on the measurement of transverse shear stress in a rectangular open channel using an optimal preston tube

The laboratory studies have been carried out in this research. determining the sensitivity analysis of the preston tube diameter in shear stress, four preston tubes with external diameters of 3.2, 3.9, 4.7 and 6.3 mm were used. the aspect ratios of 2.86 to 13.95 were examined. for measuring the pressure difference of the preston tube a 200 millibar differential pressure transducer with 0.01 accuracy of the original scale was used. laboratory results demonstrated that preston tubes with a diameter of 3.9 mm present the minimum difference in the average value of the shear stress resulting from the bechert and patel calibration equations. therefore, using the preston tube with an optimal diameter, transverse distribution of shear stress in channels bed and wall were determined. the outcome of this part of study is two dimensionless relationships for determining the local shear stress both in the bed and wall. these relationships are a function of the aspect ratio b/h and the bed relative coordinates b/b in cross section and z/h sidewall. the survey showed that the dimensionless bed shear stress distribution is considerably influenced by the aspect ratio. the transverse distribution of wall shear stress is independent from the aspect ratio for b/h>3.