Boundary Shear Stress Measurement by Preston Tube

This work confirmed the possibility of using the Preston tube technique on sand roughened boundaries but emphasized certain problems, particularly those related to obtaining a satisfactory theoretical calibration. The main conclusions were that: 1. A small plate close to the nose of the Preston tube can increase the accuracy, particularly where the tube diameter is not large than the grains; 2. the effective boundary roughness, K_s, is between the mean and maximum grain size unless the sand is very closely graded; and 3. the value of z may vary with position of the probe on the boundary and cannot be predicted, which results in uncertain stress readings.