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Keywords:

  • river bends;
  • secondary flow;
  • stratified flow

[1] We present direct numerical simulation results for both isothermal and density-stratified turbulent flow in an open channel into and around a 120° bend with a bulk Reynolds number of 7500 and Prandtl number of 1.5. The bend is sharp with a radius-to-channel breadth ratio of 1.5. The bulk Richardson number for the stratified flow is 2.4 based on overall channel depth. The gradient Richardson number (Rig) varies between 10 and 20 at the entrance to θ ≈ 60°, where θ is the angular location. Above θ ≈ 60 − 120, Rig≈ 1. In isothermal flow, the well-known helical flow structure is observed. In stratified conditions, the vertical variation in relative strength of the outward-directed baroclinic pressure gradient and the centrifugal acceleration leads to a more complex circulation structure. In the near bed region and immediately above the interface, the centrifugal acceleration is greater, driving flow radially inward, while just below the density interface the baroclinic pressure gradient is greater, leading to outward-directed flow. This produces a four layer circulation structure with potentially significant implications for sediment erosion and transport. Additionally, this produces a complex dynamic at the density interface where the shear orientation varies through approximately 200° over the mixing layer depth.