층류박리 후향계단 유동의 이중주파수 가진

김성욱,최해천,유정열,Kim, Sung-Wook,Choi, Hae-Cheon,Yoo, Jung-Yul 대한기계학회 2003 大韓機械學會論文集B Vol.27 No.8

The effect of local forcing on the separated flow over a backward-facing step is investigated through hot-wire measurements and flow visualization with multi-smoke wires. The boundary layer upstream of the separation point is laminar and the Reynolds number based on the free stream velocity and the step height is 13800. The local forcing is given from a slit located at the step edge and the forcing signal is always defined when the wind tunnel is in operation. In case of single frequency forcing, the streamwise velocity and the reattachment length are measured under forcing with various forcing frequencies. For the range of 0.010〈S $t_{\theta}$〈0.013, the forcing frequency component of the streamwise velocity fluctuation grows exponentially and is saturated at x/h = 0.75 , while its subharmonic component grows following the fundamental and is saturated at x/h = 2.0. However, the saturated value of the subharmonic is much lower than that of the fundamental. It is observed that the vortex formation is inhibited by the forcing at S $t_{\theta}$ = 0.019 . For double frequency forcing, natural instability frequency is adopted as a fundamental frequency and its subharmonic is superposed on it. The fundamental frequency component of the streamwise velocity grows exponentially and is saturated at 0.5 < x/h < 0.75, while its subharmonic component grows following the fundamental and is saturated at x/h= 1.5 . Furthermore, the saturated value of the subharmonic component is much higher than that for the single frequency forcing and is nearly the same or higher than that of the fundamental. It is observed that the subharmonic component does not grow for the narrow range of the initial phase difference. This means that there is a range of the initial phase difference where the vortex parring cannot be enhanced or amplified by double frequency forcing. In addition, this effect of the initial phase difference on the development of the shear layer and the distribution of the reattachment length shows a similar trend. From these observations, it can be inferred that the development of the shear layer and the reattachment length are closely related to the vortex paring.

압전세라믹 액추에이터를 이용한 익형 후류 제어

최진,전우평,최해천,Choi, Jin,Jeon, Woo-Pyung,Choi, Hae-Cheon 대한기계학회 2000 大韓機械學會論文集B Vol.24 No.8

The objective of this study is to increase lift and decrease drag of an airfoil at high angles of attack by delaying flow separation with piezo-ceramic actuators. The airfoil used is NACA 0012 and its chord length is 0.3m. An experiment is performed at the freestream velocity of 15m/s at which the Reynolds number based on the chord length is $2{\times}10^5$. Seven rectangular actuators are attached to the airfoil surface and move up and down based on the electric signal. Drag and lift are measured using an in-house two-dimensional force-balance and the surface pressures are also measured. At the attack angle of $16^{\circ}$, the separation point is delayed downstream due to momentum addition induced by the movement of the actuators. Lift is increased by 10%, drag is reduced by 37%, and the efficiency is increased up to 170%. The flow fields with and without control are visualized using the smoke-wire and tuft techniques.

LES에서 중심 및 상류 컴팩트 차분기법의 적합성에 관하여 (III) -동적 오차 해석 -

박노마,유정열,최해천,Park, No-Ma,Yoo, Jung-Yul,Choi, Hae-Cheon 대한기계학회 2003 大韓機械學會論文集B Vol.27 No.7

The suitability of high-order accurate, centered and upwind-biased compact difference schemes for large eddy simulation is evaluated by a dynamic analysis. Large eddy simulation of isotropic turbulence is performed with various dissipative and non-dissipative schemes to investigate the effect of numerical dissipation on the resolved solutions. It is shown by the present dynamic analysis that upwind schemes reduce the aliasing error and increase the finite differencing error. The existence of optimal upwind scheme that minimizes total numerical error is verified. It is also shown that the finite differencing error from numerical dissipation is the leading source of numerical errors by upwind schemes. Simulations of a turbulent channel flow are conducted to show the existence of the optimal upwind scheme.

공간차분도식이 점탄성 유체유동의 수치해에 미치는 영향

민태기,유정열,최해천,Min, Tae-Gee,Yoo, Jung-Yul,Choi, Hae-Cheon 대한기계학회 2000 大韓機械學會論文集B Vol.24 No.9

This study examines the effects of the discretization schemes on numerical solutions of viscoelastic fluid flows. For this purpose, a temporally evolving mixing layer, a two-dimensional vortex pair interacting with a wall, and a turbulent channel flow are selected as the test cases. We adopt a fourth-order compact scheme (COM4) for polymeric stress derivatives in the momentum equations. For convective derivatives in the constitutive equations, the first-order upwind difference scheme (UD) and artificial diffusion scheme (AD), which are commonly used in the literature, show most stable and smooth solutions even for highly extensional flows. However, the stress fields are smeared too much and the flow fields are quite different from those obtained by higher-order upwind difference schemes for the same flow parameters. Among higher-order upwind difference schemes, a third-order compact upwind difference scheme (CUD3) shows most stable and accurate solutions. Therefore, a combination of CUD3 for the convective derivatives in the constitutive equations and COM4 for the polymeric stress derivatives in the momentum equations is recommended to be used for numerical simulation of highly extensional flows.

잠자리 날개의 경로 최적화

박우성(Woo Sung Park),권지훈(Ji Hoon Kwon),최해천(Hae Cheon Choi) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.11

Numerical simulations are conducted to investigate the mechanism of hovering flight with an inclined stroke. The Reynolds number considered is 150 based on the maximum translational velocity and a wing chord length. First, we conducted mechanism analysis based on three different paths, inclined plane motion, oval motion, figure eight motion, and rowing motion under various conditions. For each pattern, we calculated mean stroke-averaged values of the force coefficients and an estimated power. Second, from the analysis I suggest possible optimized paths to maximize lit and to minimize power consumption. Finally, we are going to take a closer look in the view of biological sense.

곤충 모방 플래핑 날개의 공력 특성

한종섭(Jong-seob Han),장조원(Jo-won Chang),최해천(Hae-cheon Choi),강인모(In-mo Kang),김선태(Sun-tae Kim) 한국추진공학회 2007 한국추진공학회 학술대회논문집 Vol.2007 No.11

두개저종양 제거술 후 유리근피판을 이용한 재건

고경석,윤근철,한상훈,정복성,최해천 大韓成形外科學會 1993 Archives of Plastic Surgery Vol.20 No.6

There were several problems in skull base tumor surgery as followings. The tumor resection would be incomplete due to the complex skull base anatomy and difficult surgical approach. There was fatal complication such as meningitis due to CSF leakage and difficulty in filling the surgical defect, and the wound healing was poor in previously radiated cases. But, in recent years, the development of craniofacial and microsurgical techniques enables us to solve these problems. The microsurgical free muscle flap provides sufficient filling-out of defect and rapid healing. We experienced three cases of skull base tumour. Through craniofacial approach, the patients underwent resection of tumours by a multidisciplinary team consisting of a neurosurgeon, ENT surgeon and plastic surgeon. The resultant defects of skull base were reconstructed using microsurgical transfer of free muscle flaps; free rectus abdominis musculocutaneous flap in 2 cases and free serratus anterior muscle flap in 1 case. The follow-up period is between 4 and 24 months and all patients are free of tumour recurrence or severe complications. The esthetic and functional results are satisfactory.

비처리 연골과 냉동건조 연골의 변형에 대한 비교 연구

최해천,윤근철,강정훈,오두영 大韓成形外科學會 1998 Archives of Plastic Surgery Vol.25 No.6

Human untreated costal cartilage was compared with lyophilized human costal cartilage which was treated with defatting solution for 48 hours and freeze drying for 72 hours (-70℃, 10??bar) on the tendency of distortion. All cartilages, which were taken from six cadavers, were carved 5×5×30mm in size on principle of the balanced cross section. Their distortions were evaluated by two methods at intervals of one week, one month, three months, six months after experiment. At first, the degree of distortion was grossly graded with four steps: grade O; no distortion / grade Ⅰ; minimal distortion / grade Ⅱ; moderate distortion / grade Ⅲ; severe distortion. Second method is measurement and quantification of distortion in the horizontal and vertical plane of cartilage. Untreated cartilage is shown to be an unsatisfactory material, with only three(12%) of the 25 cartilages being cosmetically acceptable(grade 0 and 1) after 6 months. In lyophilized cartilage, 18(94%) of the 19 cartilages were cosmetically acceptable(grade 0 and 1) at 6 months. This figure is highly significant(p<0.01). In another method, distortion in the horizontal(h) and vertical (v) planes of cartilage were measured, and mean values of ???? were calculated. In untreated group, the mean values of ???? were 0.82 at 1 week, 0.91 at 1 month, 1.13 at 3 months, and 1.31 at 6 months. In lyophilized group, the mean values were 0.27 at 1 week, 0.29 at 1 month, 0.40 at 3 months and 0.47 at 6 months. All values were statistically significant(p<0.01).

Titanium micro-mesh를 이용한 광범위한 안와상벽결손의 재건

최해천,윤근철 大韓成形外科學會 1994 Archives of Plastic Surgery Vol.21 No.4

In the reconstruction of the massive orbital wall defect, the accuracy in three dimensional anatomic restoration is a paramount importance to prevent enophthalmos and dystopia. Although autogenous bone graft is theoretically ideal, its later resorption and difficulties in shaping make the goal seldom accompanished. Inorganic implants are not welcomed for high rate of infection. Meanwhile, the advent of titanium implants afforded us more dexterity in facial bone fixation. The titanium, micro-mesh was introduced and used with bone grafts for massive orbital floor reconstruction to ensure perfect restoration of the orbital volume and the position of eyeball. Likewise the floor, reconstruction of the orbital roof defect is not easy because of its complex domed contour especially when the defect is so massive as adjacent frontal bone, lateral and medial orbital wall are included. We experienced two cases of successful reconstruction of the massive orbital roof defect with titanium micro mesh. The meshes were prefabricated on a dry skull and used as a carrier for rib bone graft or a support for fractured bone fragments.

PMMA Micropheres (Artecoll)의 임상적 이용과 조직학적 소견

강유미,전용욱,최한영,최해천,곽동렬 大韓成形外科學會 1998 Archives of Plastic Surgery Vol.25 No.7

Many attempts have been made to correct wrinkles or scar artificial or biological materials during the past decades. However artificial implants(fluid silicone, bioplastique, collagen) might be appeared in the form of dislocation, granuloma formation, or late allergic reaction many years later. Biological materials (bone, tendons, cartilage, fat) tend to be resorbed, and same injections are repeated. Recently, PMMA microspheres (Artecoll) have been applied to correct deep glabellar frowns, wrinkles, and scar. Artecoll is an injectable implant for the long-lasting correction of wrinkles and the other irregular surfaces. We obtained specimens of Artecoll for histopathology which injected in the upper for the correction of triple fold deformity. As a results, the histological findings of imbedded PMNA microspheres resemble that of fat cell closely and showed capsule formation around Artecoll compartments. Each microsphere was covered with collagen fibers, so that the distance between microspheres was decreased. The volume of monocytes seemed to diminish, and the active fibrosis seemed to end. Vascularization was complete and the number of foreign body cells decreased to a stable amount. No evidence of significant degradation of PMMA in our specimens was found. With above findings, we think that PMMA microsphere is a stable and reliable implant for soft tissue augmentation.