Flow characteristics of unit artificial reefs in 3-D placement models

Dongha Kim1), Sol Han2), Quynh T.N. Le3) and *Won-Bae Na4)

1), 2), 3), 4) Dept. of Ocean Engineering, Pukyong National University, Busan 609-737, Korea

4) wna@pknu.ac.kr

ABSTRACT

To quantitatively evaluate flow characteristics (wake and upwelling regions) of unit artificial reefs (ARs), three 3-D models such as Hexa, Rect, and Ryra were investigated. The element-based finite volume method was used for numerical flow analyses, by facilitating ANSYS-CFX software. From the analyses results, the following conclusions were made. First, each placement model has a different wake volume up to 58% variation, showing that Hexa is the most advantageous due to its size of 140m3 while Rect has the largest volume of 1970m3 in terms of upwelling volume. Second, considering the wake and upwelling distributions, Pyra has a unique characteristic showing that 77.04% of the wake volume concentrates on a sub-region, which is beneficial to demersal fish. 1. INTRODUCTION

Wake and upwelling regions around an artificial reef (AR) are believed to be important to marine lives because these regions contain a variety of organic, dead plankton, nutrient salt and accordingly attract more marine species such as fish (Oh et al. 2011). For example, fish (predators) have a strong chance to get preys which stay the wake and upwelling regions. Although the attraction and production debate has not been solved yet, there are scientific clues supporting the production, attraction, or both, which are depend on various parameters such as the age, size, gender, and other ecological factors of fish (Bortone 2011, Grossman et al. 1997, Pickering and Whitmarsh 1997, Pitcher and Seaman 2000). Thus, many marine ecologists support the fact that the primary producers around ARs are dominant in the wake and upwelling regions and accordingly the food chain is formed in the areas (Oh 2004). In a sense, wake and upwelling regions are the critical factors in estimating the fishing grounds, which are often facilitated by ARs.

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3. RESU

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is found thke volume ating) direshown in Fe 140.0, 11 (Hexa), 4

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Wake volum

he inlet bohe flow frory conditiodependingcannot beding controty; hence, he conservn, in the mstruct fine he followind Newtoniamodel. Foue of cube wgth of each

ke volume–0.33m s-1

respect tnsider the 88.4m3, re), and 30.4hoice in t–0.21 (Heies and wa

me of AR1 (

oundary prom the doon is a no-sg on the wae vanishedol volume no-slip wa

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to the maiunit-ARs,

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(a) iso-view

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all boundahod was uprocess, ond save a eses wered, the wateesign watees, and thto 0.2m.

s 2.91m3 (e negativen flow direthe wake

y, as showmes the corthe wake 1 (Rect), oes of AR1

w, (b) side-

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only half ocomputatio

e made. Fer temperar velocity is

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Vw). The ae sign indicection. Thvolumes on in Figs. rrespondinvolume s

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-view, (c) p

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ns that thevelocity at d, the cons

the bottomon may noestigate thof each plonal time. First, the ature is 25℃s 2m/s. Fin

um size of

average vecates the e wake voof Hexa, R6, 7, and 8

ng AR volusize. The s-1 (Pyra).unit-AR.

plan-view

and the ssure. In e velocity the wall, servative m, and it ot assign e bottom acement Through water is

℃. Third, nally, the a grid is

elocity of opposite olume of

Rect, and 8. These

ume (Vw). average Table 2

Fig

Fig

Fig

g. 6 Wake v

g. 7 Wake

g. 8 Wake

volume of

volume of

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Pyra unit-

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side-view,

side-view,

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(c) plan-v

(c) plan-vi

(c) plan-vi

iew

iew

iew

Table 2 Wake regions of AR1 and unit-ARs

AR1

Hexa unit-AR

Rect unit-AR

Pyra unit-AR

Wake region

Volume (m3) 2.9 (Vw)140

(48.1Vw) 119.7

(41.1Vw) 88.4

(30.4Vw)

Ave. velocity (m/s) -0.33 -0.21 -0.41 -0.23

Table 3 Upwelling region of AR1 and unit-ARs

AR1

Hexa unit-AR

Rect unit-AR

Pyra unit-AR

Upwelling region

Volume (m3) 24.9 (Vu)

1276.7 (60.3Vu)

1969.5 (93.0Vu)

1574.6 (74.4Vu)

Ave. velocity (m/s) 0.22 0.25 0.29 0.24

In addition, the wake regions of the placement models were characterized by their horizontal profiles to investigate the planar distributions. The results show that Pyra has a unique characteristic showing that 77.04% of the wake volume concentrates on a sub-region near the seabed, which is beneficial to demersal fish.

Similarly, it is found that the upwelling volume in AR1 was 24.9m3 (Vu, the upwelling volume of AR1) and the average velocity was 0.22m/s. In addition, it is found that the upwelling volumes of Hexa, Rect, and Pyra unit-ARs were 1276.7, 1969.5, 1574.6m3, respectively. Each upwelling volume is 60.3, 93.0, or 74.4 times the corresponding the upwelling volume of AR1 i.e., Vu. The average upwelling velocity is 0.25 (Hexa), 0.29 (Rect), or 0.24m/s (Pyra). The results are shown in Table 3.

4. CONCLUSIONS

In this study, in order to quantitatively evaluate flow characteristics (wake region and upwelling region) of unit ARs, three 3-D placement models (Hexa, Rect, and Pyra) were investigated. The element-based finite volume method was used for numerical flow analyses, by facilitating ANSYS-CFX, a general purpose CFD software package. From the flow analyses, the following conclusions were found.

First, the AR1 has the wake region quantified by the wake volume (Vw) of 2.9m3. In addition, the units have the wake volumes of 140 (Hexa), 119.7 (Rect), and 88.4m3 (Pyra), which are 48.1, 41.1, and 30.4 times the reference wake volume (Vw). This result shows that Hexa has the most desirable in terms of the size of wake volume, followed by Rect. In terms of the average recirculating flow velocity, it is found the

sequence: Rect (0.41m/s), Pyra (0.23m/s), and Hexa (0.21m/s). Thus, considering the quality of recirculating zone or wake region (a so-called tranquility), Hexa has a better quality. However, it should be noted here that the average counter velocity (0.41m/s, the highest average among three placement models) of Rect is about 20% of the inlet velocity (2m/s); hence, the tranquility of the corresponding wake region is likely secured. From the observations above, it is shown that each placement model has different wake volume up to 58%. In overall, Hexa unit-AR is the most advantageous based on wake volume. In addition, the wake regions of the placement models were characterized by their horizontal profiles to investigate the planar distributions. The results show that Pyra has a unique characteristic showing that 77.04% of the wake volume concentrates on a sub-region near the seabed, which is beneficial to demersal fish.

Second, the AR1 has the upwelling region quantified by the upwelling volume (Vu) of 24.9m3. In addition, the units have the upwelling volumes of 1969.5 (Rect), 1574.6 (Pyra), and 1276.7m3 (Hexa), which are 93.0, 74.4, and 60.3 times the reference upwelling volume (Vu). This result shows that Rect has the most desirable in terms of the size of upwelling volume, followed by Hexa. In terms of the average upwelling flow velocity, it is found the sequence: Rect (0.29m/s), Hexa (0.25m/s), and Pyra (0.24m/s). Thus, considering the quality of upwelling zone (a so-called tranquility), there is no considerable difference. It should be noted here that the upwelling region is defined by 5% of and more than the inlet velocity (2m/s). Thus, the average velocities are between 5% and 14.5% of the reference velocity. From the observations above, it is shown that each placement model has different upwelling volume up to 54%. In overall, Rect unit-AR is the most advantageous based on upwelling volume.

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