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The increase in FO frequency due to the use of BFS is accompanied by an increase in pressure losses. The study was conducted using the URANS governing equation and the SST k-ω turbulence model. Double BFS exhibited the highest frequency, with an average increase of 25.78% over the prototype. In contrast, the average frequency increases of single and triple BFS were 20.29% and 19.6%, respectively. The frequency increase is influenced by the momentum of the backflow in the feedback channel. Double BFS had a lower pressure loss than the prototype model, with 4.54% reduction. The average pressure loss of the single BFS model was 24.9% higher than that of the prototype model, whereas the triple BFS model showed a 0.039% increase. The pressure loss is influenced by the recirculation bubble in the FO chamber. Nondimensional analysis using Strouhal and Euler numbers also showed that double BFS exhibited the best performance. The prototype model and single BFS had a velocity profile shape that is closer to a homogeneous shape. The double and triple BFS exhibited a velocity profile shape that is closer to the bifurcated jet shape. Bifurcated jets, which exhibit a wider spread, are characteristic of oscillatory flows. Thus, it can be concluded that the double BFS FO is more recommended.

Laminar, transient forced convection problem over a 2D backward facing step (BFS) at an inlet Reynolds number (Re) of 400 is investigated numerically using OpenFOAM. To increase the Nusselt number (Nu) along the bottom wall, active flow control is applied by zero-net-mass-flux (ZNMF) combinations of suction and injection through three thin slits which are placed on the top, step and the bottom walls in the vicinity of the BFS. The combinations of each jet velocity is determined by jet to inlet mean velocity ratios which are limited to integer numbers between -2 and 2 and satisfying ZNMF condition where negative and positive values indicate suction and injection, respectively. All 19 cases which satisfy these rules are investigated. Average Nusselt number, friction coefficient and recirculation zone lengths are calculated along the bottom wall from time averaged flow fields. Among 19 cases with each having different jet configuration, some cases converged to steady state solution while others indicated temporal effects and converged to periodic solutions. To understand these transient effects, velocity oscillation magnitude and Strouhal number which are monitored at a selected critical point are evaluated. It is shown that temporal interaction of chosen active flow control methodology has significant effect on enhancing mixing which results in an increase of Nusselt number. Among all cases, the best case concerning thermal improvement has an increase of 78.5% in Nu number while the best aerodynamic improvement is achieved for another case with a decrease of 81% in total recirculation zone length compared to the reference case where no control is applied.

Modifying the geometrical structures is a potential strategy that targets the compactness of any new devices in order to produce greater cooling performance. The heat transfer enhancement over a BFS with unique square-rectangular ribs as FFS in a two-dimensional channel is investigated numerically in this study. Each of the structures features a pair of square-rectangular adiabatic ribs, and both its height and width are adjustable. The ribs are positioned uniformly on the bottom wall heated with uniform heat flux. The impacts of varying number as well as space in between the pair of ribs are also analyzed. Fresh external fluid is entering into the channel from the left and leaving the channel from the right. The methods of solution of the mathematical models are solved numerically following the finite element method along with the Galerkin technique. Through the rigorous computation, the results are obtained and presented systematically over wide range of parametric variations like: with height and width of square-rectangular ribs, space between each pair of ribs, number of ribs, heat flux strength, and flow Reynolds number. In order to compare the thermal performance of BFS with ribs structure, the case of no-ribs channel is also investigated. The results indicate that geometric parameters have major influences on the thermo-fluid flow as well as heat transfer characteristics. It is found that lesser number of ribs with moderate height and width with lesser spacing corresponds to the superior thermal performance compared to no-ribbed channel. Furthermore, lower number of ribs with higher height and width with higher spacing resembles to the worst thermal performance.

In the present paper, the energy gradient method is implemented to study the instability of 2-D laminar backward-facing step (BFS) flow under different Reynolds numbers and expansion ratios. For this purpose, six different Reynolds numbers (50 ≤ Re ≤ 1000) and two various expansion ratios of 1.9423 and 3 are considered. We compared our results of the present study with existing experimental and numerical data and good agreement is achieved. To study of fluid flow instability, we evaluated the distributions of velocity, vorticity and energy gradient function K. The results of our study show that as the expansion ratio decreases the flow becomes more stable. We also found that the origin of instability in the entire flow field is located on the separated shear layer nearby the step edge. In addition, we approved that the inflection point on the profile of velocity corresponds to the maximum of vorticity resulted to the instability.

Thrombosis is a complex biological process which involves many biochemical reactions and is influenced by blood flow. Various computational models have been developed to simulate natural thrombosis in diseases such as aortic dissection (AD), and device-induced thrombosis in blood-contacting biomedical devices. While most hemodynamics-based models consider the role of low shear stress in the initiation and growth of thrombus, they often ignore the effect of thrombus breakdown induced by elevated shear stress. In this study, a new shear stress-induced thrombus breakdown function is proposed and implemented in our previously published thrombosis model. The performance of the refined model is assessed by quantitative comparison with experimental data on thrombus formation in a backward-facing step geometry, and qualitative comparison with in vivo data obtained from an AD patient. Our results show that incorporating thrombus breakdown improves accuracy in predicted thrombus volume and captures the same pattern of thrombus evolution as measured experimentally and in vivo. In the backward-facing step geometry, thrombus breakdown impedes growth over the step and downstream, allowing a stable thrombus to be reached more quickly. Moreover, the predicted thrombus volume, height and length are in better agreement with the experimental measurements compared to the original model which does not consider thrombus breakdown. In the patient-specific AD, the refined model outperforms the original model in predicting the extent and location of thrombosis. In conclusion, the effect of thrombus breakdown is not negligible and should be included in computational models of thrombosis.

The application of the nanofluid is highly increased due to its high heat rate in the attendance of the magnetic field. In current investigation, CFD is employed to model the impact of the non-homogenous magnetic force on the heat performance of backward-facing step nanomaterial flow. Current attempt tries to investigate the influence of the variable magnetic source on the flow feature of the nanofluid stream in the backward-facing step model. To study this problem, a vertical electrical wire is chosen to produce non-homogenous magnetic field. In our model, 4 Vol% nanoparticles ( Fe3O4 ) are added to pure water to attain nanofluid. To simulate the nanofluid in our work, the FVM technique is selected. Our results initially validated with experimental studies and good agreement is achieved. Inclusive parametric evaluation is made to find the impact of non-uniform magnetic field and inflow nanofluid velocity on the thermal efficiency of backward-facing step flow.. Our results show that local heat transfer could increase up to 300% with impose of such source. Moreover, the rising number of the magnetic sources could extensively enhance the thermal feature.

A computational model of inhomogeneous pressure-fluctuation fields in the vicinity of a forward-facing step–backward-facing step configuration taking into account the high degree of their mutual correlation (global correlation) is generalized from experimental data. It is shown that when determining the characteristics of pressure fluctuations that act on an elastic structure, the global correlation is represented by an additional inhomogeneous field. It is demonstrated that a high degree of correlation may lead to a significant change in the main characteristics of the pressure-fluctuation field in the wake behind the configuration. This is taken into consideration in the model by correcting the local properties of this field.

Presented, a study on the heat transfer performance and pressure loss of bio-based functionalized multi-walled nanocarbon tubes nanofluid (C-MWCNT) dispersed in distilled water through circular backward-facing step passage under constant heat flux. The experiment encompasses the evaluation of thermos-physical properties of this environment-friendly working nanofluids in a circular backward-facing step flow passage. The carbon nanotubes particles were functionalized using the clove-extract functional group and then dispersed to make nanofluid samples of different mass concentrations. Clove extracts contain eugenol, phenolic acid, and gallic acid, which act as effective reactants. Heat transfer performance result showed that the C-MWCNT nanofluids showed higher thermal conductivity than those of pure water. The heat transfer coefficient of the nanofluid through the backward-facing step flow passage increased with the increase of concentration of the nanoparticles. The observed enhancement of heat transfer coefficient of the C-MWCNT found to be 2.7, 6.5, 8.3 and 9.3% for the concentrations 0.025, 0.05, 0.075 and 0.1mass%, respectively. However, there is an inversely proportional relationship between the Nusselt number and the concentration of nanoparticles. The maximum heat transfer coefficient occurred at the reattachment point of the nanofluid to the flow passage surface after the separation.

Porous baffles can be used to enhance heat transfer in various engineering applications, including electronic cooling, gas turbine blades, and chemical reactors. Also, the backward-facing step is a widely used configuration in fluid dynamics studies due to its simplicity and relevance to real-world geometries. This study examines heat transfer and flow characteristics in a backward-facing step channel featuring a heated bottom wall and two porous baffles. A computational fluid dynamics model, validated against prior research, is used to investigate flow and temperature fields. The innovation of this work lies in the application of multi-objective optimisation to search for a set of solutions that establish a trade-off between the average Nusselt number and the pressure drop. The optimisation specifically considers various parameters of the porous baffles, including height, width, distance from the step, and Darcy number, to identify optimal design configurations. Results show that porous baffles significantly improve heat transfer compared to a backward-facing step channel without them, despite an increase in pressure drop due to their presence. This work offers valuable insights into the trade-off between heat transfer performance and pressure drop, crucial for designing efficient heat transfer systems. By exploring the Pareto-Frontier, which represents various optimal design solutions, the study provides practical guidance when seeking to optimise heat transfer in backward-facing step channels with porous baffles. The findings contribute to advancing the understanding of heat transfer enhancement, highlighting the potential of porous baffles as a viable solution for improving thermal management in engineering systems.

As high heat dissipation has increasingly become the primary factor restricting the capability of electronic elements, and the high temperature of the equipment operation will affect its reliability. In this study, a backward-facing step (BFS) model to explore the flow and heat transfer characteristics of water and 5% mass fraction of Cu/Ni-water hybrid nanofluids has established. This paper focused on the influence of the number of step layers and the step expansion ratio (ER) on the flow and heat transfer characteristics of hybrid nanofluids. The increase in the number of steps will increase the Nu of the downstream wall of the steps and bring the peak position of Nu forward. The heat exchange effect of the hybrid nanofluids is better than that of water. The increase in flow velocity has the greatest influence on Nu of the lower wall of the step, followed by the increase in the step expansion ratio and the increase in the number of steps.