The backward-facing step flow (BFSF) is a classical problem in fluid mechanics, hydraulic engineering, and environmental hydraulics. The nature of this flow, consisting of separation and reattachment, makes it a problem worthy of investigation. In this study, divided into two parts, the effect of a cylinder placed downstream of the step on the 2D flow structure was investigated. In Part 1, the classical 2D BFSF was validated by using OpenFOAM. The BFSF characteristics (reattachment, recirculation zone, velocity profile, skin friction coefficient, and pressure coefficient) were validated for a step-height Reynolds number in the range from 75 to 9000, covering both laminar and turbulent flow. The numerical results at different Reynolds numbers of laminar flow and four RANS turbulence models (standard k-ε, RNG k-ε, standard k-ω, and SST k-ω) were found to be in good agreement with the literature data. In laminar flow, the average error between the numerical results and experimental data for velocity profiles and reattachment lengths and the skin friction coefficient were lower than 8.1, 18, and 20%, respectively. In turbulent flow, the standard k-ε was the most accurate model in predicting pressure coefficients, skin friction coefficient, and reattachment length with an average error lower than 20.5, 17.5, and 6%, respectively. In Part 2, the effect on the 2D flow structure of a cylinder placed at different horizontal and vertical locations downstream of the step was investigated.

Passive flow control techniques are needed to reduce flow separation and enhance aerodynamic performance. In this work, the effect of a knitted wire mesh on the flow separation of a backward-facing ramp was numerically investigated for a Reynolds number of 3000. A grid independence study and a RANS turbulence model sensitivity analysis were conducted. The CFD simulations exhibited counter-rotating streamwise vortices emerging from the knitted wire mesh, and the reattachment length was significantly reduced. A variation of the knitted wire rows revealed a maximum reduction of the reattachment length of 25.7% for the case of four rows. A comparison with a different knitted wire mesh geometry yielded a decreased reattachment length reduction.

This study investigates the effect on the flow structure in a backward-facing step (BFSF) due to a cylinder placed downstream of the step. Numerical simulations were carried out using OpenFOAM with several turbulence models (standard k-ɛ, RNG k-ɛ, standard k-ω, and SST k-ω). The recirculating flow, the skin friction coefficient (C_f), and the pressure coefficient (C_p) of the bottom wall were comparatively analysed. The added cylinder modified the structure of flow and increased the skin friction coefficient (C_f) in the recirculation zone. Additionally, the pressure coefficient of the bottom wall increased immediately downstream of the cylinder and farther downstream of the reattachment point remained stable in the flow recovery process.

The most widely investigated model for the study of flow separation is the Backward Facing Step (BFS). The separation phenomenon can be found in several aerodynamic and heat transfer systems. The sudden decrement in the pressure gradient due to expansion of the flow leads to the separation of the shear layer at the step edge resulting in the formation of a separation bubble. The extent of the recirculation region is devised from the measurement of the re-attachment length. These measures are dependent on the flow and geometric properties. The chaotic interactions of the vortices shredded from the step edge and the upper wall surface open a wide domain to explore and study its characteristics. Several researchers in the past have theoretically, experimentally, and computationally obtained outcomes to study these kinds of flows. The present study was performed to investigate the complex interactions between the vortices formed at the step edge, the upper wall, and the secondary vortices forming in the secondary re-circulation region. The simulation was performed using an open-source CFD software package; OpenFOAM® using a transient incompressible solver which incorporates the PISO (Pressure Implicit with Splitting of operators) algorithm to solve the pressure-velocity linked equations. The visualisation outcomes at the uniform time interval of 0.001 seconds were presented along with several energy modes identified using the mathematical technique of POD (Proper Orthogonal Decomposition) were also presented in this script.