Turbulence Characteristics of Vegetated Channel With Downward Seepage

Abstract

Experimental studies were carried out for investigating changes in flow characteristics with the presence of flexible vegetation in a channel. The study focuses on the effect of introducing downward seepage on velocity profiles, Reynolds shear stress (RSS), and different turbulent length scales in a vegetative channel. The presence of vegetation provides drag and reduces the flow velocity. The turbulence generation mainly comes from the oscillations occurring near the top of the vegetation as is evident from the achievement of maximum Reynolds stress near the top of the vegetation. Application of downward seepage results in a higher velocity zone in the lower vegetation zone and a higher Reynolds stress. Quadrant analysis shows that sweep and ejection contribute most to Reynolds stress. The dominance of sweep event over ejection event is more with the application of downward seepage which means more bed transport. Different turbulent length and time scales increase with increase in downward seepage percentage due to reduction in energy dissipation. The increase in the length scale and time scale with downward seepage infers that higher level of turbulence prevail near the bed with an increased eddy size resulting in higher Reynolds stresses with downward seepage. The universal probability distribution functions (PDFs) of velocity fluctuations, RSS, and conditional RSS of vegetative channel follow Gram Charlier (GC) series based on exponential distribution except that a slight departure of inward and outward interactions of conditional RSS is observed which may be due to weaker events.