Author(s): Armaghan Severi, Stefan Felder
Linked Author(s): Armaghan Severi
Keywords: Boundary layer, flat-sloped spillway, flow properties, flow resistance, shear stress
Abstract:
Spillways are often associated with air entrainment attributed to turbulent velocity fluctuations close to the free-surface. At the upstream end of the spillway, a turbulent boundary layer is generated due to bed friction developing in downstream direction. In small dams, the boundary layer may not reach the free-surface at the downstream end of the spillway. In order to efficiently design an energy dissipator at the downstream end of the spillway, it is required to determine the flow resistance over the spillway. Detailed laboratory experiments were conducted on a large scale uncontrolled flat sloped (θ = 11°) spillway with smooth bed. The present study aimed to investigate the developing turbulent boundary layer properties and shear stresses over the spillway. The non-aerated flow properties were measured with a Pitot tube, identifying the growth rate of the boundary layer, displacement and momentum thicknesses as δ ∼ x0.86, δ1 ∼ x0.57 and δ2 ∼ x0.70, respectively. Despite reaching fully developed flow conditions, no free-surface aeration was observed and the surface remained rough with fast fluctuations and entrapped air at the air-water interface. The shear stresses were calculated using the logarithmic law of the wall, the outer flow region as well as the direct step method. The dimensionless shear stresses were in a range between 0.029 < τo/(ρ×g×dc) < 0.065 corresponding to Darcy friction factors 0.024 < f < 0.027. The present study results provided new insights into the turbulent boundary layer properties in high-velocity flows over a flat-sloped spillway which may be useful for a hydraulic design of spillway and downstream energy dissipator. (2626, 61, 302)
Year: 2017