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Small-scale tank testing of proposed wave energy converters is a useful way to predict how a full-scale device would behave, and the power such a device would likely absorb from the ocean waves. In this practical, a 1:50 scale model of a slice of a fix, oscillating water column device is to be tested in the narrow tank at DkIT. During the testing, the wave height within the water column will be measured, as will the gauge pressure within the chamber, for a number of incident wave amplitude/frequency pairings. Using this data, the flow rate of air into and out of the chamber through a orifice can be found, and power absorbed by the device estimated.

The model will be installed in the tank and tested for a number of incident waves. Data obtained during the testing will be subsequently analysed using the theory outlined below in MATLAB.

Theory

The power absorbed by airflow through an orifice is given by:

where:

Pow = Q xAP

Pow = power absorbed, Watts

Q         = volumetric flow rate of air through the orifice, m3/s

AP     = pressure difference across the orifice, N/m2.

AP is measured during the test and the volumetric flow rate may be calculated a number of ways. Standard SI units should be used throughout. Using orifice theory, the mass flow rate, m,

where:

CD = coefficient of discharge of the orifice, typically 0.5-0.6

v                                                                                                                                ~

A         = cross-sectional area of the orifice, m2

p = density of air kg/m3.

The mass flow rate and the volumetric flow rates are related by:

Q = ¾

Pair

A second method by which the volumetric flow rate may be found is to assume that the surface of the moonpool remains level and acts like a piston. Under this condition:

where:

Q = A v

owc

Aowc    = the cross-sectional area of the water column m3 v               = the velocity of the water column m/s.

The velocity of the water column may be found by differentiating the water column displacement time series.

Finally, to time average a signal, it is common to find the root mean square (RMS) of this signal. The RMS of a time series X(/), comprising n samples is given by:

RMS =           x (X W

i=1         n

Deliverable

Submit a written report on this practical in both hardcopy and softcopy (uploaded to

Moodle) to include a discussion of the theory and practice of testing an oscillating water column device using the DkIT narrow tank, presentation of the results obtained during the practical and discussion of same. The results presented should include (but are not limited to) time- and frequency-domain plots of the water motions and the pressure variation within the chambers, and the power absorbed by the device. Further, estimates of the response amplitude operator (RAO) and the capture width of the device should be included. Use either LaTeX or MS Word to prepare the report.

Assignment Guidelines and Criteria for Written Work (DkIT) regulations will be applied to missed deadlines.