• 한국항공우주학회지
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• 제35권12호
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• pp.1075-1081
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• 2007

온도예조건화 방법을 Navier-Stokes 방정식에 적용하여 보았다. 그리고 Navier-Stokes 방정식에 적절한 기준마하수를 결정하기 위해 확산마하수 개념을 도입하였다. 다양한 레이놀즈수에 대한 실린더 주위의 2차원 유동을 계산하였다. 온도예조건화를 Navier-Stokes 방정식에 적용한 결과, 수렴특성이 주목할 만큼 개선되는 것을 보였다. 그리고 확산마하수로 수정한 기준마하수는 국부적으로 마하수가 낮은 영역의 수렴성 문제를 상당히 완화시키는 것을 확인하였다.

• 자원환경지질
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• 제52권4호
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• pp.291-297
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• 2019

단열을 통한 유체의 유동은 선형유동이 우세하다는 가정아래 Navier-Stokes 방정식에서 유도된 Stokes 방정식, Reynolds 식(또는 local cubic law), cubic law 와 같은 방정식을 이용하여 해석되고 있다. 하지만 이러한 방정식은 선형 흐름에 국한되며, 비선형 유동영역에 적용하게 되면 오류가 발생한다. 본 연구에서는 레이저 계측기를 이용하여 정밀하게 측정한 3차원 단열 자료와 Navier-Stokes 방정식과 Stokes 방정식을 지배방정식으로 한 수치모델링을 수행함으로써 비선형 유동이 일어나는 현상과 임계 레이놀즈수를 제시하였다. 레이놀즈수가 10이상이 되면 유속의 제곱에 비례하는 관성력이 점성력을 충분히 압도할 정도로 커지면서 지하수 유동이 선형영역에서 비선형 유동영역으로 전환되는 것으로 분석되었다. 이는 평균 간극과 거친 정도가 다른 두 단열에서 모두 동일하게 나타났다. 비선형 유동의 발생기작은 소용돌이 구조의 발생과 성장에 의한 것으로 알려져 있지만, 본 연구결과 단순히 소용돌이 구조가 비선형 유동을 일으키는 아니라 유속이 증가하면서 관성력의 영향이 훨씬 큰 영향을 끼치게 되어 비선형 유동이 발생하는 것으로 나타났다.

• 한국항해학회지
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• 제9권2호
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• pp.43-63
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• 1985

As a circular cylinder has a comparatively simple shape and becomes a basic problem for flows around other various shapes of bodies, the problem of two-dimensional viscous flow around the circular cylinder has been investigated, both theoretically and experimentally. But not a few problems are left unsolved. It is well known that the calculations are successfully made with the approximations of Stokes or Oseen for very low Reynolds numbers, but as Reynolds number is increased, Oseen's approximations as well as Stokes's ones become more and more remote from the exact solution of the Navier-Stokes equations. Therefore, in this paper, the authors transform the Navier-Stokes equations into the finite difference equations in the steady two-dimensional viscous flow at Reynolds number up to 45, and then solve the solution of the Navier-Stokes equations numerically. Also, the authors examine the accuracy of the solution by means of flow visualization with aluminum powder. The main results are as follows; (1) The critical Reynolds number at which twin vortices begin to form in the rear of the circular cylinder is found to be 6 in the experiment and 4 in the numerical solution. (2) As Reynolds number is increased, it is proved that the ratio of the length of the twin vortices to the diameter is grown almost linearly, both experimentally and numerically. (3) Separation angle is also increased according to reynolds number. But it is found that it would converge into 101.3 degrees, both experimentally and numerically.

• 한국항공우주학회지
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• 제36권2호
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• pp.123-130
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• 2008

예조건화 나비어스톡스 방정식의 수렴 특성에 미치는 특성 조건수의 영향을 조사하였다. Choi와 Merkle 예조건화를 적용한 경우와 온도 예조건화를 적용한 경우의 수렴 특성을 분석하였다. 공간차분을 위해 예조건화 Roe의 FDS 기법을 적용하였고, 시간적분을 위해 LU-SGS 기법을 적용하였다. 나비어스톡스 방정식의 수렴 특성은 특성 조건수에 크게 영향을 받으며, 최적의 특성 조건수가 존재하는 것을 보였다. 그리고 점성 유동의 최적 특성 조건수는 비점성 유동에 비해 큰 것으로 나타났다.

• 대한기계학회논문집B
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• 제41권4호
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• pp.239-248
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• 2017

원형 실린더 주위의 유동에 부유된 입자가 실린더 근처에서 분산되거나 실린더에 부착되는 특성을 이해하는 것이 중요하다. 본 연구에서는 입자의 부착을 조절하는 방안으로 실린더의 회전을 고려해 보았고, 실린더의 회전 속도 및 입자의 Stokes 수가 실린더 주위의 유동과 입자의 분산 및 부착 특성에 미치는 영향을 수치해석적으로 연구하였다. 해석 결과 4보다 작은 Stokes 수에서는 회전속도가 증가함에 따라 부착효율이 크게 감소하였고, 4보다 큰 Stokes 수에서는 회전속도가 증가함에 따라 부착효율이 다소 증가하였다. 한편, 회전속도가 일정한 경우에는 Stokes 수가 증가함에 따라 입자의 부착효율이 증가하고, 입자의 부착 위치도 넓어졌다.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 1998년도 춘계 학술대회논문집
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• pp.35-42
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• 1998

Time marching algorithms applied to compressible Navier-Stokes equation have a convergence problem at low Mach number. It is mainly due to the eigenvalue stiffness and pressure singularity as Mach number approaches to zero. Among the several methods to overcome the shortcomings of time marching scheme, time derivative preconditioning method have been used successfully. In this numerical analysis, we adopted a preconditioner of K.H. Chen and developed a two-dimensional, axisymmetric Navier-Stokes program. The steady state driven cavity flow and backward facing step flow problems were computed to confirm the accuracy and the robustness of preconditioned algorithm for low Mach number flows. And the transonic and supersonic flows insice the JPL axisymmetric nozzle internal flow is exampled to investigate the effects of preconditioning at high Mach number flow regime. Test results showed excellent agreement with the experimental data.

• 한국전산유체공학회지
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• 제21권2호
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• pp.81-89
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• 2016

Numerical simulations are carried out for the fluid flow and particle transport around two nearby circular cylinders in a side-by-side arrangement. The present study aims to understand the effects of the particle Stokes number and the spacing between two cylinders on particle dispersion and deposition characteristics. Simulations are based on an Eulerian-Lagrangian approach where the motion of particles is calculated by a Lagrangian approach based on one-way coupling. Results show that the flow structure is very different depending on the cylinder spacing, eventually affecting the overall pattern of particle dispersion significantly. It is also found that particles with smaller Stokes number tend to be distributed more uniformly in the wake of two cylinders, being located even inside the vortex cores. Meanwhile, particle deposition is analyzed in terms of the deposition efficiency and deposition location. The deposition efficiency of particles strongly depends on the Stokes number, whereas it is slightly affected by the cylinder spacing. The deposition location gets wider as the Stokes number increases, and it becomes asymmetric about the center of each cylinder as the cylinders get close.

• 대한기계학회논문집B
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• 제30권11호
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• pp.1101-1106
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• 2006

To investigate a relation between fine scale eddies and particle dispersion in a near-wall turbulence, direct numerical simulations of turbulent channel flow laden particle are performed for $Re_{\tau}$=180. The motions of 0,8 million particles are calculated for several particle response times ($t_p$) which is the particle response time based on stokes’ friction law. The number density of particles has a tendency to increase with approaching the near-wall regions ($y^+$<20) except for cases of very small and large particle response times (i.e. $t_p$=0.02 and 15). Near the wall, the behavior and distribution of particles are deeply associated with the fine scale eddies, and are dependent on particle response times and a distance from the wall. The Stokes number that causes preferential distribution in turbulence is changed by a distance from the wall. The influential Stokes number based on the Burgers' vortex model is derived by using the time scale of the fine scale eddies. The influential Stokes number is also dependent on a distance from the wall and shows large value in the buffer layer.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2008년도 추계학술대회B
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• pp.2396-2400
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• 2008

The motion of small heavy particles in homogeneous isotropic turbulence in the present of gravity is investigated using Direct Numerical Simulations (DNS) at moderate Reynolds number. The Lagrangian velocity and acceleration statistics of particles and of flow for a wide range of Stokes number, defined as the ratio of the particle response time to Kolmogorov time scale of turbulence, were obtained for the direction of the gravity and normal direction, respectively. It is found that particles lose their correction faster than the case without gravity. Then, a significant increase in the average settling velocity was observed for a certain range of Stokes number. Our focus is placed on gravitational effect on very small particles. Our simulations show that as the Stokes number reduces to zero, their mean settling velocity approaches the terminal velocity in still fluid.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.35-38
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• 2002

The convergence acceleration methods for the compressible Wavier-Stokes equations are studied ,which are multigrid method and implicit preconditioned multistage time stepping method. In this paper, the performance of implicit preconditioning methods are studied for the full-coarsening multigrid methods on the high Reynolds number compressible flow computations. The effect of numerical flux on the convergence are investigated for the inviscid and viscous calculations.