Effect of Preston Tube Diameter to Accurate Estimation of Flow Resistance in Smooth Rectangular Open Channel

In this paper, the effect of tube diameter (Dp) in measurement of shear stress (tb) of the boundary layer wall was investigated under a zero pressure gradient. For this purpose, four different tubes with outside diameters of 3.2, 3.9, 4.7 and 6.3 mm were used which are corresponded to Dp/d of 0.039, 0.047, 0.057 and 0.076, respectively. Measurement of the pressure difference of static point and total in Preston tube was implemented using a differential pressure transducer with accuracy of 0.2 percent of the original scale. The results of the shear stress measurements around Reynolds numbers from 6.4×104 to 39.87×104 using Patel and Bechert calibration equations showed that the Preston tube diameter has a significant impact on the shear stress estimation and both equations are sensitive to the tube diameter. The maximum value of difference in the shear stress measurements tb of different tubes using Patel calibration is about 9%, while the Bechert calibration gives a maximum difference of approximately 14%. Comparing results obtained from both Patel and Bechert calibration equations showed that variation in result of the two equations is minimum when the diameter Preston tube is 3.9 mm.