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AbstractUnsteady laminar magnetohydrodynamic forced convection and entropy generation inside a backward-facing step (BFS) porous channel with Cu-H2O nanofluid under a tilted magnetic field is numerically handled using a thermal lattice Boltzmann method (TLBM) with two distribution functions. The Darcy-Brinkman-Forchheimer (DBF) equations filled up with the energy equation (with local thermal equilibrium assumption) have been drawn up as the governing equations model. Their numerical solutions unveiled streamlines, Nusselt number, pumping power, performance index and entropy generation, and impacts of influential parameters (Hartmann number (Ha = 0; 10; 20; 100, 200), Darcy number (Da = 10−3; 10−2; 10−1), magnetic tilt angle (γ = 0, π∕4, π∕2), nanoparticles’ volume fraction (φ = 1 − 4%), medium porosity (ε = 0.6, 0.7, 0.8). The irreversibility analysis is also addressed to highlight the flow behavior. Regarding the validation of the modeling approach deemed, a consensus has been achieved with results available in the literature. Computations revealed that the volume fraction provides a fair performance rating with moderate pumping power. Likewise, the average Nusselt number, the average entropy generation, the pumping power and the performance index have a direct relationship with the Hartmann number, the nanoparticles volume fraction and the magnetic field tilt. Moreover, it can be stated the Cu-H2O nanofluid that seems worthwhile in terms of heat transfer efficiency compared to other examined nanofluids. Entropy generation due to the magnetic field was identified as the main source of irreversibility processes in all cases. Based on the outcomes, it is thought that applying a magnetic field can help minimize entropy generation in practical applications. [ABSTRACT FROM AUTHOR]

The aim of this research work was to explore how modifying the design of small-scale HAWT rotor blades through the backward-facing step technique affects their efficiency under varying wind speeds. The study involved altering step parameters such as location, length, and depth to create four distinct stepped blade shapes and enhance the aerodynamic performance of a rotor with a diameter of 280 mm. A specific blade profile, NREL S822, was selected to meet both aerodynamic and structural criteria. The rotor models were examined at a Reynolds number of 4.7 × 104 for wind speeds between 8.5 and 15.5 m/s and tip-speed ratios between 2 and 5. The experimental results indicated that for certain geometric step parameter values, the efficiency of the rotor model (B3) increased by approximately 47% compared to the base model (B1), particularly for tip-speed ratios lower than around 3.2. However, beyond this point, the rotor efficiency dropped significantly, reaching approximately 60% in one case. Additionally, a hybrid rotor model (B6) was generated by combining the shape of the rotor model (B4) with the most efficient rotor model from the literature, generated using the leading-edge wavy shape technique. This hybrid rotor model enhanced rotor efficiency for specific values of tip-speed ratio and also ensured its smoother operation. Overall, the rotor model (B2), distinguished by smaller step parameter values and a shift as well as broadening of the power coefficient curve towards lower tip-speed ratio values, exhibited a higher peak power coefficient, approximately 1.4% greater than the base rotor (B1). This increase occurred at a lower tip-speed ratio, allowing the rotor to operate with higher efficiency across a broader range of tip-speed ratios. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Three large-eddy simulations of open channel flow over a backward-facing step are performed to investigate the effect of submergence on the turbulence, hydrodynamics, and water surface deformation downstream of the step. The deformation of the water surface, the extent of the recirculation zone as well as the strength of the shear layer are a function of relative submergence. All flows downstream of the step exhibit elevated levels of turbulent shear stress and contain significant amounts of turbulent kinetic energy. The instantaneous flow features rollers immediately behind the step and horseshoe-shaped vortices shed from the shear layer, the latter being advected towards the water surface where they cause deformations. It is shown that these vortices can originate from any location along the dividing streamline; however, they contain more energy the closer to the mean attachment location they originate. [ABSTRACT FROM AUTHOR]

The expanded wall channel backward-facing step (BFS) and axisymmetric diffuser plays an important role in the society of fluid dynamics. Using a cut-cell technique is an established new method to treat the inclined wall of an axisymmetric diffuser. Cut-cell handle to reach the shape of the inclined wall, an axisymmetric diffuser and complex geometry. It helps treat the boundary condition at the wall in an accurate physical way. The turbulent flow through the geometries is solved by using Reynolds averaged Navier-Stokes equations (RANS) with the standard k-ε model. A self-built FOTRAN code based on the finite volume method with the Semi-Implicit Method for Pressure Linked Equations (SIMPLE) algorithm for pressure velocity coupling is established and examined with published experimental data for two different geometries backward-facing step (BFS) and axisymmetric diffuser. The results of the new technique reflect good agreement between the numerical results and the experimental data. A parametric study of the impact of area ratios (2, 2.5, 3, 3.5) in a backward-facing step on pressure, velocity, and turbulent kinetic energy. The angles (7°, 10°, 14°) and area ratios (2, 2.5, 3, 3.5) effect of an axisymmetric diffuser on the streamlines, local skin friction, pressure, velocity, turbulent kinetic energy, and separation zone. [ABSTRACT FROM AUTHOR]

A backward facing duct are present in various industrial applications especially those focused on heat transfer. The flow through a curved backward facing duct especially in the presence of nanofluid presents complexities compared to a straight backward facing step (BFS) duct. Therefore, the present numerical study deals a nanofluid flow (Fe3O4-H2O) forced convection in a curved backward facing duct. The objective of this investigation is to visualize at different curvature angles g (0°, 30°, 45°, 60°, 90°) imposed on the top wall of the duct, the effect of Hartmann number Ha (0, 50, 100), magnetic field inclination angle = (0°, 60°, 90°), Reynolds number Re (10, 100, 200) and nanoparticle volume fraction f (0 %, 0.05 %, 0.1 %). The dimensionless governing equations are solved using the multigrid finite element method. The results showed that the heat transfer was enhanced at the curved angle g = 90° for large Hartman numbers, thus, the average Nusselt number increased with a ratio of 240.74 % in the case of Hartmann number (Ha = 100). [ABSTRACT FROM AUTHOR]

In this paper, we propose a new stabilization technique for numerical simulation of incompressible turbulent flow by solving Reynolds-averaged Navier-Stokes equations closed by the SST k-ω turbulence model. The stabilization scheme is constructed such that it is consistent in the sense used in the finite element method, artificial diffusion is added only in the direction of convection and it is based on a purely nonlinear approach. We present numerical results obtained by our in-house incompressible fluid flow solver based on isogeometric analysis (IgA) for the benchmark problem of a wall bounded turbulent fluid flow simulation over a backward-facing step. Pressure coefficient and reattachment length are compared to experimental data acquired by Driver and Seegmiller, to the computational results obtained by open source software OpenFOAM and to the NASA numerical results. [ABSTRACT FROM AUTHOR]

A finite-difference unsteady coupled heat transfer solver was developed. This solver was utilized to simulate the hypersonic flow over a backward-facing step with a transverse gap, and the unsteady thermal conduction in an infinite circular pipe. The backward-facing step leads to local dramatically changing distributions of aerodynamic parameters and wall heat fluxes. The gas flow in the gap decelerates rapidly along with the increase of the gap depth, and there is rather weak convective heat transfer at the bottom of the gap. In the case of hypersonic flow around the infinitely long circular pipe, there exists large temperature gradient in the boundary layer, and the wall temperature increases with time, otherwise the aerodynamic parameters outside the boundary layer change quite slightly. The predicted results are in good agreement with the tested data. The comparison between numerical simulation results and tested data verifies the calculation ability of the developed solver. [ABSTRACT FROM AUTHOR]

We carry out experimental study on turbulent backward-facing step flow at the step Reynolds number of Re = 1.0 × 103 by planar particle image velocimetry (PIV). Based on the time-resolved velocity fields in the central vertical plane, we analyze the vortex growth as well as the convection within the separated/reattaching shear layer. By visualizing the fluid transport of non-inertial color-dyed flow tracers in the field of view, we find the characteristics of vortex growth governed by the two distinct mechanisms, i.e., the shear-layer mode in the initial part of the shear layer and the wake mode in the middle of the recirculation region. The shear-layer mode of instability causes the vortex rolling-up and pairing process while moving fast downstream, whereas the wake mode governs the further vortex growth to the scale of the step height while the vortex core remains stationary approximately in the middle of the reattachment length. Strong reverse flow is clearly visualized by the vortex-induced flow tracers beneath the vortex core, as the key to the onset of the absolute instability in the wake flow. As a result, large-scale, unsteady and vertical flapping motions occur in the latter half of the shear layer. [ABSTRACT FROM AUTHOR]

The safe operation and performance of a mixed compression air intake critically depend on the nature of shock wave/boundary layer interactions (SBLIs). The interaction between the ramp boundary layer and the cowl shock at the ramp–isolator junction plays a key role. In this experimental study, a modified backward-facing step called "notch" is used at the ramp–isolator junction to control the SBLI in a rectangular intake at Mach 3. The unstart and performance characteristics are evaluated and compared with the baseline, "faceted" configuration. The intake was unstarted by varying the back-pressure using a choke flap located at the exit in a quasi-steady manner. The surface and rake pressure measurements in addition to the shadowgraph and oil flow visualizations were taken to characterize the effect of flow control. The results showed that the notch anchors the separation bubble at the ramp–isolator junction and helps mitigate the strength of SBLI. The notched intake static pressures are relatively lower as compared to the baseline configuration suggesting reduced severity of structural loads. The floor boundary layer is energized by the notch leading to better efficiency and flow uniformity. There is an increase in the margin of unstart due to the presence of the notch by 7–10%. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper focuses on the off-surface flow control with a circular cylinder. Numerical investigation of the effectiveness and mechanisms of this method for flow separation control is conducted using two benchmark test cases. They are turbulent flow over a backward-facing step and the NASA wall-mounted hump separated flow. Firstly, Reynolds-averaged Navier-Stokes simulations are performed to analyze the influence of position and diameter of the cylinder on flow reattachment. The optimal control parameters for the cylinder are determined by a parametric study. Then, delayed detached eddy simulations are carried out for the separated flows under control using a cylinder with the optimal parameters, in order to investigate the flow control mechanisms in depth. Numerical simulation results show that an off-surface circular cylinder can significantly affect turbulent flow separation and reattachment. When the cylinder is placed at a suitable location outside the core region of the separated shear layer, the shear layer instability characteristics are enhanced with reduced sizes of turbulent vortex in the both reattachment zone and the recirculation region. With the application of control, the curving degree of the separated shear layer downward the lower wall is slightly increased, and the flow reattachment point moves upstream with a reduced recirculation region. • The role of position and diameter of a circular cylinder for off-surface flow control is discussed in detail. • Both RANS and DDES simulations are performed to investigate the off-surface flow control mechanisms in depth. • The reattachment length is significantly reduced by at least 11.0% with the optimal control parameters. • There is an obvious interaction between the separated shear layer and the shedding vortices from the cylinder. [ABSTRACT FROM AUTHOR]

• Investigating turbulence and heat transfer characteristics in SCO 2 flows over a vertical backward-facing step. • Revealing the influence of large thermophysical property variations on heat transfer. • Uncovering the impact of buoyancy on the flow and heat transfer characteristics of SCO 2 backward-facing step flow. • Providing valuable reference data for developing and testing turbulent heat transport models in SCO 2 flows under complex geometric conditions. This study employed direct numerical simulation to investigate the turbulent flow and heat transfer characteristics of supercritical carbon dioxide (SCO 2) in a vertical backward-facing step (BFS). Three cases were considered: constant property flow, SCO 2 forced convection, and SCO 2 mixed convection. All cases were conducted with an inlet Reynolds number of 4805, and the expansion rate of BFS is set at 1.5. Mean statistics like velocity, temperature, Reynolds stress, turbulent heat flux, and thermophysical properties are given and discussed. The results show that the turbulent and heat transfer characteristics of SCO 2 over a backward-facing step are significantly different from those of conventional fluids due to the dramatic changes in its thermophysical properties near the pseudo-critical temperature. In the case of SCO 2 forced convection, though the large thermophysical property variations do not have an obvious impact on the mean flow field, the near-wall mean density decreases largely and thus Reynolds stress and turbulent heat flux near the wall are reduced significantly, which in turn leads to a substantial reduction in the local skin friction coefficient and Nusselt number. In the SCO 2 mixed convection, buoyancy substantially distorts the flow, resulting in a significant reduction in the size of the recirculation zone. Simultaneously, buoyancy acts as an additional driving force leading to the formation of a wall jet. These combined effects induce intensified turbulence near the heated wall, which enhances the turbulent heat flux and facilitates efficient heat transfer. The instantaneous vortical structures based on Q criterion also clearly show the intensified turbulence in the near-wall region. [ABSTRACT FROM AUTHOR]

To address the closure problem of Reynolds-averaged Navier–Stokes in numerical simulations of turbulence, the method of solving Reynolds-averaged Navier–Stokes equations based on artificial neural network is introduced in this paper. We establish the nonlinear mapping relationship between the average flow field and the steady-state eddy viscosity field. The machine learning (ML) surrogate model for the shear stress transport turbulence model is constructed. The solution process of replacing the original turbulence model equations with the predicted field variables is realized by coupling the ML algorithm with the CFD solver. The classical backward facing step problem is selected in our study to verify the simulation accuracy of the surrogate model. The comparative analysis is carried out on the six backward facing step flows simulations at different Reynolds numbers. The results of simulations show that the testing flows with the Reynolds numbers closest training datasets Reynolds numbers can obtain the best simulation accuracy. Then for the Reynolds number that is lower than the training datasets, the simulation accuracy will decrease as the Reynolds number decreases. On the contrary, the simulation accuracy of the test flow will increase as the Reynolds number increases. These results indicate the feasibility of the ML surrogate model to simulate at higher Reynolds number. It shows the great potential of applying ML algorithms to Reynolds-averaged Navier–Stokes simulation (RANS) turbulence model and also provides a new idea for industrial simulations of turbulent flows. [ABSTRACT FROM AUTHOR]

In this paper, we study steady‐state conjugate heat transfer over a backward‐facing step flow using a combination of a compact finite difference scheme for the ψ‐v form of the Navier–Stokes equations and a higher‐order compact scheme for the temperature equations on nonuniform grids. We investigate the effect of Reynolds number (200 ≤ Re ≤ 800), conductivity ratio (1 ≤ k ≤ 1000), Prandtl number (0.1 ≤ Pr ≤ 15), and slab thickness (h ≤ b ≤ 6h) on the heat transfer characteristics. Isotherms remain clustered near the reattachment point in the fluid, while the temperature in the solid decreases vertically, with the minima at the reattachment point. Heat transfer rate (HTR) increases with Re, the maximum at the reattachment point. The HTR increases with k till k = 100 after, which it becomes invariant as k → ∞. Isotherms at the inlet become more disorderly with increasing Pr, and progressively clustered near the interface, indicating an increase in HTR, while the temperature in the solid region decreases with Pr. Increasing b decreases the HTR. In addition to obtaining an excellent match with results previously reported in the literature, we offer more comprehensive and previously unreported insights on flow physics. [ABSTRACT FROM AUTHOR]

The combined effect of leading-edge bluntness and backward-facing step (BFS) is critical because they affect the surface thermal loads and boundary layer transition of hypersonic vehicles, which involves engine design, passive thermal protection and aero-optical effect correction of infrared imaging window. This paper mainly studies heat flux distribution characteristics and flow structure over the backward-facing step on a flat plate with various leading-edge bluntness. The experiments were conducted in the Mach 6 hypersonic wind tunnel at various unit Reynolds numbers (from 6 × 106 to 1.8 × 107 m–1). The wall temperature was measured by temperature-sensitive paints (TSP) technique and the flow field was visualized by the nano-particles planar laser scattering (NPLS) system and schlieren technique. The results of heat flux distribution and flow structure showed that the leading-edge bluntness had little influence on recirculation region size but could effectively delay transition of the redeveloping boundary layer and reduce the surface heat flux intensity downstream of the step, and the reduction was most significant when the leading-edge diameter was equal to the step height. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In the present study, the effects of the square, circular and equilateral triangular cross-sectional obstacles placed on a backward-facing step on the flow-field were investigated numerically. Assumed to be fully turbulent, three-dimensional, steady and incompressible flow was solved by modified k-omega turbulence model. Obtained results were exhibited in terms of non-dimensional friction and non-dimensional pressure coefficients in addition to non-dimensional reattachment length. Apart from the geometric shapes of the obstacles, the influence of their heights (h) on the relevant parameters were presented for H/h=0.125, 0.25, 0.5 and 1 by dimensionalizing with the step height (H). Results obtained for the backward-facing step without any obstacles were compared with the experimental data of Driver and Seegmiller, 1985 to show the accuracy of the model. It was shown that in comparison with the step without the obstacles, not only the existence of the obstacles but their cross-sectional shapes also affect friction and pressure coefficients and the reattachment length. Regardless of the cross-sectional geometry of the obstacles, it was detected that the existence of the obstacles increases the length of the recirculation, however, the longest regions were obtained when equilateral triangular cross-sectional one was used. It was revealed that the effect of the height of the obstacle on the detached flow region is quite low for H/h=0.25. [ABSTRACT FROM AUTHOR]

Peatlands often experience turbulent sheltering from their surrounding upland forests, which results in spatially variable surface–atmosphere exchanges of momentum, heat, and moisture produced by flow-separation dynamics, which suppresses the transport of such scalars in the sheltered region while promoting transport in the reattachment zone. With evapotranspiration being the dominant source of water loss in the Boreal Plains, it is necessary to understand the dynamics and controls on evapotranspiration within these peatlands. We used the regional atmospheric forest large-eddy simulation (RAFLES) model to study the impact of flow separation and surface roughness on microclimates leeward of a forest-to-peatland roughness transition. We parametrized our simulation with observed vegetation characteristics and meteorological data from three natural peatlands to accurately estimate natural ranges of peatland roughnesses and energy dynamics. Our simulations show that changes to peatland roughness do not affect the distances required for flow reattachment, and therefore the size of the sheltered region. However, increasing the surface roughness produces greater surface turbulence, quicker flow recovery, and decreased flow reversals within the sheltered region. Further, substantial microclimatic differences are observed throughout the flow regions of the roughness transition. Our results show that turbulence, aerodynamic resistance, and the microclimate vary throughout the backward-facing step transition and should be taken into account when estimating the spatial dynamics of evaporative demand leeward of a roughness transition. Furthermore, increasing the surface roughness of a peatland minimizes the spatial variability of turbulent drivers of evapotranspiration across a roughness transition. That is, flow separation and the surface roughness of the peatland should be accounted for when estimating the spatial variability and total evaporative potential across a peatland. [ABSTRACT FROM AUTHOR]

This paper examines the combustion dynamics of a methane/air premixed flame establishing past a backward-facing step placed in a ducted combustor. Spectral analysis performed on high-speed recordings of the flame allows to show that the flame dynamics is governed by two main components. The first one is an acoustic longitudinal resonance, which imposes a flapping motion of the flame around the step. The second one is a hydrodynamic instability, which corresponds to the roll-up of the flame by the recirculation region in the trail of the step. When the frequencies associated to each mechanism are far apart, the flame is relatively stable. When the two frequencies get closer, the roll-up process is amplified and leads to a more pronounced dilatation of the recirculation bubble, as well as an enhanced vertical flame flapping. When the two frequencies coincide, a fully-coupled hydro-acoustic instability is established and prompts the flame to periodically flashback in that it is favored by the fact that the fluctuations of the hydro-acoustically modulated flow velocity are so large that the flow velocity drops below the flame speed. Besides, an acoustic analysis of the ducted combustor reveals the presence of an acoustic velocity anti-node at the position of the step, which is exacerbating the amplitude of the fluctuations. Time series analyses of the flame position, reduced to a 1D description of its rotational motion, reveals that the flashback dynamics is locked to a limit-cycle attractor. The flashback motion is intermittently accompanied with the stochastic generation of an autoignition front developing from a hot spot located on the top wall. [ABSTRACT FROM AUTHOR]

The influences of the backward-facing step and injection schemes on the spray characteristics of a liquid jet in a tandem backward-facing step cavity were investigated experimentally using high-speed photography and Schlieren techniques under a Mach 2.0 supersonic crossflow for the first time. The instantaneous structures of the spray and wave structures of the air-flow field were accurately captured by the high temporal resolution experimental technology. It is found that exist complex wave structures in the tandem backward-facing step cavity. A dimensionless spray factor was defined to describe the concentration of spray inside the cavity qualitatively. As a result, it is revealed that for the short backward-facing step cavity, the injection scheme near the upstream inlet has a higher penetration depth but a lower spray distribution, while the injection scheme near the cavity has a more spray distribution. For the long backward-facing step cavity, the injection scheme near the upstream inlet also has a higher penetration depth and the injection scheme near the sharp corner of the backward-facing step has a more sufficient spray distribution. The long backward-facing step cavity-based combustor with the upstream wall transverse injection is an optimized injection scheme to both improve penetration and spray distribution inside the cavity. Finally, a penetration depth formula is proposed to explain the spray distribution behaviors in a tandem backward-facing step cavity. [ABSTRACT FROM AUTHOR]

The application of the acoustic Doppler velocimetry (ADV) technique to backward-facing step flows is very scarce in the literature. This work aims at investigating the applicability of the ADV technique to flows over a backward-facing step, at various Reynolds numbers to capture different flow regimes, and to provide reliable data which can be used to validate numerical models. The ADV V e c t r i n o + was used in a laboratory flume and a customized numerical code was developed, which implements a methodology to post-process the ADV records. The normalized mean streamwise velocity and turbulence profiles were compared with classical results measured using a laser Doppler velocimeter and a high correlation was found. The ADV measurements showed accurate results regarding the streamwise velocity profiles, the turbulent shear stress and the reattachment length. Thus, ADVs can be used to characterize separated flows with good accuracy providing that an adequate post-processing is made to the acquired data. The errors obtained by the uncertainty analysis were relatively low, in general. [ABSTRACT FROM AUTHOR]

The injection of the fuel is a highly important process for the enhancement of the scramjets. In this article, the presence of the backward-facing step on the mixing of the multi-fuel jets is expansively studied. The primary attention of this article is to scrutinize the flow feature of the fuel jet under the backward-facing step. The mixing mechanism of the fuel is also studied to compare this injection system with conventional methods. To do this, a 3-dimensional model is chosen to consider the real physic of the problem. Reynolds Average Navier-Stocks equations are solved with a computational fluid dynamic method to visualize the flow pattern of the fuel jet at the free stream Mach number of 4. SST turbulence model is also used for the calculation of the viscosity. Our results indicate that increasing the jet space from 4 to 10 times of jet diameter in the presence of the backward-facing step increases the mixing efficiency up to 20% in the downstream. Our findings depict that augmenting the number of fuel injectors from 4 to 8 augments the mixing rate up to 15% inside the combustor. • The influence of backward step on the mixing characteristics is studied. • CFD is applied for the simulation of multi jet at supersonic flow. • The impact of jet space on mixing zone is studied. • Increasing the fuel jet space improves the fuel mixing in the far downstream. [ABSTRACT FROM AUTHOR]

The backward-facing step or the sudden expansion in internal flows is an important problem in different areas. In this study, a porous baffle is mounted on the opposite wall of a sudden expansion to enhance heat transfer near the step. Unlike the solid baffle, which is extensively studied in the literature, the porous baffle has a lower pressure drop, and its properties can be tuned to reach the optimal prospected performance. Effects of different porous baffle geometrical parameters including its normalized height (Hb = 0.5, 1.0, 1.5, 1.75), width (Wb = 0.5, 1, 1.5, 2.0, 2.5), porous baffle-step relative distance (D = 1, 2, 3, 4), Darcy number (10−2, 10−3, 10−4, 10−6), and Reynolds number (100, 200, 300, 400, 500) on the heat transfer and pressure drop are investigated. The simulation indicates that higher Reynolds numbers enhance more the heat transfer (35% improvement at Re = 500 with respect to 10% at Re = 100). Also, longer baffles can lead to higher heat transfer rates (5% improvement in Hb = 0.5 with respect to 32% at Hb = 1.5). [ABSTRACT FROM AUTHOR]

The forced convective heat transfer and friction factor of nanofluids flow over a backward-facing step (BFS) in a channel with the base wall dissipating a uniform heat flux are experimentally investigated in this paper. Nanoparticles such as CuO and MgO dispersed in pure ethylene glycol (EG) with average diameters of 40 nm, and volume concentrations of 0, 1, 3, and 5% are considered as the working nanofluids. The results indicate that the heat transfer rate increases as the volume concentration of nanoparticles increased. The Nusselt number is enhanced up to 11% at 0.05 volume concentration compared to pure EG. The friction factor increases up to approximately 15% at Reynolds number (Re) of 5000 and volume concentrations of 0.01 and 0.03. The maximum value of the PEC in a BFS channel is 1.5 for the CuO-EG and 1.2 for the MgO-EG at a volume concentrations of 0.03 and Re = 20,000. Unlabelled Image • Fluid flow and heat transfer of CuO-EG over BFS are experimentally investigated. • The effect CuO and MgO concentration on Nu and f were discussed. • The average increase in Nu and f in the experiment up to 11% and 15% respectively. • Finding will be a useful reference to implement nanofluids into various system. [ABSTRACT FROM AUTHOR]

In the present three-dimensional study, laminar flow over a backward-facing step was investigated in the framework of the finite volume method for various Reynolds numbers ranging from 100 to 1000. Attempts were made to clarify the effects of the slope of the downstream heated bottom wall behind the step on the reattachment length and heat transfer. It was found that the length of the recirculation region changes slightly with the slope angles at lower Reynolds numbers while it changes dramatically as Reynolds number increases. The longest length of the recirculation region was obtained at the highest slope angle in the negative vertical-direction while the shortest one was determined at the maximum slope angle in the positive vertical direction. Increasing slope angles in both directions also provide a higher local Nusselt number in some distance behind the step. [ABSTRACT FROM AUTHOR]

• Bubble dynamics in sudden expansion: after the expansion, smaller bubbles tend to migrate toward the wall while larger ones rise in a core region. • Liquid-phase flow statistics in sudden expansion: the flow acceleration and enhancement of turbulence are locally induced near the separation edge and are widened toward the wall as the flow develops downstream. • Locally enhanced turbulence in the separating shear layer is found to be the main reason for reattachment length reduction in the two-phase flow: we estimated the interfacial forces acting on bubbles and suggested an empirical relation for the reattachment length of two-phase flows. We investigate the bubble dynamics and subsequent changes in the liquid-phase flow characteristics of upward bubbly flows in a square pipe with a sudden expansion (expansion ratio of 4.0). The experiments are conducted under three liquid-phase Reynolds numbers of 0 (stationary), 420 (laminar) and 6000 (turbulent). The inlet volume void fraction ranges about 0 − 1.0 % , and we use a high-speed two-phase particle image velocimetry and shadowgraphy to measure two phases simultaneously. It is observed that after the expansion, smaller bubbles tend to migrate toward the wall and larger bubbles rise in a core region, which is more encouraged by the steeper velocity gradient with increasing the Reynolds number. This bubble distribution is further analyzed by estimating the interfacial forces acting on rising bubbles. Affected by this bubble distribution, the enhanced turbulence in the inlet flow energizes the separating shear layer, resulting in the reduction of reattachment length behind the edge. Finally, we suggest an empirical relation for a two-phase flow reattachment length in terms of Reynolds number and mean void fraction, which explains the contribution of added dispersed phase on the mixing enhancement in the backstep flows. [ABSTRACT FROM AUTHOR]

A mathematical model consisting of quasi-hydrodynamic equations and Dong's outflow boundary conditions is proposed for solving fluid dynamics problems in a truncated computational domain. A solution algorithm based on finite-element and control-volume methods is developed. The Kovasznay flow and the flow over a backward-facing step in truncated computational domains are numerically simulated. A comparative analysis of the numerical results shows that the proposed mathematical model adequately describes the hydrodynamic flows in a truncated domain. [ABSTRACT FROM AUTHOR]

Forced convection hybrid nanofluid flow over a backward-facing step under a non-uniform magnetic field is numerically studied using a finite volume method. The external magnetic source is placed in the step edge. The study is performed for a range of nanoparticles volume fraction, φ, from 0 to 2%, Hartmann number, Ha, from 0 to 50, and Reynolds number, Re, from 100 to 300. Results show that the reattachment length reduces by increasing volume fraction of nanoparticles and by decreasing Reynolds number. The recirculation bubble weakens and the conductive heat transfer mode growth by increasing Hartmann number at weak magnetic field intensity. It totally disappears at high Hartmann number when the convective mode dominates. The average Nusselt number increases by increasing volume fraction of nanoparticles and varies with the Hartmann number. The effects of Lorentz force and hybrid nanoparticles on the heat transfer enhancement rates are strongly linked with volume fraction of nanoparticles and Hartmann and Reynolds numbers. [ABSTRACT FROM AUTHOR]

Al2O3/water forced convection nanofluid flow was numerically studied in a duct with backward-facing step. Nanoparticle volume fraction was changed between 1%-5%. Diameter of nanoparticle was constant (dp=40 nm). The Reynolds number was increased from 100 to 500. The step and total height of the duct were 4.8 mm and 9.6 mm, respectively. The bottom wall, which was positioned after the step, was heated with 2000 W/m² and the rest of the walls were adiabatic. Nusselt number, velocity profiles and friction factor were investigated in detail. It was obtained that Nusselt number increases with increasing nanoparticle volume fraction and Reynolds number. [ABSTRACT FROM AUTHOR]

Highlights • DNS of LBE flow over backward-facing step is conducted to study buoyancy impact. • Statistics like velocity, temperature, Reynolds stress and heat flux are discussed. • Buoyancy has substantially altered the flow field for LBE flow at moderate Ri. • Recirculation zone decreases and heat transfer is improved in buoyancy-aided flow. • Recirculation zone largely increases and heat transfer is weakened in opposed flow. Abstract Studies of turbulent mixed convection over a vertical backward-facing step for liquid metals are indispensable to research the buoyancy effects in flow separation and reattachment scenarios that occurs in innovative nuclear and solar power facilities. Direct numerical simulations of turbulent buoyancy-aided flow at two moderate Richardson numbers (Ri = 0.1, 0.2) and buoyancy-opposed flow at Ri = −0.04 for lead–bismuth eutectic (LBE, Prandtl number Pr = 0.025) are performed. The forced convection and air mixed convection (Pr = 0.7, Ri = 0.1) are also simulated for comparison. In all the cases Reynolds number is 4805 and the expansion ratio is 1.5. Mean statistics like velocity components, temperature and heat fluxes are given. Second-order quantities such as Reynolds stresses, temperature fluctuations, and turbulent heat fluxes are also discussed. The results show that buoyancy has substantially altered the flow field for LBE flow compared to air flow at moderate Ri. For buoyancy-aided LBE flow, the recirculation zone is reduced in size and the reattachment length is shortened. With Ri increasing, the second vortex gradually prevails over the main vortex and finally pushes it detach from the wall, leading to positive skin-friction coefficient. Buoyancy weakens the turbulent quantities such as Reynolds stresses, temperature fluctuations and turbulent heat fluxes. Both skin-friction coefficient and Nusselt number increase due to the reduced reversed flow resulting from buoyancy acceleration near the wall. However, it is opposite for the case of buoyancy-opposed flow. The skin-friction coefficient and Nusselt number decrease due to the increased reversed flow though the turbulence is enhanced in buoyancy-opposed flow. These high-resolved data could promote the understanding of this scenario and improve turbulent modelling for flow separation and reattachment of low Prandtl fluids. [ABSTRACT FROM AUTHOR]

The present study investigates the effect of backward facing step in a micro combustor on blowout limit and outer wall temperature. Effect of equivalence ratio of hydrogen-air mixture and combustor material (i.e. conductive heat transfer coefficient) on emitter efficiency and radiation efficiency perused. In addition, the effect of step height and diameter of combustor are studied. The results show that the presence of backward facing step leads to increase blowout limit and mean wall temperature. Temperature distribution along external wall of micro combustor shows a uniform distribution. It is also observed that combustor with lower step height has higher (mean) wall temperature. Maximum mean wall temperature and radiation efficiency achieved when equivalence ratio is located in lean values. It has been also observed that the wall thermal conductivity has an optimum value to obtain maximum radiation efficiency. [ABSTRACT FROM AUTHOR]

In this work a Reynolds-Averaged Two-Fluid fully coupled model (RA-TFM) for modelling of turbulent fluid-particle flow is implemented in OpenFOAM and applied to a vertically orientated backward-facing step. Three particle classes with varying mass loadings (10–40%) and different Stokes number are investigated. Details of the implementation and solution procedure are provided with special attention given to challenging terms. The prediction of mean flow statistics are in good agreement with the data from literature and show a distinct improvement over current model predictions. This improvement was due to the separation of the particle turbulent kinetic energy k p , and the granular temperature Θ p , in which the large scale correlated motion and small scale uncorrelated motion are governed by separate transport equations. For each case simulated in this work, turbulence attenuation was accurately predicted, a finding that is attributed to separate coupling terms in both transport equations of k p and ε p . [ABSTRACT FROM AUTHOR]

The influence of buoyancy-aided mixed convection on the heat transfer downstream of a sudden expansion of a plane channel is investigated by means of the steady-state Reynolds averaged Navier Stokes (RANS) simulation. A linear eddy viscosity model is used to compute the Reynolds stresses. The turbulent heat fluxes are modelled with a single gradient diffusion hypotheses using a local correlation to evaluate the turbulent Prandtl number. The velocity, turbulent kinetic energy and Reynolds shear stress profiles predicted are in good agreement with available DNS data. The local Stanton number trend is qualitatively well captured, however, its magnitude is underestimated for the mixed convection cases. Compared to the case of forced convection, the flow field and heat transfer change significantly. An influence of buoyancy is reported at very low Richardson numbers (Ri). A steady increase in heat transfer with rising influence of buoyancy is observed. [ABSTRACT FROM AUTHOR]

In this paper, flow and heat transfer characteristics of SiO2-water nanofluid flow over a corrugated backward-facing step are numerically investigated. The numerical study is performed by solving governing equations (continuity, momentum, and energy) with finite volume method. The duct inlet and step heights are 4.8 mm. The expansion ratio is 2. The upstream wall, Lu, and downstream wall, Ld, lengths are 48 cm and 96 cm, respectively. The downstream wall of the duct is subjected to a constant and uniform heat flux of 2000 W/m². The ranges of the volume fraction of nanoparticles and Reynolds number are 0%-3.0% and 135-240, respectively. The effects of the volume fraction of nanoparticles on the average Nusselt number, average Darcy friction factor, and velocity distribution are investigated under laminar forced convective nanofluid flow condition. It is revealed that the nanoparticle volume fraction substantially influences the heat transfer and flow characteristics. The volume fraction of 3.0% shows the highest heat transfer performance. [ABSTRACT FROM AUTHOR]

The flame stabilization and blowout velocity of lean premixed turbulent flames in a backward-facing step combustor were investigated experimentally with well-defined stable flames. The dynamic pressure in the combustor was monitored to select a stable regime among overall flame behaviors. The responses in the flame behaviors to changes in the initial conditions of equivalence ratio, inlet temperature, and Reynolds number were simultaneously captured using high-speed CH ∗ chemiluminescence. The results showed that the flame modes were categorized into four representative regimes according to their dynamic behavior, overall sound pressure level, and frequency spectrum: (1) stable regime, (2) unstable regime, (3) quasi-unstable regime, and (4) transition regime. For lean premixed turbulent flames in the stable regime, the flame distance, defined as distance x from the step to flame appearance, increased with the decrease in the initial equivalence ratio, and blowout occurred near the lower flammability limits. The blowout velocities linearly correlated with both the inverse of chemical reaction time ( α 0 / S L,0 2 ) −1 and extinction strain rate ε ext,0 under different initial conditions. The two chemical time scales were obtained using different flame models based on either the premixed laminar flame-speed or premixed opposed-flow flame model. In addition, the flame distances showed a linear correlation of d f  ∝  U 0 / S L,0 2 for all the data in the stable regime. This result implies that the lean premixed turbulent flames could be stabilized at flame distances where the local mean flow velocity is balanced with the turbulent flame speed, which is proportional to the square of laminar burning velocity. [ABSTRACT FROM AUTHOR]

This paper presents the results of investigations of the influence of the separated flow structure behind a rib located before the edge of the backward-facing step on the parameters of the separation region behind it. Numerical investigations were carried out by the method of large eddy similution in a plane air channel. The velocity of the developed turbulent flow at the inlet to the channel was assumed to be 25 m/s with a level of initial turbulence for three velocity components of 5%. The Reynolds number calculated by the initial velocity and the step height was 15,500. It has been shown that the creation of a preseparation region causes the two-dimensional flow to transform into a three-dimensional one before the edge of the back-facing step with the formation of nonuniform verticity along its length. This restructuring of the flow leads to an increase in the inhomogeneity of velocity parameters in the separated mixing layer. The flow expansion behind the step increased and was accompanied by local separation on the upper wall of the channel. The two separation regions caused a rapid growth of pulsations throughout the channel behind the step. [ABSTRACT FROM AUTHOR]

The effect of local forcing on the separated, three-dimensional shear layer downstream of a backward-facing step is investigated by means of large-eddy simulation for a Reynolds number based on the step height of 10,700. The step edge is either oriented normal to the approaching turbulent boundary layer or swept at an angle of 40°. Oblique vortices with different orientation and spacing are generated by wavelike suction and blowing of fluid through an edge parallel slot. The vortices exhibit a complex three-dimensional structure, but they can be characterized by a wavevector in a horizontal section plane. In order to determine the step-normal component of the wavevector, a method is developed based on phase averages. The dependence of the wavevector on the forcing parameters can be described in terms of a dispersion relation, the structure of which indicates that the disturbances are mainly convected through the fluid. The introduced vortices reduce the size of the recirculation region by up to 38%. In both the planar and the swept case, the most efficient of the studied forcings consists of vortices which propagate in a direction that deviates by more than 50° from the step normal. These vortices exhibit a spacing in the order of 2.5 step heights. The upstream shift of the reattachment line can be explained by increased mixing and momentum transport inside the shear layer which is reflected in high levels of the Reynolds shear stress -ρu'v'. The position of the maximum of the coherent shear stress is found to depend linearly on the wavelength, similar to two-dimensional free shear layers. [ABSTRACT FROM AUTHOR]

This study aims at investigating the mixing layer located at the interface between the free stream and recirculation zone downstream from a sudden lateral open-channel expansion. Specific attention is paid to the interaction of the shallowness of the flow, characterized by the bed-friction number, with the lateral confinement due to the side wall. The velocity field for four flows with the same geometry but very different bed-friction numbers is measured in detail in order to characterize the mean velocity fields and Reynolds stresses across the mixing layers and evaluate the width of the mixing layers and their growth rates along with the typical oscillation frequencies. In the upstream region of the recirculation zone, the mixing-layer characteristics for the examined configurations are analogous to the ones of classical-laterally unbounded-mixing layers. In this region, the shallowness modifies the shape of the streamwise velocity profiles, extends the mean velocity gradient magnitudes and lowers the Reynolds stress terms, but hardly affects the mixing layer's expansion rate. On the other hand, in the region near the flow reattachment, the mixing layer adopts very different behavior, with an abrupt drop of the its expansion. This change in behavior is linked to the dynamics of the two-dimensional (2D) vortices within the mixing layer, not a damping effect of the bed friction on these vortices, as the local bed-friction numbers remain much lower than the critical values reported in the literature. It is rather due to the interaction of the coherent structures with the side wall; the characteristics of this interaction itself are influenced by the flow shallowness. It is also shown that the damping effect due to bed friction is not responsible for the huge variations reported on the recirculation zones length. This imposes a distinction between local mixing-layer shallowness-derived using the mixing-layer width as length scale and governing the mixing layer's characteristics-and a global flow shallowness-derived using the expansion width and governing the recirculation length [ABSTRACT FROM AUTHOR]

One of the promising and simplest methods of passively controlling separating flow behind a backward-facing step with the aid of slotted ribs is considered. The size and number of slots were varied in wide ranges in experiments so that the relative area of the slots amounted to R = F/F = 0-1. The influence of the longitudinal vortices induced by the teeth in a rib on the distribution of pressure and heat transfer rate in the recirculation region in the case of varying the sizes of slots in a rib, the spacing between them, and the position of the rib relative to the backwardfacing step has been studied. The effectiveness of using slotted ribs for reducing the length and intensity of the region of reverse flow behind the step is shown. [ABSTRACT FROM AUTHOR]

Near space has been paid more and more attentions in recent years due to its militarily application value. Direct simulation Monte Carlo (DSMC) which is one of the most successful particle simulation methods in treating rarefied gas dynamics is employed to investigate the flow characteristics of a hypersonic backward-facing step (BFS) under active flow control using supersonic jet in near space. The numerical tool is validated by an experimental flow of dual cusped-plate model, shock wave structures from the numerical simulation are shown in quite good agreement with the experimental result. The influence of altitude and active flow control on BFS flow are then studied in detail. Three parameters, i.e. boundary layer thickness, recirculation region length, and lean angle of the primary recirculation region that is first defined to describe recirculation region shape, are used to evaluate the flow characteristics of every case computed. The numerical results indicate that the main effect of vertical jet upstream of the step is the enhancement of boundary layer thickness downstream of the jet slot, then, it shows a weak influence on recirculation region length and a negligible effect on lean angle. Conversely, the horizontal jet near the step edge can greatly change the recirculation region length by adjusting jetting angle, but it only has a weak influence both on boundary layer thickness and on lean angle for every jetting angle considered. A significant finding is that the recirculation region length is decreased severely in near space compared with experimental and numerical results presented in the open literature. [ABSTRACT FROM AUTHOR]

The structure and heat transfer in a turbulent separated flow in a suddenly expanding channel with injection (suction) through a porous wall are numerically simulated with the use of two-dimensional averaged Navier-Stokes equations, energy equations, and v - f turbulence model. It is shown that enhancement of the intensity of the transverse mass flux on the wall reduces the separation region length in the case of suction and increases the separation region length in the case of injection up to complete boundary layer displacement. The maximum heat transfer coefficient as a function of permeability is accurately described by the asymptotic theory of a turbulent boundary layer. [ABSTRACT FROM AUTHOR]

In this study, a direct numerical simulation of the fluid flow and heat transfer characteristics of separation and reattachment flow at a backward-facing step is presented. A computer program of FORTRAN code is used to solve the governing equations according to finite volume method. The effects of the Reynolds number and expansion ratio on the fluid flow and heat transfer characteristics are investigated. The size of the primary recirculation zone increases with the reduction of expansion ratio and the fluctuation of isotherms increased with the increase of Reynolds number. The periodic characteristics and the dissimilarity between Nu and Cf appear in the transitional flow regime. The rotating fluids in the reattachment region increase the flow instability and the interchange of the hot and cold fluids increases heat transfer instability. The combined effects of flow instability and heat transfer instability play an important role in the formation of the dissimilarity between Nu and Cf. [ABSTRACT FROM AUTHOR]

This paper presents a direct numerical simulation of the vortical structure along the bottom wall and its effect on heat transfer enhancement in the transitional flow over a backward-facing step. The governing equations are solved with a computer program using FORTRAN code according to finite volume method. The heat transfer of the bottom wall is enhanced greatly for the transitional Reynolds numbers, especially in the range 6 ≤ x/H ≤ 14. In this range, the local Nusselt number distributions for various time instants of the periodic cycle exist three peak values at Re = 1200. The corresponding locations of Nu peak values in the vorticity contours are all between the negative vortex and the adjacent downstream positive vortex. This vortical structure appears due to the flow instability which causes the heat transfer enhancement of the bottom wall in the transitional flow. [ABSTRACT FROM AUTHOR]

Dual-mode scramjet combustor is the crucial component for the combined cycle engine of space mission vehicles. The Reynolds Averaged Navier-Stokes (RANS) equations, Spalart-Allmaras turbulence model and the finite-rate reaction model have been utilized to investigate the mixing and combustion in a dual-mode combustor with backward-facing steps, and the species distributions and the Mach number profile of a turbulent diffusion combustion problem have been employed to validate the numerical approach. Moreover, the influences of the injection strategy and the fuel equivalence ratio arrangement on its mixing and combustion flow fields have been explored. The obtained results show that the vertical injection is beneficial for mode transition, and an obvious high pressure region is generated with the vertical injection strategy. Accordingly, its mass-weighted average Mach number drops more sharply. The lower total equivalence ratio is beneficial for the mixing augmentation, as well the smaller fuel equivalence ratio discrepancy. However, the larger total equivalence ratio is beneficial for the mode transition. [ABSTRACT FROM AUTHOR]

A non-thermal surface plasma discharge (dielectric barrier discharge) is installed at the step corner of a backward-facing step ( U 0 = 15 m/s, Re h = 30,000, Re θ = 1650). Wall pressure sensors are used to estimate the reattaching location downstream of the step and also to measure the global wall pressure fluctuation coefficients. A parametric study is performed where the voltage amplitude, the burst frequency and the duty-cycle of the input high voltage are investigated separately regarding their performance. Mean and phase-averaged velocity fields are obtained by time-resolved PIV and they are used to analyze the impact on the dynamic of the flow. The minimum reattaching position is achieved by forcing the flow at the shear layer mode through 2D periodic perturbations in the initial region at St h = 0.25 (St θ = 0.013), where a large spreading rate is obtained by increasing the organization and periodicity of the vortex street. A mean recirculation reduction by 22% is observed. Pressure fluctuations are maximized for an actuation at half the shear layer mode (St h = 0.125 or St θ = 0.006). For this subharmonic forcing mode by single-frequency perturbation, the vortex pairing is enhanced. The present study shows that the subharmonic mode corresponds to the well-known shedding mode that promotes flow structures remaining coherent over a long distance and dominating all the flow dynamics, this including the dynamics of the reattaching point. [ABSTRACT FROM AUTHOR]

To gain insight into characteristic wake flow modes, which among others are responsible for asymmetrical loads on the engine extensions of space launchers at transonic speeds, combined experimental and numerical investigations of a turbulent wake flow are performed at M a ∞ = 0.8 and R e h = 1.8 ⋅ 10 5 . The experiments are conducted at the Bundeswehr University Munich using planar PIV and wall pressure measurements, while the numerical investigations are performed by the Institute of Aerodynamics at RWTH Aachen University using a zonal RANS–LES approach and dynamic mode decomposition (DMD). The analysis is done on a planar space launcher configuration that consists of a backward-facing step with a long shock-free forebody avoiding undesired shock-boundary-layer interactions upstream of the analyzed wake flow region. The investigated wake flow is characterized by a highly unsteady behavior of the shear layer shedding from the forebody and subsequently reattaching onto the splitter plate. The strong variation of the reattachment positions in the spanwise and the streamwise direction leads to pronounced wall pressure oscillations and consequently, structural loads. By means of a classical statistical analysis, i.e., spatial and temporal Fourier transforms and two-point correlations of experimental and numerical data, one spatial coherent scale with a spanwise wavelength of λ z ≈ 2 h and two characteristic frequencies of S r h ≈ 0.01 and S r h ≈ 0.07 have been obtained for the investigated wake flow problem. To clarify the coherent fluid motion dominating the detected spatio-temporal behavior, a DMD algorithm is applied to the time-resolved three-dimensional streamwise velocity field. The frequencies of the first two extracted stable sparsity DMD modes closely coincide with the characteristic peaks in the wall pressure spectra. The analysis of the three-dimensional shape of the extracted DMD modes reveals that the detected wake flow behavior is caused by a pronounced low-frequency cross-pumping motion of the recirculation region and a high-frequency cross-flapping motion of the shear layer, respectively. Both wake flow modes feature a pronounced nearly-periodical variation in the spanwise direction with an approximate spanwise wavelength of two step heights. Thus, the extracted underlying wake modes clearly explain the occurrence of corresponding peaks in the wall pressure spectra, the periodical formation of wedge-shaped reattachment regions on the splitter plate, and the detection of finger-like structures in the PIV experiments. [ABSTRACT FROM AUTHOR]