Author(s): Ana M. Ricardo; Rui M. L. Ferreira; Giovanni Di Lollo; Rodrigo Farias; Luis Gil; Moises Brito
Linked Author(s): Ana M. Ricardo, Moisés Brito
Keywords: Density currents; Vertical cylider; Large eddy simulation (LES); Validation; Flow-structure interaction
Abstract: Density currents are an important class of fluid flows that arise in numerous environmental scenarios in which buoyancy forces, generated by density differences, produce motions with large changes in the flow velocity and areas of intense turbulence. The interaction with bluff bodies increases the drag force acting on the current and provide additional mechanisms for entrainment, mixing, and energy dissipation. Large eddy simulation (LES) models have been used recently to study these mechanisms, however, verification and validation still a challenging task, since both the error induced by the sub-grid scale (SGS) model and the numerical discretization error are dependent on the grid resolution. This paper provides an extensive validation of 3D-LES of lock-exchange density currents interacting with an emergent circular cylinder. Numerical modelling was performed in OpenFOAM using different SGS models and VoF (volume-of-fluid) method to simulate the fluid phase. The discussion is restricted to the current propagation in the slumping phase. This study is of paramount importance to be able to confidently use LES model to understand the physics of lock-exchange density currents interacting with bluff obstacles. The validation is performed by comparing LES results and a new independent experimental data of both density and velocity fields. The model is validated using both density field measurements in two layouts: side (i.e., transversal cross-channel averaged) and plan (i.e., vertical cross-channel averaged) views. Measurements were performed in both side and plan views for cylinder Reynolds number range from 1,300 to 3,475 in a horizontal and rectangular cross-section channel with 3.0 m long and 0.175 m wide. The gravity current was generated using the classic lock-exchange configuration with both lock and ambient regions filled up at same depth of 0.2 m. The emergent cylinder with a diameter of 2.5 cm is positioned in the ambient fluid region at 0.6 m beyond the gate with its axes vertical and perpendicular to the streamwise direction of the current propagation in the middle of the channel. The lock-release experiments were carried out for four reduced velocities of 0.06, 0.12, 0.24 and 0.48 m s-2. Insights on the impact of the SGS models and grid refinement are obtained with comparisons of the concentration isosurfaces, density fields, vertical and transversal profiles. The comparisons show that the current propagation is predicted with reasonable accuracy, including the entrainment, and mixing at a local level (near the cylinder) during the impact stage. No monotonical convergence of the mesh was found. On the other hand, the accuracy of the results is affected by changes of the Reynolds number. This work was funded by national funds through Portuguese Foundation for Science and Technology (FCT) WinTherface project PTDC/CTA-OHR/30561/2017.