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Investigation of Wave Characteristics with Rotor Type Water Wave Generator

Badhan Saha, Mazharul Islam, Abu Torab and Dewan Hasan Ahmed

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Water wave flume; wave power; wave height; wave frequency.

PUBLISHED DATE December 2017
PUBLISHER The Author(s) 2017. This article is published with open access at www.chitkara.edu.in/publications

Wave energy is the most available energy associated in deep water seas and oceans. Therefore, many attempts have been applied to capture these energies. This paper describes the design, construction and testing of water wave flume. The water wave flume contains an electromechanically driven rotor type wave maker to generate water wave powers. The waves are constructed by different sizes and arrangements of blades which are connected to the rotor. The rotor is driven by an ac motor to generate wave. At the end of the tank a force measuring device is attached opposite to the rotor to measure the thrust of the wave. Experimental results are validated with available literature and wave theory. The results also show that the width of the blade play major role in generating wave sizes including frequency, amplitude and the power. Wider blade displaces much water to generate wave but reduces the blade speed.


Among the renewable energy resources, ocean energy is the one, in sufficiency around the universe. The ocean wave energy in the form of surface gravity waves are formed due to the imbalance between gravitational force and shear due to wind. Water waves are always present on the ocean surface as long as there is wind blowing over the ocean, thus offering an infinite source of wave energy. The power flow in the waves is up to five times compared to the wind that generates the waves, making wave energy more persistent than wind energy. The common factors that determine the characteristics of the wind-generated waves are the distance, the wind velocity over which the wind is in contact with the sea and for how long they are in contact. As the waves travel from deep to shallower water region, certain amount of energy (potential energy + kinetic energy) dissipates, particularly in the breaking zone. The level of dissipation as well as an increase in its amount depends on several parameters, of which, the most important being the seabed friction, bathymetry, and presence of obstructions.

Several reviews, chapters and pioneer books are available on water wave generator. However, Yoshio Masuda (1925-2009), a former Japanese navy officer, may be regarded as the father of modern wave energy technology, with studies in Japan since the 1940s. He developed a navigation buoy powered by wave energy, equipped with an air turbine which was in fact what was later named as a (floating) oscillating water column (OWC). These buoys were commercialized in Japan since 1965 (and later in USA). Later, in Japan, Masuda promoted the construction, in 1976, of a much larger device: a barge, named Kaimei, used as a floating testing platform housing several OWCs equipped with different types of air turbines. Probably because this was done at an early stage when the science and the technology of wave energy conversion were in their infancy, the power output levels achieved in the Kaimei testing program were not a great success. Brown [1] carried out a study to estimate of surface wind speeds using satellite-borne radar measurements at normal incidence. Atlas et al. [2] constructed the grid based global surface stress fields from SEASAT scatter meter data and conventional meteorological data using a data assimilation method. Young [3] carried out an estimation of the Geosat altimeter wind speed algorithm at high wind speed. Offiler [4] carried out the calibration of ERS-1 satellite scatter meter winds.

Takahashi et al. [5] defined the development of wave energy extracting caisson breakwater in Japan. The wave energy extracting device is combined in the form of air chamber attached with an ordinary caisson. The dynamic pressure excited on the sloped front wall well compared with the theory of Goda (1985). The sliding tests on the caisson breakwater proved that sloped front wall have a higher stability than the other caisson types tested. It was inferred by Malmo and Reitan [6] that the natural frequency of an OWC system primarily depends on its front lip depth. McIver and Evans (1988) observed that the reaction of OWC system depends on the extent of the dynamic pressure and its excitation period, whereas, Zheng et al. [7] proved that flared harbour walls in an OWC enhanced its efficiency compared to the one with rectangular walls. Muller and Whittaker [8] tested a 1:36 physical model of the Isle of Islay to obtain the wave-induced pressures on the lip wall. It was observed that the suggestions given by the Coastal Engineering Research Center (CERC 1984) for the estimation of design pressures were conservative. Jayakumar [9] conducted experimental study on OWC caisson model and found that the wave forces on OWC caisson model were less than the conventional rectangular caisson when air damping inside the OWC model maintained is less.

Through a detailed experimental study on OWC, Thiruvenkatasamy and Neelamani [10] found that an increase in wave steepness causes a decrease in its performance in terms of its efficiency and for a/A (ratio of air hole area (a) to plan area (A)) larger than 0.81 %, a considerable reduction in energy absorption capability of the device was reported. Ruo-Shan [11] reported that the experimental investigation on multi-resonant oscillating water column yields only 28.5 % of efficiency because of high-energy loss. Wang [12] studied analytically and experimentally the change in bottom slope in front of the shoreline mounted OWC model and observed that an increase in the slope of the bottom leads to a shift in the capture-width ratio at lower frequencies.

Sudheesh et al. [13] carried out the comparison of the NCEP re-analysis winds with the data of deepwater buoys to ascertain the accuracy of NCEP winds for a summer monsoon for north Indian Ocean. The result shows that NCEP winds match closely with buoy winds. However, Swail and Cox [14] used a state-of-the-art, third-generation wave model to evaluate the marine surface wind fields produced in the NCEP-NCAR reanalysis project. They found that storm peak wave height in extra tropical storms were systematically underestimated at higher sea states due to underestimation of peak wind speeds in major jets treak features propagating about intense extra tropical cyclones. In addition, in situ data were incorrectly assimilated and tropical cyclones were poorly resolved.

Ashlin et al. [15] presented a comprehensive review on the possible approaches that can make use of the OWC as part of breakwaters and coastal defence systems for the harbour formation. The concept of integration of OWC with breakwaters that can reduce the total cost significantly to bring forth economic security in project planning was highlighted. Zhang et al. [16] observed that the efficiency of OWC centred on a resonant frequency. This clearly shows the importance of phase lag between the dynamic excitation pressure and the corresponding air pressure being developed. Wilbert [17] have considered the parameters such as water depth inside the wave energy converter (d) and opening in the bottom of the wave energy converter (o) and, found that effective energy conversion capacity of OWC was found to be increasing with an increase in its bottom opening, o/d. It reached a maximum efficiency of 94 % closer to the natural frequency for o/d = 0.80. However, at the same time, the peak efficiency was found to shift towards the higher frequency with an increase in opening depth. Recently, the chamber bottom profile configuration (i.e., Flat, Circular curve, Slope 1 in 1 and Slope 1 in 5 bottom profiles) has been optimized by Ashlin et al. [15]. Faizal et al. [18] have found that in wave motion, the water particles are known to follow orbital paths. This orbital motion was studied and a five bladed Savonius rotor was built to extract energy from the orbiting particles. Experiments were performed on a rotor placed parallel to the incoming waves in a two dimensional wave channel by varying the frequency of the wave generator, which produced sinusoidal waves. The rotor submergence below the mean level was varied. The flow around the rotor was studied with particle image velocimetry (PIV) measurements. Tutar and Veci [19] carried out experimental study on rotor type wave generator and found that placement of the rotor on the water surface and number of blades makes significant contribution on water wave energy. In another study Tutar and Veci [20] showed that the wave height and wave period along the submergible condition like shallow or deep water depth influence the wave energy conversion efficiency. However, the blade width and rotor rotation are the two issues which are not considered in their studies. As derived by McCormick, within the sea waves there are two components of energy one of which is the potential energy and another one is the kinetic energy. The total energy for regular (sinusoidal) waves is

E = Ep+ EK = (ρ g H2 λ)/8 (1)
Where, EP= Potential energy
EK= Kinetic energy
ρ = mass density of water
λ= the wave length
H= the wave height
The output power for deep water is also given as P = (ρ g2 H2T)/32π (2)

The purpose of this paper is to design and construct of a small water wave generator, which was built and instrumented with a limited budget, is described. The design feature and constructing methods are explained to investigate the general water wave properties and validate with general wave theory. The article is also tried to investigate the effect of the blade number, blade width and rotor rotation on wave formation and related energy outcome.

Page(s) 123-140
URL http://dspace.chitkara.edu.in/jspui/bitstream/123456789/702/1/3%20-%20Investigation%20of%20Wave%20Characterstics%20with%20Rotor%20Type%20Water%20Wave%20-%20Dewan%20Ahmed.pdf
ISSN Print : 2321-3906, Online : 2321-7146
DOI https://doi.org/10.15415/jotitt.2017.52003

A water wave flume is constructed as a part of major project for harvesting energy from the waves and tides. The coastal line the Bay of Bangle which is at the south of Bangladesh can be useful as the source of wave energy and will be able to fulfil the demand. The wave flume is constructed with a length of 16 feet and rotor type water wave generator is used. The experiments are conducted with different sizes of the rotor blades with different combinations. The constructed wave flume gives the considerable results and validated with the available literature and the general wave theory. The results show that the blade size has significant impact on water wave generation and hence the force and the power. Wider blade displaces much water and at the same time due to the larger momentum leads to reduce the motor speed. Constant rpm is necessary to investigate the high tide and low tide effects.

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