Journal of Marine Science and Engineering Article Numerical Study of Efficiency Indices to Evaluate the Effect of Layout Mode of Artificial Reef Unit on Flow Field Jiating Zhang 1, Lixin Zhu 1, Zhenlin Liang 1, Liyuan Sun 2, Zhaoyi Nie 1, Jiahao Wang 1, Wude Xie 1 and Zhaoyang Jiang 1,* 1 Marine College, Shandong University, Weihai 264209, China; [email protected] (J.Z.); [email protected] (L.Z.); [email protected] (Z.L.); [email protected] (Z.N.); [email protected] (J.W.); [email protected] (W.X.) 2 Shandong Hydrobios Resources Conservation and Management Center, Yantai 264003, China; [email protected] * Correspondence: [email protected] Abstract: Artificial reefs (ARs) have been widely used to restore the seabed habitat and protect biodiversity. They can effectively increase the dissolved oxygen content in the bottom water layer by their disturbing effect of upwelling and downwelling. The bottom water is prone to hypoxia in summer due to the extreme weather of the global climate and excessive biomass in some marine ranching in northern China. Therefore, how to effectively use the upwelling effect of artificial reefs to alleviate this problem is a necessary subject of research. Generally, ARs are arranged by different intervals in a unit form on the seafloor, and the flow field effect is different from that of the individual reefs. However, few studies have been focused on the effect of layout mode on the flow field of a unit reef (UR). In this paper, we selected the interval between reefs (IR) and the angle of inflow (AI) as the Citation: Jiating, Z.; Lixin, Z.; influencing factors to study the flow field effect of UR. The upwelling and wake regions of 64 URs Zhenlin, L.; Liyuan, S.; Zhaoyi, N.; Jiahao, W.; Wude, X.; Zhaoyang, J. were presented by the efficiency and disturbance indices related to the flow characteristics and Numerical Study of Efficiency Indices proposed an optimal layout mode having the best performance of the upwelling effect. The results to Evaluate the Effect of Layout Mode showed that the interactions among the AI, the transverse, and longitudinal IRs were significant, and of Artificial Reef Unit on Flow Field. J. the AI has a significant influence on the flow field. These indices were effective and contribute to Mar. Sci. Eng. 2021, 9, 770. https:// the layout optimization of UR. The AI close to 45◦ has a significant influence on the flow field effect doi.org/10.3390/jmse9070770 of UR. Academic Editors: Kamal Djidjeli and Keywords: unit reef; interval between reefs; angle of inflow; efficiency index; disturbance index Apostolos Papanikolaou Received: 12 May 2021 Accepted: 11 July 2021 1. Introduction Published: 15 July 2021 Artificial reefs (ARs), as man-made permeable structures, are widely used in biodi- Publisher’s Note: MDPI stays neutral versity conservation and coastal habitat restoration in marine ranching engineering [1–4]. with regard to jurisdictional claims in After the settlement of ARs, a complex hydrodynamic condition is formed in a certain published maps and institutional affil- range around the ARs. For example, due to the block of an AR, a slow flow region full of iations. complex eddies will form behind the AR, also known as the wake region. This turbulent region full of eddies can cause the physical aggregation of plankton, crustaceans, and fish [5–10]. Simultaneously, there is an upwelling region above the AR, which can enhance the exchange and transport of nutrients between the upper and lower water layers and is conducive to the improvement of primary productivity. This is one of the basic principles Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. for AR to perform its function on the flow field. This article is an open access article Moreover, the upwelling has higher chlorophyll concentration (Chl a) and lower sea distributed under the terms and surface temperature (SST), which can also promote the improvement of surface primary conditions of the Creative Commons productivity [11]. For a small-scale area such as marine ranching, nutrient exchange in Attribution (CC BY) license (https:// regional water also needs a continuous driving force. ARs can change the original seabed creativecommons.org/licenses/by/ topography, and the upwelling formed by them plays a necessary role in regulating the 4.0/). flow in a small sea area [12–15]. J. Mar. Sci. Eng. 2021, 9, 770. https://doi.org/10.3390/jmse9070770 https://www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2021, 9, 770 2 of 16 At present, many research results have been obtained based on the analysis of flow field effect of individual AR, mainly focusing on opening ratio, inlet velocity, angle of inflow (AI), and characterization of drag coefficient [9,16–21]. However, AR is generally arranged and released in the form of a combination of several ARs, which is often called AR module, AR set or AR group [22]. In order to facilitate the statistics and expression of the number of ARs, it also called unit reef (UR). Different combinations of UR will produce different flow field effects. Therefore, a reasonable layout mode can effectively promote water exchange, improve water quality, and enhance biodiversity in the areas surrounding ARs [23–27]. The common layout modes of UR were usually in the form of piles, such as rock reefs, which were used to multiply sea cucumber in northern China. Li et al. [28] studied the flow field effect of three kinds of tube artificial reef piles under different inlet velocities. However, this form of layout mode is prone to collapse due to the impact of the ocean current. For large ARs, such as those made of concrete, they were usually arranged in an array combination. Woo et al. [8] presented effective layout modes of labyrinth artificial concrete reef (LTACRs) based on the wake volume. Kim et al. [29] selected four marine forest artificial reefs (MFARs) frequently used in South Korea to simulate the flow field in order to analyze the spreading pattern and range of seaweed spores. The interval between reefs (IR) was determined by the wake length of the single-type AR, and the optimal flat layout mode was selected based on the wake volume and flow characteristics. It was concluded that the major wake length has important significance to guide further research on the IRs. At present, the studies on upwelling only focus on the height and the upwelling flow rate of the individual reefs [9,21], and rarely consider the influence of AR layout mode. In recent years, during high temperatures and rainy summers, the bottom water of AR areas dominated by rock reefs in northern China has formed hypoxic areas. This causes the death of sea cucumbers and other benthic organisms that proliferate in AR areas, which induces ecological and environmental disasters [30–32]. Our previous research [9,13,21] showed that structural optimization can significantly improve the upwelling effect of AR. However, if we want to solve the regional hypoxia phenomenon in marine ranching, it needs to be solved by the upwelling effect of UR, and it is also necessary to optimize the layout mode of UR. A reasonable combination of ARs, such as IRs and AIs, can improve the overall flow field effect of UR and enhance the disturbance efficiency between the bottom and the upper water layer in the artificial reef area. In this article, we adopted the CFD method to analyze the influence of IR and AI on the formation of the flow field of UR, focusing on the effect of the upwelling and wake region with the efficiency index and the disturbance index. This study can provide a scientific reference for the layout optimization of UR. 2. Materials and Methods 2.1. Reef Module In this article, the upwelling reef designed in our previous research [9] was used as the research object, which is composed of four isosceles triangles and right triangles as shown in Figure1. The flow field effect produced by the upwelling reef has a wider range, higher upwelling height, and more complex flow patterns. J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 3 of 17 J. Mar. Sci. Eng. 2021, 9, 770 3 of 16 J. Mar. Sci. Eng. 2021, 9, x FOR PEER REVIEW 3 of 17 Figure 1. TheThe upwelling upwelling reef. reef. Figure 1. The upwelling reef. 2.2. Numerical Numerical Simulation Simulation 2.2.1. Layout Mode 2.2. Numerical 2.2.1.Simulation Layout Mode In this study, nine upwelling reefs were combined into a UR, which is shown in 2.2.1. Layout ModeIn this study, nine upwelling reefs were combined into a UR, which is shown in Fig- Figure2. The IR is measured by the length of the upwelling reef (L). Both the longitudinal In this study,ure 2. nine The upwelling IR is measured reefs wereby the combined length of intothe upwelling a UR, which reef is (L).shown Both in the Fig- longitudinal IR IR (h1) and the transverse IR (h2) were set to 1 L, 4/3 L 5/3 L, and 2 L, respectively. The ure 2. The IR is(h measured1) and the transverseby the length IR (ofh2 )the were upwelling set to 1 L,reef 4/3 (L). L 5/3 Both L, andthe longitudinal2 L, respectively. IR The AI was AI was set to 0◦, 15◦, 30◦, and 45◦ respectively (Figure3). These three influence factors (h1) and the transverseset to 0°, IR15°, (h 230°,) were and set 45° to respectively1 L, 4/3 L 5/3 (FigL, andure 23). L, Theserespectively. three influence The AI was factors and their and their levels produced a total of 64 combinations of ARs.