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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.

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.

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.

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.

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.

Integrated radial basis function based on finite difference (IRBF–FD) method is presented in this paper for the solution of incompressible Navier–Stokes equations. A semi-implicit temporal scheme is first used to discretize the time variable of the incompressible Navier–Stokes (NS) equations. We consider the same discrete scheme for the time variable for both pressure–Poisson equation and vorticity stream function formulation. For solving lid-driven cavity flow and backward-facing step flow, we used the vorticity-stream formulation. The proposed method approximates the function derivatives at a knot in terms of the function values on a collection of nodes existing in the support domain of the node. We also utilize an algorithm for finding the optimal shape parameter for each stencil based on the range of condition numbers. It can be seen that no special treatment is needed to impose the essential boundary conditions. The efficiency, accuracy and robustness of the presented method are demonstrated by comparing the current method with existing methods.

Purpose This paper aims to study passive control techniques for transonic flow over a backward-facing step (BFS) using square-lobed trailing edges. The study investigates the efficacy of upward and downward lobe patterns, different lobe widths and deflection angles on flow separation, aiming for a deeper understanding of the flow physics behind the passive flow control system. Design/methodology/approach Large Eddy Simulation and Reynolds-averaged Navier–Stokes were used to evaluate the results of the study. The research explores the impact of upward and downward patterns of lobes on flow separation through the effects of different lobe widths and deflection angles. Numerical methods are used to analyse the behaviour of transonic flow over BFS and compared it to existing experimental results. Findings The square-lobed trailing edges significantly enhance the reduction of mean reattachment length by up to 80%. At Ma = 0.8, the up-downward configuration demonstrates increased effectiveness in reducing the root mean square of pressure fluctuations at a proximity of 5-step height in the wake region, with a reduction of 50%, while the flat-downward configuration proves to be more efficient in reducing the root mean square of pressure fluctuations at a proximity of 1-step height in the near wake region, achieving a reduction of 71%. Furthermore, the study shows that the up-downward configuration triggers early spanwise velocity fluctuations, whereas the standalone flat-downward configuration displays less intense crosswise velocity fluctuations within the wake region. Practical implications The findings demonstrate the effectiveness of square-lobed trailing edges as passive control techniques, showing significant implications for improving efficiency, performance and safety of the design in aerospace and industrial systems. Originality/value This paper demonstrates that the square-lobed trailing edges are effective in reducing the mean reattachment length and pressure fluctuations in transonic conditions. The study evaluates the efficacy of different configurations, deflection angles and lobe widths on flow and provides insights into the flow physics of passive flow control systems.

Backward-facing step flow is a benchmark problem that has been studied in various fields, such as airfoils, diffusers, boilers, nuclear reactors, electronic devices, and air-conditioning ducts. In this study, a rigid rectangular flapping longitudinal vortex generator was mounted at the step of the channel to investigate the fluid flow and heat transfer characteristics at three flapping frequencies (0.167, 0.25, and 0.5 Hz) in the Reynolds number range of 3000 to 8000, while maintaining at a constant heat flux. When the fluid flowed over the backward-facing step with flapping longitudinal vortex generator, a train of longitudinal vortices developed simultaneously. At a flapping frequency of 0.167 Hz, the developed high-intensity longitudinal vortices were stable and augmented the heat transfer by 38.54 % more than the smooth channel. The friction factor at 0.167 Hz was found to be 19.47 % and 25.33 % greater than at the higher frequencies of 0.25 and 0.5 Hz, respectively.

Curved surfaces are a feature of many engineering applications, and as such, the accurate prediction of separation and reattachment from a curved surface is of great engineering importance. In this study, improved delayed detached eddy simulation (IDDES) is used, in conjunction with synthetic turbulence injection using the synthetic eddy method (SEM), to investigate the boundary layer separation from a curved backward-facing step for which large eddy simulation (LES) results are available. The commercial code Star CCM+ was used with the k-ω shear stress transport (SST) variation of the IDDES model to assess the accuracy of the code for this class of problem. The IDDES model predicted the separation length within 10.4% of the LES value for the finest mesh and 25.5% for the coarsest mesh, compared to 36.2% for the RANS simulation. Good agreement between the IDDES and LES was also found in terms of the distribution of skin friction, velocity, and Reynolds stress, demonstrating an acceptable level of accuracy, as has the prediction of the separation and reattachment location. The model has, however, found it difficult to capture the pressure coefficient accurately in the region of separation and reattachment. Overall, the IDDES model has performed well against a type of geometry that is typically a challenge to the hybrid RANS-LES method (HRLM).