Instantaneous Turbulent Structure Relating to Momentum and Scalar Transport in a Reducing Channel Flow With Surfactant Additives

2017 ◽  
Author(s):  
Takuya Matsumoto ◽  
Shumpei Hara ◽  
Takahiro Tsukahara ◽  
Yasuo Kawaguchi
Keyword(s):  
Reynolds Shear Stress ◽  
Surfactant Solution ◽  
Outer Edge ◽  
Wall Region ◽  
Turbulent Structures ◽  
Solution Flow ◽  
Partial Explanation ◽  
Vortex Model ◽  
Planar Laser ◽  
High Reynolds

Turbulent surfactant solution flows dramatically suppress turbulent scalar and momentum transports with changes to turbulent structures near the wall. In this study, particle image velocimetry and planar laser induced fluorescence concentration measurement method were used simultaneously to analyze turbulent mass transfer experimentally in surfactant channel flows at high Reynolds number. When compared against the instantaneous flow fields of the water case, the results showed a decrease in the magnitude of elementary vortices in the near-wall region. Momentum and scalar transports are caused by the combination of elementary vortices that are irregularly arranged at the outer edge of the shear layer. A conceptual vortex model is proposed for turbulent scalar transfer that provides a partial explanation for the turbulence statistics of a surfactant solution flow, such as the Reynolds shear stress, turbulent mass flux, and mean concentration distribution.

scholarly journals Study on the Drag Reducing Channel Fluids by Experiments and DNS Using Giesekus Model

2014 ◽  
Vol 6 ◽  
pp. 175059 ◽  
Author(s):  
Weiguo Gu ◽  
Dezhong Wang ◽  
Yasuo Kawaguchi
Keyword(s):  
Shear Stress ◽  
Channel Flow ◽  
Mass Concentration ◽  
Reynolds Shear Stress ◽  
Surfactant Solution ◽  
Solution Flow ◽  
Velocity Fluctuations ◽  
Giesekus Model ◽  
Numerical Studies ◽  
Fluid Characteristics

Both experimental and numerical studies are simultaneously performed for fully developed water and surfactant solution channel flow. The comparison aims at the surfactant solution flow in experiment with mass concentration of 25 ppm at Re = 1000 and Giesekus model with Weissenberg numbers of 10 and 40 at Reτ = 150. Big differences are found between the experimental and DNS results by comparing the distributions of velocity fluctuations, Reynolds shear stress, and so on. Although large drag reduction appears in DNS, Giesekus model has some limitations in describing the fluid characteristics and viscoelasticity of the surfactant solution.

Spectral Analysis and Discussion on the Velocity Fluctuation in Drag Reducing Channel Flow by Surfactant Additives

2016 ◽  
Author(s):  
Yuichi Kaiho ◽  
Shumpei Hara ◽  
Takahiro Tsukahara ◽  
Yasuo Kawaguchi
Keyword(s):  
Channel Flow ◽  
Time Scale ◽  
Velocity Fluctuation ◽  
Surfactant Solution ◽  
Turbulent Velocity ◽  
Laser Doppler Velocimeter ◽  
Wall Region ◽  
Solution Flow ◽  
Velocity Fluctuations ◽  
Turbulent Velocity Fluctuation

It is known as the Toms effect that the wall friction coefficient is reduced by adding a small amount of polymer or surfactant into a water flow. In the drag-reducing flow, it is expected that a time scale of turbulent velocity fluctuation is changed by relaxation time due to viscoelasticity. In the present study, experimental analysis of the turbulent velocity fluctuation was performed with temporal characteristics in surfactant solution flow. The velocity fluctuations were measured by using a two-component laser Doppler velocimeter system on turbulent channel flow. And then, we performed statistical operation on those data and examined the time scale. From spectra analysis, it was found that very low frequency velocity fluctuations existed near the wall region in the surfactant solution flow. It was also revealed that the strong anisotropy occurred not only with the intensity but also with frequency distribution in turbulent velocity fluctuations. Moreover, the turbulence contributes nothing to the Reynolds shear stress and behaves as a wave motion. It was concluded that the turbulent eddies and viscoelasticity were two factors contributing to turbulent generation in the viscoelastic turbulent flow, with each factor having its own time scale.

scholarly journals Asymmetrical Velocity Distribution in the Drag-Reducing Channel Flow of Surfactant Solution Caused by an Injected Ultrathin Water Layer

Symmetry ◽  
2020 ◽  
Vol 12 (5) ◽  
pp. 846
Author(s):  
Zaiguo Fu ◽  
Xiaotian Liang ◽  
Kang Zhang
Keyword(s):  
Velocity Distribution ◽  
Channel Flow ◽  
Reynolds Shear Stress ◽  
Surfactant Solution ◽  
Water Layer ◽  
Statistical Characteristics ◽  
Wall Region ◽  
Experimental Conditions ◽  
Mean Velocity ◽  
Near Wall

Although the turbulent intensity is suppressed in the drag-reducing channel flow by viscoelastic additives, the mean velocity distribution in the channel flow is symmetrical and tends to be similar to the laminar flow. In the study of near-wall modulation of the drag-reducing flow with an injected ultrathin water layer, an asymmetrical mean velocity distribution was found. To further investigate this phenomenon and the underlying cause, an experiment was carried out with the water injected from a porous channel wall at a small velocity (~10−4 m/s) into the drag-reducing flow of surfactant solution. The instantaneous concentration and flow fields were measured by using planar laser-induced fluorescence (PLIF) and particle imaging velocimetry (PIV) techniques, respectively. Moreover, analyses on turbulent statistical characteristics and spatial distribution of viscoelastic structures were carried out on the basis of comparison among various flow cases. The results showed that the injected ultrathin water layer under present experimental conditions affected the anisotropy of the drag-reducing flow. The characteristics, such as turbulence intensity, showed the zonal feature in the wall-normal direction. The Reynolds shear stress was enhanced in the near-wall region, and the viscoelastic structure was modified severely due to the redistributed stress. These results may provide experimental supports for the near-wall modulation of turbulence and the exploration of the drag-reducing mechanism by viscoelastic additives.

Simultaneous PIV and PLIF Measurement on the Drag Reducing Channel Flow of the Homogeneous Surfactant Solution

2011 ◽  
Author(s):  
Takahiro Watanabe ◽  
Kohei Tanaka ◽  
Masaaki Motozawa ◽  
Yasuo Kawaguchi
Keyword(s):  
Drag Reduction ◽  
Reduction Rate ◽  
Surfactant Solution ◽  
Coherent Motion ◽  
Wall Region ◽  
Frictional Drag ◽  
Piv Measurement ◽  
Planar Laser ◽  
The Relationship ◽  
Plif Measurement

Simultaneous Particle Image Velocimetry (PIV) measurement and Planar Laser Induced Fluorescence (PLIF) measurement at the same position were performed to clarify the relationship between spatial structure and mass transfer in the drag reducing surfactant flow. In the drag reducing flow, mass flux is largely suppressed in the near-wall region with increasing drag reduction rate. To discuss the relationship between coherent motion and drag reduction more detail, weighted probability density function was also calculated. As a result of simultaneous measurement, diffusion of wall-normal direction is largely suppressed and this indicated that turbulent coherent structure changes and sweep and ejection which produce the skin frictional drag are suppressed.

A Study on Heat Transport Phenomena in a Developed Thermal Boundary Layer of Drag Reducing Channel Flow

2016 ◽  
Author(s):  
K. Watanabe ◽  
Y. Kaiho ◽  
S. Hara ◽  
T. Tsukahara ◽  
Y. Kawaguchi
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Heat Transport ◽  
Thermal Boundary Layer ◽  
Transport Phenomena ◽  
Surfactant Solution ◽  
Temperature Fluctuations ◽  
Large Temperature ◽  
Wall Region ◽  
Solution Flow

The heat transport phenomena in a developed thermal boundary layer of surfactant solution flow were investigated experimentally. The experiment was conducted under different surfactant additive concentrations. The temperature fluctuations in a thermal boundary layer in a smooth channel flow were measured by fine-wire thermocouple probe. Heat transfer reducing rate and temperature fluctuation characteristics including mean temperature distribution, intensity, wave form, spectral density function, and skewness factor were studied. The results showed that the turbulent transport is obstructed by additives, and the temperature field shows dramatic changes. High frequency component of temperature fluctuation of surfactant solution flow was decreased due to suppression of turbulence and viscoelasticity. Large temperature fluctuations occur in the thermal boundary layer because the development of the thermal boundary layer is obstructed, and large temperature fluctuations appear to concentrate the temperature gradient in the near-wall region (10 < y+ < 60). In contrast, viscous sublayer expands due to viscoelasticity, and the temperature gradient and turbulence fluctuation are small in the near-wall region of y+ < 10. As a result, two layers having significantly different characteristics seem to coexist. The heat transfer reduction is constant with variation of additive concentration condition, but heat transport phenomena were microscopically influenced by viscoelasticity.

Large Eddy Structure Appearing in Meandering Motion of High Reynolds Number Viscoelastic Flow Past a Backward-Facing Step

2017 ◽  
Author(s):  
Shohei Onishi ◽  
Ryusuke Ii ◽  
Shumpei Hara ◽  
Takahiro Tsukahara ◽  
Yasuo Kawaguchi
Keyword(s):  
Shear Stress ◽  
High Reynolds Number ◽  
Reynolds Shear Stress ◽  
Pressure Rise ◽  
Surfactant Solution ◽  
Main Flow ◽  
Recirculation Region ◽  
The Mean ◽  
Surfactant Additive ◽  
High Reynolds

The addition of a small amount of surfactant into water induces viscoelasticity. In turbulent channel and pipe flows, the wall friction significantly decreases with the surfactant additive, but a heat-transfer reduction also occurs. On the other hand, a phenomenon is reported that the instantaneous main flow broadly meanders in space and time under a certain condition in the surfactant solution flow past a backward-facing step (BFS); thus, the mean Reynolds shear stress remarkably increases. The influence of such a phenomenon cannot be neglected in thermal fluid equipment with usually complicated configurations. Therefore, understanding this phenomenon in detail is important both academically and industrially. In this study, particle image velocimetry measurements were carried out in a viscoelastic BFS flow with meandering phenomenon at high Reynolds number. We focused on the turbulent structures in the flow fields and investigated the interrelationship among the spatial scales of the eddy structures, Reynolds shear stress, and meandering motion using a spatial two-point correlation function or conditional averaging. In the meandering condition, we revealed that the Reynolds shear stress due to the low-speed fluid that departs from the upper wall opposite the step across the main flow contributes the largest to the mean Reynolds shear stress value. Then, a secondary recirculation region appears near the upper wall, in addition to the primary recirculation region. A reverse flow is apt to occur when rapid deceleration and pressure rise happens due to the sudden expansion of the channel cross section. Therefore, flow separation occurs at the upper wall, and a large-scale circulation appears there. Such flow is caused by the relationship between the pressure rise and the momentum transfer between the flow and the wall. We can conclude that the condition where an unstable motion occurs is influenced by the concentration of the surfactant solution because the surfactant additive acts on the shear stress.

Experimental Study of Turbulence Transport in a Dilute Surfactant Solution Flow Investigated by PIV

2010 ◽  
Vol 132 (5) ◽  
Author(s):  
Weiguo Gu ◽  
Yasuo Kawaguchi ◽  
Dezhong Wang ◽  
Saito Akihiro
Keyword(s):  
Particle Image ◽  
Reynolds Shear Stress ◽  
Surfactant Solution ◽  
Instantaneous Velocity ◽  
Normal Velocity ◽  
Solution Flow ◽  
Velocity Fluctuations ◽  
Image Velocimetry ◽  
Turbulence Transport ◽  
Main Factor

Drag-reducing flow of dilute surfactant solution in the two-dimensional channel is investigated experimentally by using particle image velocimetry (PIV) system. Five hundred instantaneous velocity frames of u-v in the x-y plane are taken by PIV for every condition. Fluctuation intensity and instantaneous velocity distributions are discussed in order to study the turbulence transport in the drag-reducing flow. As compared with water, the results show that wall-normal velocity fluctuations in the drag-reducing flow are suppressed significantly, and instantaneous velocity distributions display different features. Moreover, the drag-reducing flow exhibits the reduced inclination angle of turbulence transport and appearance of “zero Reynolds shear stress.” High shear dissipation also appears in some solutions. Based on the analysis of the balance of mean and mean turbulent kinetic energies, it is found that the complex rheology, i.e., the elasticity and viscosity of the solution, is considered as the main factor that change the characteristics of turbulence transport.

The Effects of Adverse Pressure Gradients on Momentum and Thermal Structures in Transitional Boundary Layers: Part 2—Fluctuation Quantities

1996 ◽  
Vol 118 (4) ◽  
pp. 728-736 ◽  
Author(s):  
S. P. Mislevy ◽  
T. Wang
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Transition Region ◽  
Boundary Layers ◽  
Reynolds Shear Stress ◽  
Turbulent Shear ◽  
Wall Region ◽  
Pressure Gradients ◽  
Baseline Case ◽  
Near Wall

The effects of adverse pressure gradients on the thermal and momentum characteristics of a heated transitional boundary layer were investigated with free-stream turbulence ranging from 0.3 to 0.6 percent. Boundary layer measurements were conducted for two constant-K cases, K1 = −0.51 × 10−6 and K2 = −1.05 × 10−6. The fluctuation quantities, u′, ν′, t′, the Reynolds shear stress (uν), and the Reynolds heat fluxes (νt and ut) were measured. In general, u′/U∞, ν′/U∞, and νt have higher values across the boundary layer for the adverse pressure-gradient cases than they do for the baseline case (K = 0). The development of ν′ for the adverse pressure gradients was more actively involved than that of the baseline. In the early transition region, the Reynolds shear stress distribution for the K2 case showed a near-wall region of high-turbulent shear generated at Y+ = 7. At stations farther downstream, this near-wall shear reduced in magnitude, while a second region of high-turbulent shear developed at Y+ = 70. For the baseline case, however, the maximum turbulent shear in the transition region was generated at Y+ = 70, and no near-wall high-shear region was seen. Stronger adverse pressure gradients appear to produce more uniform and higher t′ in the near-wall region (Y+ < 20) in both transitional and turbulent boundary layers. The instantaneous velocity signals did not show any clear turbulent/nonturbulent demarcations in the transition region. Increasingly stronger adverse pressure gradients seemed to produce large non turbulent unsteadiness (or instability waves) at a similar magnitude as the turbulent fluctuations such that the production of turbulent spots was obscured. The turbulent spots could not be identified visually or through conventional conditional-sampling schemes. In addition, the streamwise evolution of eddy viscosity, turbulent thermal diffusivity, and Prt, are also presented.

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