Turbulence measurements in the boundary layer over a sand wave field

Abstract

To assist in elucidating the turbulent structure of flow over regular sets of bed forms, data procured with small mechanical current meters above 2 m high 96 m long sand waves in the Columbia River are analyzed. The high‐frequency resolution of the flow sensors and the half hour records that were collected with them are shown to be sufficient to yield reliable Reynolds stresses. In addition, an expression from which the sampling error in Reynolds shear stresses can be estimated from the sampling frequency, the local flow speed, and the distance from the boundary is presented. Although the κ−5/3 region of the energy spectrum was found, the conditions for isotropy were not satisfied anywhere within the measured band. Spatial structure of the Reynolds stress components was found to be complicated in character but explicable in basic fluid mechanical terms. Owing to the near boundary effects of form drag induced by the sand waves, the spatially averaged shear stress increased from 20 dyn/cm2 at 35 cm to 40 dyn/cm2 at 100 cm. Furthermore, the Reynolds shear stress was found to vary with the structure of the variance of the vertical velocity component rather than with that of turbulent kinetic energy as assumed in the simple closure models that have been used so far in theories for flow over such bed forms. The data also were examined for flow intermittency, and the results of Gordon and Witting [1977] were not confirmed.