난류채널유동에서 움직이는 벽면에 대한 수치연구

황준혁(Jun Hyuk Hwang),이재화(Jae Hwa Lee) 한국가시화정보학회 2017 한국가시화정보학회지 Vol.15 No.3

Direct numerical simulations of turbulent channel flows with moving wall conditions on the top wall are performed to examine the effects of the moving wall on the turbulent characteristics. The moving wall velocity only applied to the top wall with the opposite direction to the main flow is systematically varied to reveal the sustained-mechanism for turbulence. The turbulence statistics for the Couette-Poiseuille flow, such as mean velocity, root mean square of the velocity fluctuations, Reynolds shear stress and pre-multiplied energy spectra of the velocity fluctuations, are compared with those of canonical turbulent channel flows. The comparison suggests that although the turbulent activity on the top wall increases with increasing the Reynolds number, that on the bottom wall decreases, contrary to the previous finding for the canonical turbulent channel flows. The increase of the turbulent energy on the top wall is attributed to not only the increase of the Reynolds number but also elongation of the logarithmic layer due to increase of the wall layer on the top wall. However, because the logarithmic layer is shortened on the bottom wall due to the decrease of the wall layer, the turbulence energy on the bottom wall decreases despite of the increase of the Reynolds number.

상류 센서를 이용한 Reτ=395의 난류 채널 유동 반대 제어

김금영,윤진혁,이정일 대한기계학회 2023 大韓機械學會論文集B Vol.47 No.3

In this study, opposition control with upstream sensors is applied to the turbulent channel flow at Reτ =395. As a control input, the amplitude of the actuation at the wall is proportional to the transverse velocity at an upstream sensing location in the opposite direction and varies over time. We perform direct numerical simulations of turbulent channel flow and evaluate the control performance of skin friction reduction based on the locations of upstream sensor. It is shown that the opposition control with upstream sensors exhibits a better control performance than that with a sensor directly above the actuator. The use of upstream sensors for the opposition control effectively decreases the strength of streamwise vortices and velocity fluctuations near the wall, resulting in the maximum skin friction reduction of 26% for the turbulent channel flow with the optimal upstream sensor location. 본 연구에서는 Reτ =395에서 상류 센서 기반 반대 제어를 난류 채널 유동에 적용하였다. 제어 입력으로서, 벽에서의 가진 크기는 가진의 상류에 위치한 센싱 위치의 수직 방향 속도에 비례하고 반대 방향의 값으로 결정되며 시간에 따라 변화한다. 난류 채널 유동의 직접 수치 모사를 수행하고, 상류 센서 위치에 따른 마찰 저항 감소의 제어 성능을 평가하였다. 이를 통해 가진 위치 바로 위의 센서를 활용하는 것보다 상류 센서를 사용하는 반대 제어가 더 좋은 성능을 보임을 확인하였다. 상류 센서를 활용한 반대 제어는 벽 근처 주유동 방향 와류의 강도와 속도 섭동을 효과적으로 약화시켰으며, 최적의 상류 센서 위치에서 난류 채널 유동의 마찰 저항을 최대 26% 감소시켰다.

평행한 두 사각유로를 연결하는 협소유로내의 난류유동 특성에 관한 대형 와 수치 모사

홍성호(Seong-Ho Hong),서정식(Jeong-Sik Seo),신종근(Jong-Keun Shin),최영돈(Young-Don Choi) 대한설비공학회 2008 대한설비공학회 학술발표대회논문집 Vol.2008 No.2

Turbulent flow characteristics on the gap of two parallel channels are investigated using LES(large eddy simulation) approach. Two parallel channels have the same cross-section area and are connected by the narrow channel named the gap. Turbulent flow near the gap makes the flow pulsation along the streamwise direction of two channels. The flow condition is the Reynolds number of 2.5× 10?? . We compared the predicted results with the previous experimental results and presented the axial mean velocity, turbulent intensities, Reynolds shear stresses and turbulent kinetic energy.

난류 채널 유동 내부의 레이놀즈 전단 응력 분포

김경연(Kyoungyoun Kim) 대한기계학회 2012 大韓機械學會論文集B Vol.36 No.8

벽면 난류의 항력과 밀접한 관련이 있는 유동구조를 조사하기 위해 Reτ = 180, 395, 590 의 난류채널유동에 대한 직접수치모사를 수행하였다. 확률밀도함수를 조사하여 레이놀즈 전단응력에 가장 큰 기여를 하는 Q2 이벤트를 파악하였으며 Q2 이벤트의 각도의 변화가 y+ < 50 에서는 벽 단위로 스케일링되며, y/h > 0.5 에서는 채널의 높이로 스케일링 됨을 확인하였다. Q2 이벤트를 조건으로 하는 조건부 평균 유동장을 조사하여 레이놀즈 전단응력의 발생과 관련이 있는 유동구조는 주 유동방향의 보텍스 및 헤어핀 형상의 보텍스임을 보였다. 또한, 순간 유동장을 관찰하여 높은 레이놀즈 전단 응력의 분포가 이러한 보텍스 구조와 관련이 있으며 1.5 ~ 3h 의 크기를 갖는 대형유동구조를 구성함을 확인하였다. Direct numerical simulations were carried out for turbulent channel flows with Reτ = 180, 395 and 590 to investigate the turbulent flow structure related to the Reynolds shear stress. By examining the probability density function, the second quadrant (Q2) events with the largest contribution to the mean Reynolds shear stress were identified. The change in the inclination angle of Q2 events varies with wall units in y+ < 50 and with the channel half height in y/h > 0.5. Conditionally averaged flow fields for the Q2 event show that the flow structures associated with Reynolds shear stress are a quasi-streamwise vortex in the buffer layer and a hairpin-shaped vortex in the outer layer. Three-dimensional visualization of the distribution of high Reynolds shear stress reveals that the organization of hairpin vortices in the outer layer having a size of 1.5~3 h is associated with large-scale motions with high Reynolds shear stress in the outer layer.

주기변동하는 합성입구유동이 난류경계층에 미치는 영향

이영우,임희창 한국풍력에너지학회 2020 풍력에너지저널 Vol.11 No.4

Large eddy simulation (LES) has been popularly applied and used in the last several decades to implement the atmospheric environment of a turbulent boundary layer in the numerical domain. One of its representative applications would be the development of wind turbines and farms under the turbulent boundary layer. A fully developed turbulent boundary layer is also applied to predict the flow around the wake behind bluff bodies such as wind turbines and aerodynamic structures. In this study, we aimed to generate an artificial turbulent boundary layer in the numerical domain, which can be made by a synthetic method of inflow generation. In order to get an appropriate, well-developed boundary layer in a three-dimensional domain, this generation method has been coupled with oscillating flow, which was expected to make faster convergence in the calculation. To make an effective flow analysis, a hexahedral mesh was been and Cholesky decomposition was applied to possess turbulent statistics such as the randomness and correlation of turbulent flow. As a result, the flow characteristics in the domain are very close to the calculation without the oscillating flow.

Wall Shear-Free Control을 이용한 채널 유동의 난류 항력 감소

김정훈(Jung Hoon Kim),이재화(Jae Hwa Lee) 한국가시화정보학회 2016 한국가시화정보학회 학술발표대회 논문집 Vol.2016 No.4

Active flow control of wall-bounded flow for turbulent skin-friction drag reduction (DR) has been received great attention in recent years due to large economical and ecological attention. In the present study, direct numerical simulations (DNSs) of turbulent channel flows are utilized to explore an active flow control concept using streamwise shear-free control (SSFC) at the wall. The control is only applied to half of the entire wall comprised of spanwise-alternating longitudinal regions of no-control and control surfaces for simplicity, and the simulations are performed with changing the spanwise periodicity (P/h) of the control surface. In addition, an amplitude parameter (A) imposing the strength of the actuating streamwise velocity at the wall is introduced to enhance the DR. Significant DR is observed with increasing the two parameters with accompanying reduction of the Reynolds stresses and turbulent structures, although further increase of the parameters amplifies the turbulence activity in the near-wall region.

난류 채널 유동에서 무거운 입자의 거동에 관한 연구

이정훈(Junghoon Lee),이창훈(Changhoon Lee) 대한기계학회 2010 대한기계학회 춘추학술대회 Vol.2010 No.4

The motion of heavy particles in inhomogeneous turbulence is investigated by using direct numerical simulation of turbulent channel flow. The equation of motion of inertial particle including Stokes drag was used and elastic collision of inertial particle at the wall was considered. In turbulent channel flow, inertial particles tend to accumulate at the solid wall because of inhomogeneity in the wall normal direction. We discuss relevant velocity, acceleration statistics and particle distribution statistics for a wide range of Stokes numbers.

난류 채널 유동 내 역류 현상에 대한 횡방향 도메인 크기 영향

박익찬(Ikchan Park),황진율(Jinyul Hwang) 대한기계학회 2022 대한기계학회 춘추학술대회 Vol.2022 No.11

The region of backflow occurs intermittently in fully-developed turbulent channel flow. Although the backflow event is rare and characterized by viscous scales, it is expected to have an essential role in the self-sustaining cycle and to be affected by outer large-scale motions. In order to examine the structure and the dynamical role of the backflow event, three-dimensional flow fields with high spatial and temporal resolutions are required, which is obtained by direct numerical simulation (DNS). This paper aims to explore the influence of the computational domain sizes on the backflow event. We conduct DNSs of the turbulent channel flow at the low Reynolds number (Re<SUB>τ</SUB> = 180) with five different spanwise domain sizes. We find that relatively narrow boxes possibly mispredict the distribution of the negative wall-shear stress events and tend to underpredict the size of backflow regions.

합성난류경계층이 벽면에서의 변동압력에 미치는 영향

이영우(Y.W. Yi),이동섭(D.S. Lee),신구균(K.K. Shin),홍진숙(C.S. Hong),임희창(H.C. Lim) 한국가시화정보학회 2021 한국가시화정보학회지 Vol.19 No.3

Large Eddy Simulation (LES) has been popularly applied and used in the last several decades to simulate turbulent boundary layer in the numerical domain. A fully developed turbulent boundary layer has also been applied to predict the complicated wake flow behind bluff bodies. In this study we aimed to generate an artificial turbulent boundary layer, which is based on an exponential correlation function, and generates a series of realistic three-dimensional velocity data in two-dimensional inlet section which are correlated both in space and in time. The results suggest its excellent capability for high Reynolds number flows. To make an effective generation, a hexahedral mesh has been used and Cholesky decomposition was applied to possess suitable turbulent statistics such as the randomness and correlation of turbulent flow. As a result, the flow characteristics in the domain and fluctuating pressure near the wall are very close to those of fully developed turbulent boundary layers.

난류 채널 유동에서 관성 입자의 라그란지안 통계

이정훈(Junghoon Lee),이창훈(Changhoon Lee) 대한기계학회 2014 대한기계학회 춘추학술대회 Vol.2014 No.11

Understanding particle-turbulence interactions near a wall is relevant to a variety of engineering applications, such as material transfer, energy conversion and propulsion. Despite numerous studies regarding particles in near-wall turbulence, detailed investigation on Lagrangian nature of particles near the wall is very rare. Our main purpose is to investigate Lagrangian statistics of inertial particles starting from the plane at specific wall-normal locations in turbulent channel flow for a wide range of Stokes number. To do so, we perform direct numerical simulation of turbulent channel flow and calculate large numbers of Lagrangian individual trajectories of inertial particles suspended in the flow assuming the particles to be point sources.