Flow over a rounded backward-facing step, using a z-coordinate model and a σ-coordinate model

Abstract

Homogeneous, nonrotating flow over a backward-facing rounded step is simulated using the 2D vertical version of two general circulation models, a z-coordinate model-the Massachusetts Institute of Technology general circulation model (MITgcm)-and a σ -coordinate model-the Bergen Ocean Model (BOM). The backward-facing step is a well-known testcase since it is geometrically simple but still embodies important flow characteristics such as separation point, reattachment length, and recirculation of the flow. The study compares the core of the two models and uses constant eddy viscosities and diffusivities. The Reynolds numbers ranges from 2·10 2 to 2·10 6 . The results correspond with previously published results having a relatively stationary separation point and a fluctuating reattachment length due to downslope propagating eddies released from the reattachment zone for Reynolds numbers higher than or equal to 2 · 10 4 . For Reynolds number within the laminar regime, the flow is stationary. The discrepancies between the models increase by enhancing Reynolds numbers. The σ -coordinate model experiences a reduction in eddy sizes with increasing resolution and Reynolds numbers in correspondence with published experiments, while the size of the eddies are independent of the Reynolds number using the MITgcm. Due to mixing generated by the staircase topography, the z-coordinate model gives a better convergence of the separation point and reattachment length compared with the BOM; however, this conclusion might change with the inclusion of a relevant turbulence scheme.

Passive Flow Control of Rotary-Wing and Fixed-Wing Aircraft Airfoils Via Surface-Based Trapped Vortex Generators

“…Although the BFS is a well-known test case because of the simple geometry, it still embodies flow characteristics such as the separation point, reattachment as well as recirculating flow, which with only simple modifications can serve for other practical applications such as modeling of ocean waves [7]. Rygg et al [7] used a rounded BFS for studying the nature of the flow of an ocean wave; the step was a 2D vertical version of two general circulation models, a z-coordinate model invented by the Massachusetts Institute of Technology, a general circulation model (MITgcm), and a σ-coordinate model originated by the Bergen Ocean Model (BOM). The flow was assumed to be homogeneous and irrotational.…”

“…These were an open cavity with length to depth ratio of L/D = 6, a transitional cavity with L/D = 10 and a closed cavity with L/D = 15. All three cavities were simulated under two Mach numbers, 0.6 and 0.8 and Reynolds numbers per meter of 1.23×10 7 and1.55×107 , respectively. The computational domain and mesh is shown inFig.…”

Large-eddy simulations of the ice-shelf–ocean boundary layer near the ice front of Nansen Ice Shelf, Antarctica

“…However, we cannot observe the exact horizontal scale of the outer overturning cell owing to the limitation of the domain scale in this study. Because the flow with the sub-ice-shelf plume beneath the ice shelf is a backward-facing step flow (reattachment flow with geometry), we can estimate the horizontal scale of the outer overturning cell based on the reattachment length (Rygg et al, 2011). For the oceanic flow at a high Reynolds number (2 × 10 6 ), we observe the development of the outer cell (5.3-8.4 h (h is geometry height)) and inner cell (1.3-1.5 h).…”