• KSCE Journal of Civil and Environmental Engineering Research
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• v.28 no.6B
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• pp.643-651
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• 2008

In the present paper, turbulent open-channel flows over longitudinal bedforms are numerically simulated. The Reynolds- averaged Navier-Stokes equations in curvilinear coordinates are solved with the non-linear $k-{\varepsilon}$ model by Speziale( 1987). First, the developed model is applied to rectangular open channel flows for purposes of model validation and parameter sensitivity studies. It is found that the parameters $C_D$ and $C_E$ are important to the intensity of secondary currents and the level of turbulent anisotropy, respectively. It is found that the non-linear $k-{\varepsilon}$ model can hardly reproduce the turbulence anisotropy near the free surface. However, the overall pattern of the secondary currents by the present model is seen to coincide with measured data. Then, numerical simulations of turbulent flows over longitudinal bedforms are performed, and the simulated results are compared with the experimental data in the literature. The simulated secondary currents clearly show upflows and downflows over the ridges and troughs, respectively. The numerical results of secondary currents, streamwise mean velocity, and turbulence structures compare favorably with the measured data. However, it is observed that the secondary currents towards the troughs were significantly weak compared with the measured data.

• Journal of the Korean Society of Food Science and Nutrition
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• v.37 no.10
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• pp.1278-1286
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• 2008

The effect of Tween 20 addition on changes in the stability of methylcellulose (MC) emulsions (1 wt% MC, 10 wt% n-tetradecane, 20 mM bis-tris buffer, pH 7) was investigated by creaming stability and orthokinetic stability measurements. In the case of MC emulsions containing varying amounts of oil (1$\sim$30 wt%) and no Tween 20 added, creaming stability, judged by mean migration velocity of fat globules ($V_m$), was found to depend on droplet size: the larger the droplet size, the worse the stability [$V_m$: 0.326 $\mu$m $min^{-1}$ ($d_{32}$: 0.32 $\mu$m) ${\rightarrow}V_m$: 0.551 $\mu$m $min^{-1}$ ($d_{32}$: 0.53 $\mu$m)]. With Tween 20, creaming stability was found to be worse than the one without Tween 20, except for MC emulsion containing 0.2 wt% Tween 20. In addition, cream stability was the lowest with the lowest concentration of Tween 20 and a tendency to recover with increasing Tween 20 concentration [$V_m$: 0.598 $\mu$m $min^{-1}$ (0.01 wt%)${\rightarrow}V_m$: 0.389 $\mu$m $min^{-1}$ (0.2 wt%)] was found. From viscosity measurement for aqueous bulk phase of MC emulsions, such a change in the creaming stability was found to coincide well with the results of viscosity measurement. Therefore, it was reasonable to say that creaming stability of MC emulsions containing Tween 20 depended on MC concentration in aqueous bulk phase, which was in turn varied by competitive adsorption between MC and Tween 20 at the oil droplet surface. In case of orthokinetic stability, judged by destabilization time ($t_d$), it was found that the addition of Tween 20 resulted in lowered stability with more pronounce tendency at higher concentrations [$t_d$: 160 min (0.03 wt%)${\rightarrow}t_d$: 100 min (0.2 wt%)]. Moreover, combined with previous results, the orthokinetic stability of MC emulsions containing Tween 20 was found to be exponentially proportional to MC load. In conclusion, competitive adsorption between MC and Tween 20 may affect the stability properties of MC emulsion to varying extents, depending on the concentration of Tween 20.