINE/Marine and STAR-CCM+. Firstly, unsteady Reynolds Averaged Navier-Stokes (URANS) simulation was built in FINE/Marine with volume of fluid (VOF) model to simulate the flow around the surface boat and the tandem hydrofoils as a system, followed by the high-fidelity simulation of the passive eccentric rotation of the underwater tandem hydrofoils in STAR-CCM + using overset mesh. By taking the advantages of both softwares, manual iteration was conducted to achieve a converged result. Consequently, by analyzing these results, the surge force acting on the surface boat and the passive eccentric rotation law of the hydrofoils have been achieved which are proved to be the main factors affecting the propulsion efficiency of the wave glider

The Correction of Recovered Spectral Images in a Hadamard Transform Spectral Imager Based on a Digital Micro-Mirror Device

The Hadamard transform spectral imager based on a digital micro-mirror device (DMD) is a novel type of spectral imager developed in recent years. This paper describes the designing scheme of the Hadamard encoding mask and analyzes the encoding process of the detector pixels. Generally the Hadamard encoding mask constructed by DMD cannot completely encode the dispersed spectrum of all the pixels according to the Hadamard matrix; therefore, the spectral images recovered by inverse Hadamard transform inevitably have errors. A correction method for the recovered spectral images is proposed. The experimental results show that this method improves the quality of the recovered spectral images

Analysis and study of the interlaced encoding pixels in Hadamard transform spectral imager based on DMD

The key innovation in Hadamard transform spectral imager (HTSI) introduced recently is the use of digital micro-mirror device (DMD) to encode spectral information. However, because the size of individual micro-mirrors does not match the detector pixel size or for other unavoidable errors in the optical design and the system assembling, an interlaced encoding phenomenon appears on some pixels of the encoded images obtained from the detector. These interlaced encoding pixels are not encoded based on Hadamard transform, so they should be processed specially in spectrum recovery. This paper analyzes the interlaced encoding phenomenon and proposes a positioning method and a decoding method for the interlaced encoding pixels on the encoded images. In our experiment, we direct a beam of laser into our HTSI and fill the entire field of view: by observing the column vector, which is made up of the gray values of a pixel on the encoded images from the detector in sequence, the interlaced encoding pixels can be distinguished easily and a coefficient is obtained simultaneously, which denotes the ratio of the area between the left part and the right part of the interlaced encoding pixel. By substituting the coefficient and the encoded gray values of the interlaced pixel into its encoding equation, we can recover the spectral elements of the interlaced pixel with ease. By comparing the spectral curve of the interlaced encoding pixels recovered by the method mentioned in this paper and the spectral curves of its two adjacent pixels, we find the decoding results are quite effective. (C) 2011 Elsevier Ltd. All rights reserved

Performance evaluation of a submerged tidal energy device with a single mooring line

A submerged tidal energy device with contra-rotating Diffuser Augmented Tidal Turbines (DATTs) has been investigated in this paper. The device is moored to the seabed with a single mooring line, which limit it to operate at mean water depth, but otherwise allows it to float freely with the tidal current, like a kite in the wind, to harness tidal current energy. This research focuses on the evaluation of stability and power generation of the submerged tidal energy device based on 1:5th scaled model tests and the full-scaled prototype sea trials. A 1:5th scaled model had been manufactured and tested in a circulating water channel to observe the power generation performance and working attitude around the designed inflow velocity. A full-scaled prototype was manufactured and tested near the CHU Island in Shandong Province, China. The results show that the device can change its direction automatically to make the DATTs face the tidal inflow, as the tidal current changes direction. The device has a good stability in pitch and roll motions. But the device's stability in yaw motions is worse than the other two, which will significantly affect the power generation performance and introduce more demanding structural requirements

A Fast Algorithm for 2D DOA Estimation Using an Omnidirectional Sensor Array

The traditional 2D MUSIC algorithm fixes the azimuth or the elevation, and searches for the other without considering the directions of sources. A spectrum peak diffusion effect phenomenon is observed and may be utilized to detect the approximate directions of sources. Accordingly, a fast 2D MUSIC algorithm, which performs azimuth and elevation simultaneous searches (henceforth referred to as AESS) based on only three rounds of search is proposed. Firstly, AESS searches along a circle to detect the approximate source directions. Then, a subsequent search is launched along several straight lines based on these approximate directions. Finally, the 2D Direction of Arrival (DOA) of each source is derived by searching on several small concentric circles. Unlike the 2D MUSIC algorithm, AESS does not fix any azimuth and elevation parameters. Instead, the adjacent point of each search possesses different azimuth and elevation, i.e., azimuth and elevation are simultaneously searched to ensure that the search path is minimized, and hence the total spectral search over the angular field of view is avoided. Simulation results demonstrate the performance characters of the proposed AESS over some existing algorithms

Research of stress assessment for the 135-degree sheet corner based on singularity strength theory

This paper will research the singularity strength theory to stress distribution of a 135-degree sheet corner which bases on notch stress intensity factor (N-SIF) theory. The N-SIF formula is simplified through singularity strength 'as' which is related to structure size. Numerical simulations by ANSYS and regression analysis by MathCAD are done for a number of cases. Finally a simple assessment algorithms and the empirical formula for the notch stress are proposed and verified. The results can be used for structural strength and fatigue analysis

Effect of microstructural characteristics on the impact fracture behavior of cryogenic 9Ni steel

The percentage of shear fracture largely determines the service performance of 9Ni steel used in low-temperature pressure vessels. Through elemental analysis, microstructural characterization, and mechanical property tests, this study investigates why the percentage of shear fractures is low in 9Ni steel and clarifies the mechanism by which the microstructural characteristics influence the low-temperature impact behavior of 9Ni steel. It was found that cleavage fracture zones, formed when segregation bands appear in the microstructure, decrease the percentage of shear fractures at the impact fracture surface. Specifically, as the segregation area increases from 0.9% to 7.1%, the shear-fracture percentage in 9Ni steel sharply decreases from 100% to 65%, accompanied by a deterioration in low-temperature toughness. The segregation zone is enriched in austenite-forming elements (Ni, C, Mn), leading to a tempered martensite microstructure with a lath shape. The small number of high-angle grain boundaries and low interface bonding strength cannot effectively prevent crack initiation and propagation, resulting in brittle cleavage fracture. In contrast, the non-segregated zone is tempered sorbite with a uniform structure, several high-angle grain boundaries, and a high interface bonding strength. These features hinder crack initiation and propagation. Furthermore, the shear-fracture zone generated in the non-segregated zone exhibits ductile fracture characteristics