Yoon Chul Rhim

Clinical and experimental investigation of pseudoaneurysm in the anterior communicating artery area on 3-dimensional time-of-flight cerebral magnetic resonance angiography.

Objective: To investigate the hemodynamic mechanism of pseudoaneurysm in the anterior communicating artery (AcoA) area in magnetic resonance (MR) angiography. Methods: For the clinical study, a total of 62 patients who undertook digital subtraction angiography (DSA) because of the rupture of an aneurysm originating from a location other than the AcoA area were examined with MR angiography. The relation between signal defect at the AcoA in MR angiography and anatomic variation of the anterior cerebral artery (ACA) was evaluated. For the experimental study, MR angiography and DSA were performed on elastic silicon vascular phantoms with 2 different bifurcation angles (70° and 140°). Hemodynamic factors producing signal defects were evaluated, and the results were compared by computational fluid dynamics (CFD). Results: In a clinical study, 21 of 62 patients had a hypogenetic A1 segment on either side of the ACA. Their MR angiography showed signal defects in the axilla area of the bifurcated AcoA complex in 14 patients, 7 of which could make the residual normal vessel seem to be an aneurysm. All the cases with an intact AcoA complex showed no signal defect. In an experimental study, MR angiography of vascular phantoms with broad-angle bifurcation (140°) showed signal defects at the axilla areas of bifurcation, and these were shown as turbulent flow in DSA and CFD. Phantoms with narrow-angle bifurcation (70°) did not show a significant signal defect, however. Conclusions: A hypoplastic A1 segment accompanying a broad bifurcation angle of the contralateral A1 segment may cause a pseudoaneurysm in MR angiography because of signal defect in the AcoA area.

Numerical Simulations of a Flexible Disk Rotating Close to a Rigid Rotating Wall

In this paper, we present a numerical study about the dynamics of a flexible disk rotating close to a rigid rotating wall. Two new types of flat stabilizers, co-rotating and counter-rotating flat stabilizers, are introduced besides the well-known fixed-stabilizer type which has been studied extensively. The disk is modeled using linear plate theory and the air flow between the flexible disk and the rigid wall is modeled using Navier–Stokes and continuity equations. The flow equations are discretized using finite volume method (FVM) and solved numerically with semi-implicit method for pressure-linked equations (SIMPLE) algorithm, while the spatial terms in the disk model are discretized using finite difference method (FDM) and time integration is performed using fourth-order Runge–Kutta method. The transient numerical simulation is performed to compare the stability boundaries of the different types of flat-stabilizer at a wide range of circumferential mode numbers. The numerical results showed an improved stability of the flexible disk when rotating close to a counter-rotating flat-stabilizer compared with co-rotating and fixed flat-stabilizers.

Towards a Flat Rotating Flexible Disk for High Speed Optical Data Storage

A flexible optical disk system, which consists of a thin optical disk and a rigid stabilizer, has recently introduced as the next-generation optical storage media. The present work introduces a new design for the stabilizer that helps to hold the rotating flexible optical disk almost flat and thereby reducing its axial run-out at high rotational speeds; the new design incorporates an axisymmetrically curved active surface of the stabilizer. The combination of the stabilizer curvature and disk rotation generates moderate air-film forces that balance the disk mechanical forces and reduces the disk axial run-out considerably. With a proper combination of the stabilizer geometrical parameters, the out-of-flatness as well as the axial run-out of the disk could be reduced to less than 10 µm. The significant decrease in the axial run-out at rotational speed of 10,000 rpm is primarily due to the flatness of the disk.

Noise-Level Reduction of a Slim-Type Optical Disk Drive Using the Idea of a Helmholtz Resonator

In this study, we have introduced a new attempt to reduce the noise level of a slim-type optical disk drive (ODD) using the concept of a Helmholtz resonator, which is a container with a neck through which the air can flow in and out. The Helmholtz resonator is known to generate a resonant sound when we apply disturbances across the opening of the neck but it is also known to absorb the acoustic noise to some extent, especially the noise of resonant frequency. The resonator is realized in the ODD using a tray, which carries a disk. A tray is prepared with various sizes of holes and hole configurations. The experimental results show that the total sound pressure level (SPL) can be reduced by more than 1.5 dB when a compound resonator type is employed.

Passive-Damping of the Axial Run-Out for High Speed Rotating Flexible Optical Disk Using the Idea of Damping Orifice

In the present work, the idea of damping orifice is applied so as to reduce the axial run-out of a high speed rotating flexible optical disk. A track or more of rectangular-edge orifices is inscribed in a rigid flat stabilizer near the outer region of the disk that exhibits large vibration amplitudes. The effects of the orifice geometry, number of orifices per track, and the number of tracks are investigated experimentally. The results from this study show that the introduced new design of the stabilizer can reduce the axial run-out of the disk at 10,000 rpm to within 10 µm over its entire span using two tracks of damping orifices near the disk rim. The study proved that the introduced orifices in the flat stabilizer effectively enhance the damping capability of the air-film to dissipate the vibration energy of the rotating disk.

Experimental and Numerical Study on the Dynamic Behavior of a Spinning Flexible Disk Close to a Rotating Rigid Wall

In the present work, the behavior of a flexible disk rotating close to a fixed, a co-rotating, and a counter rotating flat-stabilizers in open air is investigated both experimentally and numerically. The Navier–Stokes equations along with the continuity equation representing the flow in the air-film are discretized using the finite volume method and solved numerically with the simple algorithm. An experimental test-rig is designed to investigate the effects of the rotation speed, the initial gap height and the inlet-hole size on the flexible disk displacement and its vibration amplitude. Finally, a comparison between the experimental and the numerical results is made.

Validation of compliance zone at cerebral arterial bifurcation using phantom and computational fluid dynamics simulation.

Objective A zone compliant to pulsatile flow (compliance zone) showing evagination and flattening at the apex of the cerebral arterial bifurcation was documented in our previous report using electrocardiogram-gated computed tomographic and magnetic resonance angiography. We aimed to validate the existence of compliance zones and examine their relationship to local thin-elastic walls. Methods We examined different bifurcating vascular models: a phantom with a thin elastic region at the apex and computational fluid dynamics models with either an elastic or rigid region at the apex of a bifurcation. Results In the phantom, the elastic region at the apex of the bifurcation showed evagination and flattening in time with the pulsatile circulating fluids. The size of the evaginations increased when the outlet side was tilted down below the level of the flow-generating pump. Pulsatile evagination could be simulated in the computational fluid dynamics model with an elastic region at the bifurcation apex, and the pressure gradient was highest in the evaginating apex in peak systolic phase. Conclusions We were able to demonstrate a compliance zone, which responds to pressure gradients, experimentally, in the form of a thin elastic region at an arterial bifurcation.

Application of Herringbone Pattern to a Slim-Type Optical Disk Drive for the Reduction of Warpage and Vibration of the Rotating Disk

A herringbone pattern is applied beneath the top cover of a slim-type optical disk drive to suppress the warpage and vibration of a rotating disk since the pattern can modify the pressure generation over the disk by changing its geometric parameters. The effect of the herringbone geometries on the pressure generation is evaluated for various geometric parameters. The flow field and the pressure distribution in the cavity are calculated by using a commercial program with the k–e model for the turbulent computation. To confirm the numerical results, the axial vibrations of the rotating disk as well as the pressure distribution of the air gap between the disk and the top cover are measured at various points. In this study, the application of the herringbone pattern reduces the disk warpage and axial vibration by 34 and 10% at most, respectively.

Analysis of slip flow considering surface topology and accommodation factor

This paper proposes flow factor for arbitrary Knudsen number considering accommodation factor. Numerical analyses of rarefied gas are conducted by using proposed slip model. An analysis model is a slider of HDD, which is one of the typical cases of rarefied gas regime. The slider surface is measured by a confocal laser microscope. And then we confirm the effect of surface roughness in rarefied gas regime.

Investigation of unloading performance for change of FDB transient behavior in low temperature

This research investigated unloading performance for change of disk RPM with temperature during emergency parking. First, disk RPM drop was measured by experiment during emergency parking process and simulated by 6-DOF of spindle system. And slider flying attitude variation was investigated by change in the disk RPM drop.