Byoung-Kwon Ahn

A Numerical Analysis of the Supercavitating Flow around Three-Dimensional Axisymmetric Cavitators

Recently submerged objects moving at high speed such as a supercavitating torpedo have been studied for their practical advantage of the dramatic drag reduction. In this study, we are focusing our attention on supercavitating flows around axisymmetric cavitators; a numerical method based on inviscid flow is developed and predicted supercavities around several shapes of 2D and 3D cavitators are presented. The results are validated by comparison of existing theoretical and empirical results. In addition, characteristic features of supercavity shapes and drag forces acting on a real scale torpedo are evaluated according to practically appropriate operating conditions.

Numerical Analysis of Axisymmetric Supercavitating Flows

Recently submerged objects moving at high speed such as a supercavitating torpedo have been studied for their practical advantage of the dramatic drag reduction. In this study we are focusing our attention on supercavitating flows around axisymmetric cavitators; a numerical method based on inviscid flow is developed and predicted supercavities around several shapes of cavitators are presented. The results are validated by comparison of existing theoretical and empirical results. In addition, characteristic features of supercavity shapes and drag forces acting on the cavitator are evaluated according to practically appropriate operating conditions.

An Experimental Study on Noise Characteristics of Propeller Cavitation Inception

Cavitation is the formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapor pressure. Various types of cavitations are generated on the propeller blades. As cavity bubbles passing the blade are forced to oscillate in size or shape and come to collapse, they cause very strong local acoustic waves in the fluid and radiate noise. Comparing the Sound Pressure Level(SPL) before and after cavitation, SPL increases 2dB per 1 knot increase in ship speed above the cavitation inception speed(CIS). Consequently, the CIS is an important criteria to design silent propellers. In this work, experimental measurements of radiated noise according to various types of cavitations from the model propeller are carried out in a large cavitation tunnel and their acoustical characteristics are extensively investigated.

Propeller Tip Vortex Cavitation Control Using Water Injection

As considerable interests in noise emission from the ships have been increased, control of the propeller cavitation generating vibration and radiating noise is looming large. In general, the tip vortex cavitation is first produced in case of full scale propellers, and noise levels rise dramatically from that moment. In order to reduce induced noise from the tip vortex cavitation and hence increase the cavity inception speed, we propose the mass injection method. Water injected from the propeller tip decreases rotating speed of the tip flow, and it restrains growing the tip vortex cavity. Experimental investigations of the model tests carried out in a large cavitation tunnel show that the tip vortex cavitation is effectively controled by water injection from the propeller tip.

A numerical and experimental study on the drag of a cavitating underwater vehicle in cavitation tunnel

Abstract For Super-Cavitating Underwater Vehicles (SCUV), the numerical analyses and experiments in a large cavitation tunnel are carried out at relatively large Reynolds numbers. The numerical results agree well with experiments and the drag coefficient of SCUV is rarely changed by the Reynolds number. As the cavitation number is decreased, the cavity occurs and grows, the cavitator drag decreases and the body drag is affected by the degree of covering the body with the cavity. The tunnel effects, i.e. the blockage and the friction pressure drop of the tunnel, on the drag and the cavitation of SCUV are examined from the numerical results in between the tunnel and unbounded flows. In the tunnel, a minimum cavitation number exists and the drag of SCUV appears larger than that in unbounded flow. When the super-cavity covers the entire body, the friction drag almost disappears and the total drag of SCUV can be regarded as the pressure drag of cavitator.

Wake Roll-up Modeling and Steady Hydrodynamic Analysis of Marine Propellers Using a B-Spline Based Higher-Order Panel Method

AbstractA numerical model for the analysis of the marine propeller incl uding wake roll-up is presented. In this study, we apply a higher-order panel method, which is based on a B-spine representation for both generations of the propeller ge ometry and hydrodynamic solutions, to predict the flow around the propeller blades. The present model is validated by comparison of the experimental measurements. The results show that the present method is able to predict the improved pressure distributions on the blad e surface, especially very close to propeller tip regions, where other panel methods witho ut the wake roll-up model give erroneous results. ※Keywords: B-spline(B-스플라인), Higher-order panel method(고차패널법), Propeller steady flow(프로펠러 정상유동), Propeller wake(프로펠러 후류), Wake roll-up(후류감김) 1. 서 론 Hess and Smith(1964)의 선구적인 연구 이후프로펠러 유동을 해석하기 위해 많은 종류의 포텐 접수일: 2008년 2월 19일, 승인일: 2008년 6월 19일✝교신저자: csleepro@cnu.ac.kr, 042-821-6623 셜 유동 해석법이 개발되어 널리 사용되고 있으며 현재 대부분의 해석법들은 속도 포텐셜을 기저로한 패널법을 사용하고 있다. Kerwin and Lee(1978), Hoshino (1989), Kim et al.(1993)은 정상 상태의 프로펠러 문제에 적용하였으며, Hsin(1990), Moon et al.(1994)은 비정상 프로펠

A Study on Natural Supercavitation and Drag Characteristics of Axisymmetric Cavitators

수중에서 이동하는 운동체의 속도가 빨라지면 운동체 주위의 국부압력이 유체의 증기압(vapor pressure)보다 낮아지고, 이로 인해 그 주변 유체가 기화되는 공동(cavitation, 캐비테이션) 현 상이 발생한다. 여기서 운동체의 이동 속도가 더욱 증가하여 공 동이 물체의 형상을 덮을 만큼 성장하게 되면 이를 자연적으로 발생한 초월공동, 즉 자연 초월공동(natural supercavitation)이라 한다. 수중 운동체가 초월공동으로 둘러싸임에 따라 몸체는 물과 의 직접적인 접촉으로 인한 마찰저항의 영향을 받지 않게 되고 전체 항력이 극적으로 감소함에 따라 운동체는 고속 운항이 가능 하게 된다. 이러한 초월공동을 이용한 수중 운동체의 고속화 기 술에 대한 개념은 2차 세계대전 직후인 1940년대 말부터 정립되 기 시작하였으며, 1950년대에 걸쳐 기초적인 연구가 수행되었 다. Self and Ripken (1955)은 자유 낙하하는 수류 터널(free-jet water tunnel) 실험을 통해 축대칭 물체에서 발생하는 초월공동 의 발생 과정을 관측하였고, Waid (1957)는 2차원 캐비테이터에 서 발생하는 부분공동과 초월공동의 형상을 계측하였다. 또한 Tulin (1953), Wu (1955), Garabedian (1956) 등은 초월공동 특 성에 관한 이론해석 방법들을 제시한 바 있다. 그 후 초월공동 기술이 어뢰와 같은 군사적인 목적으로 개발 되기 시작하면서 이와 관련된 연구 결과들이 제한적으로 공개되 어 왔다 (Serebryakov, 1972). 러시아는 1990년대 후반에 Shkval이라는 초월공동 어뢰를 개발하여 실전에 배치 한 것으로 알려져 있다. Shkval의 속도는 최소 200 knots 이상으로 현재 운 용되고 있는 어뢰의 약 5배 이상의 속도이다. 때문에 초월공동을 이용한 수중운동체는 기술 경쟁 국가들 사이에서 차세대 전장 환 경을 변화시킬 미래 무기기술로 인식되고 있다. 또한 상대적으로 저속인 주행 단계에서도 캐비테이터 후방에서 압축가스를 분사 시킴으로써 초월공동을 생성시키는 인공 초월공동(artificial supercavitation) 기술은 기존의 기술들과 함께 최근 들어 활발 축대칭 캐비테이터에서 발생하는 자연 초월공동과

Experimental Study on Artificial Supercavitation of the High Speed Torpedo

Recently supercavitating underwater torpedo moving at high speed (over 200 knots) has been interested for their practical advantage of the dramatic drag reduction. Cavitator located in front of the torpedo plays an important role to generate a natural supercavity and control the motion of the object. Supercavity can be created artificially by injection of compressed gas from the rear of the cavitator at a relatively low speed. In this paper, we investigated physical characteristics of artificial supercavities through cavitation tunnel experiments. One of the main focuses of the study was to measure pressure inside the cavity, and examined variation of the gravity effects appearing according to different amount of injected air. It was also found that a stable supercavity could be sustained at injection rates less than that required to form the stable supercavity because of hysteresis effect.

Numerical Analysis of Supercavitating Flows Based on Viscous/Inviscid Method

Recently supercavitating torpedo has been studied because of its high-speed performance as the next generation of underwater weapon systems. In this study, we present a numerical method based on the potential flow. Characteristic features of the shape of supercavities and drag forces are investigated. In addition, we introduce a viscous-potential method to compensate for the effects of viscosity. The results are compared with viscous calculations using a commercial package, FLUENT V13.

Numerical Analysis of Unsteady Cavitating Vortex around Two-dimensional Wedge-shaped Submerged Body

Unlike a slender body, vortices are shed off alternately in the wake of a blunt body. In the case of liquid flows, when the pressure falls below the vapor pressure, cavitation occurs in the vortex core and affects the formation of the vortex street. This phenomenon is of major importance in many practical cases because the alternate shedding of vortices creates imbalanced forces on the body. Hence, it is very important to determine the shedding frequency of cavitating vortices. In this paper, the unsteady cavitating flow around a two-dimensional wedge-shaped submerged body was simulated using the commercial code STAR-CCM+. A numerical investigation of the structure of cavitating vortices was performed for a model with an apex angle of 20°. The results were validated by comparing them with experimental measurements carried out at a cavitation tunnel of Chungnam National University (CNU-CT). It was found that the shedding frequency of the vortex increased by up to 18%, which was strongly affected by the development of cavitation. Received 19 December 2017, revised 18 January 2018, accepted 1 February 2018 Corresponding author Byoung-Kwon Ahn: +82-42-821-6625, bkahn@cnu.ac.kr, ORCID: https://orcid.org/0000-0002-0339-6069 It is noted that this paper is revised edition based on proceedings of SNAK 2017 in Yeosu c 2018, The Korean Society of Ocean Engineers This is an open access article distributed under the terms of the creative commons attribution non-commercial license (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.