Towards an understanding of marine fouling effects on VIV of circular cylinders: Aggregation effects
Introduction
For a structure placed in seawater, it is only a matter of time before its immersed surface is covered by marine fouling. Fouling expands the outer dimensions of the structure, increases its surface roughness, alters the flow regime around the structure and changes the hydrodynamic forces acting on it. As a result, the dynamics of vortices shed past the structure and its vortex-induced vibrations (VIVs) change.
Numerous studies have already been conducted on the VIV of circular smooth cylinders. Comprehensive reviews on the subject can be found in Khalak and Williamson, 1999, Blackburn et al., 2001, Sarpkaya, 2004, Gabbai and Benaroya, 2005, Williamson and Govardhan, 2008, Bearman, 2011 and Sumner (2013).
A number of researchers also addressed vortex shedding around rough cylinders. These studies can be grouped into stationary and oscillatory cylinders. For example, pressure distribution, mean and fluctuating force coefficients, boundary layers, flow separation and Strouhal numbers in the stationary rough cylinders were studied by Achenbach, 1971, Sarpkaya, 1976, Güven et al., 1980, Buresti, 1981, Achenbach and Heinecke, 1981, Bearman and Harvey, 1993, Schoefs and Boukinda, 2004, Yamagishi and Oki, 2004, Yamagishi and Oki, 2005, Fuss, 2011 and Zhou et al., 2015a, Zhou et al., 2015b.
Previous studies on VIV of oscillatory rough cylinders, which is more relevant to the subject of the current study, are less frequent. For example, Wooton (1969) conducted a series of wind-tunnel tests on freely oscillating smooth and rough cylinders in subcritical and supercritical Reynolds numbers. Okajima et al. (1999) studied the aeroelastic instability of a roughened cylinder in a wind-tunnel and reported that the amplitude of oscillations reduced over a range of critical Reynolds numbers. However, larger amplitudes were observed at Reynolds numbers higher than the critical value.
VIV of circular cylinders, with tripping wires on their surface, was studied by Hover et al. (2001). The results indicated that the wires reduce the maximum amplitude of the oscillations. They also reported a plateau of constant amplitude over the synchronisation range. Bernitsas and Raghavan (2008) carried out VIV experiments on a circular cylinder with strips of sandpaper and reported reductions in the oscillations amplitude. The marine fouling effects on the VIV of circular cylinders with helical strakes were studied by Skaugset and Baarholm (2008) and Resvanis et al. (2014). They found that marine fouling increases the amplitudes of oscillations in cylinders with suppression device and greatly reduces the suppression effectiveness of helical strakes.
Kiu et al. (2011) studied the VIV of cylinders covered by sandpapers at subcritical Reynolds numbers. The relative roughness, i.e. roughness height over diameter, varied from 0.28 × 10−4 to 1.38 × 10−2. They reported that the width of the lock-in region, maximum amplitude, and maximum drag coefficient reduce as the roughness increases. On the contrary, Nedrebø (2014) and Henry et al. (2016) reported an increase in the mean drag coefficient for cylinders with high relative roughness.
A comprehensive review on vortex shedding behind stationary and non-stationary cylinders with surface roughness can be found in Zeinoddini et al. (2016). They also conducted VIV experiments on circular cylinders entirely covered by regular single-size pyramidal protrusions to model the biofouling effects. Their results showed that the maximum amplitude, the lock-in range and the maximum lift and drag coefficients are reduced by the surface protrusions. In another study, Zeinoddini et al. (2017a) conducted VIV tests on cylinders covered by regular single-size hemispherical protrusions. Three different coverage ratios were considered. Differences in the coverage ratio had, reportedly, small influences on the maximum lift and drag coefficients, however, cylinders with lower coverage ratios had lower peak amplitudes and had narrower lock-in ranges. Shape of the protrusions (pyramidal or hemispherical) was reported to have also small impacts on the peak oscillation amplitude and on the maximum lift and drag coefficients. They also stated that despite previous efforts, a systematic understanding of the fundamental mechanism of VIV in biofouled circular cylinders is still missing. Further studies, therefore, are required to better understand the impacts of the marine fouling (growth) on the VIV of bluff bodies (Zeinoddini et al., 2017a).
The current study is aimed at getting a further insight into the effect of marine biofouling on the VIV of circular cylinders and extends the previous research conducted by Zeinoddini et al., 2016, Zeinoddini et al., 2017a, Zeinoddini et al., 2017b. The main advances in the current study are that, in spite of regular patterns used in the two previous studies, an aggregated spatial distribution is considered for the artificial marine fouling. This refers to cases where individuals in a space are neither regular nor random. The surface protrusions occur in clusters too dense to be explained by chance. The aggregated spatial distributions are believed to better simulate the natural colonisation of the marine biofouling.
The paper first describes the mathematical model for generating data points with aggregated spatial distribution. It also explains the approach considered for the physical synthesis of the artificial marine fouling on the test cylinders. The arrangements for the VIV towing tank experiments on small-scale circular cylinders covered by aggregated artificial barnacles are then discussed. The test results on the oscillation amplitudes, oscillation frequencies, and force coefficients are reported and discussed.
Section snippets
Laboratory modelling of aggregated biofouling
Generating spatial distributions analogous to the natural conditions requires insights into the fouling diversity and statistics. By combining the relevant physical data and statistics, the spatial distribution of artificial barnacles is mathematically modelled. Procedures are then taken to synthesise the mathematical models to physical samples.
Set-up overview
A mass-spring rigging system comprising flexibly mounted rigid cylinders is used to conduct the in-water VIV tests. The system is adopted from Assi et al. (2006) and also used by Zeinoddini et al., 2015, Zeinoddini et al., 2014, Zeinoddini et al., 2013. The tests are carried out in the towing tank of Marine Engineering Laboratory at Sharif University of Technology. Two parallel leaf springs of 320 mm × 100 mm × 0.4 mm, provide a lateral stiffness of around 180 N/m and allow the system to
Effects of the coverage ratio
Fig. 12a illustrates the test results for the smooth cylinder (S) and sample artificially biofouled cylinders with coverage ratios of 33% (AG-33-TC-1), 66% (AG-66-TC-1) and 100% (RG-100-TC-0). The figure displays the non-dimensional transverse amplitude (A*) against the reduced velocity (U*). As it may be noticed, in general, the peak magnitude (A*max) is notably reduced by the artificial biofouling. The reduction increases as the coverage ratio decreases. This appears aligned with Zeinoddini
Closing remarks
For a structure immersed in the seawater, it is only a matter of time before its surface is covered by marine fouling. Fouling changes the fluid-structure interactions as well as the VIV. Despite previous efforts, a systematic understanding of the fundamental mechanism of VIV in biofouled circular cylinders is still missing. The current study is aimed at getting further insights into the VIV of circular biofouled cylinders. The main advances in the current study are that, in spite of regular
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