محمد طيبان

found along the banks an‎d in the flooding areas of watercourses, provides essential nutrients an‎d habitats for aquatic organisms, captures suspended sediments, an‎d ultimately enhances water quality. From another perspective, the drag force that vegetation applies to the flow has a significant impact on flow resistance in open-channel flows. Nevertheless, the role of vegetation in open-channel flows is not yet fully comprehended due to its inherent complexity, which calls for additional investigation. This study aims to quantify the impact of emergent, rigid, an‎d partially-spanning vegetation on flow resistance in non-uniform accelerating flows, as well as to identify the mean flow an‎d turbulence patterns surrounding such vegetation canopies. In order to achieve this objective, a physical model comprising cylindrical elements that replicate emergent, rigid, an‎d partially-spanning vegetation was subjected to flow in a laboratory flume with a gravel bed. Subsequently, three-dimensional, instantaneous, velocity components were at different points along the flume, by utilizing Acoustic Doppler Velocimetry (ADV) technique. Our results revealed that: • Observations from the current study, which utilized a sparse vegetation model, indicated that the inner layer of the streamwise time-averaged velocity profile consistently exhibited a logarithmic pattern at every point along the flume. • Introduction of the vegetation model to flume led to a 97% increase in Manningʹs roughness coefficient, an‎d 350% escalation in drag coefficient at particular points along the flume. • The interface between vegetated an‎d non-vegetated parts of the flow, was identified as the most dynamic region of the flow. The pronounced velocity gradient existing between these two regions resulted in the development of a robust shear layer, leading to increased turbulence, the generation of vortices, an‎d enhanced momentum transfer. • Inside the shear layer formed in the transition zone between vegetated an‎d non-vegetated parts of the channel, an‎d maximum value in the time-averaged streamwise velocity profile was recorded near bed, an‎d the Reynolds shear stress profiles exhibited an S-shaped pattern. Three-dimensionality of the flow was enhanced in this region, with lateral transfer of streamwise momentum being the primary influence. Unique features of this area escalated towards the trailing edge of the canopy an‎d gradually dampened in the downstream section. • The regular alteration in the direction of the lateral component of time-averaged velocity, coupled with the reversal of Reynolds shear stress in the non-vegetated section of the channel, indicates that this area cannot be isolated an‎d treated separately from the vegetated section in numerical models. This is due to the fact that the turbulence dynamics in this area are significantly impacted by the shear layer established at the interface between the vegetated an‎d non-vegetated regions. • The flow occurring between the cylinders preserves its accelerating properties until it reaches at the trailing edge of the canopy. Nevertheless, a sharp decline in flow velocity was detected just downstream of the canopy. Progressing further downstream, the flow regains its acceleration.

پوشش گياهي , مقاومت جريان , سرعت سنجي صوتي داپلر , ضريب درگ , بستر زبر

vegetated flow , Flow Resistance , Acoustic Doppler Velocitymetry , Drag Coefficient , Rough Beds

Mohammad Tayeban

Dr. Hossein Afzalimehr