人工鱼礁水动力的实验研究与流场的数值模拟
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摘要
由于捕捞强度增强和环境污染,我国近海渔业资源严重衰退,修复近海水域生态环境、保护渔业资源、保持海洋渔业的可持续发展是当前面临的迫切任务。建设人工鱼礁,发展海洋牧场是缓解渔业资源衰退、渔场“荒漠化”、促进沿海渔场建设的一项重要措施。由于人工鱼礁处于海洋环境中,同时受到波浪和水流的共同作用,但是一般意义上的人工鱼礁都放置在海底,属于底置鱼礁,因此,大部分有关人工鱼礁的水动力学性能的研究不考虑波浪影响。人工鱼礁的基础研究主要是鱼礁材料、结构、稳定性、鱼礁合理布局、投放鱼礁的密度以及鱼礁区生态系、鱼礁区流态变化、鱼礁区的渔具渔法等。鱼礁的强度和稳定性设计要考虑选用的鱼礁材料和海域的水流环境。鱼礁流场的影响范围、流态分布都要受到鱼礁结构形状、规模大小的影响,鱼礁流场分布特征直接影响到人工鱼礁的投放布局。为了提高人工鱼礁的可靠性和鱼礁效能,在人工鱼礁设计和投放之前,应该首先运用工程分析的方法研究人工鱼礁的水动力性能和流场效应,研究这类结构力学性能的通常方法有模型实验、数值模拟。随着计算机技术的迅速发展,数值模拟广泛应用到各行各业,并且越来越受到重视。因此,本文应用水槽实验和数值模拟2种方法研究人工鱼礁的水动力和流场特征。
     本文以3m×3m×3m的方型鱼礁做为实物礁体,取尺度比λ_L=20,制作了2种15cm×15cm×15cm的鱼礁模型。在回流水槽中测量不同水流速度下的鱼礁水阻力,并计算阻力系数(C_d)与雷诺数(R_e)。结果表明:无盖礁体模型正面迎流时(θ=0~o),当R_e超过7.5×10~4时,C_d为1.64,其自动模型区域为R_e>7.5×10~4;45°方向迎流时(θ=45°),当Re超过6.36×10~4时,C_d为1.3,其自动模型区域为R_e>6.36×10~4。有盖礁体模型正面迎流时,当R_e超过6.0×10~4时,C_d为1.69,其自动模型区域为R_e>6.0×10~4;45°方向迎流时,当R_e超过8.48×10~4时,C_d为1.43,其自动模型区域为Re>8.48×10~4。一种礁体模型在不同迎流方式下所受的阻力不同,45°方向迎流比正面迎流时的阻力大。在相同的迎流方式下,有盖礁体所受的阻力比无盖礁体大。
     本文通过FLUENT和ANSYS对水流经过人工鱼礁的流场变化进行单向耦合分析。其中,几何建模及网格划分分别在DM Geometry及Mesh中进行,流体分析在FLUENT中完成,结构分析在Static Structural(ANSYS)中设置。利用FLUENT的后处理,得到流场中不同断面的速度云图。从水流速度云图上,可以明显看出流体在经过鱼礁后,周围流场受到的影响是显著的。通过实验数据和模拟结果对比分析可以看出,利用ANSYS WORKBENCH做单向流固耦合分析对人工鱼礁流场进行数值模拟是可行的。进而可以通过数值模拟来预测水流经过鱼礁后,在距离鱼礁多远距离流场会得到恢复,也就可以据此推断,下一个鱼礁投放的合适位置。从水流速度云图上可以看出,在水流方向上距离礁体17倍处(A19断面),外部流场恢复。从模拟值和测量值比对可以看出,A19断面以后水流速度基本稳定,如果按照进口流速50cm/s计算,稳定后的流速误差在0.7-2.94%。同样,从水流速度云图上也可以看出,位于礁体两侧的C断面,距离礁体3.5倍距离处(C8断面)流场得到恢复。
     本文研究了柔性结构的人工鱼礁水动力性能,柔性结构的人工鱼礁主要由支撑框架和网衣组成,支撑框架为圆柱形结构材料,网衣为普通聚乙烯网衣。人工鱼礁的单体模型为椭圆形,高为175mm,最大外径480mm,四周被均匀分为8份,其中,4份用聚乙烯网片(2a=20mm,d=1mm)缝制,其他4份为镂空结构。将柔性结构的人工鱼礁看着是静止在水中的渔具构件,根据渔具或渔具构件水动力的性质和种类,一个物体所承受的水阻力的大小取决于来流的速度。将鱼礁单体按照2种方式进行组合,组合方式1就是将1-4个单体鱼礁分别垂直组合成圆柱型结构,共形成4个组合体;组合方式2就是将4个单体鱼礁水平组合一起,然后再分别垂直组合1-4层,共形成4个组合体。在循环水槽中取V=0.12、0.2、0.3、0.4、0.5、0.6、0.7m/s共7个流速,测量不同鱼礁组合结构在以上7种水流情况下的阻力值(R)。结果表明:(1)当其45o迎流时(=45o)所受到的水阻力比正面迎流(=0o)时大,对于组合方式1的4种组合体(单体、双体、三体、四体),差值在0.3-23.3%之间;对于组合方式2的4种组合体(一层4个、二层8个、三层12个、4层16个),差值在5.9-35.4%之间。(2)水阻力的计算值比测量值大,对于组合方式1,其差值在4.1-10.8%之间;对于组合方式2,其差值在2.1-10.7%之间。所以,在海上进行实际投放时应该选择正面迎流投放,以减少水流对礁体的影响。
     总体而言,本文研究的方型和柔性结构的人工鱼礁的水动力可以为礁体设计、阻力计算、稳定性校核等提供理论支持。流场的模拟方法可以有效地计算礁体周围流场分布,计算结果可以为人工鱼礁的投放提供理论上的参考依据。
Due to fishing intensity enhanced and environmental pollution, our offshorefishery resources is in a serious recession. Therefore, the restoring of the ecologicalenvironment of the offshore waters, the protection of fishery resources andmaintenances of the sustainable development of marine fishery are the urgent tasks atpresent. Construction of artificial reef and development of ocean pasture ranching areconsidered as effective approaches to ease recession of fishery resources anddesertification of fishing ground, to promote the construction of coastal fishingground. As the artificial reef is in marine environment, it suffered the interaction ofwaves and currents. It is also generally placed at the bottom of the sea which belongsto bottom fish reef; as a result, most of the studies on hydrodynamic performance ofartificial reefs were taken no account of the wave. The fundamental researches ofartificial reefs are about materials, structure, stability, rational layout, density placed,ecosystem, fishing technology, etc. We should consider material of reef and sea waterenvironment to design of strength and stability of artificial reef. The influence rangeand distribution of flow field of artificial reef are mainly influenced by the fish reefstructure and the scale size, and distribution of flow field directly affects the layout ofartificial fish reef. In order to improve the efficiency and reliability of artificial fishreef, we should apply engineering analysis method to study the hydrodynamicperformance of it and the effect of the flow field before the design and delivery of theartificial reef. The research methods of this kind of structure mechanics performanceusually have model experiment and numerical simulation method. With the rapiddevelopment of computer technology, numerical simulation is widely applied to allwalks of life, and more and more be taken seriously. Therefore, flume experiment andnumerical simulation are applied to study reef hydrodynamic and flow fieldcharacteristics of artificial fish reefs in this article.
     Basing on3m×3m×3m square fish reef as a physical body, we taken a scale20(λ_L=20)to make two kinds of15cm×15cm×15cm square fish reef model in thisarticle. We measured water resistance of reefs under different flow rates in the refluxtank, and calculated the drag coefficient (C_d) and R_eynolds number (R_e). R_esultsshowed that R_ewas over7.5×10~4, C_dwas1.64and R_ewas greater than7.5×10~4in theautomatic model area when the model reef without lid was meeting flow head-on; R_ewas over6.36×10~4, C_dwas1.30and R_ewas more than6.36×10~4in the automaticmodel area when it was meeting flow at45°direction. R_ewas over6.0×10~4, C_dwas1.69and R_ewas more than6.0×10~4in the automatic model area when the model reefwith lid was meeting flow head-on; R_ewas over8.48×10~4, C_dwas1.43and R_ewasmore than8.48×10~4in the automatic model area when it was meeting flow at45°direction. A model of the reef body suffered different flow resistance under thedifferent incident flow way. The model reef was subjected to greater flow resistance at45°direction than hand-on. In the same incident flow, the reef body with lid sufferedlarger resistance than one without lid.
     Based on the FLUENT and ANSYS, this paper analysis the flow changesthrough an artificial reef using the one-way coupling analysis. Geometric modelingand meshing were deal with DM Geometry and Mesh, respectively. Fluid analysis inFLUENT and structure analysis was setting in the Static Structural (ANSYS). Thevelocity contour of different sections in flow field was achieved by the FLUENTpost-processing. From the velocity contour, flow field around the artificial reef wasimpact significantly after the fluid pass through the reef. By comparison withexperimental data and simulation results analysis, we can be seen that it is feasible todo numerical modeling using ANSYS WORKBENCH for flow field of artificial reefs.Therefore through the numerical modeling we can predict the distance when the flowfield can recover after the current pass the reef, or simply, we can calculate theappropriate position of the next reef delivered. As can be seen from the velocitycontour, the external flow field recovers when the distance is17times of the reef inthe direction of flow (section A19). From the comparison of the simulated values andmeasured values, the flow rate basically stable after the section A19. If the import flow was50cm/s, and the velocity error was in0.7to2.94%when the flow stability.Also, from the velocity contour, we can see that in the section C the distance is about3.5times of the reef (section C8) the flow can be restoration.
     This paper studies the hydrodynamic performance of artificial reefs of theflexible structure, and artificial fish reefs of the flexible structure is mainly composedof the supporting framework and the net panel. The support framework wascylindrical structure materials and the netting was ordinary polyethylene mesh. Themonomer model of artificial reef was oval (high is175mm, maximum diameter is480mm), and the surrounding was evenly divided into eight pieces, including4partsof PE mesh (2a=20mm,d=1mm) and4parts hollow structure. R_egarded the flexiblestructure of artificial reef as a stationary in the water fishing gear components andaccording to the nature and type of the fishing gear and fishing gear hydrodynamic,the water resistance of an object depends on the velocity of the incomingflow.Combination the reef monomer in two ways, and one was that vertical combining1to4of the monomer reefs into a cylindrical structure, in all, forming four combinations;the other one was combined4monomer reef horizontal together and then verticalcombination1-4layer, in all, forming four combinations. In the circulation channel,we choose seven flow rates: V=0.12m/s,0.2m/s,0.3m/s,0.2m/s,0.5m/s,0.6m/s,and0.7m/s, and measured seven different fish reef combination structure resistancevalue(R) of the current situation. The results show that:(1) The water resistance wasbigger when it was meeting flow=45o than=0o, for the4kinds in the firstcombination (mono-reef, bi-reefs, tri-reefs, quadri-reefs), the difference with in the0.3-23.3%. For the4kinds in the second combination (4in one layer,8in two layer,12in three layer,16in four layer), the difference was between5.9%and35.4%.(2)The calculated value of water resistance was larger than measured values, and thedifference was between4.1and10.8%for the first combination, and the difference isbetween2.1and10.7%for the second combination. In order to reduce the effect ofwater for the reef, we should choose head-on flow during the actual delivery.
     Overall, this study on hydrodynamic of artificial reefs with square and flexiblestructure can provide theoretical basis for the design of the reef, resistance calculationand stability checking, etc. The analogy procedure of the flow field can effectivelycalculate the flow field distribution around the reef, and the results can providetheoretical reference for the arrangement of artificial reef.
引文
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