Hydrodynamic pressure and effective mixing length in vegetated free-flow region

Abstract

Submerged vegetation acts as large-scale roughness element that changes flow structure and promotes flow subdivision, thereby affecting scalar transport and energy transfer in the river. This article explores vertical distributions of both the hydrodynamic pressure P D and effective mixing length L in vegetated free- flow region through flume experiments. Firstly, formulas have been proposed in the present study to calculate the P D and L values by some theoretical analyses. Then, we discuss the P D and L profiles under different vegetation density λ, submergence degree H/h v and mean bulk velocity U m conditions. The P D value is positive and attains its vertical maximum around the middle position of this region, indicating that shear turbulence promotes the upwards release of the hydro pressure. The vertical variation coefficient for P D is nearly 1.15 and independent of λ and U m in this study. The dimensionless L value varies little in the lower part of this region and increases linearly upwards in the upper part, reflecting vertical variation of shear turbulence movement. Since the vegetation density λ and submergence H/h v control turbulence coherent motion, the P D and L/(h o - h v) values change monotonically with them in this study. The high-velocity flow accelerates turbulence waves and promotes an increase in P D.