Hyung-Il Kwon

A Preliminary Study of Open Rotor Design Using a Harmonic Balance Method

An open rotor is one of the next generation aero-engines as it has 30% higher efficiency compared to the conventional turbofan engines. However, a high level of noise loudness has been a major drawback of the open rotor for its commercial use in aviation market. Although there have been a number of efforts to reduce its noise level, an accurate prediction of unsteady and complex flow field of the open rotor makes it difficult for the design methodologies to be applied. This paper introduces one of the state-of-the-art design methodologies to handle the unsteady problem of low-noise open rotor design. A harmonic balance method which is an order of magnitude more efficient than the conventional timeaccurate CFD method is used to analyze open rotor flows. To demonstrate the accuracy of the harmonic balance method, a wind-tunnel experiment of the scaled model of the open rotor is carried out and aerodynamic performances are compared with the harmonic balance predictions. With the steady formulation of the flow governing equations through the harmonic balance method, a design method using a surrogate model is employed to find an optimum configuration that minimizes the noise level and total power at a constant thrust level. A noise prediction is computed using the Farassat formula, derived from the FfowcsWillimas Hawkings equation. Design variables of the blade radii, rotor spacing, and the pitch angle variation of the aft rotor are chosen. A parameter study to investigate the sensitivities of the design parameters to thrust and torque/power levels as well as to the noise loudness is carried out. A genetic algorithm to handle multi-objectives is used in combination with the surrogate model of Kriging response surface. An optimum configuration is obtained from the pareto front of the optimization results and shows the reduction of noise level by 7dB and power level by 4% from the baseline values.

Design of Efficient Propellers Using Variable-Fidelity Aerodynamic Analysis and Multilevel Optimization

A multilevel design optimization framework was developed for the aerodynamic design of an electric aerial vehicle propeller in cruise conditions. The objective was to determine the optimum propeller shape to minimize torque at a given required thrust level and thus maximize overall propeller efficiency. A key concept of the design is the sequential application of a three-dimensional planform and two-dimensional section designs iteratively to make the best use of the complementary characteristics of gradient-free and gradient-based optimization strategies and the corresponding parameterization of the design space. Variable-fidelity aerodynamic analyses of blade element momentum theory and Navier–Stokes solutions were used to achieve computational efficiency and high accuracy. First, the optimal planform shape was determined by adjusting radius, twist angle, and chord lengths of the blade. Subsequently, the sectional airfoil design was performed at several spanwise locations. Given the new airfoil sections,...

Surrogate-Based Robust Optimization and Design to Unsteady Low-Noise Open Rotors

An efficient, robust design optimization method based on surrogate models was developed and applied to a practical design problem of a low-noise, coaxial contrarotating rotor. A classical Monte Carlo simulation was carried out at greatly reduced computational cost through dual-level Kriging surrogate models, and it determines the variance of the simulation for uncertainty quantification. The accuracy and numerical stability of the Kriging model were improved by a mathematical indicator of the function trend. The numerical instabilities of the Kriging model in finding distribution parameters were also reduced by the approaches of penalization and cross-validation in a maximum likelihood evaluation. Finally, the robust design optimization framework was applied to a low-noise DLR, German Aerospace Center contrarotating open rotor at a nominal takeoff condition. The objective function was to minimize an overall sound power level at the baseline thrust level. An efficient computational-fluid-dynamics approach ...

Efficient Global Optimization using a Multi-point and Multi-objective Infill Sampling Criteria

An efficient optimization framework for the practical application of aerodynamic design is presented in this study. A multi-point and multi-objective approach infill sampling criteria (MPMO ISC) is proposed to enhance efficient global optimization (EGO) performances and best utilize parallel computing resources. A new infill sampling strategy is devised on consideration of efficient way to infill the design space is appropriate tradeoff between exploitation and exploration. It can be solved with the multi-objective approach that one objective is to search the region with high uncertainty and the other is to find the optimum point. A strategy of MPMO ISC is to determine the multiple additional points for training of surrogate model by picking the several populations on the pareto-frontier set. The EGO design framework containing the Kriging model as surrogate model, the non-dominated sorting genetic algorithm as optimizer and the MPMO ISC method is validated using the analytic function to prove accuracy and efficiency of the framework. The proposed design framework is applied for the aerodynamic design of airfoil which objective is to minimize drag of RAE 2822 airfoil, maintaining the reference lift. A total 10 design variables are used to provide variations to the airfoil shape. Consequently, an optimal airfoil shape is obtained and 25% reduction of drag is observed.

Aerodynamic Design of EAV Propeller using a Multi-Level Design Optimization Framework

본 연구에서는 고효율 풍력 발전 시스템 개발을 위하여 풍력 발전용 블레이드의 토크를 향상 시키도록 단면 최적 설계를 수행하였다. 특히 블레이드 주위의 공기역학적 특성이 만들어내는 탄성 구조 변형을 고려할 수 있도록 전산구조해석과 전산공력해석을 연계한 공력-구조 연계 해석을 수행하였다. 이 연계 해석 프로그램을 이용하여 구조 변형 정도와 내리씻김 효과를 예측하고 이를 바탕으로 설계 단면의 유동 조건을 산출하였다. 또한 설계 후에는 이 공력-구조 연계 해석 프로그램을 이용하여 최적 설계안에 대한 검증 해석을 수행하였다. 단면 최적 설계 프레임워크에는 천이 효과를 고려한 2차원 비압축성 N-S 해석 프로그램과 구배율 기반의 최적화 알고리즘, PARSEC 형상함수를 포함시켜, 반복 설계 과정에서 정확한 해석을 바탕으로 효율적으로 최적점의 탐색이 수행되도록 하였다. NREL Phase VI 로터 블레이드의 실험 데이터와의 비교를 통해 전산공력해석과 구조해석 결과에 대한 검증을 수행하였으며 동일한 블레이드와 NREL 5MW급 블레이드의 단면 최적 설계를 수행하였다. 설계 결과, NREL Phase VI와 5MW 블레이드의 토크 성능이 각각 14.8%, 4.2% 향상되었다.

Development of an Engineering Education Framework for Aerodynamic Shape Optimization

Design optimization is a mathematical process to find an optimal solution through the use of formal optimization algorithms. Design plays a vital role in the engineering field; therefore, using design tools in education and research is becoming more and more important. Recently, numerical design optimization in fluid mechanics, which uses computational fluid dynamics (CFD), has numerous applications in the engineering field, because of the rapid development of high-performance computing resources. However, it is difficult to find design optimization software and contents for educational purposes in aerospace engineering. In the present study, we have developed an aerodynamic design framework specifically for an airfoil, based on the EDucation-research Integration through Simulation On the Net (EDISON) portal. The airfoil design framework is composed of three subparts: a geometry kernel, CFD flow analysis, and an optimization algorithm. Through a seamless interface among the subparts, an iterative design process is conducted. In addition, the CFD flow analysis and the design framework are provided through a web-based portal system, while the computation is taken care of by a supercomputing facility. In addition to the software development, educational contents are developed for lectures associated with design optimization in aerospace and mechanical engineering education programs. The software and content developed in this study is expected to be used as a tool for e-learning material, for education and research in universities.