A study of the flow characteristics of developing turbulent pulsating flows in a curved duct

Hyun-Chull Sohn,Haeng-Nam Lee,Gil-Moon Park 대한기계학회 2007 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.21 No.12

Flow characteristics of turbulent pulsating flows in a square-sectional curved duct were experimentally investigated. Experimental studies for air flow were conducted to measure axial velocity profiles, secondary flow and pressure distributions in a square-sectional 180° curved duct by using an LDV system with a data acquisition and processing system which includes a Rotating Machinery Resolve (RMR) and PHASE software. Measurements were made at the seven cross-sections from the inlet (ø=0°) to the outlet (ø=180°) of the duct with 30° intervals. Pressure was measured by using a magnetic differential pressure gage. The experiment was conducted in nineteen sections from the inlet to the outlet of the duct at 10° intervals. Velocity profiles for turbulent pulsating flows were large at the outer wall for a bend angle of ø=30° because of the centrifugal force. The velocity profiles were similar to those of turbulent steady flows. The secondary flow of the turbulent pulsating flow had a positive value at a bend angle of 150° without regarding the phase. The dimensionless value of the secondary flow became gradually weak and approached to zero in the region of a bend angle of 180° regardless of the ratio of velocity amplitude. The pressure difference of turbulent pulsating flows was the largest near the region of a bend of angle of 90° in the case of the middle region and became small beyond 90.

Non-invasive estimation of relative pressure in turbulent flow using virtual work-energy

Marlevi, David,Ha, Hojin,Dillon-Murphy, Desmond,Fernandes, Joao F.,Fovargue, Daniel,Colarieti-Tosti, Massimiliano,Larsson, Matilda,Lamata, Pablo,Figueroa, C. Alberto,Ebbers, Tino,Nordsletten, David A. Elsevier 2020 Medical image analysis Vol.60 No.-

<P><B>Abstract</B></P> <P>Vascular pressure differences are established risk markers for a number of cardiovascular diseases. Relative pressures are, however, often driven by turbulence-induced flow fluctuations, where conventional non-invasive methods may yield inaccurate results. Recently, we proposed a novel method for non-turbulent flows, <I>ν</I>WERP, utilizing the concept of virtual work-energy to accurately probe relative pressure through complex branching vasculature. Here, we present an extension of this approach for turbulent flows: <I>ν</I>WERP-t. We present a theoretical method derivation based on flow covariance, quantifying the impact of flow fluctuations on relative pressure. <I>ν</I>WERP-t is tested on a set of <I>in-vitro</I> stenotic flow phantoms with data acquired by 4D flow MRI with six-directional flow encoding, as well as on a patient-specific <I>in-silico</I> model of an acute aortic dissection. Over all tests <I>ν</I>WERP-t shows improved accuracy over alternative energy-based approaches, with excellent recovery of estimated relative pressures. In particular, the use of a guaranteed divergence-free virtual field improves accuracy in cases where turbulent flows skew the apparent divergence of the acquired field. With the original <I>ν</I>WERP allowing for assessment of relative pressure into previously inaccessible vasculatures, the extended <I>ν</I>WERP-t further enlarges the method's clinical scope, underlining its potential as a novel tool for assessing relative pressure <I>in-vivo</I>.</P> <P><B>Highlights</B></P> <P> <UL> <LI> vWERP-t uses virtual work-energy to accurately assess turbulent relative pressure. </LI> <LI> In-vitro, vWERP-t shows 1:1 agreement with invasive measurements of relative pressure. </LI> <LI> In transient flow, vWERP-t shows significant improvement compared to other approaches. </LI> <LI> vWERP-t guarantees divergence free flow even in turbulent fields, improving accuracy. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

배열회수보일러(HRSG)의 입구유동 경계조건에 따른 유동특성 변화에 관한 연구

김태권(Tae Kwon Kim),이부윤(Boo Yoon Lee),하지수(Ji Soo Ha) 한국가스학회 2011 한국가스학회지 Vol.15 No.3

본 연구는 배열회수보일러(HRSG)에서의 유동특성을 유동수치해석을 통하여 분석하였다. HRSG 입구영역은 가스터빈 후류의 출구에 해당하고 가스터빈 후류는 강한 선회 및 난류 유동이다. 따라서 HRSG 입구 유동은 가스터빈 출구 유동 특성이 고려되어야 한다. 본 연구에서는 HRSG 입구 유동 경계조건을 가스터빈 출구 유동 해석을 통하여 도출된 결과를 이용하였다. 가스터빈 출구 유동해석 결과를 보면 축방향 속도가 가장 크게 나타나는 곳이 원형 덕트의 벽면 측이고 난류운동에너지와 소산율이 크게 나타나는 곳이 속도 구배가 급격한 곳으로 축방향 속도가 최대가 되는 곳과 차이가 있다. 본 연구에서는 HRSG 입구영역에서의 난류 성분을 가스터빈 출구 유동을 계산 한 결과를 이용한 경우와 난류강도를 속도의 10%를 이용하고 원형 덕트의 직경을 특성 길이로 사용한 두 가지 경우에 대하여 유동해석을 통하여 유동 특성을 비교하였다. 본 연구를 통하여 HRSG 입구 유동 경계조건은 반드시 난류성분이 올바르게 적용되어야 HRSG 유동 특성 해석의 정확성을 기할 수 있음을 알았다. The present study has been carried out to analyze the flow characteristics of a heat recovery steam generator with the change of inlet flow conditions by using numerical flow analysis. The inlet of HRSG corresponds the outlet of gas turbine exit and the flow after gas turbine has strong swirl flow and turbulence. The inlet flow condition of HRSG should be included the exit flow characteristics of gas turbine. The present numerical analysis adopted the flow analysis result of gas turbine exit flow as a inlet flow condition of HRSG analysis. The computational flow analysis result of gas turbine exit shows that the maximum axial velocity appears near circular duct wall and the maximum turbulent kinetic energy and dissipation rate exist relatively higher gradient region of axial velocity. The comparison of flow analysis will be executed with change of inlet turbulent flow condition. The first case is using the inlet turbulent properties from the result of computational analysis of gas turbine exit flow, and the second case is using the assumed turbulent intensity with the magnitude proportional to the velocity magnitude and length scale. The computational results of flow characteristics for two cases show great difference especially in the velocity field and turbulent properties. The main conclusion of the present study is that the flow inlet condition of HRSG should be included the turbulent properties for the accurate computational result of flow analysis.

Turbulent heat transfer characteristics in compound channels with gap

Hong, Seong-Ho,Seo, Jeong-Sik,Byun, Jae-Ki,Choi, Young-Don,Shin, Jong-Keun 대한기계학회 2011 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.25 No.9

To examine the characteristics of turbulent flow and heat transfer about the compound channel with gap, the present study has performed a numerical analysis by using large eddy simulation (LES). Our first aim was to analyze the cause of pulsating flow by investigating the turbulent flow through the unsteady analysis of compound channel. Next, to understand the correlations between gap width and heat transfer enhancement, we tried to verify the enhancement effect of heat transfer when a gap exists within the compound channel. From this numerical analysis, we could see that the lateral velocity occurs periodically around the gap and this is developed in the pulsating flow. The pulsating flow has generated a strong turbulent flow mixing within the compound channel. And the turbulent flow mixing in the pulsating flow plays an effective role of enhancing heat transfer effect by making the fluid temperature uniformly within the compound channel.

Numerical predictions of detailed flow structural characteristics in a channel with angled rib turbulators

박종명,박삼규,P. M. Ligrani 대한기계학회 2015 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.29 No.11

Turbulent air flows within a channel with angled rib turbulators (45 degrees) on one wall are numerically predicted using the numericalcode ANSYS CFX with a Shear-stress transport (SST) κ-ω turbulence model, and a hexahedral grid with 7115346 cells and no wallfunction. Three-dimensional turbulent transport, and detailed flow structural characteristics are considered to provide new insight into themechanisms which result in surface heat transfer augmentations. Time-averaged turbulent flow characteristics and surface Nusselt numberdistributions are presented for an inlet turbulence intensity level of 1.0 percent, and for Reynolds numbers based upon channel heightof 18300 and 48000. Overall, the numerically-predicted results show that large-scale, secondary flow induces a collection of small-scalevortical flows in the channel, as a result of local interactions with individual rib turbulators. These are often associated with differentsized and highly skewed vortex pairs, which also induce secondary advection and increased turbulent mixing near ribbed channel surfaces. Within the flow separation regions, just downstream of each rib, surface Nusselt number ratios which are locally lower than forother surface locations, and local secondary flows are generally and partially characterized as upwash flows, with large positive magnitudesof spanwise vorticity, large static pressure deficits, and large static pressure augmentation regions. As the shear layers (initiallylocated above the recirculation zones) impinge onto the test surface, increased mixing develops, as well as local thinning of the reattachingboundary layers, which lead to local Nusselt numbers which are generally higher than for other locations along the test surface.

Axisymmetric Swirling Flow Simulation of the Draft Tube Vortex in Francis Turbines at Partial Discharge

Susan-Resiga, Romeo,Muntean, Sebastian,Stein, Peter,Avellan, Francois Korean Society for Fluid machinery 2009 International journal of fluid machinery and syste Vol.2 No.4

The flow in the draft tube cone of Francis turbines operated at partial discharge is a complex hydrodynamic phenomenon where an incoming steady axisymmetric swirling flow evolves into a three-dimensional unsteady flow field with precessing helical vortex (also called vortex rope) and associated pressure fluctuations. The paper addresses the following fundamental question: is it possible to compute the circumferentially averaged flow field induced by the precessing vortex rope by using an axisymmetric turbulent swirling flow model? In other words, instead of averaging the measured or computed 3D velocity and pressure fields we would like to solve directly the circumferentially averaged governing equations. As a result, one could use a 2D axi-symmetric model instead of the full 3D flow simulation, with huge savings in both computing time and resources. In order to answer this question we first compute the axisymmetric turbulent swirling flow using available solvers by introducing a stagnant region model (SRM), essentially enforcing a unidirectional circumferentially averaged meridian flow as suggested by the experimental data. Numerical results obtained with both models are compared against measured axial and circumferential velocity profiles, as well as for the vortex rope location. Although the circumferentially averaged flow field cannot capture the unsteadiness of the 3D flow, it can be reliably used for further stability analysis, as well as for assessing and optimizing various techniques to stabilize the swirling flow. In particular, the methodology presented and validated in this paper is particularly useful in optimizing the blade design in order to reduce the stagnant region extent, thus mitigating the vortex rope and expending the operating range for Francis turbines.

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.

Analysis of Two Dimensional and Three Dimensional Supersonic Turbulence Flow around Tandem Cavities

Chel Hun Woo,Jae Soo Kim,Kyung Hwan Lee 대한기계학회 2006 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.20 No.8

The supersonic flows around tandem cavities were investigated by two-dimensional and three-dimensional numerical simulations using the Reynolds-Averaged Navier-Stokes (RANS) equation with the k-ω turbulence model. The flow around a cavity is characterized as unsteady flow because of the formation and dissipation of vortices due to the interaction between the freestream shear layer and cavity internal flow, the generation of shock and expansion waves, and the acoustic effect transmitted from wake flow to upstream. The upwind TVD scheme based on the flux vector split with van Leer's limiter was used as the numerical method. Numerical calculations were performed by the parallel processing with time discretizations carried out by the 4th-order Runge-Kutta method. The aspect ratios of cavities are 3 for the first cavity and 1 for the second cavity. The ratio of cavity interval to depth is 1. The ratio of cavity width to depth is 1 in the case of three dimensional flow. The Mach number and the Reynolds number were 1.5 and 4.5 × 10?, respectively. The characteristics of the dominant frequency between two-dimensional and three-dimensional flows were compared, and the characteristics of the second cavity flow due to the first cavity flow was analyzed. Both two dimensional and three dimensional flow oscillations were in the 'shear layer mode', which is based on the feedback mechanism of Rossiter's formula. However, three dimensional flow was much less turbulent than two dimensional flow, depending on whether it could inflow and outflow laterally. The dominant frequencies of the two dimensional flow and three dimensional flows coincided with Rossiter's 2nd mode frequency. The another dominant frequency of the three dimensional flow corresponded to Rossiter's 1st mode frequency.

Analysis of Two Dimensional and Three Dimensional Supersonic Turbulence Flow around Tandem Cavities

Woo Chel-Hun,Kim Jae-Soo,Lee Kyung-Hwan The Korean Society of Mechanical Engineers 2006 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.20 No.8

The supersonic flows around tandem cavities were investigated by two-dimensional and three-dimensional numerical simulations using the Reynolds-Averaged Navier-Stokes (RANS) equation with the k- ω turbulence model. The flow around a cavity is characterized as unsteady flow because of the formation and dissipation of vortices due to the interaction between the freestream shear layer and cavity internal flow, the generation of shock and expansion waves, and the acoustic effect transmitted from wake flow to upstream. The upwind TVD scheme based on the flux vector split with van Leer's limiter was used as the numerical method. Numerical calculations were performed by the parallel processing with time discretizations carried out by the 4th-order Runge- Kutta method. The aspect ratios of cavities are 3 for the first cavity and 1 for the second cavity. The ratio of cavity interval to depth is 1. The ratio of cavity width to depth is 1 in the case of three dimensional flow. The Mach number and the Reynolds number were 1.5 and $4.5{\times}10^5$, respectively. The characteristics of the dominant frequency between two- dimensional and three-dimensional flows were compared, and the characteristics of the second cavity flow due to the first cavity flow was analyzed. Both two dimensional and three dimensional flow oscillations were in the 'shear layer mode', which is based on the feedback mechanism of Rossiter's formula. However, three dimensional flow was much less turbulent than two dimensional flow, depending on whether it could inflow and outflow laterally. The dominant frequencies of the two dimensional flow and three dimensional flows coincided with Rossiter's 2nd mode frequency. The another dominant frequency of the three dimensional flow corresponded to Rossiter's 1st mode frequency.

국내 중소하천에 설치된 아이스하버 어도 내부 흐름 특성 규명

백경오,민병조 한국수자원학회 2022 한국수자원학회논문집 Vol.55 No.1

In this study, flow patterns in the ice-harbor fishway were analyzed according to fluctuations of the upstream water level, an increase of weir interval, and the presence or absence of orifices using a three-dimensional commercial numerical model, Flow-3D. In order to prove the suitability of the numerical simulation results, the flow velocity and flow rate at the exit of the fishway were observed using a 3D ultrasonic velocimetry on an actual ice-harbor fishway installed downstream of the Daegok bridge in Gyeongan-Cheon. Four types of turbulence modules can be selected for the Flow-3D model. As a result of verification with observation data, the RNG model best described the flow characteristics in the ice-harbor fishway. The velocity structure in the fishway according to fluctuations of the upstream water level was simulated. The results showed that the plunging flow and the streaming flow were mixed at the lowest water level. When the water level increased about 10 cm or more from the lowest water level, the plunging flow disappeared in all pools and only the streaming flow occurred. Contrary to expectations, even when the water level is rose a little, the flow simply occurred mainly on the streaming flow. If the interval between the weirs is increased, both the plunging flow and the streaming flow are showed continued even if the water level rises. In addition, compared to the case where there are no orifices at the bottom of the weirs, the plunging flow tends to be generated in several pools. It is necessary to prevent blocking orifices through active management so that various flow patterns in the fishway can be generated in multiple pools. 본 연구에서는 3차원 상용 수치모형 Flow-3D를 활용하여 상류 수위(유입유량) 변동, 격벽의 간격(pool의 길이) 변화, 잠공 유무 등에 따른 아이스하버 어도 내 흐름 양상을 분석하였다. 수치모의 결과의 적합성을 입증하기 위해 경안천 대곡교 하류에 설치된 실제 어도에서 3차원 초음파 유속계를 사용하여 어도 출구부 단면 유속과 유량을 관측하였다. Flow-3D 모형에는 난류 모듈로 4가지를 선택할 수 있는데, 관측 자료로 검증 결과 RNG 모형이 아이스하버 어도 내 흐름 특성을 가장 잘 재현하였다. 하천의 유량 전량이 어도로만 유입되어 흐른다는 조건에서 수위 변화에 따른 어도 내 유속구조를 모의해 보았다. 그 결과 최저수위에서는 잠입류와 표면류가 혼재하여 발생하였는데, 최저수위에서 약 10 cm 이상만 수위가 상승하여도 모든 pool에서 잠입류가 사라지고 오직 표면류만 발생하였다. 예상과 달리 수위가 조금 상승해도 흐름이 꽤 단순하게 표면류 위주로 발생하였다. 격벽간 간격을 늘려주면 수위가 상승하더라도 잠입류와 표면류가 혼재되어 나타나는 현상이 지속되었다. 그리고 격벽 하단에 잠공이 있는 경우가 없는 경우에 비해 잠입류가 여러 pool에서 생성되는 경향을 보였다. 보다 적극적인 어도 사후 관리로 잠공 폐색을 막아 어도 내 다양한 흐름이 생성될 수 있도록 유도하는 것이 필요해 보인다.