Byung Joon Lee

Optimizing a Boundary-Layer-Ingestion Offset Inlet by Discrete Adjoint Approach

A large amount of low-momentum boundary-layer flow ingesting into a flush-mounted inlet can cause significant total pressure loss and distortion to the extent beyond operability of a fan/compressor. To improve the quality of incoming flow into the engine, shape optimization of the surface geometry at the inlet entrance has been carried out using the discrete adjoint method; the inlet-floor shape is parameterized by the use of control points on B-spline surface patches. To resolve the complicated geometry flexibly and wall-bounded turbulent flow accurately, an overset mesh system is well-suited for integrating the flow analysis code, sensitivity analysis code, and grid modification tools. To enhance the convergence characteristics of the sensitivity analysis code, additional numerical dissipation for the discrete adjoint formulation is introduced. After using this optimization procedure, the new inlet yields a significant improvement in performance: a more than 50% reduction in flow distortion and a 3% increase in total pressure recovery. High performance at off-design conditions is also realized with only slight degradation, confirming the capability of the adjoint method for a practical design problem. Finally, the physical meaning and implication of the performance improvement are elaborated upon in relation to the flow characteristics resulting from the new design.

Aerodynamic Redesign Using Discrete Adjoint Approach on Overset Mesh System

An adjoint-based design approach for the delicate treatment of complex geometry is presented by using an overset mesh technique. Overset blocks, such as collar and tipcap grids, which are commonly used in accurate drag prediction, are employed to evaluate the applicability of the proposed design approach to practical problems. Various pre- and postprocessing techniques for overset flow and sensitivity analyses are implemented to develop a robust gradient-based optimization method on an overset mesh topology. In preprocessing, overlap optimization, which can provide an accurate overset solution and enhance convergence characteristics, is adopted to automatically construct the block connectivity. A new postprocessing method, the spline-boundary intersecting grid scheme, is introduced by reflecting the ratio of the surface cell area for accurate prediction of aerodynamic coefficients and a convenient evaluation of sensitivities under a parallel computing environment. For the sensitivity analysis, the adjoint formulation for the overset boundary condition is implemented in the fully hand-differentiated sensitivity analysis code. A three-dimensional discrete adjoint solver on the overset mesh system is developed by exploiting the overset flow analysis techniques. Good convergence characteristics of the adjoint solver can be achieved by using the automatic construction process of block connectivity. The derivatives of aerodynamic coefficients can be obtained by an efficient and accurate postprocessing technique. The present overset adjoint formulation and flow analysis techniques are validated by comparing the flow and sensitivity analyses, as well as the design results, with those of a single-block case for a transonic wing. Finally, careful designs are carried out by minimizing the drag of a three-dimensional wing-body configuration. The design results successfully demonstrate the capability of the present design approach.

Adjoint-Based Design Optimization of Vortex Generator in an S-Shaped Subsonic Inlet

This paper deals with an adjoint-based design optimization of vortex generators for the performance improvement of an S-shaped subsonic inlet, the Royal Aircraft Establishment intake model 2129. To enhance the flow quality entering the engine face maximally, the vortex generators are independently optimized with five design parameters per each vortex generator (a total of 55 design variables). To increase the design efficiency, the source term model of the vortex generator is employed. The original source term model, which does not reflect a small change in position and thus has difficulties in differentiation for sensitivity analysis, is modified into a differentiable source term model. To deal with a large number of design variables, the gradient-based design optimization method using the discrete adjoint approach is employed to minimize the distortion coefficient while maintaining the baseline total pressure recovery ratio. A total of five design cases are conducted to validate the proposed design approach, to obtain the optimized vortex generators, and to confirm their enhanced performance. Through the proposed design process,theperformanceofthetargetinletisremarkablyimproved,showingthatthedistortioncoefficientdecreases well over 70% while maintaining the total pressure recovery ratio.

Adjoint Based Design Approach for Boundary Layer Ingestion Offset Intake

The design of boundary layer ingestion offset intake is conducted by using a gradientbased optimization method. A large amount of boundary layer ingestion into flush-mounted intake on a configuration surface can cause total pressure loss and distortion. For improvement of the quality of the incoming flow into the engine, shape optimization of surface geometry at the inlet entrance is performed by using control points of B-Spline surface patch. In order to resolve the complicated flow phenomenon over a complicated geometry, the flow analysis code, sensitivity analysis code and grid modification tools are prepared for overset mesh system. Especially, overset based adjoint method is presented to deal with a lot of geometrical design parameters. For enhancing the convergence characteristics of sensitivity analysis code, a numerical dissipation for the discrete adjoint formulation is introduced. The performance of the designed geometry is evaluated by offdesign condition tests to show the capability of adjoint method for practical design problem. The physical meaning of the performance improvement by the design is elaborately investigated as well.

Unsteady Adjoint Approach for Design Optimization of Flapping Airfoils

This paper describes the work for optimizing the propulsive efficiency of flapping airfoils, i.e., improving the thrust under constraining aerodynamic work during the flapping flights by changing their shape and trajectory of motion with the unsteady discrete adjoint approach. For unsteady problems, it is essential to properly resolving time scales of motion under consideration and it must be compatible with the objective sought after. We include both the instantaneous and time-averaged (periodic) formulations in this study. For the design optimization with shape parameters or motion parameters, the time-averaged objective function is found to be more useful, while the instantaneous one is more suitable for flow control. The instantaneous objective function is operationally straightforward. On the other hand, the time-averaged objective function requires additional steps in the adjoint approach; the unsteady discrete adjoint equations for a periodic flow must be reformulated and the corresponding system of equations solved iteratively. We compare the design results from shape and trajectory optimizations and investigate the physical relevance of design variables to the flapping motion at on- and off-design conditions.

Multi-Stage Aerodynamic Design of Multi-Body Geometries via Global and Local Optimization Methods

*† ‡ § An efficient and high-fidelity design approach is proposed by combining global and local optimization methods for wing planform and surface design. For enhanced design results, aerodynamic shape optimization process is carried out via 2-stage with different optimization strategy. In the first stage, global optimization techniques are applied to planform design with a few geometric design variables. In the second stage, local optimization techniques are used for wing surface design with a lot of design variables to maintain a sufficient design space with high DOF (Degree of Freedom) geometric change. For global optimization, meta-modeling techniques such as RS (Response Surface) and Kriging methods are used in conjunction with Genetic Algorithm (GA). For local optimization, a discrete adjoint variable method is used. By the successive combination of global and local optimization techniques, drag minimization is performed for a multi-body aircraft configuration while maintaining the baseline lift and the wing weight at the same time. Through the design process, performances of the test models are remarkably improved in comparison with the single stage design approach. The capability of proposed design framework including wing planform design variables can be evaluated by the drag decomposition method which can provide improvement of induced drag and wave drag, respectively.

Characterization of Aerodynamic Performance of Boundary-Layer-Ingesting Inlet Under Crosswind

NASA has been studying future transport concepts, envisioned to be technically realizable in the timeframe of 2020–2030, to meet environmental and performance goals. One concept receiving considerable interest involves a propulsion system embedded into a hybrid wingbody aircraft. While offering significant advantages in fuel savings and noise reduction by this concept, there are several technical challenges that are not encountered in the current fleet and must be overcome so as to deliver target performance and operability. One of these challenges is associated with an inlet system that ingests a significantly thick boundary layer, developing along the wingbody surface, into a serpentine diffuser before the flow meeting fan blades. The flow is subject to considerable total pressure loss and distorted at the fan face, much more significantly than in the inlet system of conventional aircraft. In our previous studies [1, 2], we have shown that through innovative design changes on the airframe surface, it is possible to simultaneously increase total pressure recovery and decrease distortion in the flow, without resorting to conventional penalty-ridden flow control concepts, such as vortex generator or boundary layer bleeding/suction. In the current study, we are interested in understanding the following issues: how the embedded propulsion system performs under a crosswind condition by studying in detail the flow characteristics of two inlets, the baseline and another optimized previously under the cruise condition. With the insight, it is hoped that it can help in the follow-on study by devising effective strategies to minimize flow distortion arising from the integration of an embedded-engine system into an airframe to the level acceptable to the operation of engine fan.

Efficient Design Optimization of Vortex Generators in Subsonic Offset Inlet by Discrete Adjoint Approach

This paper deals with the adjoint-based design optimization of vortex generators for the performance improvement of the subsonic inlet. The RAE M2129, an S-shaped inlet, was selected as the target geometry. Because of the differences in radius of curvature between the port side (outer wall) and starboard side (inner wall), a centrifugal pressure gradient that induces circulatory flow and secondary flow was developed. To minimize these detrimental aerodynamic phenomena, a design optimization of vortex generators installed in subsonic inlet was conducted. To maximize the flow quality enhancement, the effectiveness and position of vortex generator are independently optimized by using the design parameters of height, chord length, angle of incidence, axial position, and the circumferential position of each vortex generator. To increase the design efficiency, the source term model of vortex generator was employed instead of the gridded vortex generator. Since the original source term model cannot reflect a small change in position and has difficulties in differentiation for adjoint method, a differentiable source term model was developed. To deal with a large number of design variables, the gradient-based design optimization method using the discrete adjoint approach was employed to minimize the distortion coefficient while maintaining the baseline total pressure recovery ratio. As a validation of the proposed design approach, optimization with three kinds of design parameters representing the effectiveness of each vortex generator (a total of 33 design variables) was carried out with the original source term model. The designed vortex generator configuration yields a substantial improvement in inlet performance, which confirms the validity of the proposed design approach. After the validation process, a complete design with five design parameters per each vortex generator (a total of 55 design variables) was performed by using the differentiable source term model. Through the complete design process, the performance of target inlet was remarkably improved; the distortion coefficient was decreased over 79% while maintaining the total pressure recovery ratio.

Multi-Stage Aerodynamic Design of Multi-body Geometries by Kriging-based models and Adjoint Variable Approach

An efficient and high-fidelity design approach for wing-body shape optimization is presented. Depending on the size of design space and the number of design of variable, aerodynamic shape optimization process is carried out via different optimization strategies at each design stage. In the first stage, global optimization techniques are applied to planform design with a few geometric design variables. In the second stage, local optimization techniques are used for wing surface design with a lot of design variables to maintain a sufficient design space with a high DOF (Degree of Freedom) geometric change. For global optimization, Kriging method in conjunction with Genetic Algorithm (GA) is used. A searching algorithm of EI (Expected Improvement) points is introduced to enhance the quality of global optimization for the wing-planform design. For local optimization, a discrete adjoint method is adopted. By the successive combination of global and local optimization techniques, drag minimization is performed for a multi-body aircraft configuration while maintaining the baseline lift and the wing weight at the same time. Through the design process, performances of the test models are remarkably improved in comparison with the single stage design approach. The performance of the proposed design framework including wing planform design variables can be efficiently evaluated by the drag decomposition method, which can examine the improvement of various drag components, such as induced drag, wave drag, viscous drag and profile drag.

Aerodyamic Optimization for the Subsonic S-Shaped Diffuser Using Two-Equation Turbulence Models

A parallelized design optimization approach is presented for a subsonic S-shaped inlet using aerodynamic sensitivity analysis. Two-equation turbulence model is adopted to predict the strong counter vortices in the S-shaped duct more precisely. Sensitivity analysis is performed for the three-dimensional NavierStokes equations coupled with two-equation turbulence models using a discrete adjoint method. For code validation, the result of the flow solver is compared with experiment data and bench marking data of other computation researches. To study the influence of turbulence models and grid refinement in the duct flow analysis, the results using several turbulence models are compared with each other on various grid systems. The adjoint variable code is validated by comparison with the finite difference results. The capability and the efficiency of the present design tools are successfully demonstrated in three-dimensional subsonic inlet flow analysis and design optimization.