Jung-Chun Suh

Function approximations by superimposing genetic programming trees: with applications to engineering problems

Abstract This paper concerns fundamental issues regarding genetic programming (GP) as a tool for real-valued function approximations. Standard GP suffers from the lack of estimation techniques for numerical parameters of a functional tree. Unlike other research activities, where non-linear optimization techniques are employed, we adopt the use of a linear associative memory for the estimation of these parameters under the GP algorithm. Instead of dealing with a large associative matrix, we present the method of building several associative matrices in small size, each of which is responsible for determining the value for different small portions of the whole parameter. This approach can significantly reduce computational cost, and a reasonably accurate value for parameters can be obtained. Due to the fact that the GP algorithm is likely to fall into a local minimum, the GP algorithm often fails to generate the functional tree with the desired accuracy. This motivates us to devise a group of additive genetic programming trees (GAGPT) which consists of a primary tree and a set of auxiliary trees. The output of the GAGPT is the summation of outputs of the primary tree and all auxiliary trees. The addition of auxiliary trees makes it possible to improve both the learning and generalization capability of the GAGPT, since the auxiliary tree evolves towards refining the quality of the GAGPT by optimizing its fitness function. The effectiveness of our approach is verified by applying the GAGPT to the estimation of the principal dimensions of a bulk cargo ship and engine torque of a passenger car.

A vorticity–velocity formulation for solving the two-dimensional Navier–Stokes equations

This paper describes a vorticity-based integro-differential formulation for the numerical solution of the two-dimensional incompressible Navier–Stokes equations. A finite volume scheme is implemented to solve the vorticity transport equation with a vorticity boundary condition. The Biot–Savart integral is evaluated to compute the velocity field from a vorticity distribution over a fluid domain. The Greens scalar identity is employed to solve the total pressure in an integral approach. Global coupling between the vorticity and the pressure boundary conditions is considered when this integro-differential approach is employed. For the early stage development of the flow about an impulsively started circular cylinder, the computational results with our numerical method are compared with known analytical solutions in order to validate the present formulation.

Numerical investigation on vortex shedding from a hydrofoil with a beveled trailing edge

To better understand the vortex shedding mechanism and to assess the capability of our numerical methodology, we conducted numerical investigations of vortex shedding from truncated and oblique trailing edges of a modified NACA 0009 hydrofoil. The hybrid particle-mesh method and the vorticity-based subgrid scale model were employed to simulate these turbulent wake flows. The hybrid particle-mesh method combines the vortex-in-cell and the penalization methods. We have implemented numerical schemes to more efficiently use available computational resources. In this study, we numerically investigated vortex shedding from various beveled trailing edges at a Reynolds number of 106. We then compared the numerical results with the experimental data, which show good agreement. We also conducted numerical simulations of wakes behind the hydrofoil at rest in periodically varying flows. Results reveal that vortex shedding is affected by the periodicity of a free-stream flow, as well as the trailing-edge shape.

Computation of pressure fields around a two-dimensional circular cylinder using the vortex-in-cell and penalization methods

The vorticity-velocity formulation of the Navier-Stokes equations allows purely kinematical problems to be decoupled from the pressure term, since the pressure is eliminated by applying the curl operator. The Vortex-In-Cell (VIC) method, which is based on the vorticity-velocity formulation, offers particle-mesh algorithms to numerically simulate flows past a solid body. The penalization method is used to enforce boundary conditions at a body surface with a decoupling between body boundaries and computational grids. Its main advantage is a highly efficient implementation for solid boundaries of arbitrary complexity on Cartesian grids. We present an efficient algorithm to numerically implement the vorticity-velocity-pressure formulation including a penalty term to simulate the pressure fields around a solid body. In vorticity-based methods, pressure field can be independently computed from the solution procedure for vorticity. This clearly simplifies the implementation and reduces the computational cost. Obtaining the pressure field at any fixed time represents the most challenging goal of this study. We validate the implementation by numerical simulations of an incompressible viscous flow around an impulsively started circular cylinder in a wide range of Reynolds numbers: Re = 40, 550, 3000, and 9500.

Rudder Gap Cavitation and Its Suppression Devices

Development of rudder gap flow blocking device for lift augmentation and cavitation suppression is presented. In order to verify the performance of this device, cavitation visualization and surface pressure measurements were carried out in a cavitation tunnel. Numerical simulations were conducted using a computational fluid dynamics code for more rigorous verification. The new rudder system is equipped with cam devices, which effectively close the gap between the horn/pintle and movable wing parts. The experimental and computational results show that the proposed rudder system is superior to the conventional rudder systems in terms of the lift augmentation and cavitation suppression.© 2008 ASME

Numerical Study of Non-Cavitating Underwater Propeller Noise

Non-cavitating noise of underwater propeller is numerically investigated. The main purpose is to analyze non-cavitating noise in various operating conditions with different configurations. The noise is predicted using time-domain acoustic analogy and boundary element method. The flow field is analyzed with potential based panel method, and then the time-dependent pressure data are used as the input for Ffowcs-Williams Hawkings formulation to predict the far-field acoustics. Boundary element method is also considered to investigate the effect of ducted propellers. Sound deflection and scattering effect on the duct is considered with the BEM. The governing equations are based on the assumption that all acoustic pressure is linear. A scattering approach is applied in which the acoustic pressure field is split into the known incident component and the unknown scattered component. Noise prediction results are presented for single propeller and ducted propeller in non-uniform flow conditions. The investigation reveals that the effect of a duct on the acoustic performance of propellers is small in the far field under non-cavitating situations since the noise directivities of single and ducted propeller are almost the same. Only the high order BPFs are influenced by the existence of the duct.

Further validation of the hybrid particle-mesh method for vortex shedding flow simulations

Abstract This is the continuation of a numerical study on vortex shedding from a blunt trailing-edge of a hydrofoil. In our previous work (Lee et al., 2015), numerical schemes for efficient computations were successfully implemented; i.e. multiple domains, the approximation of domain boundary conditions using cubic spline functions, and particle-based domain decomposition for better load balancing. In this study, numerical results through a hybrid particle-mesh method which adopts the Vortex-In-Cell (VIC) method and the Brinkman penalization model are further rigorously validated through comparison to experimental data at the Reynolds number of 2 × 106. The effects of changes in numerical parameters are also explored herein. We find that the present numerical method enables us to reasonably simulate vortex shedding phenomenon, as well as turbulent wakes of a hydrofoil.

Design of Propeller Geometry Using Blade Sections Adapted to Surface Streamlines

In this paper, we suggest a design concept of defining the propeller geometry by stacking up the blade sections aligned with propeller surface streamlines. Numerical and experimental propeller open water(P.O.W.) characteristics of a newly designed propeller are presented. The surface streamlines for a propeller are obtained by using the panel method. Redefinition of the blade sections aligned with the streamlines is provided together with 8-spline modeling, by which we manufacture model propellers. We carried out the P.O.W, tests in a towing tank in order to show the effect of the present method on P.O.W. characteristics.

Vortex Shedding Frequency for a 2D Hydrofoil with a Truncated Trailing Edge

프로펠러 날개 뒷날에서 발생하는 명음(singing) 현상에 대한 연구는 선박유체 분야에서 오랜 시간 동안 다루어지고 있는 연구 주제 중의 하나이다. 프로펠러 명음 현상은 날개 뒷날에서 박리 되는 와도 흘림(vortex shedding)과 밀접한 관계가 있다. 명음 현 상은 와도 박리의 발생 주파수와 날개 고유 주파수가 서로 일치 하는 경우에 공진 현상에 의해 발생한다. 일반적으로 진동계는 외부 가진에 의해 진동이 발생하지만, 외부 가진 없이 진동이 유 발되기도 하며 이러한 진동을 자려 진동(self-excited vibration) 이라 한다. 대표적인 예는 전선이나 해저 송유관 등에서 발생하 는 갤러핑(galloping) 현상이며 명음 현상 역시 자려 진동에 해당 된다. 와도 흘림 현상에 의한 자려 진동은 일정한 유입속도 영역 에서 특정 주파수 특성을 가지는 구속(lock-in) 현상이 발생함이 보고되고 있다 (Ausoni, 2009; Ahn, et al., 2009). 명음을 방지하기 위해서는 날개의 고유 진동수가 와류 발생 주파수와 불일치되도록 하여야 한다. 최근 수중익 선박이나 프로 펠러 추진 선박의 경우 특정 운용속도 영역에서 이러한 명음이 발생하는 사례가 보고되고 있다. 명음 발생은 날개의 국부 공진 현상으로 기계적으로 동일하게 가공된 프로펠러 날개일지라도 특정 날개에서만 발생하기도 한다. 명음의 발생 조건은 아직 명 확히 밝혀지지 않아 설계단계에서 이를 방지할 수 있는 대책이 반영되지 못하고 있다 (Ahn, et al., 2009). 명음이 발생하였을 때는 경험적으로 연삭하거나 재가공하여 박리 와류의 특성을 바 꾸어 문제를 해결하고 있다 (Kim & Chung, 1994). 본 논문에서는 명음 현상의 발생 메커니즘을 이해하기 위한 연 구의 일환으로 2차원 날개 뒷날 근처에서의 와류 유동을 수치적 으로 모사하고 이를 통해 와도 흘림 주파수에 대한 특성을 파악하 고자 한다. 유동 모사에 사용된 수치해석 방법은 라그랑지안 보텍 스 방법(Lagrangian vortex method)을 기반으로 한 하이브리드 입자-격자법(hybrid particle-mesh method)를 사용한다. 보텍스 방법은 와도를 기본변수로 사용하기 때문에 강한 와류 유동 해석 에 있어서 유리한 점이 있으며, 무한 원방 조건이 자동으로 만족 pISSN:1225-1143, Vol. 51, No. 6, pp. 480-488, December 2014

An Experimental Study on Tip Vortex Cavitation Suppression in a Marine Propeller

Normally, tip vortex cavitation (TVC) is first observed at a certain location behind the tips of propeller blades. Therefore, TVC is important for naval ships and research vessels that require raising the cavitation inception speed to maximum possible values. The concepts for alleviating the tip vortex are summarized by Platzer and Souders (1979), who carried out a thorough literature survey. Active control of TVC involves the injection of a polymer or water from the blade tip. The main effect of such mass injection (both water and polymer solutions) into the vortex core is an increase in the core radius, consequently delaying TVC inception. However, the location of the injection port needs to be selected with great care to ensure that the mass injection is effective in delaying TVC inception. In the present study, the authors propose a semiactive control scheme that is achieved by attaching a thread at the propeller tip. The main idea of a semiactive control is that because of its flexibility, the attached thread can be sucked into the low-pressure region closer to the vortex core center. An experimental study using a scale model was carried out in the cavitation tunnel at the Seoul National University. It was found that a flexible thread can effectively suppress the occurrence of TVC under the design condition for a model propeller.