• Title/Summary/Keyword: Turbulent Shear Stress

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Reduction of Skin Friction Force for Turbulent Boundary Layer (난류 경계층의 표면 마찰력 감소화)

  • Kim, Si-Young
    • Journal of Fisheries and Marine Sciences Education
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    • v.5 no.2
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    • pp.128-137
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    • 1993
  • This paper presents a new concept to reduce turbulent frictional drag by injecting micro-bubble into buffer layer of turbulent boundary layer on flat plate. The buffer layer of boundary was specified by minus velocity gradient of law of the wall. When the buffer layer region of turbulent boundary layer is filled with micro-bubble of air and viscous of the region is kept low, the velocity profile in the region should be changed substantially. Then the Reynolds stress in the buffer layer region becomes less, which guide to higher velocity gradient there. It results in reduction of velocity gradient at the viscous sublayer, which gives the reduction of shear stress at the wall.

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A RANS modeling of backward-facing step turbulent flow in an open channel (개수로에서의 후향단차 난류 흐름 RANS 수치모의)

  • Kim, Byungjoo;Paik, Joongcheol
    • Journal of Korea Water Resources Association
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    • v.55 no.2
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    • pp.147-157
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    • 2022
  • The backward-facing step (BFS) is a benchmark geometry for analyzing flow separation occurred at the edge and resulting development of shear layer and recirculation zone that are occupied by turbulent flow. It is important to accurately reproduce and analyze the mean flow and turbulence statistics of such flows to design physically stable and performance assurance structure. We carried out 3D RANS computations with widely used, two representative turbulence models, k-ω SST and RNG k-ε, to reproduce BFS flow at the Reynolds number of 23,000 and the Froude number of 0.22. The performance of RANS computations is evaluated by comparing numerical results with an experimental measurement. Both RANS computations with two turbulence models appear to reasonably well reproduce mean flow in the shear layer and recirculation zone, while RNG k-ε computation results in about 5% larger velocity between the outer edge of boundary layer and the free surface above the recirculation zone than k-ω SST computation and experiment. Both turbulence models underestimate the shear stress distribution experimentally observed just downstream of the sharp edge of BFS, while shear stresses computed in the boundary layer downstream of reattachment point are agree reasonably well with experimental measurement. RNG k-ε modeling reproduces better shear stress distribution along the bottom boundary layer, but overestimates shear shear stress in the approaching boundary layer and above the bottom boundary layer downstream of the BFS.

Experimental investigation on the turbulent elliptic jets by using a 3-D LDV system (3-D LDV 시스템을 이용한 타원제트의 난류특성에 관한 연구)

  • 권영철;이상준
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.15 no.6
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    • pp.2160-2170
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    • 1991
  • Three-dimensional turbulent structures in the near field of elliptic jet were experimentally investigated by using a three-color, three-component Laser Doppler Velocimeter. The Reynolds number based on the nozzle exit velocity and nozzle equivalent diameter(De) was about 4*10$^{4}$. The turbulent characteristics of a sharp-edged elliptic nozzle with aspect ratio of 2 were analyzed along major and minor axis at X/De=2,3,5,7 and along the centerline up to X/De=14. Quantities measured at each point with the 3-D LDV system were three orthogonal velocity components, turbulent intensity, skewness, flatness, and Reynolds shear stress. The nondimensional mean velocities coincided well with the Schlichting's empirical curve with going downstream. Elliptic jet of AR=2 had two switching points at about X/De=2 and 16. The turbulent intensity along the minor axis was distributed widely than that along the major axis. In the near field, X/De<5, the Reynolds shear stresses of the inner part of the elliptic jet had negative value, which indicated the enhancement of entrainment toward the inner part.

CHARACTERISTlCS OF PLANE JETS IN THE TRANSITION REGION

  • Seo, Il-Won;Ahn, Jung-Kyu;Kwon, Seok-Jae
    • Water Engineering Research
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    • v.3 no.3
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    • pp.163-176
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    • 2002
  • In this study, laboratory experiments have been performed to investigate characteristics of the velocity fields and turbulence for non-buoyant plane jet in the vicinity of the jet nozzle using PIV system. The experimental results show that, in the transition region, the lateral velocity profile is in good agreement with Gaussian distribution. However, the coefficient of Gaussian distribution, $\K_{u,}$, decreases with longitudinal distance in the transition region. The existing theoretical equation for the centerline velocity tends to overestimate the measured data in the transition region. A new equation for the centerline velocity derived by incorporating varying $k_{u}$ gives better agreement with the measured data than the previous equation. The results of the turbulence characteristics show peak values are concentrated on the shear layers. The Reynolds shear stress profile shows the positive peak in the upper layer and negative peak in the lower layer. The turbulent kinetic energy also provides double peaks at the shear layers. The peak of the Reynolds shear stress and the turbulent kinetic energy increases until x/B=8, and then it decreases afterwards.s.

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Evaluation of the K-Epsilon-VV-F Turbulence Model for Natural Convection in a Rectangular Cavity (직사각형 공동 내부 자연연대류 문제에 대한 k-epsilon-vv-f 난류모델의 평가)

  • Choi Seok-Ki;Kim Seong-O;Kim Eui-Kwang;Choi Hoon-Ki
    • Journal of computational fluids engineering
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    • v.7 no.4
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    • pp.8-18
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    • 2002
  • The primary objective of the present study is evaluation of the k-ε-vv-f turbulence model for prediction of natural convection in a rectangular cavity. As a comparative study, the two-layer k-ε model is also considered. Both models, with and without algebraic heat flux model, are applied to the analysis of natural convection in a rectangular cavity. The performances of turbulence models are investigated through comparison with available experimental data. The predicted results of vertical velocity component, turbulent heat fluxes, turbulent shear stress, local Nusselt number and wall shear stress are compared with experimental data. It is shown that, among the turbulence models considered in the present study, the k-ε-vv-f model with an algebraic heat flux model predicts best the vertical mean velocity and velocity fluctuation, and the inclusion of algebraic heat flux model slightly improves the accuracy of results.

The Equilibrium Solution and the Stability Analysis of Reynolds Stress Equations for a Homogeneous Turbulent Shear Flow (난류 균일전단유동에 대한 레이놀즈 응력 모형방정식의 평형해와 안정성 해석)

  • 이원근;정명균
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.19 no.3
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    • pp.820-833
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    • 1995
  • An analysis is performed to examine the equilibrium state and the stability of modeled Reynolds stress equations for homogeneous turbulent shear flows. The system of the governing equations consists of four coupled ordinary differential equations. The equilibrium states are found by the steady state solution of the governing equations. In order to investigate the stability of the system about its state in equilibrium, and eigenvalue problem is constructed. As a result, constraints for the coeffieients in the model equations are obtained by the stability condition of the equilibrium state as well as by their physically realizable bounds. It is observed that the models with pressure-strain rate correlation that are linear in the anisotropy tensor are stable and produce reasonable equilibrium tensor do not behave properly. Stability considerations about three most commonly used models are given in detail in the final section.

Flow Characteristics of Turbulent Flow in the Exit Region of Join Stream Curved Duct (합류 곡관덕트 출구영역에서 난류유동의 유동특성)

  • Sohn, Hyun-Chull;Park, Sang-Kyoo
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.27 no.5
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    • pp.569-578
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    • 2003
  • In the present steady the flow characteristics of turbulent steady flows were experimentally investigated in the exit region of join stream. The experimental was carry out to measure the velocity profiles of air in a square duct. For the measurement of velocity profiles, a hot-wire anemometer was used. The experimental results shows that the velocity profiles do not change behind the fully developed flow region , which is defined as dimensionless axial direction x/Dh=50. In addition, the gradient of shear stress distribution became stable as the flow reached progress downstream.

Wave-Current Friction in Rough Turbulent Flow (전난류에서 파랑과 해류의 마찰력)

  • 유동훈
    • Journal of Korean Society of Coastal and Ocean Engineers
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    • v.6 no.3
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    • pp.226-233
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    • 1994
  • The present paper considers the method to estimate the bottom friction driven by waves and current on rough turbulent flow. Parameter adjusting technique is suggested for the computation of bed shear stress driven by uni-directional flow. and the value of parameter is determined by comparing the computational results against Bijker's laboratory data. For the computation of combined flow bottom shear stress, two methods are presented; one is the modified Bijker approach (BYO Model) and the other is the modified Fredsoe approach (FY Model). both of which are refined by the present writer. Both models are again refined in two aspects, and tested against the Bijker's laboratory data.

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Experimental Investigation on the Flow in Concentric Annuli with Both Rough Walls (내·외벽에 거칠기가 있는 이중동심관 유동에 대한 실험적 연구)

  • Ahn, S.W.;Jung, Y.B.;Kim, K.C.
    • Korean Journal of Air-Conditioning and Refrigeration Engineering
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    • v.7 no.1
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    • pp.81-88
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    • 1995
  • Fully developed turbulent flow through three concentric annuli with both the rough inner and outer walls was experimentally investigated for a Reynolds number range Re=15,000-85,000. Measurements were made of the pressure drop, the positions of zero shear stress and maximum velocity, and the velocity distributions in annuli of radius ratios, ${\alpha}=0.26$, 0.4 and 0.56, respectively. The experimental results showed that the positions of zero shear streess and maximum velocity were only weakly dependent on the Reynolds number. It was also found that the position of zero shear stress was not coincident with that of maximum velocity. Furthmore, the former was influenced more sensitively than the latter on the square-ribbed roughness along the axial direction.

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Prediction of Hemolysis in Intra-Cardiac Axial Flow Blood Pumps for Optimization of the Impellers (심장 내 이식형 축류 혈액펌프의 임펠러 최적화를 위한 용혈량 예측)

  • Kim, Dong-Uk;Mitamura, Yoshinori
    • The Transactions of the Korean Institute of Electrical Engineers D
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    • v.51 no.9
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    • pp.431-437
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    • 2002
  • Low hemolysis is one of the key factors in the production of successful rotary blood pumps. It is, however, difficult to identify the areas where hemolysis occurs. Computational fluid dynamics(CFD) analysis enables the engineer to predict hemolysis on a computer Fluid dynamics in five different axial flow pumps was analyzed 3-dimensionally using CFD software. The impeller was rotated at a speed which supplied a flow of 5L/min at a pressure difference of 100mmHg. Changes in the turbulent kinetic energy along streamlines through the pumps were computed. Reynolds' shear stress( (equation omitted) ) was calculated using the turbulent kinetic energy. Hemolysis was evaluated based on Reynolds'shear stress and its exposure time(t) : dHb/Hb=3.62$\times$10$^{-5}$ $t^{0.785}$$\tau$$^{2.416}$ . Hemolysis of the pumps was measured in vitro using fresh bovine blood to which citrate phosphate dextrose was added to prevent clotting. A pump flow of 5L/min was maintained at a pressure difference of 100mmHg for 3h. The normalized index of hemolysis(NIH) as measured. Reynolds' shear stress was high behind the impellers. The measured NIH and the calculated hemolysis(dHb/Hb) shoed a good correlation; NIH=0.0003(dHb/Hb) (r=0.90, n=6) in the range of NIH between 0.003 and 1.1. CFD analysis can predict the in vitro results of hemolysis as well as the areas where hemolysis occurs.ysis occurs.