Dong-Kyu Lee’s research while affiliated with Korea National University of Transportation and other places

This review paper examines the technological evolution, applications, and scientific achievements of surface drifters in measuring ocean currents. Since their inception in the mid-twentieth century, drifters have developed from simple floating devices to sophisticated instruments equipped with GPS and various environmental sensors. After succeeding the WOCE Surface Velocity Program, the Global Drifter Program has played a crucial role in maintaining a worldwide network of these platforms, significantly advancing our understanding of ocean dynamics and providing real-time data for multiple applications. At the same time, interpretation of drifter trajectories faces challenges including separation of the signal on currents, wave-induced motion, effects of vertical shear and down-wind slippage as well as changes of geometry due to drogue loss, biofouling, and design modification. This review discusses ongoing efforts to address these issues through improved drifter designs and data processing techniques. It also explores future directions, including the integration of drifter data with other observing systems and the application of machine learning to the data analysis. Key scientific findings from drifter observations include the outlining of the large-scale ocean surface circulation and in such dynamically important regions as the Equatorial Pacific, quantification of wind-driven currents, providing a reference for high-resolution mapping of mean dynamic topography, and improved understanding of boundary current systems, eddies and convergence zones. Drifters have also been instrumental in studying ocean dispersion, connectivity, and extreme events such as hurricanes.

This paper presents a novel methodology for designing the flight control system of a micro aerial vehicle (MAV); it allows the MAV to learn how to fly by itself, without the risk of damage. The proposed methodology uses a magnetic levitation–based safety-guaranteed flight test environment and deep reinforcement learning techniques to avoid the current problems in flight control system design procedures, such as the reality gap issue of simulations, and the safety issues inherent to real flight testing. The safety-guaranteed flight test environment was achieved using a developed magnetic suspension and balance system, which dynamically adjusts magnetic forces interacting with a magnetically levitated MAV. As a result, the MAV can perform in either a free-flight condition for reinforcement learning or can be constrained for safety. This learning environment was developed to permit safe reinforcement learning of MAV, since trial-and-error-based learning typically makes the MAV unstable. In this regard, the MAV can learn to fly by itself based on trial and error, by interacting with the emulated free-flight test environment. Notably, the entire learning process can be conducted without numerical models for both the MAV itself and the flight environment, and the safety of the MAV is guaranteed, even when attempting undesirable actions that might cause the model to become unstable. By avoiding the modeling errors typical of computational simulations and preventing the risk of damage in real flight tests, this approach could enhance the use of reinforcement learning in the development of advanced flight control systems.

The magnetic suspension and balance system (MSBS) uses magnetic force and moment to precisely control the movement of the test object located at the center of the test section without mechanical contact, and at the same time measure the external force acting on the test object. If such an MSBS is installed around the test section of the wind tunnel so that the position and attitude angle of the test object follow the harmonic function, various vibration tests can be performed on structures subjected to aerodynamic loads without the influence of the mechanical support. Because the control force and moment in the MSBS is generated by a number of electromagnets located around the test section, it is necessary to apply the adaptive control algorithm to the position and attitude control system so that the experiment can be carried out stably despite the sudden performance change of each electromagnet and electric power supply. In this study, a fault-tolerant position and attitude angle control system was designed through an adaptive control algorithm, and the effectiveness was verified through simulation under the condition that the electric power supply of MSBS failed.

In case of a fire at a high-rise building which is densely populated, an extension ladder is used to rescue people who have yet to evacuate to a safe place away from the fire, whereas those who are stranded at a height that is unreachable with the ladder should be promptly saved with different rescue methods. In this case, an application of the tethered flight system capable of receiving power over a power cable from the ground to a multicopter may guarantee effective execution of the rescue plan at the scene where fire is raging without any restrictions of the flight time. This article identified restrictions that should be considered in the design of a multicopter capable of tethered flight aimed to rescue stranded people at an inaccessible location with an extension ladder at a fire-ravaged high-rise building and assessed its feasibility. A power cable capable of providing dozens of kilowatts of electricity should be installed to enable the implementation of the rescue mission using the tethered multicopter. A flexible multi-body dynamics modeling and simulation with viscoelastic characteristics and heavy weight of power cable were carried out to evaluate the effects of such cable of the tethered flight system on the dynamic characteristics of the multicopter. The results indicate that as for a heavy-lift tethered multicopter designed to be utilized for rescue operations, the properties of the power cable, such as weight, rigidity and length, have a major impact on the position and attitude control performance.

A cue for long-range vision allows mosquitoes to identify hosts and differentiate the ecological niches (e.g., habitats). However, the visual factors involved in attracting mosquitoes to a host are complex and have not been fully understood. Therefore, we assessed color preference to Aedes albopictus (Skuse) and Culex pipiens (Conquillett) as diurnal and nocturnal species, respectively, using seven fundamental colors including black, white, red, yellow, green, blue, and purple with each trap at 100 lux in a laboratory. We used a binary behavioral assay using the Mosquito Preference Index (MPI) as a preference ratio with a range of 0–1. Our analyses showed that Ae. albopictus had a greater response to black (MPIs, 0.7), followed closely by red, blue, and purple (MPIs, 0.6). We also found that red, blue, and purple were significantly higher (P < 0.05) than those of green (MPI, 0.5), white (MPI, 0.3), and yellow (MPI, 0.2). Similarly, the MPIs for Cx. pipiens were significantly higher at black and red (MPIs, 0.7; P < 0.05) compare to those of white and yellow (MPIs, 0.3; P < 0.05). The color preference of Ae. albopictus showed significant correlation to luminous intensities (L-value) (r = −0.640; P = 0.000) and blue intensities (b-value) (r = −0.372; P = 0.000) for all seven colors. In addition, Cx. pipiens negatively correlated (r = −0.703; P = 0.000) between color preference and L-value. Our analyses provide a greater understanding of how color plays a role in visual sensory stimuli, and how that could potentially affect mosquito host-seeking behavior.

This study examined Culex pipiens pallens responses to different combinations of colors and chemicals employed via a mosquito trap under semifield conditions. Our results indicated that Cx. p. pallens has color and chemical concentration preferences. Culex p. pallens had a 38.0% greater response to white than black color treated traps. Further, Cx. p. pallens showed differences in olfactory attraction depending on the chemical and concentration. Culex p. pallens was 107.6% more attracted to traps employing 500 ppm ammonia than control (i.e., unscented). Similarly, Cx. p. pallens was 117.5%, 128.8%, and 140.3% more attracted to traps employing, respectively, 1,000, 10,000, and 20,000 ppm of ammonia hydrogen carbonate compared to controls. And the response to lactic acid showed that Cx. p. pallens was most attracted to concentrations of 100 and 500 ppm (135.7% and 142.9%, respectively) compared to controls.

In this study, we report the precise shape control of crystalline cerium oxide, whose morphology changes between nanorods and nanoparticles in a short time. The proposed synthetic route of cerium oxide nanorods was highly dependent on the reaction time, and 10 min was determined to be the optimum synthetic condition. The cerium oxide nanorods were further converted into nanoparticles by the spontaneous assembly of cerium oxide nanoparticles into nanorods. The transmission electron microscopy results showed that the synthesized nanorods grew with high crystallinity along the 〈110〉 direction. The cerium oxide nanorods have been proven to be very efficient electron mediators for use as excellent photocatalytic materials and highly sensitive chemical sensors. The chemical sensor fabricated on a carbon paper substrate showed the high sensitivity of 1.81 μA mM ⁻¹ cm ⁻² and the detection limit of 6.4 μM with the correlation coefficient of 0.950.