• Journal of Ocean Engineering and Technology
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• v.28 no.4
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• pp.288-293
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• 2014

The IEC standard for onshore or offshore wind turbines requires additional dummy simulations (at least 5 s) for the transient responses due to initial conditions. An increase in the dummy time causes a considerable increase in the computational cost considering multiple design spirals with several thousand design load analysis cases. A time of 30 s is typically used in practical simulations for a wind turbine design with a fixed platform. However, 30 s may be insufficient for floating offshore wind turbines (FOWT) because the platforms have lower natural frequencies, and the transient responses will last much longer. In this paper, an initial condition application algorithm is implemented for WindHydro, and the appropriate dummy simulation time is investigated based on a series of dynamic simulations of a FOWT. As a result, it is found that more than 300 s is required for the platform to have stationary motion after the initial transient responses for the FOWT under the conditions considered.

• The KSFM Journal of Fluid Machinery
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• v.18 no.4
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• pp.36-42
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• 2015

This is a study about one of the most widely used hydro machinery all over the world - pump-turbine. The system has an impeller which pumps water to an upper reservoir during the night and the same impeller acts as a runner for turbine mode during the day for providing stable electrical power to the grid. The internal flow analysis is investigated in this study to help understand how the water passes through the passage of the vanes and blades, providing the designer with useful information on the behavior of recirculation flows which could reduce the efficiency of the pump-turbine. The 100 kW pump-turbine model has H = 32 m, $Q=0.336m^3/s$ and $N=1200min^{-1}$. For this study there are 7 blades, 19 stay vanes and 20 guide vanes. From this study, it was observed that this pump-turbine design showed very good internal flow characteristics with no flow separation and no recirculation flows in normal operation mode.

• International Journal of Fluid Machinery and Systems
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• v.10 no.1
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• pp.1-8
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• 2017

Bifurcation refers to wye division of penstock to divide the flow symmetrically or unsymmetrically into two units of turbine for maintaining economical, technical and geological substrates. Particularly, water shows irrelevant behavior when there is a sudden change in flow direction, which results into the transition of the static and dynamic behavior of the flow. Hence, special care and design considerations are required both hydraulically and structurally. The transition induced losses and extra stresses are major features to be examined. The research on design and analysis of bifurcation is one of the oldest topics related to R&D of hydro-mechanical components for hydropower plants. As far as the earlier approaches are concerned, the hydraulic designs were performed based on graphical data sheet, head loss considerations and the mechanical analysis through simplified beam approach. In this paper, the multi prospect approach for design of Bifurcation, incorporating the modern day's tools and technology is identified. The hydraulic design of bifurcation is a major function of dynamic characteristics of the flow, which is performed with CFD analysis for minimum losses and better hydraulic performances. Additionally, for the mechanical design, a simplified conventional design method as pre-estimation and Finite Element Method for a relevant result projections were used.

• Steel and Composite Structures
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• v.33 no.1
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• pp.93-109
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• 2019

In the present study, vibration analysis of double bonded micro sandwich cylindrical shells with saturated porous core and carbon/boron nitride nanotubes (CNT/BNNT) reinforced composite face sheets under multi-physical loadings based on Cooper-Naghdi theory is investigated. The material properties of the micro structure are assumed to be temperature dependent, and each of the micro-tubes is placed on the Pasternak elastic foundations, and mechanical, moisture, thermal, electrical, and magnetic forces are effective on the structural behavior. The distributions of porous materials in three distributions such as non-linear non-symmetric, nonlinear-symmetric, and uniform are considered. The relationship including electro-magneto-hydro-thermo-mechanical loadings based on modified couple stress theory is obtained and moreover the governing equations of motion using the energy method and the Hamilton's principle are derived. Also, Navier's type solution is also used to solve the governing equations of motion. The effects of various parameters such as material length scale parameter, temperature change, various distributions of nanotube, volume fraction of nanotubes, porosity and Skempton coefficients, and geometric parameters on the natural frequency of double bonded micro sandwich cylindrical shells are investigated. Increasing the porosity and the Skempton coefficients of the core in micro sandwich cylindrical shell lead to increase the natural frequency of the structure. Cylindrical shells and porous materials in the industry of filters and separators, heat exchangers and coolers are widely used and are generally accepted today.

• Nuclear Engineering and Technology
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• v.55 no.12
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• pp.4647-4658
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• 2023

In this paper, elastic-plastic fracture mechanics analysis is performed to determine the critical crack sizes of the multipurpose canister (MPC) manufactured using austenitic stainless steel under dynamic loading conditions that simulate drop accidents. Firstly, dynamic finite element (FE) analysis is performed using Abaqus v.2018 with the KORAD (Korea Radioactive Waste Agency)-21 model under two drop accident conditions. Through the FE analysis, critical locations and through-thickness stress distributions in the MPC are identified, where the maximum plastic strain occurs during impact loadings. Then, the evaluation using the failure assessment diagram (FAD) is performed by postulating an external surface crack at the critical location to determine the critical crack depth. It is found that, for the drop cases considered in this paper, the principal failure mechanism for the circumferential surface crack is found to be the plastic collapse due to dominant high bending axial stress in the thickness. For axial cracks, the plastic collapse is also the dominant failure mechanism due to high membrane hoop stress, followed by the ductile tearing analysis. When incorporating the strain rate effect on yield strength and fracture toughness, the critical crack depth increases from 10 to 20%.

• Transactions on Electrical and Electronic Materials
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• v.16 no.2
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• pp.53-61
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• 2015

High-energy ball milling was tested as a method for producing Ultra High Molecular Weight Polyethylene (UHMWPE)- based nanodielectrics containing 1 wt% and 5 wt% OctaIsoButylPOSS (OibPOSS). Qualitative and quantitative evaluations were used to explore the compatibility between OibPOSS and PE. Several ball milling variables were optimized in a bid to achieve UHMWPE/OibPOSS nanodielectrics. The morphology, as well as the thermal and the dielectric properties of the samples, were characterized by scanning electron microscopy, thermogravimetric analysis, broadband dielectric spectroscopy, and progressive-stress breakdown tests. The results showed that (i) ball milling was an effective method for producing UHMWPE/OibPOSS dielectric composites, but appeared ineffective in dispersing OibPOSS at the nanoscale, and (ii) the resulting UHMWPE/OibPOSS dielectric composites presented thermal and dielectric properties similar to those of neat UHMWPE.

• International Journal of Fluid Machinery and Systems
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• v.3 no.2
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• pp.129-136
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• 2010

The attachment of inducer upstream of main impeller is an effective method to improve the suction performance of turbopump. However, various types of cavitation instabilities are known to occur even at the designed flow rate as well as in the partial flow rate region. The cavitation surge occurring at partial flow rates is known to be strongly associated with the inlet back flow. In the present study, in order to understand the detailed structure of internal flow of inducer, we firstly carried out the experimental and numerical studies of non-cavitating flow, focusing on the flow field near the inlet throat section and inside the blade passage of a two bladed inducer at a partial flow rate. The steady flow simulation with cavitation model was also made to investigate the difference of flow field between in the cavitating and no-cavitating conditions.

• International Journal of Fluid Machinery and Systems
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• v.3 no.2
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• pp.122-128
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• 2010

The attachment of inducer in front of main impeller is a powerful method to improve cavitation performance; however, cavitation surge oscillation with low frequency occurs with blade cavity growing to each throat section of blade passage simultaneously. Then, one conceptual method of installing suction axi-asymmetrical plate has been proposed so as to keep every throat passage away from being unstable at once, and the effect on suppression of the oscillation were investigated. In the present study, cavitation behaviors in the inducer is observed with distributing multi-cameras circumferentially, recording simultaneously and reconstructing multi-photos on one plane field as moving a linear cascade. Observed results are utilized for discussion with other measuring results as casing wall pressure distribution. Then the suppression mechanism of oscillation by installing axi-asymmetrical inlet plate will be clarified in more details.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.25 no.11
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• pp.1854-1862
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• 2001

In this paper, power flow characteristics of a hydromechanical transmission(HMT) are investigated using network analysis. The HMT used in this study consist of a hydrostatic unit(HSU), planetary gear sets, clutches and brakes providing forward 4 speeds and backward 2 speeds. Since the HMT power flows showing a closed loop and the HSU efficiency varies depending on the pressure and speed, a systematic approach is required to analyze the power transmission characteristics of the HMT. In order to analyze the closed loop power flow and the HSU power loss which changes depending on the pressure and speed, network model is constructed fur each speed range. In addition, an algorithm is proposed to calculate an accurate HSU loss corresponding to the experimental results. It is found from the network analysis that the torque and speed of each transmission element including the HSU can be obtained as well as direction of the power flow by the proposed algorithm. It is expected that the network analysis can be used in the design of relatively complicated transmission system such as HMT.

• Nuclear Engineering and Technology
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• v.50 no.4
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• pp.570-581
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• 2018

Recently, a software program has been used in nuclear power plants (NPPs) to digitalize many instrumentation and control systems. To guarantee NPP safety, the reliability of the software used in safetycritical instrumentation and control systems must be quantified and verified with proper test cases and test environment. In this study, a software testing method using a simulation-based software test bed is proposed. The test bed is developed by emulating the microprocessor architecture of the programmable logic controller used in NPP safety-critical applications and capturing its behavior at each machine instruction. The effectiveness of the proposed method is demonstrated via a case study. To represent the possible states of software input and the internal variables that contribute to generating a dedicated safety signal, the software test cases are developed in consideration of the digital characteristics of the target system and the plant dynamics. The method provides a practical way to conduct exhaustive software testing, which can prove the software to be error free and minimize the uncertainty in software reliability quantification. Compared with existing testing methods, it can effectively reduce the software testing effort by emulating the programmable logic controller behavior at the machine level.