Review of Fluid Structure Interaction Methods Application to Floating Wave Energy Converter

International Journal of Fluid Machinery and Systems DOI: http://dx.doi.org/10.5293/IJFMS.2018.11.1.063 Vol. 11, No. 1, January-March 2018 ISSN (Online): 1882-9554 Original Paper Review of Fluid Structure Interaction Methods Application to Floating Wave Energy Converter Mohammed Asid Zullah1, Young-Ho Lee2 1Geoscience, Energy and Maritime Division, Pacific Community, Private Mail Bag, Suva, Fiji, [email protected] 2Division of Mechanical Engineering, College of Engineering, Korea Maritime and Ocean University, 727 Taejong-ro, Yeongdo-Gu, Busan 49112, South Korea, [email protected] Abstract Computational fluid dynamics (CFD) is a highly efficient paradigm that is used extensively in marine renewable energy research studies and commercial applications. The CFD paradigm is ideal for simulating the complex dynamics of Fluid-Structure Interactions (FSI) and can capture all kinds of nonlinear fluid motions. While nonlinear simulations are considered more expensive and resource intensive compared to the frequency domain approaches, they are much more accurate and ideal for commercial applications. This review study presents a comprehensive overview of the computation fluid dynamics paradigm in context of wave energy converter (WEC) and highlights different CFD tools that are available today for commercial and research applications. State-of-the-art CFD codes such as ANSYS CFX that are highly ideal for WEC simulation problems are highlighted and aspects such as time and frequency domains are also thoroughly discussed along with efficacy of the nonlinear simulations compared to the linear models. The paper presents a comparative evaluation of different WEC modelling codes available today and illustrates the code framework of different CFD simulation software suites. Keywords: CFD, FSI, Navier-Stokes equation, WEC, nonlinear, simulations. 1. Introduction The concept of modern computational fluid dynamics first took shape in the early seventies when it essentially represented a conglomeration of domains such as physics, mathematics and computer science used for the purpose of simulating fluid flow dynamics [1]. A study carried out by Kopal (1947) is probably the very first significant example of CFD where he deduced supersonic flow values that flowed over geometrical cone structures [2]. With the development of fast mainframes in the late fifties and sixties the first generation of computational fluid dynamics solutions started becoming available. Computational fluid dynamic solutions that came next started implementing governing equations such as the Navier–Stokes equations to solve fluid hydrodynamic problems. Today, the relevance of computational fluid dynamics with regards to hydrodynamic problem solving and simulations has become very high and with the progress in computational technology the stature of CFD is defined as a third dimension in the area of fluid dynamics [1]. In the recent years researchers have extensively used the CFD (Computational Fluid Dynamics) paradigm to understand the underlying hydrodynamic properties of WECs. In a recent study, researchers used CFD to analyze the impact of chamber-duct on the inner water surface amplitude and nozzle air flow [3]. Another CFD study conducted in 2012 focused on an experiment carried out by a group of researchers in 2007 and reported that there has been an overestimation of efficiency by up to 30% because the several aspects such as complicated variations in pressure were never taken into account [4][5]. The experiment carried out by Thomas et al. (2007) was further repeated by Kamath et al. (2015) where they carried out complicated computational fluid dynamics simulations to understand if there is any effect on the operational efficiency of wave energy converters such as the Oscillating Water Column device mediated by power take-off damping [5][6]. In essence, researchers today fully appreciate the efficacy and applicability of the computational fluid dynamics paradigm for oceanic wave energy research and for better understanding of the interaction of oceanic waves with the wave energy converters in a simulated environment. A common apprehension regarding the usage of computation fluid dynamics in marine engineering is that it is not very cost- effectiveness and for that reason many commercial organizations prefer to go for the tried and tested frequency-based methods. As a matter of fact, setting up massive turbines to capture energy from tidal streams could be very difficult [7]. However, given the broad applicability and efficacy of computational fluid dynamics it is definitely a more desirable option compared to the traditional methods. Computational fluid dynamics offer a highly detailed overview of the interaction between oceanic waves and structure and is definitely not restricted to the linear motions alone unlike the traditional frequency-based methods. Computational fluid dynamics can also be Received December 12 2016; revised January 9 2017; accepted for publication Agust 10 2017: Review conducted by Abdus Samad. (Paper number O16042K) Corresponding author: Young-ho Lee, [email protected] 63 used to simulate the physics and the dynamics of waves breaking and overtopping and given the exceptional pace of development in computational technologies more robust algorithms are being conceptualized and developed, making them highly ideal for sectors such as marine engineering and wave energy conversions [8]. Algorithms implementing computational fluid dynamics executes the Navier- Stokes equations that can handle both non-linear and rotational dynamics. This make that highly intensive computationally and their implementation becomes extremely resource intensive. However this can definitely be considered an acceptable trade-off because computational fluid dynamics allow simulation of analysis of complex aspects such as wave drag, viscosity and other forces that are entirely non-linear in nature. Furthermore, numerical simulations can be carried out at minimal costs and are almost entirely risk-free. They can be excellent tools to ascertain the impact of the tidal stream turbines on the environment and can also be very instrumental in the modelling of the turbine blades [9, 10]. Other options that can be very useful for offsetting the high computation costs of CFD applications are the nonlinear model-reduction methods that are projection-based [11, 12]. These projection-based approaches are very ideal for systems that are linear and essentially time-invariant and also for those systems that are linear and stationary [13, 14]. The projection based model reduction paradigms are also ideal for nonlinear systems based on quadratic nonlinearities and to solve problems of structural dynamics [15, 16, 17]. These paradigms function by approximating a functional model with minimal dimension and are highly ideal for aerodynamic problem solving [18,19,20,21,22]. As a result, the project-based models become ideal for applications that are highly time-sensitive such as flow control, optimization of structure designs and estimation of structure uncertainties [23,24,25]. The projection-based methods have also been found to be effective for problem solving in the aero elasticity domain. [26,27,28]. In the recent times a wide range of studies have used complex computational fluid dynamics for device simulation in experimental tanks. A recent study by Bhinder et al. (2011) successfully used computational fluid dynamics to calculate the viscous drag coefficients of wave energy conversion devices [29]. Similarly multiple research studies are currently underway to further streamline the applicability of computational fluid dynamics in wave energy conversion array problems and establish the efficacy of the same [30]. 2. The Core of CFD: Reynolds-Averaged Navier-Stokes Equations In any given engineering problem scenario, substantial fluctuations can be seen in the fluid flow levels, both in the time and space domain. These fluctuations are mediated largely by turbulence but it needs to be noted that the fluctuation values are stable and stays within a fixed range. The Reynolds averaged Navier-Stokes equations utilize the average fluid flow quantities and properly address the influence or the effect of turbulence. The average flow quantities are deduced in Reynold’s averaged quantities and this allows expression of every dependent variable. Each dependent variable is made up of average time value and the value of the fluctuating component [31]. Advanced computational fluid dynamics address the engineering problem of fluid flow by executing the governing Reynolds averaged Navier-Stokes equations shown below. (1) (2) In the above governing equations, u stands for the average velocity value over a specific time period t. The fluid density value is represented by ρ and the pressure is represented by p. The fluid kinematic viscosity value is represented by ν and the eddy viscosity is represented by νt. Finally, the fluid acceleration due to the gravitational pull is represented by g [6]. To analyze the effects of diverse hydrodynamic forces during any kind of wave energy converter simulation, it is important that factors such as turbulence, overtopping and oceanic wave breaking are properly taken into consideration. All of these are not possible to account for using the standard potential flow paradigm and that is one of the main reasons why researchers prefer the Navier-Stoke equation methods [32]. While implementing CFD approaches based on Navier-Stokes equations to simulate hydrodynamic models of

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