ii) Long floats in weave should be avoided.
Justification
Answers
Answer:
long float in weave should be avoided because the clothe we are wearing will be torn easily
The hydrodynamic behavior of floating axi-symmetric point absorbers capturing energy from heave oscillations has been studied since the early 1970s, see, e.g., McCormick (1981). Control is particularly desirable for such devices due to their narrow-band frequency response and short resonant periods. For such devices, Budal and Falnes (1980) introduced a latching type control approach using real-time application of clutching or braking forces. Since then, this approach has been investigated by many authors, see Hoskin et al. (1985), Falcao and Justino (1999), Perdigao and Sarmento (1989), Korde (2001), and Babarit and Clement (2006), to name a few. A heaving buoy type device driving a hydraulic power take-off was considered more recently by Falcao (2008) for optimized conversion in the time domain. Frequency-domain ‘complex-conjugate control’ approaches comprising adjustable reactive loading for selective tuning to changing wave spectra have been studied since the mid-seventies, see, e.g., Salter (1978), Nebel (1992) and Korde (1991). Such an approach was tested recently on the Wavestar device in Denmark (e.g., Hansen and Kramer 2011).
Real-time control for optimum velocity operation in irregular waves presents fundamental difficulties, as outlined in Naito and Nakamura (1985), and later in Falnes (1995). Since wave radiation from body oscillation is causal, the radiation impulse response function is also causal. Therefore, the frequency-dependent added mass and radiation damping satisfy the Kramers–Kronig relations. For optimum velocity in irregular waves, the impulse response functions corresponding to these two quantities need to be synthesized and used independently, both of which are non-causal, one being symmetric and the other anti-symmetric. In part for this reason, control force synthesis at a given time instant requires knowledge or prediction of velocity into the future. Compromise solutions using velocity estimation based on time-series analysis of past velocities were reported several years ago by Korde (1999) and Korde et al. (2002). Recently, systematic studies were reported Fusco and Ringwood (2012) on the relation between the device geometry and the required ‘prediction horizon’. More recent work followed an approach discussed in Naito and Nakamura (1985) and Falnes (1995) and used up-wave surface elevation measurements with right-shifted impulse response functions to generate approximate instantaneous control forces, see Korde (2014). Other approaches such as model predictive control and adaptive control have also been considered. Many of these were evaluated in a recent paper by Hals et al. (2011). Other more recently reported approaches include the simple and effective control strategy, Fusco and Ringwood (2013), and a robust controller suitable in the presence of model uncertainties and oscillation constraints, Fusco and Ringwood (2014). Force and displacement constraints were considered within a single framework in Bacelli and Ringwood (2012).
It is easy to see that control seeking tuning over an entire spectrum would enable better energy conversion in bi- or multi-modal spectra. However, for uni-modal and narrow-band spectra, and especially for small heaving buoys, it can be argued that peak-frequency tuning (such as reported in Hansen and Kramer 2011) could produce comparable performance with less effort. The comparative assessment of different strategies in Hals et al. (2011) shows that, in longer periods, the peak-frequency tuning approach would perform comparably with other approaches seeking tuning over a range of frequencies. This paper investigates this question further, for floating and submerged heaving devices. The findings are interpreted using an approximate analysis in the frequency domain. Effect of oscillation constraints is not difficult to study by extending the approach of this paper, but is not addressed in this paper. Only uni-modal spectra are considered in this work. As discussed in Falnes and Hals (2012), the floating and submerged configurations have considerably different wave radiation properties impacting their performance, especially in long waves.
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