In the present study, a single particle settlement was studied using a developed unresolved computational fluid dynamics (CFD) and discrete element method (DEM) coupling solver. The solver was implemented by coupling OpenFOAM, the open-source computational fluid dynamics libraries, with LIGGGHTS, the open-source discrete element method libraries. An averaging method using a kernel function was considered to decrease the grid dependency. For the drag model of a single particle, a revised volume fraction with a particle volume expansion coefficient was applied. Falling particles in a water tank were simulated and compared with the empirical correlation. A parametric study using several integrated added mass coefficients and volume expansion coefficients from low to high Reynolds numbers was carried out. The simulations which used the developed numerical methods showed significantly improved predictions of particle settlement.
1. 서 론 해상풍력시장의 대부분을 차지하고 있는 유럽과 더불어 국내 에서 고려하고 있는 해상풍력발전단지는 비교적 낮은 수심인 수심 60m 이하에 조성되었기 때문에 대부분이 고정식 구조물의 형태를 가지고 있다. 이러한 고정식 해상풍력발전기에서는 터 빈의 크기가 커짐에 따라 그에 따른 하중이 증가하게 되고, 해 상의 열악한 환경에서 발전기를 안정적으로 지지할 수 있는 해 상기초구조물의 설치가 중요하게 작용한다. 고정식 해상풍력발전기와 같이 기초가 해저 면에 고정되어 있는 구조물에서 발생하는 문제점 중의 하나는 Fig. 1과 같은 세굴(Scour)에 의한 침식현상이다. 즉, 해류 및 파랑에 노출된 기초에서 발생하는 세굴은 유체 흐름과 구조물 사이의 상호작 용으로 인해 해저 입자가 유실되는 현상으로 정의할 수 있으며, 설계 시 고려하는 하나의 인자이다. 이러한 세굴 현상은 터빈에 작용하는 큰 전복 모멘트를 견뎌야 하는 구조물의 안정성을 저 하시키는 요인이며 이에 대한 대응 연구는 매우 중요하다. 또 한, 이러한 현상을 물리적인 방법으로 보호하는 방법은 해저지 반의 다짐, 기초 부근의 해저지반을 돌로 덮는 방법, 모니터링 과 때로는 수리가 필요하다. 하지만, 이는 발전기가 대형화될수 록 설치 및 유지보수 비용 증가의 요인이 된다. 해상풍력발전기의 경우 교량교각의 경우처럼 세굴 현상에 대한 연구가 광범위하게 진행되지는 않았지만, 고정식 해상풍력발전기 의 운용 기간이 점차 증가함에 따라 관련 세굴 문제에 대한 연구 가 지속적으로 진행되고 있다(Whitehouse, 1998; Sumer and Fredsoe, 2002). 고정식 해상구조물의 형태는 크게 모노파일 (Monopile), 중력식 기초(Gravity-based foundation), 자켓(Jacket), 트 라이포드(Tripod), 트리파일(Tri-pile)로 나눌 수 있다. 특히, 가장 단순한 형태인 모노파일은 유럽에서 가장 많이 설치된 구조물이 며, 모노파일 주위에서 발생하는 세굴현상에 대한 실험 및 수치적 ABSTRACT: In the case of fixed offshore wind turbines, scouring phenomena have been reported around sub-structures as a result of currents, which seriously damage the structural stability. A parametric study of the various sub-structures of a fixed offshore wind turbine was performed to investigate their effects on the scouring phenomena. For a suction bucket foundation and monopile, the effects of the stick-up heights and water depth were studied, respectively. The open source libraries, called OpenFOAM, were used to simulate a violent flow around a foundation. The numerical methods were selected based on a two-dimensional analysis of a suction bucket. Based on the results for various stick-up heights, a larger scouring region was observed with an increase in the stick-up height because of the down-wash flow around a foundation. Based on the results for various monopile water depths, the water depth had an insignificant effect on the scouring.
In advance of building moored floating offshore platforms, in recent years, there has been a greater demand for two-way coupled simulations between a motion solver based on the viscous flow theory and a mooring line model, including cable dynamics. This paper introduces open-source libraries such as MoorDyn (the lumped-mass mooring line model) and OpenFOAM (the computational fluid dynamics libraries). It describes the methods by which they can be coupled bi-directionally. In each time step, the platform motions calculated by OpenFOAM are transferred to MoorDyn as the boundary conditions for the mooring system analysis. In contrast, MoorDyn calculates the restoring force and moment due to the mooring system and transfers them to OpenFOAM. The restoring force and moment act on the platform as the external force and moment for the platform motions in the next time step. The static tension and profile of the mooring system, dynamic tension of the mooring system, and free decay motions of the floating buoy in the still water were simulated to check the accuracy of OpenFOAM and MoorDyn. The coupled solver was used to produce simulations of the moored decay motions of the floating buoy in the still water and the moored motions with the Stokes 5th order wave. All simulation results were compared and showed good agreement with the numerical solution and experiment results. In addition, the characteristics of each solver were investigated.
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