• Journal of Korea Water Resources Association
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• v.42 no.8
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• pp.619-629
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• 2009

The objective of this study is to estimate bed-resistance in gravel-bed rivers using the equivalent roughness height($k_s$). We calculated the friction factor(f) with the measured data from 8 domestic gravel-bed rivers and investigated the size distributions of the bed materials. The averaged $k_s$ in each cross-section, which is determined under the hypothesis that the vertical velocity distribution follows the logarithmic law, is compared with the reach $k_s$ which is calculated with the cumulative grain diameter distribution curve of bed materials. Moreover, the applicability of existing formulae, such as Strickler type equations, is examined by comparing with Manning's n value converted from the $k_s$. According to the results, the reach $k_s$ proves to be a good indicator of representative characteristic of bed materials in a reach, and the Manning's n based on the reach $k_s$ is appropriate for practical estimation of the bed-resistance, for RMS errors between calculated and measured Manning's n is less than 0.003. The correlation between the $k_s$ and specified bed-material size($D_i$) is very low, so it is difficult to select a proper one among the existing empirical equations.

• Wind and Structures
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• v.11 no.4
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• pp.275-289
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• 2008

Large eddy simulations have been performed within and over different types of urban building arrays. This paper adopted three dimensionless parameters, building frontal area density (${\lambda}_f$) the variation degree of building height (${\sigma}_h$), and the staggered degree of building range ($r_s$), to study the systematic influence of building spacing, height and layout on wind and turbulent characteristics. The following results have been achieved: (1) As ${\lambda}_f$ decrease from 0.25 to 0.18, the mean flow patterns transfer from "skimming" flow to "wake interference" flow, and as ${\lambda}_f$ decrease from 0.06 to 0.04, the mean flow patterns transfer from "wake interference" flow to "isolated roughness" flow. With increasing ${\lambda}_f$, wind velocity within arrays increases, and the vortexes in front of low buildings would break, even disappear, whereas the vortexes in front of tall buildings would strengthen and expand. Tall buildings have greater disturbance on wind than low buildings do. (2) All the wind velocity profiles and the upstream profile converge at the height of 2.5H approximately. The decay of wind velocity within the building canopy was in positive correlation with ${\lambda}_f$ and $r_s$. If the height of building arrays is variable, Macdonald's wind velocity model should be modified through introducing ${\sigma}_h$, because wind velocity decreases at the upper layers of the canopy and increases at the lower layers of the canopy. (3) The maximum of turbulence kinetic energy (TKE) always locates at 1.2 times as high as the buildings. TKE within the canopy decreases with increasing ${\lambda}_f$ and $r_s$ but the maximum of TKE are very close though ${\sigma}_h$ varies. (4) Wind velocity profile follows the logarithmic law approximately above the building canopy. The Zero-plane displacement $z_d$ heighten with increasing ${\lambda}_f$, whereas the maximum of and Roughness length $z_0$ occurs when ${\lambda}_f$ is about 0.14. $z_d$ and $z_0$ heighten linearly with ${\sigma}_h$ and $r_s$, If ${\sigma}_h$ is large enough, $z_d$ may become higher than the average height of buildings.

• Korean Journal of Food Science and Technology
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• v.27 no.5
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• pp.799-806
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• 1995

The steady shear and small amplitude oscillatory dynamic rheological properties of citrus pectin $([\eta]=3.75\;dL/g)$ were characterized for a wide range of pectin concentrations $({\sim}6%)$. The typical power-law flow was observed above 2.0% concentration, and the shear rate dependence of viscosity increased with pectin concentration. The transition from dilute to concentrated regime, determined from the double logarithmic plot of ${\eta_{sp.o}}\;vs\;C[\eta]$, occurred at a critical coil overlap parameter $C^{*}[\eta]\approx4.0$, at which ${\eta_{sp.o}}$ corresponded to approximately 10.0. The slopes of ${\eta_{sp.o}}\;vs\;C[\eta]$, at $C[\eta]\;at\;C[\eta]C^{*}[\eta]$were 1.1 and 4.5, respectively. The steady viscosity $(\eta)$ displayed a good superposition at ${\eta}/{\eta}_o\;vs\;{\gamma}/{\gamma}_{0.8}$ relation with an exception of high concentration (6%), which arised from the significant deviation of flow behavior index (n values of $\eta_{a}=K\gamma^{n-1}$) at high concentration. Dynamic measurements showed that the loss modulus $(G^{\prime\prime})$ was much higher than the storage modulus $(G^\prime)$for all concentrations studied, indicating predominant viscoelastic liquid-like behavior of pectin solutions. The frequency dependence of $G^\prime$ was higher than that of $G^\prime\prime$ at the same concentration, whose trend was more pronounced with decreasing pectin concentration. The shear viscosity $(\eta)$ was almost identical to the complex viscosity $(\eta^{*})$ at low concentration, following the Cox-Merz rule, but they became increasingly different at high concentration.