• Journal of the Society of Naval Architects of Korea
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• v.55 no.5
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• pp.385-393
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• 2018

In this study, the analysis of 2DOF (2 Degree Of Freedom) motion and added resistance of a ship in regular head waves is carried out using RANS (Reynolds Averaged Navier-Stokes) approach. In order to improve the accuracy for large amplitude motions, the dynamic overset scheme is adopted. One of the dynamic overset schemes, Suggar++ is applied to WAVIS which is the in-house RANS code of KRISO (Korea Research Institute of Ships and Ocean Engineering). The grid convergence test is carried out using the present scheme before the analysis. The target hull form is KRISO VLCC tanker (KVLCC2) and 13 wave length conditions are applied. The present scheme shows the improved results comparing with the results of WAVIS2 in the non-inertial reference frame. The dynamic overset scheme is confirmed to give the comparatively better results for the large amplitude motion cases than the non-inertial frame based scheme.

• Journal of the Society of Naval Architects of Korea
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• v.36 no.4
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• pp.1-8
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• 1999

Numerical study is carried out to investigate flow characteristics around an axisymmetric body with and without an integrated propulsor. The incompressible Reynolds-Averaged Navier-Stokes(RANS) equations are also solved using the finite volume method and the standard $k-\varepsilon$ turbulence model for turbulence closure. In order to investigate the propulsor-hull interaction, the induced velocity calculated by surface panel methods is utilized for the boundary condition at the propeller plane. The calculated results are compared to the experimental results. It is considered that the present numerical code can be used for design of an integrated propulsor.

• Journal of Ocean Engineering and Technology
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• v.32 no.5
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• pp.386-392
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• 2018

It is necessary to predict hydrodynamic derivatives when assessing the maneuverability of a submarine. The force and moment acting on the vehicle may affect its motion in various modes. Conventionally, the derivatives are determined by performing captive model tests in a towing tank or applying a system identification method to the free running model test. However, a computational fluid dynamics (CFD) method has also become a possible tool to predict the hydrodynamics. In this study, virtual captive model tests for a full-scale submarine were conducted by utilizing a Reynolds-averaged Navier-Stokes solver in ANSYS FLUENT version 18.2. The simulations were carried out at design speed for various modes of motion such as straight forward, drift, angle of attack, deflection of the rudder, circular, and combined motion. The hydrodynamic force and moment acting on the submarine appended rudders and stern stabilizers were then obtained. Finally, hydrodynamic derivatives were determined, and these could be used for evaluating the maneuvering characteristics of the submarine in a further study.

• Proceedings of the Korea Water Resources Association Conference
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• 2015.05a
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• pp.41-41
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• 2015

하천의 지형을 조사하고 계측하는 것은 하천을 연구하는 전문가들에게 필수적인 일이다. 하지만 하천의 지형을 계측하는 것은 쉽지 않으며, 조사를 하여도 유사의 이송으로 인하여 하천의 지형은 시간이 지남에 따라 변하게 된다. 그러므로 실험이나 모델링을 통하여 하천의 지형을 예측하고 모의하는 것은 중요한 연구이다. 모델링을 이용하여 유사이송에 의한 하상변동을 잘 예측하기 위해서는 하천의 복잡한 흐름을 정확히 모의하는 것이 중요하며 유사를 발생시키는 힘인 하상전단응력을 정확히 산정하는 것 또한 중요하다. 하상의 전단응력을 산정하는 방법으로는 대표적으로 로그법칙에 의한 방법, 레이놀즈응력 분포를 이용한 방법, 난류운동에너지를 이용한 방법 등이 있다. 앞서 말한 방법으로 산정된 전단응력 값은 차이를 보이며, 이는 하상변동을 정확히 모의하는 것에 문제를 발생시킬 수 있다. 따라서 본 연구에서는 곡선좌표계를 이용하여 3차원 유동 및 하상변동을 모의할 수 있는 수치모형을 이용하여 전단응력 산정 방법에 따른 하상변동량을 분석하는 것이다. 하천의 복잡한 흐름을 정확히 모의하기 위하여 본 연구에서는 RANS (Reynolds Averaged Navier-Stokes) 방정식을 3차원으로 해석하여 흐름 계산을 하였고 유사량 산정공식과 Exner 방정식을 이용하여 유사이송에 의한 하상변동을 계산하였다. 흐름 계산의 검증을 위하여 선행 연구의 실험을 대상으로 모의하였다. 그리고 곡선으로 된 실험 수로를 대상으로 전단응력 산

• Proceedings of the Korea Water Resources Association Conference
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• 2015.05a
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• pp.42-42
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• 2015

사행수로에서의 원심력은 비선형적인 압력분포를 야기하여 이차류, 편수위 등과 같은 불규칙하고 복잡한 흐름을 발생시킨다. 일반적으로 이들 흐름은 난류이고 매우 3차원적이며 자유수면과의 상호작용이 중요한 역할을 할 수도 있다. 환경, 유사이동, 지형 변화와 관련된 환경 수리학적 관점에서 사행수로에서의 흐름을 이해하고 설계하기 위해서는 이러한 복잡한 3차원 흐름을 정확하게 계산하는 것이 매우 중요하다. 이 연구에서는 유한차분법에 근거한 3차원 흐름해석 모형을 이용하여 사행수로에서의 난류 흐름을 모의하고자 한다. 지배방정식은 3차원 비정상 RANS(Reynolds averaged Navier-Stokes) 방정식이며, 난류 해석을 위해서 공학적으로 널리 이용되고 있는 난류 모형 중 k-omega 모형을 이용한다. 수치모형은 시간과 공간에 대해서 2차 정확도의 이산화 기법을 적용한다. 자유수면의 변동은 이상(two-phase) VOF (volume of fluid) 기법을 이용하여 계산한다. 수치모형의 적용 대상은 기존 문헌에서 제시되어 있는 키노시타 사인곡선을 이용하여 만든 폭 60cm의 사행수로에서 후르드수 0.23 그리고 레이놀즈수 41,700의 조건에서 발생시킨 난류 흐름이다. 적용한 난류모형들을 이용하여 해석한 결과들을 유속벡터분포와 수위의 항으로 비교분석하여 사행수로에서 발생되는 이차류와 편수위 변화 재현에 대한 수치모형의 적용성을 평가하고 각 난류모형들의 특성을 제시한다.

• Journal of the Society of Naval Architects of Korea
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• v.58 no.2
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• pp.73-83
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• 2021

In this study, the numerical investigation of the flow around the SUBOFF submarine model is performed by using the Detached Eddy Simulation (DES) method which is developed based on the SST k-ω turbulence model. At the DES analysis level, complex vortical flows around the submarine model are caused mainly by the vortices due to the appendages and their interactions with the flows from the hull boundary layer and other appendages. The complexity and scale of the vortical flow obtained from the numerical simulations are highly dependent on the grid. The computed local flow properties of the submarine model are compared with the available experimental data showing a good agreement. The DES analysis more reasonably estimates the physical phenomena inherent in the experimental result in a low radius of the propeller plane where vortical flows smaller than the RANS scale are dominant.

• Wind and Structures
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• v.39 no.1
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• pp.31-45
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• 2024

This paper examines the flow characteristics around an inclined prism with a U-shaped cross-section ("U-profile") and investigates the connection between the flow and flow-induced vibrations. The study employs a combined approach that involves wind tunnel experiments and computational fluid dynamics (CFD) using an unsteady Reynolds-averaged Navier-Stokes (RANS) turbulence model. Distinct vortex formation patterns are observed in the flow field surrounding the stationary inclined profile. When the cavity of the profile faces away from the incoming flow, large vortices develop behind the profile. Conversely, when the cavity is oriented towards the oncoming flow, these vortices form within the cavity. Notably, due to the slow movement of these large vortices through the cavity, the frequency at which vortices are shed in the negative inclination case is lower compared to the positive inclination, where they form in the wake. Wind tunnel experiments reveal an intermittent transition between the two vortex formation patterns at zero inclination. Large vortices sporadically emerge both in the cavity and behind the profile. The simulation results demonstrate that when these large vortices occur at a frequency close to the structure's natural frequency, they induce prominent pitch vibrations. This phenomenon is also sought after and presented in coupled vibration experiments. Additionally, the simulations indicate that when the natural frequency of the structure is considerably lower than the vortex shedding frequency, this type of vibration can be observed.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.34 no.7
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• pp.703-714
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• 2010

This paper describes the design optimization of a rotating rectangular channel with staggered arrays of pin-fins by Kriging metamodeling technique. Two non-dimensional variables, the ratio of the height to the diameter of the pin-fins and the ratio of the spacing between the pin-fins to the diameter of the pin-fins are chosen as the design variables. The objective function that is a linear combination of heat transfer and friction loss related terms with a weighting factor is selected for the optimization. To construct the Kriging model, objective function values at 20 training points generated by Latin hypercube sampling are evaluated by a three-dimensional Reynolds-averaged Navier-Stokes (RANS) analysis method with the SST turbulence model. The Kriging model predicts the objective function value that agrees well with the value calculated by the RANS analysis at the optimum point. The objective function is reduced by 11% by the optimization of the channel.

• Nuclear Engineering and Technology
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• v.56 no.4
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• pp.1310-1319
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• 2024

The reactor cavity cooling system (RCCS) is a passive reactor safety system commonly present in the designs of High-Temperature Gas-cooled Reactors (HTGR) that removes heat from the reactor pressure vessel by means of natural convection and radiation. It is one of the factors responsible for ensuring that the reactor does not melt down under any plausible accident scenario. For the simulation of accident scenarios, which are transient phenomena unfolding over a span of up to several days, intermediate fidelity methods and system codes must be employed to limit the models' execution time. These models can quantify radiation heat transfer well, but heat transfer caused by natural convection must be quantified with the use of correlations for the heat transfer coefficient. It is difficult to obtain reliable correlations for HTGR RCCS heat transfer coefficients experimentally due to such a system's size. They could, however, be obtained from high-fidelity steady-state simulations of RCCSs. The Rayleigh number in RCCSs is too high for using a Direct Numerical Simulation (DNS) technique; thus, a Reynolds-Averaged Navier-Stokes (RANS) approach must be employed. There are many RANS models, each performing best under different geometry and fluid flow conditions. To find the most suitable one for simulating an RCCS, the RANS models need to be validated. This work benchmarks various RANS models against three experiments performed on the HTTR RCCS Mockup by the Japanese Atomic Energy Agency (JAEA) in 1993. This facility is a 1/6 scale model of a vessel cooling system (VCS) for the High Temperature Engineering Test Reactor (HTTR), which is operated by JAEA. Multiple RANS models were evaluated on a simplified 2d-axisymmetric geometry. They were found to reproduce the experimental temperature profiles with errors of up to 22% for the lowest temperature benchmark and 15% for the higher temperature benchmarks. The results highlight that the pragmatic turbulence models need to be validated for high Rayleigh natural convection-driven flows and improved accordingly, more publicly available experimental data of RCCS resembling experiments is needed and indicate that a 2d-axisymmetric geometry approximation is likely insufficient to capture all the relevant phenomena in RCCS simulations.

• Wind and Structures
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• v.37 no.4
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• pp.255-274
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• 2023

The aim of this paper is to enhance the accuracy of predicting time-averaged external surface pressures on low-rise buildings by utilizing Computational Fluid Dynamics (CFD) simulations. To achieve this, benchmark studies of the Silsoe cube and the Texas Tech University (TTU) experimental building are employed for comparison with simulation results. The paper is structured into three main sections. In the initial part, an appropriate domain size is selected based on the precision of mean pressure coefficients on the windward face of the cube, utilizing Reynolds Averaged Navier-Stokes (RANS) turbulence models. Subsequently, recommendations regarding the optimal computational domain size for an isolated building are provided based on revised findings. Moving on to the second part, the Silsoe cube model is examined within a horizontally homogeneous computational domain using more accurate turbulence models, such as Large Eddy Simulation (LES) and hybrid RANS-LES models. For computational efficiency, transient simulation settings are employed, building upon previous studies by the authors at the Windstorm Impact, Science, and Engineering (WISE) Lab, Louisiana State University (LSU). An optimal meshing strategy is determined for LES based on a grid convergence study. Three hybrid RANS-LES cases are investigated to achieve desired enhancements in the distribution of mean pressure coefficients on the Silsoe cube. In the final part, a 1:10 scale model of the TTU building is studied, incorporating the insights gained from the second part. The generated flow characteristics, including vertical profiles of mean velocity, turbulence intensity, and velocity spectra (small and large eddies), exhibit good agreement with full-scale (TTU) measurements. The results indicate promising roof pressures achieved through the careful consideration of meshing strategy, time step, domain size, inflow turbulence, near-wall treatment, and turbulence models. Moreover, this paper demonstrates an improvement in mean roof pressures compared to other state-of-the-art studies, thus highlighting the significance of CFD simulations in building aerodynamics.