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Experimental Research on boundary shear stress in Typical Meandering Channel

China Ocean Engineering 2018年第3期32卷 365-373页

作者： CHEN Kai-hua XIA Yun-feng ZHANG Shi-zhao WEN Yun-cheng XU Hua Nanjing Hydraulic Research Institute Nanjing 210029China College of Harbor Coastal and Offshore EngineeringHohai UniversityNanjing 210098China The State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering Nanjing 210098China Key Laboratory of Port Waterway and Sedimentation Engineering of Ministry of TransportNanjing 210024China

A novel instrument named Micro-Electro-Mechanical System(MEMS) flexible hot-film shear stress sensor was used to study the boundary shear stress distribution in the generalized natural meandering open channel, and the mean sidewall shear stress distribution along the meandering channel, and the lateral boundary shear stress distribution in the typical cross-section of the meandering channel was analysed. Based on the measurement of the boundary shear stress, a semi-empirical semi-theoretical computing approach of the boundary shear stress was derived including the effects of the secondary flow, sidewall roughness factor, eddy viscosity and the additional Reynolds stress, and more importantly, for the first time, it combined the effects of the cross-section central angle and the Reynolds number into the expressions. Afterwards, a comparison between the previous research and this study was developed. Following the result, we found that the semi-empirical semi-theoretical boundary shear stress distribution algorithm can predict the boundary shear stress distribution precisely. Finally, a single factor analysis was conducted on the relationship between the average sidewall shear stress on the convex and concave bank and the flow rate, water depth, slope ratio,or the cross-section central angle of the open channel bend. The functional relationship with each of the above factors was established, and then the distance from the location of the extreme sidewall shear stress to the bottom of the open channel was deduced based on the statistical theory.

关键词： boundary shear stress flume physical model river bend MEMS flexible hot-film shear stress sensor

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Prediction of boundary shear stress distribution in straight open channels using velocity distribution

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Water Science and Engineering 2021年第2期14卷 159-166页

作者： Behzad Malvandi Mahmoud F.Maghrebi Department of Civil Engineering Ferdowsi University of MashhadMashhad 91775-1111Iran

Conventional methods for measuring local shear stress on the wetted perimeter of open channels are related to the measurement of the very low velocity close to the *** near-zero velocity values with high fluctuations has always been a difficult task for fluid flow near solid *** solve the observation problems,a new model was developed to estimate the distribution of boundary shear stress from the velocity distribution in open channels with different cross-sectional *** estimate the shear stress at a point on the wetted perimeter by the model,the velocity must be measured at a point with a known normal distance to the *** experimental work of some other researchers on channels with various cross-sectional shapes,including rectangular,trapezoidal,partially full circular,and compound shapes,was used to evaluate the performance of the proposed *** exponent coefficients for the model were found using the multivariate Newton method with the minimum of the mean absolute percentage error(MAPE)between the model and experimental data as the objective ***,the calculated shear stress distributions along the wetted perimeter were compared with the experimental *** most important advantage of the proposed model is its inherent *** mean MAPE value for the seven selected cross-sections was 6.9%.The best results were found in the cross-sections with less discontinuity of the wetted perimeter,including the compound,trapezoidal,and partially full circular *** contrast,for the rectangular cross-section with an angle between the bed and walls of 90°,MAPE increased due to the large discontinuities.

关键词： Open channel boundary shear stress Viscous shear stress Velocity distribution Velocity gradient

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DISTRIBUTION OF LOCAL FRICTION IN AN OPEN CHANNEL

DISTRIBUTION OF LOCAL FRICTION IN AN OPEN CHANNEL

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第九次河流泥沙国际学术大会

作者： Mirali MOHAMMADI Dept. of Civil Eng. Faculty of Eng. The University of Urmia Urmia 57169-33111 Iran Dept. of Civil Eng. Faculty of Eng. The Islamic Azad University Khoy Unit Khoy Iran

＜正＞In open channels, estimation of the local and global friction factor is one of the most important tasks and it sometimes is very difficult. This difficulty arises from the existence of the secondary flow cells and the boundary induced turbulence, both of which affect the distributions of depth-averaged velocity and boundary shear stress, and consequently the local friction factors. A series of measurements was undertaken for velocity and boundary shear stress in fully developed turbulent uniform flow for a smooth open channel. Equations relating the transverse distribution of apparent shear force across the channel boundary for various flow conditions are presented. The experimental overall friction factors were evaluated from the stage-discharge relationships applying the Darcy-Weisbach equation. Using the measured local depth-averaged velocity, Ud, and me local bed shear stress, τb, the local depth-averaged bed friction factor,fb, can be calculated for each lateral position by applying fb=8τb/ρUd 2 . The lateraldistribution of local depth-averaged friction factors show that the distributions are close together for low flow discharges in both sub critical and supercritical flows, but they are more variable for the higher flow discharges. In other words, the normalized local friction factors, fb/f, illustrate that the discrepancy in the higher flow rates are larger than the lower flow rates. More importantly, the Figures show that the local friction factor values tend towards a more constant distribution at high flow rates as the Froude numbers increase. It can also be seen that the local friction factor is found to be decreased from the sidewalls to the channel centerline.

关键词： Open channel flow Friction factor Depth-averaged velocity boundary shear stress

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