Direct measurement of shear stress on a water flume floor

Abstract

A floating-element-type balance was successfully built to measure small forces, such as shear stress, on the bottom of a laboratory water flume floor. The balance was tested when it was used to measure shear stress values directly on a smooth wall. The values of shear stress were compared to those obtained by the Preston tube technique. Agreement was found to be good. The surface area of the shear plate exposed to the flow was 24.2 square centimeters. Since the magnitude of shear stress varied from 0.0023 to 0.03 grams per square cm, the total measured force on this plate varied from 0.0462 grams to 0.607 grams. The balance was calibrated for a maximum range of 1.000 grams. When the directly measured shear stress readings were compared to the Preston tube values, the maximum difference did not exceed 0.06 grams or 6% of the range of the balance. Low shear stress values of order 0.0023 grams per square centimeter can be measured satisfactorily if the maximum force range of the balance is reduced according to a design procedure outline in Appendix A. Presented in this report are the experimental results, the design details, and other applications of the balance. The advantages of this d e sign over other existing ones are also discussed.