Jaeyub Hyun

Topology optimization of the shear thinning non-Newtonian fluidic systems for minimizing wall shear stress

This paper suggests the topology optimization process to minimize wall shear stress by considering shear thinning non-Newtonian fluid effects in the systematic design of fluidic systems dealing with blood. Topology optimization was originally developed for mechanical design problems, and within the last decade the method has been extended to a range of fluidic applications. In this paper, the Carreau-Yasuda constitutive equation model is used for shear thinning non-Newtonian fluid modeling. The fundamental idea is that the material density of each element or grid point is a design variable, thus, the geometry is parameterized in a pixel-like pattern. Then, material interpolation functions for inverse permeability and dynamic viscosity are used to ensure convergence of the solution and resolve non-linearity. In order to define wall shear stress on implicit boundary between solid and fluid (i.e., blood) occurring in fluidic topology optimization, the relaxation method of wall shear stress is first proposed in this study. We then apply the proposed fluidic topology optimization to actual fluidic systems dealing with blood (e.g., a femoral bypass graft). These design examples validate the efficiency of the proposed approach and show that topology optimization can be used for the initial conceptual design of various fluidic systems.

Near perfect ultrasonic omnidirectional transducer using the optimal patterning of the zero-index acoustic metamaterials

This study proposes the theoretical optimal patterning method based on the geometrical transformation acoustics to design an ultrasonic omnidirectional transducer system, which is composed of the designed near zero-index acoustic metamaterial (ZIAMM). The designed ZIAMM is made of circular rubber rods in water matrix. Meanwhile, the curved unit cell structure is necessary to arrange the designed ZIAMM effectively into the circular-shaped ultrasonic omnidirectional transducer system. To this end, we transform the square unit cell into the curved unit cell in the physical space, instead of starting from a homogeneous medium. Also the periodic boundary condition in the two-dimensional polar coordinate is proposed to calculate the dynamic characteristic (i.e., the effective material properties and the dispersion relation) according to the curvature of curved unit cell. The proposed optimal patterning method is verified through the ZIAMM-based ultrasonic omnidirectional transducer system. Especially the radiat...

Realization of an ultrathin acoustic lens for subwavelength focusing in the megasonic range.

In this study, we report the first experimental realization of an ultrathin (0.14λ, λ = 1.482 mm means wavelength at 1 MHz in the water medium) subwavelength focusing acoustic lens that can surpass the Rayleigh diffraction limit (0.61λ/NA, NA means numerical aperture). It is termed a Super-Oscillatory Acoustic Lens (SOAL), and it operates in the megasonic range. The SOAL represents an interesting feature allowing the achievement of subwavelength focusing without the need to operate in close proximity to the object to be imaged. The optimal layout of the SOAL is obtained by utilizing a systematic design approach, referred to here as topology optimization. To this end, the optimization formulation is newly defined. The optimized SOAL is fabricated using a photo-etching process and its subwavelength focusing performance is verified experimentally via an acoustic intensity measurement system. From these measurements, we found that the proposed optimized SOAL can achieve superior focusing features with a Full Width at Half Maximum (FWHM) of ~0.40λ/NA ≃ 0.84 mm (for our SOAL, NA = 0.707) with the transmission efficiency of 26.5%.

Systematic realization of double-zero-index phononic crystals with hard inclusions

A systematic process is described to realize double-zero-index phononic crystals with Dirac-like points experimentally. This type of crystal normally has softer inclusion material than its surroundings medium, allowing mapping into a zero-index medium under certain conditions but also making experimental implementation difficult. On the other hand, realizing phononic crystals with hard inclusions can be experimentally more feasible, but the mapping conditions cannot be directly applied to hard-inclusion crystals such that mapping is not systematically guaranteed in these cases. Moreover, even if such crystals become realizable, there is a lack of a systematic design process which can be used to optimize or to redesign the crystals, which largely limits their potential applications. In this paper, we discover the essential conditions for realizing phononic crystals with hard inclusions and propose a methodology for the systematic design of these crystals using homogenization based on the effective medium theory. Using the proposed method, a double-zero-index phononic crystal with hard inclusions is optimized and experimentally realized for an underwater ultrasonic wave collimator.

Topology Optimization of a Magnetic Resonator Using Finite-Difference Time-Domain Method for Wireless Energy Transfer

In this paper, a magnetic resonator for wireless energy transfer system is designed using topology optimization based on an electromagnetic wave analysis. To analyze the magnetic resonator, the finite-difference time-domain method in a 2-D transverse magnetic mode is used with a Gaussian pulse source and perfectly matched layers. The topology optimization using solid isotropic material with the penalization method of a magnetic resonator is conducted to maximize magnetic energy. After these approaches, the fast Fourier transform is used to obtain the response of the magnetic resonator system over a wide range of frequencies.