Double Backward Facing Step Research Articles

محاكاة تدفق السوائل النانوية الصفائحية على خطوة ساخنة مزدوجة مواجهة للخلف

This research focuses on heat transfer enhancement by separating flow over a double backward-facing step using nanofluid. To conduct this research, a numerical model was constructed using the ANSYS Workbench, and the governing equations were solved using the realizable k-ε model with a second-order upwind scheme. Three different types of base fluid (water, engine oil, and ethylene glycol) with nanoparticles (AL2O3, TiO2, and Cu) were used for Reynolds numbers ranging from 200 to 1000. The constant heat flux of the 2000 W/m2 boundary condition is considered on the lower wall. The effect of nanoparticle material, nanoparticle concentrations, nanoparticle sizes, and Reynolds numbers on the Nusselt number is investigated. Two different approaches for estimating nanofluid thermal conductivity and viscosity have been used. The numerical code results were verified with previous numerical data. The results showed that the peak of the Nusselt number occurs immediately after the steps. The results demonstrated a significant improvement in the heat transfer performance of nanofluids, especially Cu/engine oil nanofluids, compared to conventional base fluids with other nanoparticles. It was noticed that the Nusselt number increases with decreasing nanoparticle diameter and also with increasing Reynolds number.

Enhancement of thermal energy transfer behind a double consecutive expansion utilizing a variable magnetic field

This research focuses on utilizing non-uniform magnetic fields, induced by dipoles, to control and enhance thermal energy transfer in a two-dimensional cooling conduit including a double backward-facing step. The presence of electronic equipment along the straight channel path creates such arrangements, and cooling is often ineffective in the corners of the formed steps. The use of a non-constant magnetic field is a passive technique to improve the cooling rate in these sections without changing the internal geometry, thereby increasing the heat transfer rate. A commercial software based on the finite volume technique is employed to solve the governing equations of fluid flow and heat transfer. Multiple parameters are examined in this study, including the flow Reynolds number (12.5–50), dipole location and strength (0.1–5 A-m), and the number of dipoles (single or double). The results indicate that all of these parameters have a significant impact on the thermal energy transfer. The results of the study show that a single dipole increase the average heat transfer by about 22%, two magnetic fields by 40%, the strength of the magnetic source by 24% with respect to the non-magnetic field in the present study.

Numerical Investigation of Fluid Flow and Heat Transfer Characteristics over Double Backward-Facing Step with Obstacles

Many engineering device, including nuclear reactors, heat exchangers, and electronic circuit boards, exhibit flow separation and reattachment. To reduce thermal stress impacts, the components of these devices, which resemble stepped channel designs and are susceptible to high heat flux, must be properly cooled. The present work numerically investigates the fluid flow and heat transfer characteristics through a double backward-facing step with elliptic obstacles located after each step. For this purpose, equations governing fluid flow and heat transfer are solved in a Cartesian framework using an in-house code based on streamline upwind/Petrov-Galerkin finite element method. The effect of various parameters (axis ratios (0.25, 0.5, 0.75, 1), vertical location of obstacles (0.38, 0.577, 0.769), Reynolds number (300, 500, 800, 1000)) on fluid flow and heat transfer characteristics in the channel with and without obstacles are compared and quantified. The recirculation region length of the double backward-facing stepped channel with obstacles decreases after the first and second steps when compared with no obstacles. The local Nusselt number distribution along the corner regions of the stepped wall is enhanced due to obstacles. The axis ratio of obstacles has less influence on the convective heat transfer enhancement than the vertical location of the obstacles.

Computational fluid dynamic analysis of graphene oxide/water nanofluid heat transfer over a double backward-facing microchannel

Background: Based on the literature, increasing the heat transfer of nanofluids in the backward microchannel owing to the micro scales and significant effects of the step has been introduced as an attractive issue.Methods: The effect of an inclined step combined with different types of obstacles on the heat transfer of graphene oxide nanofluid inside a double backward-facing step (BFS) microchannel is studied in this study. The numerical model according to the finite volume method (FVM) is utilized to discretize continuity, momentum, and energy equations via ANSYS Fluent 19.0 software. The simulation of double BFS microchannel by using computational fluid dynamics (CFD) is carried out to enhance the forced convection heat transfer of graphene oxide–water nanofluid. This work aims to illuminate the influences of the Reynolds numbers (1, 50, and 100), the nanoparticles volume fraction (0, 0.02, and 0.04), the triangular obstacle existence, and the obstacle location.Significant findings: Based on the CFD results, the maximum Nusselt number of 12.7% and heat transfer coefficient of 12.3% are observed compared to the pure water in the case of triangular obstacle at the top of the first step with nanoparticle volume fraction of 4% and Reynolds numbers of 50 and 1, respectively. Furthermore, heat transfer improves with high pressure drop and friction factor is widely affected by the triangular obstacles and their locations.

The double backward-facing step: effect of forcing on interacting separated flow regions

This paper demonstrates experimentally that imposed periodic forcing can significantly alter the global flow characteristics of the flow over a double backward-facing step. The geometry consists of two equal height steps spaced up to eight step heights apart. A periodic zero-mass flux jet located at the first step's top corner was issued at frequencies ranging from below the step-mode instability frequency up to approximately five times the shear-layer instability frequency. Reattachment of the flow onto the first step was achieved for step separations as low as three single-step heights with imposed forcing; significantly shorter than the five single-step heights that occurred without forcing. A significant reduction in mean base pressure on the first step, and increase on the second step, occurred for low forcing frequencies. Even for large step separations, the effect of forcing on the flow persisted sufficiently far downstream to appreciably influence the development of the second recirculation zone. Importantly, previous forced single and unforced double backward-facing step flows provide reference cases to examine and discuss similarities and differences. This study offers insight into possibilities and potential outcomes of flow control for applications ranging from the drag reduction of ground vehicles such as pickup trucks, to enhanced mixing in industrial processes.

The double backward-facing step: interaction of multiple separated flow regions

The backward-facing step is perhaps the quintessential geometry used to study separated flow. Extensive previous research has quantified its detailed flow characteristics. However, often regions of separated flow do not exist in isolation; rather, interaction occurs between multiple regions. This motivated an experimental investigation into the time-averaged and dynamic flow features of a double backward-facing step, covering separations of zero to eight step heights between equal-height steps. Three flow regimes are identified. A single reattachment regime occurs for separations of less than four step heights, perhaps remarkable for the lack of variation in key flow characteristics from a single backward-facing step response. Next, an intermediate regime is identified for a separation of four step heights. In this case, the flow does not yet reattach on the first step, although significant differences in reattachment length, surface pressure on the vertical step faces and turbulence statistics occur. Finally, for greater step separations, a double reattachment regime, with reattachment on both steps, is identified. Downwash, induced by the first recirculation zone, reduces the reattachment length and turbulent fluctuations of the second recirculation zone. The surface pressure on the first-step vertical face is reduced, seemingly a result of an upstream influence due to the low pressure in the second-step recirculation zone. Detailed characterisation of the regimes offers insight into the fundamental interaction of regions of separated flow, revealing aspects of complex dynamics relevant to a broad range of practical scenarios.

Comparative Study of Thermal Performance of Different Nanofluids in a Double Backward-Facing Step Channel: A Numerical Approach

Two-dimensional numerical simulations are conducted to study forced convection flow of different water-based nanofluids (ZnO, Al2O3, and SiO2) with volume fractions ( ϕ ) = 0–5% and fixed nanoparticle size (dp) = 20 nm for Reynolds numbers (Re) = 50–225 over a double backward-facing step with an expansion ratio (ER) = 2 under constant heat flux (q″ = 3000 W/m2) condition using the finite volume method. Results indicate that the local Nusselt number increases with volume fraction and Reynolds number for all working fluids. In comparison to water, the maximum heat transfer augmentation of about 21.22% was achieved by using water-SiO2 nanofluid at Re = 225 with ϕ = 5% and dp = 20 nm. Under similar conditions, the Al2O3 and ZnO nanofluids demonstrated 14.23% and 11.86% augmentation in heat transfer in comparison to water. The skin friction coefficient decreases with the increase in Re for all working fluids. No significant differences are observed in the values of skin friction coefficient among all working fluids at a particular Re. These results indicate that the heat transfer enhancement has been achieved with no increased energy requirements. In addition, the velocity increases with the rise in Re, with SiO2 nanofluid exhibiting the highest velocity as compared to other working fluids.

Experimental study and passive control of the bistable dynamics of the three-dimensional air-wake flow of a finite-width double backward-facing step

Inspired by the study of Rao et al. (2019), the aerodynamics of a simplified frigate is investigated experimentally. The natural wake flow dynamics is shown to be bi-stable at zero yaw angles, at both the rear of the ship superstructure and the stern. The bi-stable dynamics is strongly weakened at the stern when the ship is lifted off the ground in the free-stream. Perimetric cavities of sufficient depth at the rear of the superstructure are able to suppress the bi-stable dynamics and to symmetrize the air-wake flow at both the superstructure and the stern. The base pressure increases with the cavity depth while the drag force slightly decreases. Our study experimentally confirms that perimetric rear cavities offer efficient devices for passive flow control strategies of the aerodynamic air-wake flow of bodies of complex geometries subject to bi-stable dynamics.

Yaw effect on bi-stable air-wakes of a generic ship using large eddy simulation

An LES (large eddy simulation) investigation at Re=8×104 has been conducted to predict the flow past a simplified frigate shape model at yaw, of which the stern is analogous to a double backward-facing step. At 0∘ yaw, the occurrence of a double bi-stable flow configuration is recorded, i.e. an asymmetrical flow topology is observed behind each step, and the flow configuration at the flight deck (top step) is anti-symmetrical as compared to the rear step. Even when the incoming flow is yawed with a small angle (e.g. ±2.5∘), the bi-stable flow configuration is still present and the level of the asymmetry is increased at each step. Moreover, the negative angle maintains the same flow configuration as what is shown at 0∘ yaw, while the positive angle leads to the corresponding anti-symmetrical flow configuration. A larger yaw angle suppresses this asymmetrical flow topology on both steps. To explore those asymmetrical phenomena, a brief analysis and discussion are included for each case, showing that the direction of the incoming flow and the length of each step are key parameters that characterise this double bi-stable behaviour.

Hydro-thermal performance of CNT nanofluid in double backward facing step with rotating tube bundle under magnetic field

In this study, a novel method for convective heat transfer control of flow past a double backward facing step with combined effects of oriented magnetic field, rotating tube bundle and inclusion of highly conductive CNT nanoparticles in the base fluid is offered. Hydro-thermal performance assessment of double backward facing step is numerically performed with finite volume method in laminar flow regime. Effects of Reynolds number, rotational Reynolds number, circular cylinder arrangement and horizontal local of the tube bundle, distance between the steps and magnetic field strength on the fluid flow, heat transfer, pressure drop and hydro-thermal performance coefficient variation are examined. The rotation of the cylinder, arrangement and location were found to alter hydro-thermal performance while the average Nu is enhanced with higher Reynolds and Hartmann numbers. The presence of the upper vortex location resulted in higher deflection of the main stream toward the hot bottom wall which resulted in higher local heat transfer rates. This is especially the case for clockwise direction rotation at the height speed and local Nu value increment is 244% as compared to non-rotating cylinder case. Best performance coefficient is obtained with MHD flow at Hartmann number of 5 while performance increase is 13% as compared to non-magnetic field configuration. The vertical size and location of the upper vortex changes with the horizontal location of the tube bundle and spacing between the steps. As compared to reference configurations, variations in the hydrothermal performance coefficients are 15% and 20% when varying the horizontal location and distance between the steps. The CNT nanoparticles inclusion in the base fluid resulted in performance coefficient enhancement of 52% at the highest solid volume fraction. As flow separation and subsequent attachments are encountered in a variety of heat transfer engineering applications, the results of the present work will be helpful in the design and optimization of various thermal engineering systems.

Simulation of shear layers interaction and unsteady evolution under different double backward-facing steps

ABSTRACT High-order accurate schemes are employed to numerically simulate the interaction of a supersonic jet and a co-directional supersonic inflow. A double backward-facing step model is proposed to investigate the interaction between the jet shear layer and the supersonic inflow shear layer. It is found that due to the interaction of the shear layer, a secondary jet is injected into the recirculation zone at the intersection of the two shear layers. The secondary jet produced by the interaction of the two shear layers has a periodicity because of shear layers interaction. The distinction in the shape of double backward-facing steps will induce changes in the period of the secondary jet. The analysis and discussion of the periodicity of the secondary jet are mainly focused in this letter.

Qualitative assessment of the bi-stable states in the wake of a finite-width double backward facing step

The flow past a simplified frigate shape model which is analogous to a finite-width double backward facing step is investigated numerically using well-resolved large eddy simulations at Re≃8×104. The geometric configuration of this model permits the occurrence of bi-stable flow states, with an asymmetrical flow topology being observed in the lateral midplane behind each step. In each flow state, the flow at the top step is anti-symmetrical to that observed on the bottom step, and the two flow states are anti-symmetrical to each other. Furthermore, incorporating a base cavity on the top step leads to the suppression of the asymmetrical flow topology on both the steps. The recirculation bubble formed by the downwash at each step is elongated when the base cavity is used, as compared to that observed in either of the two flow states, resulting in a decrease of the drag coefficient. In each of the two flow states, the unequal strength of the streamwise vortices close to the lateral edges on the top step has been identified as the likely cause of the asymmetrical flow on the bottom step.

Heat Transfer to Laminar Flow over a Double Backward-Facing Step

Abstract — Heat transfer and laminar air flow over a double backward-facing step numerically studied in this paper. The simulations was performed by using ANSYS ICEM for meshing process and using ANSYS fluent 14 (CFD) for solving. The k-ɛ standard model adopted with Reynolds number varied between 98.5 to 512 and three step height at constant heat flux (q=2000 W/m2). The top of wall and bottom of upstream are insulated with bottom of downstream is heated. The results show increase in Nusselt number with increases of Reynolds number for all cases and the maximum of Nusselt number happens at the first step in compared to the second step. Due to increase of cross section area of downstream to generate sudden expansion then Nusselt number decrease but the profile of Nusselt number keep same trend for all cases where increase after the first and second steps. Recirculation region after the first and second steps are denoted by contour of streamline velocity. The higher augmentation of heat transfer rate observed for case 1 at Reynolds number of 512 and heat flux q=2000 W/m

Flow Bistability Downstream of Three-Dimensional Double Backward Facing Steps at Zero-Degree Sideslip

The flow downstream of a three-dimensional double backward facing step (3D DBWFS) is investigated for Reynolds number Reh ranging from 5×103 to8×104 (based on the first step height h). The flow is studied both qualitatively by means of laser tomoscopy and oil-flow visualizations and quantitatively by means of particle image velocimetry (PIV) measurements. In particular, the results show a mean flow asymmetry. A sensitivity study around zero degree sideslip has shown that the flow is bistable for this geometry. This bistability has been observed in two different wind tunnels for very different upstream conditions. As a main consequence, the zero degree drift angle could be a relevant validation case of unstable flow computation. More tests are carried out to understand and control this particular flow feature.

A study of a 3-D double backward-facing step

An investigation of the flow over a three-dimensional (3-D) double backward-facing step is presented using a combination of both quantitative measurements from a particle image velocimetry (PIV) system and qualitative oil-flow visualizations. The arrangement of the PIV instrument allows for snap-shots of the (x, y) and (y, z) planes at various axial and spanwise positions. The measurements illustrate characteristics that are found in both two-dimensional (2-D) backward-facing steps and 3-D flows around wall mounted cubes. In particular, the development of a horseshoe vortex is found after each step alongside other vortical motions introduced by the geometry of the model. Large turbulence levels are found to be confined to a region in the center of the backstep; their mean square levels being much larger than what has been observed in 2-D backward-facing steps. The large turbulent fluctuations are attributed to a quasi-periodic shedding of the horseshoe vortex as it continuously draws energy from the spiral nodes of separation, which form to create the base of the horseshoe vortex. A combination of effects including the shedding of the first horseshoe vortex, the horizontal entrainment of air and the presence of two counter rotating vortices initiated at reattachment, are shown to cause the steering vector of the flow to jettison away from the surface in the first redeveloping region and along the center at z/h = 0. Oil-flow visualizations confirm these observations.

Laminar flow of a viscoelastic shear-thinning liquid over a backward-facing step preceded by a gradual contraction

Experimental observations and numerical simulations, based upon the Phan-Thien and Tanner model, are reported for the laminar flow of a series of viscoelastic liquids (0.05%, 0.1%, and 0.4% concentrations of a polyacrylamide) over a symmetrical, double backward-facing step geometry preceded by a short gradual contraction from a long (120 hydraulic diameters in length) square duct. Reynolds numbers are typically between 10 and 100 (i.e., inertia is not negligible) and Deborah numbers of order 100 for the experiments (based on a relaxation time determined from linear viscoelasticity measurements) and order 10 for the viscoelastic simulations. As the polymer concentration is increased, the combined effects of increased shear thinning and viscoelasticity are found to dramatically reduce the length of the recirculation region downstream of the step. The nature of the flow field within the contraction itself is found to be fundamentally different for the viscoelastic liquids to that for a comparable Newtonian fluid flow: large velocity overshoots with very strong gradients appear near to the sidewalls that, due to their appearance, we have dubbed “cat’s ears.” The simulations are able to reproduce these remarkable features, at least qualitatively.

Turbulent Flow Over a Plane Symmetric Sudden Expansion

Turbulent flow with separation and recirculation over a double backward facing step has been investigated experimentally. Time mean stream wise, transverse and cross-stream components of the velocity fluctuations, together with turbulence kinetic energy and Reynolds shear stresses, were measured using a laser Doppler anemometer, operating in the differential Doppler mode with forward scattering. Ordinary tap water was used in a closed loop flow system with a Reynolds number of 30,210 and significant changes of flow patterns, increases in turbulence kinetic energy, velocity fluctuations and shear stresses were observed downstream of the step expansion.

Measurements in plane separated flow

This paper reports an experimental study of turbulent separated flow in water over a double backward facing step. The dimension of the step was 2.5 mm in depth expanding symmetrically to 7.5 mm. The duct was 76 mm wide and 300 mm long and time mean streamwise, transverse and cross-stream components of velocity and turbulent fluctuation were measured using a laser Doppler anemometer incorporating frequency tracking and frequency shifting of the incident beams. For all measurements water flowed upstream of the step at an average velocity of 1.8 m/s corresponding to a Reynolds number, based on upstream duct width of 6 × 10 3. Substantial changes of flow patterns, and increases in kinetic energy of turbulence and velocity fluctuations were detected as a consequence of the double step expansion. Turbulent intensities of up to the order of 100% have been measured in the recirculation zones. Transverse turbulent intensities based on the mean duct exit velocity were in general about 50% of the stream wise components. The maximum streamwise turbulent velocity measured was 150% more than the corresponding value at the step. A contour plot of non-dimensionalised constant turbulent kinetic energy compares favourably with similar results obtained for other duct configurations. The effect on heat transfer in asymmetrical flows is discussed.