Seulgi Yi

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,...

Design of a Low-Noise Open Rotor Using an Implicit Harmonic Balance Method

A coaxial contra-rotating open rotor belongs to the next generation of aero-engines. It has an efficiency that is about 30% higher than that of a conventional turbojet engine. However, because of the high noise level, the open rotor has not been introduced into the commercial aviation market. Although there have been numerous efforts to reduce its noise level, the need to accurately predict the unsteady and complex flow field around the open rotor makes it difficult to apply the conventional design methodologies. In this paper, we introduce a state-of-the-art design methodology for solving the unsteady flow field problem of the lownoise open rotor design. A harmonic balance method that is an order of magnitude more efficient than the conventional time accurate CFD method is used to predict the aerodynamic performance of the open rotor. With the accurate formulation of the governing equations through the harmonic balance method, a design method that uses a surrogate model is employed to find optimum configuration that minimizes the noise level and total power at a constant thrust level. A noise prediction is made using the Farassat formula, derived from the Ffowcs-Williams-Hawking’s equation. To efficiently search for the optimum configuration, the design optimization is divided into the rotor topology design level and blade planform design level. In a previous study, we investigated the optimum rotor topology parameters such as the blade radii, rotor spacing, and pitch angle of the aft rotor. In this paper, an investigation is conducted to determine the optimum planform variables such as the twist angle, chord length for several design sections, and tip shape control parameters for the aft rotor. A genetic algorithm is used as a multi-objective optimization algorithm in combination with the Kriging surrogate model. Through the planform design for the aft rotor, the noise level and power consumption of the optimum rotor are reduced by 0.6 dB and 6.8% respectively.

Adaptive Variable-Fidelity Analysis and Design Using Dynamic Fidelity Indicators

The purpose of the current study is to develop a decision-making model of a dynamic fidelity indicator (DFI) for adaptive variable-fidelity (VF) analysis and to apply it to practical engineering problems, including computational design optimization. The biggest advantage is considerable computation efficiency while satisfying solution accuracy. Bayesian-based DFI and difference-based DFI are formulated as probabilistic model validation metrics to quantify model-form uncertainties attributed to the VF analysis. Two adaptive VF surrogate models of the VF kriging with regression and sample reinterpolation and the simple VF kriging with an additive bridge function are developed. The VF kriging models are integrated with the DFIs into an efficient global optimization design framework for the design problems, which was further improved by the multipoint and multi-objective infill sampling criteria. Validation of the developed framework is carried out with two analytic functions for solution optimality and compu...

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.

Gradient Based Optimization using Spectral Formulation-Based FSI and Coupled Sensitivity Analysis

In this study, gradient based optimization using efficient computational methods for fluid-structure interaction and corresponding coupled sensitivity analysis. Considering that many dynamic aeroelatic problems are periodic in nature, a spectral based formulation has been used to solve them. A harmonic balance method (HBM) is used for computational fluid dynamics and modal analysis based reduced order model is employed for computational structural dynamics. One of the advantages of the spectral based formulation is the computational efficiency obtained by eliminating transient flow solutions to reach a periodic steady state, through the solution approximation of a discrete Fourier series. Hence, dynamic aeroelastic problems are solved by the solution methods for the static aeroelastic problems. The spectral form of the governing equation also facilitate the application of the steady form of adjoint sensitivity analysis for the dynamically coupled system. Hence, the high memory and computational time required for unsteady adjoint sensitivity analysis are avoided in the current study by directly using the steady adjoint formulation. Using the adjoint sensitivity analysis formulation, the shape sensitivity of the AGARD 445.6 wing has been computed and has been validated by comparing with FDM based results. The wing surface has been parameterized using the amplitude of Hicks-Henne functions on the wing surface as the design parameters.

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.

Spectral Formulation-Based FSI and Coupled Sensitivity Analysis for Dyanmic Aeroelastic Problems

Efficient computational methods for fluid-structure interaction and corresponding coupled sensitivity analysis based on spectral formulation are introduced to solve dynamic aeroelastic problems. A time-spectral method is used for computational fluid dynamics and a modal analysis based finite element method is employed for computational structural dynamics. One of the advantages is computational efficiency by eliminating transient flow solutions to reach a periodic steady state, through the solution approximation of a discrete Fourier series. Through the spectral formulation of the FSI problems, dynamic aeroelastic problems are solved by the solution methods for the static aeroelastic problems. The biggest advantage is the availability of the steady form of the adjoint sensitivity analysis for the dynamically coupled system. Computational time and memory requirement for the unsteady adjoint sensitivity analysis are avoided in the current study by directly using the steady adjoint formulation in the spectral form of the governing equations of both fluids and structures. In this study, a practical threedimensional problem of wing flutter is be solved to show the validity of the proposed coupled-sensitivity analysis method in terms of solution accuracy and computational efficiency as well as optimality achieved for the design cases.