LES using open-channel in VOF for Multiphase Flow in a U-Shaped Channel

Description

LES using open-channel in VOF for Multiphase Flow in a U-Shaped Channel by ANSYS Fluent

Description

This study reports a transient multiphase CFD analysis of a U-shaped channel performed in ANSYS Fluent. A Volume of Fluid (VOF) formulation is used to capture the interface between two immiscible phases, and turbulence is modeled with Large-Eddy Simulation (LES) to resolve the dominant unsteady vortical structures generated by the bend and local obstacles. The objective is to quantify the velocity field and the distribution of the secondary phase (Phase-2 volume fraction) along the channel, with particular attention to recirculation, mixing, and wake development near the internal appendages.

Geometry and Mesh

The computational domain is a 3D U-shaped open channel with one straight leg serving as the inlet and the opposite leg as the outlet. Inside the lower bend, a thin plate/baffle and a short array of small cylindrical features are included to promote mixing; these features are explicitly resolved in the mesh. The geometry was built in DesignModeler and discretized in ANSYS Meshing. A body-fitted predominantly hexahedral mesh with local refinement around the baffle, the cylinders, the inner/outer walls of the bend, and the free-surface region was generated. A total of 8696641 control volumes were used to discretize the fluid domain, providing adequate resolution in shear layers and wakes while keeping the far field coarser for efficiency.

Model and Solver Settings

The case was treated as an open-channel (free-surface) air–water flow in ANSYS Fluent. A two-phase VOF formulation was used to capture the interface, with gravity enabled in the −z direction to represent the water layer topped by air. Turbulence was modeled with Large-Eddy Simulation (LES) to resolve the dominant unsteady structures in the bend and around the internal elements; sub-grid motions were handled by Fluent’s LES SGS model. The simulation was run transient with a fixed time step of 0.01 s. Boundary conditions consisted of a Mass-Flow Inlet supplying 18.7 kg/s at the upstream leg, a Pressure Outlet at the downstream leg, and no-slip walls on all solid boundaries. The inlet was initialized with a water depth of 0.12 m, ensuring an open-channel free surface throughout the domain.

Results

The LES-VOF solution produces a physically rich, unsteady flow. Velocity contours show acceleration on the outer wall and separation/recirculation near the inner wall of the bend, with a high-shear wake forming downstream of the baffle where coherent vortices are shed from both the baffle tip and the small cylinders. Representative fields indicate peak local speeds on the order of 0.3 m/s near the outlet leg, while low-velocity cores occupy the separated regions in the bend. Phase-2 volume-fraction contours reveal interface deformation driven by these vortices, including localized entrainment and streaks of alternating high/low Phase-2 content in the wake. Streamlines illustrate the periodic shedding and subsequent reattachment further downstream, confirming that the internal features enhance mixing and break up large-scale structures before the outlet.