Complexity

Volume 2017, Article ID 5392539, 21 pages

https://doi.org/10.1155/2017/5392539

## Structure Optimization of a Vibration Suppression Device for Underwater Moored Platforms Using CFD and Neural Network

School of Marine Science and Technology, Northwestern Polytechnical University, Xi’an 710072, China

Correspondence should be addressed to Zhaoyong Mao; nc.ude.upwn@gnoyoahzoam

Received 8 July 2017; Accepted 13 September 2017; Published 11 December 2017

Academic Editor: Junpei Zhong

Copyright © 2017 Zhaoyong Mao and Fuliang Zhao. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

We only consider the underwater mooring platform (UMP) and the plate moving in the transverse direction, and the plate can be relative to the UMP free rotation. In the case of constant flow rate ( m/s), the effect of different dimensionless plate length () and damping value () on the UMP was studied. We get the sample data point set by computational fluid dynamics (CFD) simulation with changing the dimensionless plate length (, 0.5, 0.75, 1.0, 1.25, 1.5) and damping value (, 75, 100, 125, 175, 250, 300 (N × s/m)). The optimal value of the vibration suppression rate is obtained by backpropagation (BP) neural network and genetic algorithm. The optimal vibration suppression rate is and the corresponding variable value is , (N × s/m). In order to verify the accuracy of the optimization, we perform the CFD numerical simulation with the optimized parameters and compare the theoretical optimization results with the CFD simulation result. The absolute error between CFD simulation and optimal is only 0.0037. Finally, we compare the results of CFD simulation based on optimal parameter with the bare UMP and analyze their dimensionless amplitude, wake structure, and lift coefficient. It is shown that BP neural network and generic algorithm are effective.

#### 1. Introduction

The underwater mooring platforms (UMPs) are a class of underwater devices and with an anchor link to work in the sea. UMPs have broad application in both civil and military missions. Pressure, water quality detection, camera and acoustic communication, and other sensors are installed on the platform. It can be used for marine environmental management, resource protection, disaster monitoring, and marine hydrological monitoring in civil areas, and it also can be used for our army surface ships and submarines for remote reconnaissance defense. UMPs require as much as possible to stabilize and impact resistance to ensure that a variety of sensors can work properly. Therefore, in order to improve the stability of the UMP, it is very meaningful to study the vibration suppression control technology of UMP. At present, the research on the characteristics of the UMP is mainly focused on the simulation of the anchor chain and the cable, while the research on the stability of the UMP is especially less. Research on the underwater vibration control is mainly concentrated in the field of offshore riser.

The suppressing method of vortex-induced vibration can be divided into two types: active control and passive control measures. Active control measures are real-time monitoring of flow field changes and structural forces. It is through the automatic technology to disturb the flow filed, which can control the cylinder vortex shedding and finally achieve the purpose of vibration suppression. Passive control measures are to change the flow field by directly modifying the external shape of the structure or other devices attached to the structure. It is to achieve the purpose of controlling the formation and shedding of the vortex. Compared with the active control measures, passive control measures are simple to design, easy to manufacture, and low cost. So it has been widely used in the field of marine engineering.

In this paper, taking into account the stability of the UMP, we have adopted a relatively simple vibration suppression device-splitter plate which can rotate freely relative to the UMP.

In the previous research literatures, there are many studies of the cylinder with the fixed splitter plate and less research on the splitter plate that is free to rotate behind the cylinder. The majority is to study the effect of fixed dividers on vortex shedding. For the elastic-supported cylinder-splitter plate system, Ding et al. [1] studied the wake characteristics and vortex-induced vibration characteristic of the splitter plate under high Reynolds number, where the length of the splitter plate is the same as the diameter of the cylinder. The results show that, with high Reynolds number, the amplitude of the cylinder with plate dose not decrease (without the effect of suppressing vibration) but increases as the number of Reynolds increases. The vibration frequency is lower than the bare cylinder and the vortex shedding model of cylinder with a splitter plate and a bare cylinder is also different. The inhibitory effect is more obvious as the velocity decreases. Tan et al. [2] carried out a two-dimensional numerical simulation with a splitter plate cylinder and a bare cylinder, and the ratio of the length of the splitter plate to the diameter of the cylinder varies from 0.25 to 2.0 (.25~2.0). The results show that the splitter plate has the most significant effect on the vortex-induced vibration control of the riser when is 1.0–2.0. Wang et al. [3] performed a two-dimensional computational fluid dynamics (CFD) numerical simulation of the fixed separation plate (variable dimensionless length). When the Reynolds numbers are 1000 and 30000, respectively, changing = 0.5~2.0 to simulation can be found to reduce the drag coefficient, lift coefficient, and vortex shedding frequency. This document of Amiraslanpour et al. [4] is used to study the two splitter plate in the upstream or downstream stages of the cylinder. They found the influence of the splitter plate on the drag force of the cylinder. They also studied the effect of the distance from the end of the splitter to the cylindrical surface without much impact on the resistance. When the ratio of amplitude to the cylinder diameter () was 0.25 and 0.5, respectively, the resistance was reduced by 57% and 36%. Qiu et al. [5] experimentally studied the fixed cylinder with frontal plate, bare cylinder, cylinder with rear splitter plate, cylinder with bilateral plates, and semicylindrical roof. They found that the cylinder with wake splitter plate () can effectively inhibit the vortex shedding, and shear layer was separated on both sides of the plate. Lou et al. [6] experimentally studied the effect of vortex-induced vibration of two adjacent risers with splitter plates and found that the splitter plate on the risers was effectively in suppressing both cross-flow (CF) and in-line (IL). Assi et al. [7] experimented with plain cylinder, cylinder with fixed splitter plate, cylinder with free-to-rotate splitter plate, and cylinder with pairs of plates and studied their vibration amplitude at different reduced speed. The results show that these devices can effectively reduce the resistance and suppress vortex vibration. Assi et al. [8] have mainly studied the stability of a free-to-rotate short-tail fairing and a splitter plate as suppressors of vortex-induced vibration. They studied the rotating (free-to-rotate suppressors in 2-dof) and nonrotating (fixed suppressors in 1-dof) at different reduced speed. It is found that the free-rotating suppressors will be deflected position to remain stable and extend the vortex shedding and suppressing vortex-induced vibration. It is also found that fixed suppressors will show serious gallop in a considerable flow rang of flow speeds. Gozmen et al. [9] and Zhu et al. [10] studied the relationship of different lengths of splitter plate to suppress vortex shedding. Gu et al. [11] studied a fixed cylinder with a free splitter plate, where the ratio of the length of plate to the diameter of the cylinder varied from 0.5 to 6 and the Reynolds number varied from 30000 to 60000. The pressure distribution, drag, lift force, and vortex shedding model are analyzed, and the study finds that the free rotation angle corresponding to the length of the longer plate is smaller. Akilli et al. [12] studied that the influence of factors such as the dimensionless thickness () on the splitter plate and the distance () between the end of the plate and the fixed cylinder vortex shedding was studied. The dimensionless thickness of the plate varies from 0.016 to 0.08, and the distance is changed from 0 to 100 (mm) in the step size of 12.5. When changed from 0 to 1.75, the plate has effect on suppression of the vortex shedding. Law and Jaiman [13] studied four kinds of vibration-proofing devices (fairing, connected-C, disconnected-C, and splitter plate) separately, which the device can not rotate freely relative to the cylinder. It was found that the connected-C device was similar to the effect of the fairing device on vortex-induced vibration. The study also found that the connected-C device and disconnected-C device can effectively prevent the cylinder galloping at high reduced velocity, but the connection between them has little effect on vibration suppression. In [14], the authors mainly studied the circular cylinders fitted with three different geometries of splatter plate (solid splitter plate , solid splitter plate , and slotted splitter plate ). He studied the response of the different vibration mechanisms of the three devices with varying reduced speeds.

With reference to the above-mentioned latest literature, there are few studies on the splitter plate parameters and the splitter plate which are rotated relative to the cylinder. So in this paper, I mainly studied the effect of the length of the splitter plate that can rotate freely around the cylinder and the damping values between the splitter plate and the cylinder on the vibration suppression technique of the UMP.

For some complex numerical simulation, if we all use CFD simulation solution, then we will greatly increase the calculation of cost and the calculation cycle. Therefore, in order to improve the computational efficiency, this paper establishes an optimization method based on the combination of CFD and backpropagation (BP) neural network and genetic algorithm. BP neural network and genetic algorithm are used at some of the nonlinear data to find the optimal value [15–17]. Adaptive control [18, 19] in the neural network has also been a very good application. Safikhani et al. [20] have studied the multiobjective optimization of nanofluid flow in flat tubs, combined with a CFD, artificial neural networks, and genetic algorithms. They got important design information about nanofluids and flat tubes by combining CFD, GMDH, and THE multiobjective optimization method. Avci et al. [21] studied the optimization of the deign parameters of a home refrigerators using CFD and artificial neural network. This paper shows that CFD simulation and the ANN can determine the best value for the refrigerator design.

So we can combine CFD simulation and BP neural network genetic algorithm to optimize the best vibration suppression rate. Through the use of CFD simulation software, we can get some data about vibration suppression rate, finally combined with BP neural network genetic algorithm to find the best suppression effect and the corresponding dimensionless plate length and damping value size.

#### 2. Geometry Configuration

One end of the cable is tethered on the UMP and the other end is fixed under the sea, so that the UMP is floating in the sea. Some sensor can not work properly, because the ocean flow easily leads to the vortex-induced vibration. Inspired from the above research, we installed a splitter plate in the wake area of the UMP, in which the splitter plate can freely rotate around the UMP. As shown in Figure 1, (a) is the overall diagram of the UMP, and (b) is a schematic diagram of the splitter plate for rotation. The vortex shedding process alternates on both sides of the UMP and will produce vortex-induced vibrations because of the UMP generated periodic pulsating forces. So, we designed a rotatable splitter plate to extend the process and achieve the effect of vibration suppression. In this paper, we mainly focus on evaluating the wake structure, force, motion (the UMP only moves in the transverse direction with a degree of freedom, and the splitter plate is rotated relative to UMP), and the transverse amplitude at different design parameters with a CFD method.