Kwanjung Yee

Seoul National University | SNU · Department of Mechanical Aerospace Engineering

Since rotor blade sections experience significantly different flow conditions along the radial locations, proper optimization of the blade shapes are essential to achieve high-efficiency. However, most low-fidelity aerodynamic solvers used in optimization are limited because they require pre-calculated aerodynamic performance tables. This requireme...

Computational fluid dynamics (CFD) has widespread application in research and industry. The quality of the mesh, particularly in the boundary layer, significantly influences the CFD accuracy. Despite its importance, the mesh generation process remains manual and time intensive, with the introduction of potential errors and inconsistencies. The limi...

This study analyzes how the propeller wake affects wing icing characteristics from both unsteady and time-averaged perspectives. To this end, a three-dimensional quasi-unsteady simulation is employed to calculate airflow, droplet impingement, and ice accretion. High-order schemes and dual-time stepping are used on overset grids to solve for flow an...

This study evaluates the predictive capabilities of ICEPAC (Ice Contour Evaluation and Performance Analysis Code), developed at Seoul National University. Focusing on the 2nd AIAA Ice Prediction Workshop cases, simulations are conducted to compare experimental values for scallop ice shapes on a 3D hybrid wing and ice shapes on a low-speed wing. Add...

This study presents a numerical simulation framework for evaluating the performance and safety of hovering rotor blades under in-flight icing conditions. The objective is to determine the maximum allowable operating time with constant thrust in various icing conditions. The performance assessment considers thrust loss due to stall or engine power i...

The engineering model determining the onset of Vortex Ring State (VRS) was applied to eVTOL aircraft, and the effect of different landing trajectories and aircraft drag was investigated. Next, the new model to compare the VRS susceptibility according to the different blade geometries and trajectories is proposed by extending Ahlin & Brown's model t...

This paper introduces ABC2, an advanced framework for rotor blade design optimization that can effectively consider the airfoil shape variations during optimization process. A major component of this framework is an reduced-order model (ROM) that leverages deep-neural-network techniques both for airfoil parameterization and performance prediction....

In rotorcraft operations, encountering the vortex ring state (VRS) during descent flight, especially during landing, is a significant safety risk. This phenomenon leads to substantial fluctuations in the aerodynamic loads on the lifting surfaces, ultimately resulting in a loss of control. Modern electric vertical takeoff and landing (eVTOL) aircraf...

This study aims to comprehensively investigate the deep ensemble approach, an approximate Bayesian inference, in the multi-output regression task for predicting the aerodynamic performance of a missile configuration. To this end, the effect of the number of neural networks used in the ensemble, which has been blindly adopted in previous studies, is...

This paper proposes novel enhancements to the electric propulsion system analysis method for electric vertical takeoff and landing (eVTOL) aircraft conceptual design by considering the electrical characteristics of each electrical device in a more accurate manner. To this end, three modules for motor, inverter, and battery analysis are constructed...

This study explores the potential of physics-informed neural networks (PINNs) for the realization of digital twins (DT) from various perspectives. First, various adaptive sampling approaches for collocation points are investigated to verify their effectiveness in the mesh-free framework of PINNs, which allows automated construction of virtual repre...

Airfoil shape optimization plays a critical role in the design of high-performance aircraft. However, the high-dimensional nature of airfoil representation causes the challenging problem known as the "curse of dimensionality". To overcome this problem, numerous airfoil parameterization methods have been developed, which can be broadly classified as...

A large amount of heat flux from aerodynamic heating acts on reusable spacecraft; thus, an accurate heat flux prediction around spacecraft reentry is essential for developing a high-performance reusable spacecraft. Although the approximate convective-heating equations can calculate the heat flux with high efficiency and sufficient fidelity, the hea...

The development of rotorcraft with multiple propulsors has increased interest in co-rotating coaxial rotors, also known as stacked rotors, which can generate high thrust while maintaining a compact aircraft design. By optimizing the pitch angle and the blade arrangement, stacked rotors can reduce aeroacoustic noise and vibration while improving aer...

div class="section abstract"> During icing wind tunnel experiments, the calibration process of the spray nozzle and aerothermal systems introduces experimental uncertainty that can potentially compromise the reliability of the test results. Therefore, performing sensitivity analysis (SA) or uncertainty quantification (UQ) studies is not only essent...

div class="section abstract"> Diagonalized alternating-direction implicit (DADI) method is implemented in the Eulerian hyperbolic droplet solver, ICEPAC, for efficient high-order accurate analysis of aircraft icing. Detailed techniques for implementing the DADI method considering hyperbolicity characteristics are discussed. For the Eulerian droplet...

This study analyzed the aerodynamic performance and the underlying physics of various stacked rotor configurations under hovering conditions using a high-fidelity numerical solver. Three design variables, namely index angle, stacked distance, and pitch angle difference, were adopted to define the stacked rotor configurations. Accordingly, two domin...

Deep ensemble is a simple and straightforward approach for approximating Bayesian inference and has been successfully applied to many classification tasks. This study aims to comprehensively investigate this approach in the multi-output regression task to predict the aerodynamic performance of a missile configuration. By scrutinizing the effect of...

A multirotor electric vertical takeoff and landing (EVTOL) aircraft is specialized for intracity point-to-point urban air mobility services due to its advantages of efficient hover performance, high gust resistance, and relatively low noise. Because of its multiple rotors, rotor–rotor interference inevitably affects flight performance, mainly depen...

Advances in computational technologies have enabled engineers to obtain large amounts of data efficiently. One of the most widely used techniques is the regression model and numerous engineers who are interested in the reliability of their predictions have used the Gaussian process regression. However, time and memory complexities of Gaussian proce...

Recently, a fuel cell-battery hybrid system (FBHS) has attracted considerable attention as a potential propulsion system for electric vertical take-off and landing (eVTOL) aircraft. For the conceptual design of FBHS-powered eVTOL aircraft, the FBHS sizing method requires parameterized weight estimation that reflects the power distribution ratio bet...

High-order spatial schemes, which effectively treat the dissipative characteristics of computational fluid dynamics solvers, are known to assure the precise prediction of rotorcraft aeroacoustics. However, such schemes may differ in numerical performance, even if they have the same theoretical accuracy. This is because the nonlinear part in a high-...

Advances in computational technology have enabled engineers to obtain large amounts of data efficiently. However, generating datasets from a single source is still burdensome and inefficient, so there have been many studies on data fusion techniques using multiple sources of data: e.g., low-fidelity data from computational fluid dynamics simulation...

Snow accumulation on the undercarriage of a train is an important issue that significantly degrades the safety and performance of the vehicle. This phenomenon is primarily attributed to snow saltation induced by train-generated wind gusts. This study numerically investigated the snow accumulation on a train by modelling the snow saltation for the i...

Excessive vibration during high-speed flights is a critical issue in modern rotorcraft. The aeroelastic design of the composite rotor blades can relieve the hub vibratory loads of the rotor that are a dominant source of these vibrations. However, it has many technical problems, such as a high-dimensional design space, narrow and discontinuous feasi...

Waverider is a hypersonic vehicle that improves the lift-to-drag ratio using the shockwave attached to the leading edge of the lifting surface. Owing to its superior aerodynamic performance, it exhibits a viable external configuration in hypersonic flight conditions. Most of the existing studies on waverider employ the inverse design method to gene...

View Video Presentation: https://doi.org/10.2514/6.2022-3660.vid This paper presents an improved technology portfolio assessment methodology to explore available technology space and help project leaders to make a decision in earlier design phases. For browsing and assessing the effect of technology portfolio on the developing aircraft system more...

View Video Presentation: https://doi.org/10.2514/6.2022-3514.vid This study develops a fuel cell/battery hybrid system sizing methodology for eVTOL aircraft. The sizing methodology is parameterized for each fuel cell/battery hybrid system component. The operating characteristics of the battery and fuel cell according to the operating conditions are...

View Video Presentation: https://doi.org/10.2514/6.2022-3489.vid In this study, an improved reduced-order model (ROM) is suggested for predicting the force distribution of the rotor efficiently. The force distribution on the rotor disk, calculated using URANS solver, compressed to the coefficients set on an NPOD basis. The improved ROM is construct...

Autoencoder-based reduced-order modeling (ROM) has recently attracted significant attention, owing to its ability to capture underlying nonlinear features. However, two critical drawbacks severely undermine its scalability to various physical applications: entangled and therefore uninterpretable latent variables (LVs) and the blindfold determinatio...

View Video Presentation: https://doi.org/10.2514/6.2022-4121.vid High-order spatial schemes are an essential numerical technique for properly analyzing the vortex-dominated compressible flowfield, such as rotorcraft flowfield. However, prediction results of the vortex-dominated compressible flowfield vary depending on the non-linear part of each sc...

A novel spectrum calibration method is proposed for high-accuracy emission spectroscopy to extract quantitative gas property information from the spontaneous photon emission spectra. This is challenging, particularly in the presence of broadband emission, which smears the characteristic local spectral features that are often used as property indica...

View Video Presentation: https://doi.org/10.2514/6.2022-3360.vid As the next-generation rotorcraft aims to achieve high-speed flight, the rotor blade structure inevitably experiences excessive vibratory loads. The hub vibration of the main rotor is one of the critical issues in modern rotorcraft that should be relieved through aeroelastic design of...

Autoencoder-based reduced-order modeling has recently attracted significant attention , owing to the ability to capture underlying nonlinear features. However, its uninterpretable latent variables (LVs) severely undermine the applicability to various physical problems. This study proposes physics-aware reduced-order modeling using a β-variational a...

This study examines the aerodynamic performances of a wing and multiple propellers through a parametric analysis of wing–propeller interactions. A flow analysis was conducted via simulations based on actuator disk method. The parameters analyzed included the number of propellers, rotating direction, and propeller interval. An increment in the numbe...

Though inverse approach is computationally efficient in aerodynamic design as the desired target performance distribution is prespecified, it has some significant limitations that prevent full efficiency from being achieved. First, the iterative procedure should be repeated whenever the specified target distribution changes. Target distribution opt...

Despite its various outdoor applications, the operation of quadrotors is restricted under strong winds due to their small size. Since the battery energy overconsumptions induced by the winds may lead to a mission failure, it is desirable to identify the tendency of excess battery energy and operable wind conditions according to ground speed, wind s...

This technical comment is based on the previous work in AIAA Journal: Shu, L., Jiang, P., Song, X., and Zhou, Q., “Novel approach for selecting low-fidelity scale factor in multifidelity metamodeling,” AIAA Journal, Vol. 57, No. 12, 2019, pp. 5320–5330. doi: 10.2514/1.J057989. While reproducing their results, it was found that the hierarchical kri...

Due to the instability of the pitching moment induced by the strong vortical flow at moderate to high angles of attack in the unmanned combat aerial vehicle (UCAV) configuration, previous design studies focused primarily on low-speed flight conditions where the longitudinal stability is a serious issue. Furthermore, because of the limitation of com...

This paper focuses on the uncertainty propagation in the flight performance of multirotor-type unmanned aerial vehicles from the systematic perspective in conceptual design phase. The multirotor performance is estimated by a conceptual design and analysis framework which is capable of predicting the performance for given mission profiles and multir...

The use of non‐intrusive reduced order modeling (NIROM) to approximate high‐fidelity computer models has been steadily increased over the past decade. Recently, local NIROM has been proposed to improve the model accuracy in highly nonlinear problems in which distinct characteristic regimes coexist. The core concept of local NIROM is the decompositi...

A hybrid airfoil is a scaled model for generating a full-scale ice shape for icing wind tunnel tests. This is possible by matching full-scale properties such as the distributions of collection efficiency and heat transfer coefficient. Previous studies have used indirect methods using full-scale stagnation point location or tangent droplet trajector...

This paper proposes a conceptual methodology for designing vertical takeoff and landing aircraft with a hybrid-electric propulsion system as an alternative to facilitate intercity missions for urban air mobility. Four new modules are developed to consider the diversity of configurations, flight mechanisms, and hybrid-electric propulsion system arch...

The recent growing interest in urban air mobility (UAM) worldwide has led to the demand for physical analyses of the aerodynamic performance and aeroacoustic characteristics of electric vertical takeoff and landing (eVTOL) rotorcraft. In a UAM eVTOL rotorcraft, the smaller vortices generated from multiple propulsors interact with lifting surfaces s...

This article proposes an advanced analysis method of the electric propulsion system for UAM vehicles. Three modules are constructed to consider the electrical characteristics of electric devices. The motor-analysis module is developed based on the permanent magnet synchronous motor (PMSM)'s control strategies: maximum torque per ampere (MTPA), fiel...

To ensure safe flight in icing conditions it is essential to understand ice accretion, its effect on aerodynamic performance and ice shedding in any aircraft certification program. Ice fragments shed from a rotorcraft windshield and other locations can be detrimental if they are ingested into the engine intake or impinge on the tail rotor. Because...

Though inverse approach is computationally efficient in aerodynamic design as the desired target performance distribution is specified, it has some significant limitations that prevent full efficiency from being achieved. First, the iterative procedure should be repeated whenever the specified target distribution changes. Target distribution optimi...

Large droplet dynamics, such as droplet–wall interactions and droplet breakup, have a significant impact on the icing process, under super-cooled large droplet (SLD) conditions. Accordingly, many studies have been conducted to numerically implement the effects of large droplet dynamics on the 2nd generation icing codes. Most of them have focused pa...

In recent years, the technical advancement of electric propulsion systems has contributed to the growing applicability of multirotor-type unmanned aerial vehicles from personal hobbies to industrial fields. Industrial fields involve various mission profiles comprising several mission-legs, such as hover and forward flight. Thus, enhancing the appli...

10.1016/j.energy.2022.124458

Appropriately modeling surface roughness is crucial for predicting in-flight icing due to its influence on convective heat transfer. Most prevalent numerical simulations to this end rely on empirical correlations, where surface roughness is assigned a constant value. This limits the accurate computation of turbulent transitions, leading to a discre...

Since inflight icing severely threatens the operational safety, it is crucial to accurately predict the ice accretion and resulting performance degradation for the design of anti‐/de‐icing devices. Both experimental and numerical approaches are used to investigate inflight icing phenomenon, however due to cost and time efficiency, the numerical sim...

In glaze ice conditions, beads on the surface usually grow to form roughness elements through coalescence, finally resulting in enhancement of local collection efficiency. However, the effects of roughness elements due to freezing of beads are not reflected on the local collection efficiency in CFD icing simulations. This is problematic for predict...

The wake vortices of an aircraft descend by self-induction that enables a pilot to identify approximate wake location based on the preceding aircraft path. A pilot taking off an aircraft can avoid a wake vortex encounter by performing early rotation and flying at or above the climb path of the preceding aircraft. Such operation results in a situati...

Naval helicopters flying at extremely low altitudes often face communication problems when the helicopter is located in the distance from the mother ship. Accordingly, new attention is being cast on the high altitude balloon (HAB) to solve this problem due to its cost-effectiveness and ability of rapid deployment to the battlefield. The balloon is...

Understanding ice accretion and aerodynamic performance degradation is essential in any aircraft certification program to ensure safe flight in icing conditions. In addition to well-known meteorological icing parameters and flight conditions, several physical and modeling parameters are known to play a critical role in the simulation of ice accreti...

Downwash effect is the interaction between rotor wake and a launched rocket. Since the effect alters the initial motion of the rocket, a comprehensive analysis of the effect is crucial in predicting the entire trajectory and range of the rocket. In addition, external winds affect the trajectory and range as the winds alter the downwash effect. Beca...

One of the critical issues in recent ice prediction studies is the modeling of roughness formation based on physical phenomena. While a number of experimental studies have been conducted to investigate the fundamental physics of roughness formation, initial bead growth is rarely studied despite its significance to determine bead size and distributi...

With slender and flexible structures, conventional helicopter rotor blades experience numerous structural dynamic problems when exposed to a dynamically extreme situation. To solve those problems, the internal structure of the rotor blade is designed to achieve a better structural dynamic performance using composite material. The cross-section of t...

A reasonable solution to retrieve a payload of a scientific balloon regardless of safety issues and recoverability is the sea recovery strategy. Sea recovery strategy is a unique retrieval method being pioneered by the Japan Aerospace Exploration Agency (JAXA) and the Institute of Space and Astronautical Science (ISAS) for decades. This strategy ha...