• Title/Summary/Keyword: Hydrodynamic coefficients

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Study on hydrodynamic performance of Heavier-than-water AUV with overlapping grid method

  • Li, Xiang;Zhao, Min;Zhao, Faming;Yuan, Qingqing;Ge, Tong
    • Ocean Systems Engineering
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    • v.4 no.1
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    • pp.1-19
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    • 2014
  • Hydrodynamic coefficients strongly affect the dynamic performance of autonomous underwater vehicles (AUVs). A novel kind of underwater vehicle (Heavier-than-water AUV) with higher density than water is presented, which is different from conventional ones. RANS method and overlapping grids are used to simulate the flow field around the vehicle. Lifts, drags and moments of different attack and drift angles in steady state are calculated. The hydrodynamic performances and how the forces change with the attitude are analyzed according to the flow field structure. The steady-state results using overlapping grid method are compared with those of software FLUENT and wind tunnel tests. The calculation results show that the overlapping grid method can well simulate the viscous flow field around the underwater vehicle. Overlapping grid skills have also been used to figure out the planar-motion-mechanism (PMM) problem of Heavier-than-water AUV and forecast its hydrodynamic performance, verifying its effectiveness in dealing with the dynamic problems, which would be quite helpful for design and control of Heavier-than-water AUV and other underwater vehicles.

Calculation of Turbulent Flows around a Submarine for the Prediction of Hydrodynamic Performance

  • Kim, Jin;Park, Il-Ryong;Van, Suak-Ho;Kim, Wu-Joan
    • Journal of Ship and Ocean Technology
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    • v.7 no.4
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    • pp.16-31
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    • 2003
  • The finite volume based multi-block RANS code, WAVIS developed at KRISO, is used to simulate the turbulent flows around a submarine with the realizable $\textsc{k}-\varepsilon$ turbulence model. RANS methods are verified and validated at the level of validation uncertainty 1.54% of the stagnation pressure coefficient for the solution of the turbulent flows around SUBOFF submarine model without appendages. Another SUBOFF configuration, axisymmetric body with four identical stem appendages, is also computed and validated with the experimental data of the nominal wake and hydrodynamic coefficients. The hydrodynamic forces and moments for SUBOFF model and a practical submarine are predicted at several drift and pitch angles. The computed results are in extremely good agreement with experimental data. Furthermore, it is noteworthy that all the computations at the present study were carried out in a PC and the CPU time required for 2.8 million grids was about 20 hours to get fully converged solution. The current study shows that CFD can be a very useful and cost effective tool for the prediction of the hydrodynamic performance of a submarine in the basic design stage.

A Study on Estimation of Manoeuvring Performance in Shallow Water using CFD in Initial Ship Design Phase (선박 초기설계단계에서 CFD를 이용한 천수 중 조종성능 추정에 관한 연구)

  • Kim, In-Tae;Kim, Sang-Hyun;Kim, Hyun-Jun;Kim, Dong-Young;Yang, Jung-Kyu
    • Journal of the Society of Naval Architects of Korea
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    • v.55 no.4
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    • pp.350-360
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    • 2018
  • Analysis of ship's manoeuverability in shallow water is an important task from the perspective of the vessels' navigational safety. Since the number of ships operated in restricted water has increased due to the enlargement of vessels and ships represent different characteristics of the manoeuverability when operated in shallow and deep water, it is significant to evaluate ship manoeuverability at initial design stage. At the initial stage of design, the estimation of manoeuverability is generally performed with hydrodynamic coefficients estimated based on empirical formula. However, the accuracy of estimating hydrodynamic coefficients by the empirical formula in shallow water is poor compared to that in deep water. Therefore, the error in the estimation of manoeuverability increases in shallow water. In this study, CFD is proposed to improve the accuracy of manoeuverability in shallow water at the initial design stage and hydrodynamic coefficients were obtained based on PMM test in shallow water. Furthermore, the ship manoeuverability was estimated both the proposed strategy and the empirical formula. At last, validity of the proposed strategy using CFD for the estimation of manoeuverability was confirmed by comparison with the manoeuverability estimation results from model test.

Comparative Study of Mass Transfer and Bubble Hydrodynamic Parameters in Bubble Column Reactor: Physical Configurations and Operating Conditions

  • Sastaravet, Prajak;Chuenchaem, Chomthisa;Thaphet, Nawaporn;Chawaloesphonsiya, Nattawin;Painmanakul, Pisut
    • Environmental Engineering Research
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    • v.19 no.4
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    • pp.345-354
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    • 2014
  • In this paper, effects of physical configurations and operating conditions on bubble column performance were analyzed in terms of bubble hydrodynamic and mass transfer parameters. Bubble column with 3 different dimensions and 7 gas diffusers (single / multiple orifice and rigid / flexible orifice) were applied. High speed camera and image analysis program were used for analyzing the bubble hydrodynamic parameters. The local liquid-side mass transfer coefficient ($k_L$) was estimated from the volumetric mass transfer coefficient ($k_La$) and the interfacial area (a), which was deduced from the bubble diameter ($D_B$) and the terminal bubble rising velocity ($U_B$). The result showed that the values of kLa and a increased with the superficial gas velocity (Vg) and the size of bubble column. Influences of gas diffuser physical property (orifice size, thickness and orifice number) can be proven on the generated bubble size and the mass transfer performance in bubble column. Concerning the variation of $k_L$ coefficients with bubble size, 3 zones (Zone A, B and C) can be observed. For Zone A and Zone C, a good agreement between the experimental and the predicted $K_L$ coefficients was obtained (average difference of ${\pm}15%$), whereas the inaccuracy result (of ${\pm}40%$) was found in Zone B. To enhance the high $k_La$ coefficient and absorption efficiency in bubble column, it was unnecessary to generate numerous fine bubbles at high superficial gas velocity since it causes high power consumption with the great decrease of $k_L$ coefficients.

Study on the Estimation of Autonomous Underwater Vehicle's Maneuverability Using Vertical Planar Motion Mechanism Test in Self-Propelled Condition (자항상태 VPMM 시험을 통한 무인잠수정 조종성능 추정에 관한 연구)

  • Park, Jongyeol;Rhee, Shin Hyung;Lee, Sungsu;Yoon, Hyeon Kyu;Seo, Jeonghwa;Lee, Phil-Yeob;Kim, Ho Sung;Lee, Hansol
    • Journal of the Society of Naval Architects of Korea
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    • v.57 no.5
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    • pp.287-296
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    • 2020
  • The present study aims to improve the accuracy of the maneuvering simulations based on captive model test results. To derive the hydrodynamic coefficients in a self-propelled condition, a mathematical maneuvering model using a whole vehicle model was established. Captive model tests were carried out using the Vertical Planar Motion Mechanism (VPMM) equipment. A motor controller was used to control the constant propeller revolution rate during pure motion tests. The resistance tests, self-propulsion tests, static drift tests, and VPMM tests were performed in the towing tank of Seoul National University. When the vertical drift angle changes, the gravity load on the sensors were changed. The hydrodynamic forces were deduced by subtracting the gravity load from the measured forces. The hydrodynamic coefficients were calculated using the least-square method. The simulation of the turning circle test was compared with the free-running model test result, and the error of the turning radius was 8.3 % compared to the free-running model test.

Estimation of Wave Energy Extraction Efficiency for a Compact Array System of Small Buoys (밀집 배열 부이시스템의 파랑에너지 추출 효율 추정)

  • Choi, Yoon-Rak
    • Journal of Ocean Engineering and Technology
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    • v.25 no.1
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    • pp.8-13
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    • 2011
  • A compact array system of small buoys is used for wave energy extraction. To evaluate the performance of this system, hydrodynamic analysis is carried out in regular waves using the higher order boundary element method. The motion response of each buoy is calculated considering hydrodynamic interactions caused by other buoys. The effect of energy extraction device is modeled as a linear damping load. The efficiencies of energy conversion are compared using the various sizes and arrangements of the array system and the damping coefficients for energy extraction. The increase in size or the packing ratio of the system gives better efficiency. However, the wave condition and the cost for the system should be considered to optimize performance from the perspective of engineering and economics. The proposed nondimensionalized damping coefficient for energy extraction is 0.1~0.5.

A Note on the Strip Methods associated with Ship Motion Problems (선체운동(船體運動)에 관(關)한 Strip Method의 일고찰(一考察))

  • Y.J.,Kwon;J.H.,Hwang
    • Bulletin of the Society of Naval Architects of Korea
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    • v.8 no.1
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    • pp.17-28
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    • 1971
  • The coefficients of equations of heave, pitch and coupled motion are evaluated for the small typical fishing boat(KIST-MARK Fishing Boat) with transom stern in regular head sea. And the results of computations based on eight models of strip theory are compared one another for the forward speed Froude number 0.30. There are some distinctive differences among those theories for the hydrodynamic and coupling coefficients. The former seems to be caused by the effects of the transom stern and the latter of the foward speed.

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Hydrodynamic forces on blocks and vertical wall on a step bottom

  • Mondal, Ramnarayan;Alam, Md. Mahbub
    • Wind and Structures
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    • v.30 no.5
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    • pp.485-497
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    • 2020
  • A study, using potential water wave theory, is conducted on the oblique water wave motion over two fixed submerged rectangular blocks (breakwaters) placed over a finite step bottom. We have considered infinite and semi-infinite fluid domains. In both domains, the Fourier expansion method is employed to obtain the velocity potentials explicitly in terms of the infinite Fourier series. The unknown coefficients appearing in the velocity potentials are determined by the eigenfunction expansion matching method at the interfaces. The derived velocity potentials are used to compute the hydrodynamic horizontal and vertical forces acting on the submerged blocks for different values of block thickness, gap spacing between the two blocks, and submergence depth of the upper block from the mean free surface. In addition, the wave load on the vertical wall is computed in the case of the semi-infinite fluid domain for different values of blocks width and the incident wave angle. It is observed that the amplitudes of hydrodynamic forces are negligible for larger values of the wavenumber. Furthermore, the upper block experiences a higher hydrodynamic force than the lower block, regardless of the gap spacing, submergence depth, and block thickness.

A Study on the Maneuvering Hydrodynamic Derivatives Estimation Applied the Stern Shape of a Vessel (선미 형상을 반영한 조종 유체력 미계수 추정에 관한 연구)

  • Yoon, Seung-Bae;Kim, Dong-Young;Kim, Sang-Hyun
    • Journal of the Society of Naval Architects of Korea
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    • v.53 no.1
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    • pp.76-83
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    • 2016
  • The various model tests are carried out to estimate and verify a ship performance in the design stage. But in view of the cost, the model test should be applied to every project vessel is very inefficient. Therefore, other methods of predicting the maneuverability with confined data are required at the initial design stage. The purpose of this study is to estimate the hydrodynamic derivatives by using the multiple regression analysis and PMM test data. The characteristics of the stern shape which has an important effect on the maneuverability are applied to the regression analysis in this study. The correlation analysis is performed to select the proper hull form coefficients and stern shape factors used as the variables in the regression analysis. The comparative analysis of estimate results and model test results is conducted on two ships to investigate the effectiveness of the maneuvering hydrodynamic derivatives estimation applied the stern shape. Through the present study, it is verified that the estimation using the stern shape factors as the variables are valid when the stern shape factors are located in the center of the database.

Prediction of Ship Manoeuvrability in Initial Design Stage Using CFD Based Calculation

  • Cho, Yu-Rim;Yoon, Bum-Sang;Yum, Deuk-Joon;Lee, Myen-Sik
    • Journal of Ship and Ocean Technology
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    • v.11 no.1
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    • pp.11-24
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    • 2007
  • Better prediction of a ship's manouevrabilty in initial design stage is becoming more, important as IMO manoeuvring criteria has been activated in the year of 2004. In the present study, in order to obtain more exact and reliable results for ship manoeuvrability in the initial design stage, numerical simulation is carried out by use of RANS equation based calculation of hydrodynamic forces exerted upon the ship hull. Other forces such as rudder force and propeller force are estimated by one of the empirical models recommended by MMG Group. Calculated hydrodynamic force coefficients are compared with those obtained by empirical models. Standard manoeuvring simulations such as turning circle and zig-zag are also carried out for a medium size Product Carrier and the results are compared with those of pure empirical models and manoeuvring sea trial. Generally good qualitative agreement is obtained in hydrodynamic forces due to steady oblique motion and steady turning motion between the results of CFD calculation and those of MMG model, which is based on empirical formulas. The results of standard manoeuvring simulation also show good agreement with sea trial results.