Secondary Recirculation Region Research Articles

A study on micro-step flow using a hybrid direct simulation Monte Carlo–Fokker–Planck approach

This study aimed to investigate the recirculation zone in a micro-step geometry using a hybrid molecular direct simulation Monte Carlo (DSMC) Fokker–Planck (FP) approach. As this hybrid approach benefits from the accuracy of the DSMC and reduced computational cost of FP, very low Knudsen number (Kn) and high Reynolds number (Re) cases were investigated for the first time. In particular, the role of Kn, specularity of walls, and Re was evaluated on the formation of concave and convex vortices. The Kn and Re ranges were from 0.0001 to 100 and from 0.04 to 5940, respectively. The latter considers a wide range of flow regimes from laminar to transitional flow. It is the first time that transitional flows have been treated in a micro-step using a rarefied flow solver. We demonstrated the formation of a vortex on the top wall of the micro-step geometry for low Kn conditions in the range of 0.0001 <Kn < 0.001. Extended positive pressure gradient in the geometry was found to be the cause of this secondary recirculation region. It was demonstrated that the recirculation zone lays on the vertical wall for Kn >1. It was shown that making the junction and bottom wall of the step specular did not eliminate the concave vortex but rather led to an increase in its strength. In addition, cold-to-hot transfer could be observed in all cases due to the competition between the higher-order term of the heat flux formula with the Fourier term.

Heat transfer enhancement for nanofluid flows over a microscale backward-facing step

In this paper, we provide a numerical study of laminar flow in a micro-sized backward-facing step channel for water-based nanofluids containing Al2O3 and TiO2 nanoparticles and investigate the impact of the temperature differences between the inlet and the downward wall temperatures. The present work is the first study to introduce a temperature-dependent separation flow model. The velocity increases with increasing concentration of nanoparticles. However, when comparing Al2O3 and TiO2 nanofluids, Al2O3 has a velocity and shear stress higher than TiO2 for 0.04 volume fraction. Increasing the volume fraction of nanofluid led to an increase in the rate of heat transfer from the wall to the fluid, as the thermal properties improved as the volume ratio increased. The performance efficiency index (PEI) decreases as the volume fraction of the particle increases after a certain amount of nanoparticles. The simulation results of the nanofluid separation flow in the recirculation and reattachment shows that the velocity increases as the temperature difference increases, the size of the primary and secondary recirculation regions behind the step is significantly influenced with the temperature difference, a larger temperature difference means a larger recirculation zone.

Investigation of hypersonic flows through a cavity with sweepback angle in near space using the DSMC method* *Project partly supported by the National Natural Science Foundation of China (Grant No. 11802264) and the Natural Science Foundation of Jiangsu Province, China (Grant No. BK20180896).

Near space has been paid more and more attentionin recent years due to its military application value. However, flow characteristics of some fundamental configurations (e.g., the cavity) in near space have rarely been investigated due to rarefied gas effects, which make the numerical simulation methods based on continuous flow hypothesis lose validity. In this work, the direct simulation Monte Carlo (DSMC), one of the most successful particle simulation methods in treating rarefied gas dynamics, is employed to explore flow characteristics of a hypersonic cavity with sweepback angle in near space by considering a variety of cases, such as the cavity at a wide range of altitudes 20–60 km, the cavity at freestream Mach numbers of 6–20, and the cavity with a sweepback angle of 30°–90°. By analyzing the simulation results, flow characteristics are obtained and meanwhile some interesting phenomena are also found. The primary recirculation region, which occupies the most area of the cavity, causes pressure and temperature stratification due to rotational motion of fluid inside it, whereas the pressure and temperature in the secondary recirculation region, which is a small vortex and locates at the lower left corner of the cavity, change slightly due to low-speed movement of fluid inside it. With the increase of altitude, both the primary and secondary recirculation regions contract greatly and it causes them to separate. A notable finding is that rotation direction of the secondary recirculation region would be reversed at a higher altitude. The overall effect of increasing the Mach number is that the velocity, pressure, and temperature within the cavity increase uniformly. The maximum pressure nearby the trailing edge of the cavity decreases rapidly as the sweepback angle increases, whereas the influence of sweepback angle on velocity distribution and maximum temperature within the cavity is slight.

Study of Conjugate Heat Transfer from Heated Plate by Turbulent Offset Jet in Presence of Freestream Motion using Low-Reynolds Number Modeling

The present study deals with conjugate heat transfer from a heated flat plate by a turbulent offset jet in presence of freestream motion. The turbulent convection in fluid and conduction in solid is solved in a coupled manner by simultaneously satisfying the equality of temperature and heat flux at the solid-fluid interface. The computations have been carried out using low-Reynolds number (LRN) k −ω SST model in the fluid region. The capability of LRN modeling have enabled to solve the entire boundary layer including the thin viscous sublayer due to which Moffatt vortices (secondary recirculation regions) have been captured near the corner of the wall where the turbulence Reynolds number is low. The bottom surface of solid plate is maintained at a constant temperature higher than the jet inlet temperature whereas the jet inlet temperature is same as that of the ambient. The present investigation reports the effects of offset ratio of jet (OR), Reynolds number of flow (Re), solid to fluid thermal conductivity ratio (K), solid slab thickness (S) and freestream velocity (U∞) on conjugate heat transfer arises due to solid and fluid interaction. The offset ratio is varied in the range OR = 3 − 11, Reynolds number in the range Re = 10000 − 25000, solid to fluid thermal conductivity ratio in the range K = 1 − 2000, solid slab thickness in the range S = 1−20 and freestream velocity in the range U∞ = 0.1 − 0.25. The effects of various parameters on the near-wall velocity profile, solid-fluid interface temperature, local Nusselt number variation along the plate, heat flux variation along the plate, etc. have been discussed in detail.

Numerical investigation on influence of diamond shaped strut on the performance of a scramjet combustor

The present work discusses the influence of diamond shaped strut profile on combustion characteristics of a supersonic combustor. The 2D DLR scramjet model was used to investigate the effects of parametric variables which include Mach number, wall pressure ratio and inlet temperatures respectively. Further, interaction between shock-shock, shock-shear layer, and shock-near wall boundary layer is also discussed. It has been observed that the fluid flow behind the strut is oscillating while the primary and secondary recirculation region is present along the centre line of the strut. The study reveals that the optimal size of recirculation region is found at Mach number 2.5 which results into stable combustion. It is observed that the combustion efficiency starts to increase at the trailing edge of the strut due to the strong interaction of shock and the performance of a scramjet combustor is found to deteriorate at very high temperatures and very low Mach number. An optimal combustion efficiency equal to 80% has been obtained at Mach = 2.5, T = 696 K and Pj/P∞ = 0.8 corresponding to minimum pressure loss.

Numerical study of the primitive equations in the small viscosity regime

In this paper we study the flow dynamics governed by the primitive equations in the small viscosity regime. We consider an initial setup consisting on two dipolar structures interacting with a no slip boundary at the bottom of the domain. The generated boundary layer is analyzed in terms of the complex singularities of the horizontal pressure gradient and of the vorticity generated at the boundary. The presence of complex singularities is correlated with the appearance of secondary recirculation regions. Two viscosity regimes, with different qualitative properties, can be distinguished in the flow dynamics.

Numerical study of turbulent flow in a rectangular T-junction

In this paper, we report on a numerical study of the interaction and merging of a turbulent crossflow with an incoming turbulent jet in a T-junction with rectangular cross section. Our study is based on wall-resolved and experimentally validated large eddy simulations. The bulk Reynolds number of the crossflow is 15 000. Further, we consider cases with two different momentum ratios, namely, MR = 2 and MR = 0.5. In the presentation of the results, we elaborate on the main features of the flow, namely, the shear layers that emanate from the corners of the entry of the jet, the large recirculation bubble downstream the incoming jet, and the mixing process beyond the reattachment point. For validation purposes, we compare our simulations with existing experimental data. This comparison shows a good agreement between our numerical predictions and the measurements. First- and second-order statistics of the flow are also presented and analyzed in detail. Our simulations reveal two features of the flow that have not been reported before in studies of T-junctions. The first one is a secondary small-scale recirculation region between the entry of the jet and the large recirculation bubble. The second one is the negative turbulent kinetic energy production that occurs in the recirculation bubble and close to the reattachment of the flow. The analysis of our results further reveals that just across the entry of the jet, the boundary layer in the wall opposite to the jet experiences a favourable pressure gradient due to a Venturi effect induced by the incoming jet. In turn, this favourable pressure gradient contributes to the local relaminarization of the flow. On the other hand, the boundary layer downstream the recirculation bubble experiences an adverse pressure gradient. In both cases, a significant deviation from the universal law of the wall is confirmed.

Characterization of Turbulent Structures in a Transonic Backward-Facing Step Flow

A transonic backward-facing step flow, at a free stream Mach number of 0.8 and a Reynolds number of 1.86 × 105 with respect to the step height, was investigated experimentally by means of planar and stereo Particle Image Velocimetry (PIV) measurements for multiple fields of view. The primary aim of this analysis is to examine whether the large temporal variations of the reattachment location is associated with the presence of large scale coherent flow structures. The mean flow reattaches ≈6.1±0.2 times the step height downstream of the step. This value fluctuates temporally as much as ±3 step heights. Measurements of the wake flow in horizontal planes show that the strong variations of the reattachment length are associated with spanwise variations of the streamwise velocity. Two-point correlations revealed large–scale coherent regions with a length of up to 7 step heights and a dominant spanwise wave-length of 1.5…2.5 step heights. Furthermore, close to the step large structures are found, which span more than 5 step heights in spanwise direction. The Reynolds stress distribution of the separated region strongly suggests that the initial streamwise momentum is transferred to the vertical component as well as to the spanwise component in comparable portions by the deformation of the initial Kelvin-Helmholtz vortices and the generation of secondary ones. As a result, the separated shear layer is characterized by eddies of various sizes and orientations. The mean flow field only shows the primary separation bubble and a secondary recirculation region. No stationary streamwise vortices could be found for the tested Reynolds number.

Global stability analysis of flow through a fusiform aneurysm: steady flows

AbstractThe global linear stability of steady axisymmetric flow through a model fusiform aneurysm is studied numerically. The aneurysm is modelled as a Gaussian-shaped inflation on a vessel of circular cross-section. The fluid is assumed to be Newtonian, and the flow far upstream and downstream of the inflation is a Hagen–Poiseuille flow. The model aneurysm is characterized by a maximum height $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}H$ and width $W$, non-dimensionalized by the upstream vessel diameter, and the steady flow is characterized by the Reynolds number of the upstream flow. The base flow through the model aneurysms is determined for non-dimensional heights and widths in the physiologically relevant ranges $0.1 \leq H \leq 1.0$ and $0.25 \leq W \leq 2.0$, and for Reynolds numbers up to 7000, corresponding to peak values recorded during pulsatile flows under physiological conditions. It is found that the base flow consists of a core of relatively fast-moving fluid, surrounded by a slowly recirculating fluid that fills the inflation; for larger values of the ratio $H/W$, a secondary recirculation region is observed. The wall shear stress (WSS) in the inflation is vanishingly small compared to the WSS in the straight vessels. The global linear stability of the base flows is analysed by determining the eigenfrequencies of a modal representation of small-amplitude perturbations and by looking at the energy transfer between the base flow and the perturbations. Relatively shallow aneurysms (of relatively large width) become unstable by the lift-up mechanism and have a perturbation flow which is characterized by stationary, growing modes. More localized aneurysms (with relatively small width) become unstable at larger Reynolds numbers, presumably by an elliptic instability mechanism; in this case the perturbation flow is characterized by oscillatory modes.

A Stable Lattice Boltzmann Method for Steady Backward-Facing Step Flow

The severe problem of the most lattice Boltzmann methods (LBM) is the instability within the solution. This paper presents the implementation and applicability of a stable finite volume (FV) formulation of LBM for simulating steady separating and reattaching flow pasts backward-facing step geometry. For simulation purpose, a cell-centered scheme is implemented to discretize the convection operator and new weighting factors are used as flux correctors. Compared with previous FV formulations, a remarkable stability improvement is achieved and the secondary recirculation region is clearly captured for Reynolds numbers higher than 400. The simulation results show good agreement with benchmark data, which demonstrate the trustworthiness of the scheme.

Comparison of two low-Reynolds number turbulence models for fluid flow study of wall bounded jets

A comparative study of flow field and turbulence characteristics of a turbulent offset jet has been done. The computational results obtained from low-Reynolds number k–ϵ models proposed by Launder and Sharma, and Yang and Shih are compared with that of the standard k–ϵ model. The existence of Moffatt vortices (secondary recirculation region) have been identified near the corner of the wall for low-Reynolds number modeling. A velocity similarity profile has been observed as expected due to the resemblance of an offset jet with that of the wall jet flow in the wall jet region. The linear law of the wall (u+=y+) has been found to be valid up to y+≈3.0 in the wall jet region which agrees with the experimental results. The integral constant of momentum flux is higher for lower offset ratio due to a smaller recirculation region. The distribution of pressure field, Reynolds shear stress, budget of turbulent kinetic energy in the recirculation region and the jet characteristic parameters such as variation of the jet half-width (Y0.5) along the streamwise direction and decay of the maximum streamwise velocity are discussed in detail.

Influence of nanofluids on mixed convective heat transfer over a horizontal backward‐facing step

AbstractPredictions are reported for laminar mixed convection using various types of nanofluids over a horizontal backward‐facing step in a duct, in which the upstream wall and the step are considered adiabatic surfaces, while the downstream wall from the step is heated to a uniform temperature that is higher than the inlet fluid temperature. The straight wall that forms the other side of the duct is maintained at constant temperature equivalent to the inlet fluid temperature. Eight different types of nanoparticles, Au, Ag, Al2O3, Cu, CuO, diamond, SiO2, and TiO2, with 5% volume fraction are used. The conservation equations along with the boundary conditions are solved using the finite volume method. Results presented in this paper are for a step height of 4.9 mm and an expansion ratio of 1.942, while the total length in the downstream of the step is 0.5 m. The Reynolds number is in the range of 75 ≤ Re ≤ 225. The downstream wall was fixed at a uniform wall temperature in the range of 0 ≤ ΔT ≤ 30 °C which is higher than the inlet flow temperature. Results reveal that there is a primary recirculation region for all nanofluids behind the step. It is noticed that nanofluids without secondary recirculation region have a higher Nusselt number and it increases with Prandtl number decrement. On the other hand, nanofluids with secondary recirculation regions are found to have a lower Nusselt number. Diamond nanofluid has the highest Nusselt number in the primary recirculation region, while SiO2 nanofluid has the highest Nusselt number downstream of the primary recirculation region. The skin friction coefficient increases as the temperature difference increases and the Reynolds number decreases. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.20344

The dynamics of the transitional flow over a backward-facing step

The internal flow over a backward-facing step in the transitional regime (ReD = 6000) was studied based on direct numerical simulations. The predictions were carried out with the help of a finite-volume Navier–Stokes solver equipped with a co-visualization facility which allows one to investigate the flow dynamics at high temporal resolution. First, grid-induced oscillations were precluded by a careful grid design. Second, the strong influence of the velocity profile approaching the step was studied and outlined. The main objective, however, was to provide a comprehensive insight into the dynamic flow behaviour, especially oscillations of the reattachment length of the primary recirculation region. The origin of this well-known flapping behaviour of the reattachment line is not yet completely understood. In the present work, the mechanisms leading to the oscillations of the reattachment length were extensively investigated by analysing the time-dependent flow. Besides the oscillations of the primary recirculation region, oscillations of the separation and the reattachment line of the secondary recirculation bubble at the upper channel wall were also observed. The results clearly show that in the present flow case the flapping of the primary reattachment and the secondary separation line is due to vortical structures in the unstable shear layers between the main flow and the recirculation bubbles. Vortices emerging in the shear layers and sweeping downstream convectively induce small zones of backward-flowing fluid at the channel walls while passing the recirculation regions. In the case of the primary recirculation region, the rotational movement of the shear-layer vortices impinging on the lower channel wall was found to cause zones of negative fluid velocity at the end of the recirculation bubble and thus flapping of the reattachment line. In contrast, in the case of the secondary recirculation region, the shear-layer vortices moved away from the upper channel wall so that their rotational movement did not reach the boundary. In this case, the pressure gradients originating from local pressure minima located in the shear-layer vortices were identified as being responsible for the oscillations of the separation line at the upper channel wall. While moving downstream with the shear-layer vortices, the pressure gradients were found to influence the top boundary of the channel and create alternating zones of forward- and backward-flowing fluid along the wall. All of these unsteady processes can best be seen from animations which are provided on the Journal of Fluid Mechanics website: journals.cambridge.org/FLM.

Characteristics of a Central Bleed Jet in Supersonic Axisymmetric Base Flow

The near-wake flow structure of a supersonic axisymmetric base flowwith base bleed is investigated in detail using acetone planar laser-induced fluorescence. Global side-view and end-view images are analyzed to detect unstirred bleed fluid, which has been found to persist from the bleed orifice through reattachment (i.e., rear stagnation point). The characteristic size, shape, and location of the bleed fluid pockets are studied. This enables a comparison of the bleed jet structure for three different cases of mass flow rate of bleed fluid. In addition, the average location of the forward stagnation point and the primary and secondary recirculation regions are linked to the distributions of several statistics. The similarities and differences between two suboptimal bleed rate cases identify possible optimal locations for the central bleed jet to be directed. A stabilizing effect caused by increasing bleed rate is documented in many of the statistical quantities. Agreement between side-view and end-view image analysis confirms the ability of the processing technique to distinguish the unstirred bleed fluid in all images.

Turbulent separated convection flow adjacent to backward-facing step—effects of step height

Simulations of turbulent convection flow adjacent to a two-dimensional backward-facing step are presented to explore the effects of step height on turbulent separated flow and heat transfer. Reynolds number and duct’s height downstream from the step are kept constant at Re 0 = 28,000 and H = 0.19 m, respectively. Uniform and constant heat flux of q w = 270 W/m 2 is specified at the stepped wall downstream from the step, while other walls are treated as adiabatic. The selection of the values for these parameters is motivated by the fact that measurements are available for this geometry and they can be used to validate the flow and heat transfer simulation code. Two-equation low-Reynolds-number model is employed to achieve the turbulent Prandtl number. The primary and secondary recirculation regions increase in size as the step height increases. The bulk temperature increases more rapidly as the step height increases. Increasing the step height causes the magnitude of the maximum turbulent kinetic energy to increase. Near the step and below the step height, the turbulent kinetic energy becomes smaller as the step height increases. Inside the recirculation region, magnitude of the peak friction coefficient does not significantly change with the increase of step height. The friction coefficient becomes smaller in magnitude with the increase of the step height. The peak Stanton number becomes smaller as the step height increases.

Visualization of a central bleed jet in an axisymmetric, compressible base flow

The central bleed jet in an axisymmetric, compressible base flow with a Mach 2.46 free stream was visualized using an acetone planar laser-induced fluorescence (PLIF) technique. Three bleed flow rates, a low-bleed case (I=0.0038) and two others that bracket the optimal bleed case (I=0.0113 and I=0.0226), were examined. The injection parameter, I, is defined here as the bleed mass flow rate normalized by the product of the base area and freestream mass flux. This study shows that the bleed jet in the low-bleed case is almost instantly deflected outward toward the outer shear layer after it exits from the bleed orifice. When I=0.0113, the bleed fluid carries enough momentum that the bleed jet remains coherent and generally aligned along the symmetry axis for approximately one base radius before it is deflected outward by the primary recirculation region. For the highest bleed rate examined (I=0.0226), the bleed jet remains coherent and aligned along the symmetry axis for approximately 2.5 base radii downstream, where the impingement of freestream fluid causes the bleed jet to symmetrically eject mass downstream. Because the behavior of the bleed jet is much different for pre- and post-optimal bleed rates, some general conclusions can be made about the effect of bleed rate on base drag. For the two lower-bleed cases, there is a base pressure increase because the bleed fluid partially isolates the primary recirculation region from the outer flow, and provides part of the mass necessary for entrainment in the outer shear layer. The bleed jet in the high-bleed case is not as effective at increasing the base pressure, because the jet is shielded from the outer shear layer by the secondary recirculation region, which exists in the annulus between the bleed orifice and the base corner.

Navier–Stokes solutions of unsteady separation induced by a vortex

Numerical solutions of the unsteady Navier–Stokes equations are considered for the flow induced by a thick-core vortex convecting along a surface in a two-dimensional incompressible flow. The presence of the vortex induces an adverse streamwise pressure gradient along the surface that leads to the formation of a secondary recirculation region followed by a narrow eruption of near-wall fluid in solutions of the unsteady boundary-layer equations. The locally thickening boundary layer in the vicinity of the eruption provokes an interaction between the viscous boundary layer and the outer inviscid flow. Numerical solutions of the Navier–Stokes equations show that the interaction occurs on two distinct streamwise length scales depending upon which of three Reynolds-number regimes is being considered. At high Reynolds numbers, the spike leads to a small-scale interaction; at moderate Reynolds numbers, the flow experiences a large-scale interaction followed by the small-scale interaction due to the spike; at low Reynolds numbers, large-scale interaction occurs, but there is no spike or subsequent small-scale interaction. The large-scale interaction is found to play an essential role in determining the overall evolution of unsteady separation in the moderate-Reynolds-number regime; it accelerates the spike formation process and leads to formation of secondary recirculation regions, splitting of the primary recirculation region into multiple corotating eddies and ejections of near-wall vorticity. These eddies later merge prior to being lifted away from the surface and causing detachment of the thick-core vortex.

Flow management in nanofiltration spiral wound modules with ladder-type spacers

The incompressible laminar and two-dimensional flow in narrow rectangular channels filled with ladder-type spacers that have the transverse filaments adjacent to a semi-permeable wall, was investigated in order to have insight on the feed flow in a nanofiltration (NF)/reverse osmosis (RO) spiral wound module. The study was based on numerical simulations and on experiments of flow visualisation with tracer injection and pressure drop measurements. The numerical simulations were performed assuming fully developed laminar flow quasi-periodically repeating in successive inter-filament regions. Ladder-type spacers with inter-filament distances/channel height ratios ( L f) of 1.9, 3.8 and 5.7 were investigated. The ratio ( P f) between the transverse filaments height and channel height was investigated between 0.25 and 0.75 in the numerical study and was fixed to 0.5 in the experiments. Flow visualisation with tracer injection and friction factor measurements were made for the water flow in a spacer-filled rectangular channel with 2 mm height, 30 mm wide and 200 mm long, in the range of Reynolds numbers (based on the channel height and on the average inlet velocity) between 50 and 1000. These results show that for increasing inter-filament distances the transition critical Reynolds number decreases from 300 to 150. Below this critical value, the flow is observed to be laminar and two-dimensional. The numerical simulations show that the flow structures are associated to the occurrence of recirculation zones downstream each transverse filament. For low values of the Reynolds number and P f and high values of L f, the recirculation region does not reach the second filament, while for high values of the Reynolds number and P f and low values of L f the recirculation region extends from the first to the second filament. Additionally, for P f=0.75, a secondary recirculation region can be formed inside the main recirculation region. Each of these situations is associated to different patterns of concentration boundary layers growth and subsequently different solute concentration distributions at the membrane surface.

Numerical simulation of buoyancy-assisting, backward-facing step flow and heat transfer in a rectangular duct

Two- and three-dimensional numerical simulations have been performed for mixed convective upward flows over a backward-facing step in a duct. The Reynolds number, expansion ratio, and aspect ratio (in 3-D simulations) were kept constant at Re = 125, ER = 2, and AR = 16, respectively. The heat flux at the wall downstream of the step was uniform. The straight wall, the step, and the side walls (in 3-D simulations) were assumed to be adiabatic. The effect of the buoyancy level, Ri*, was the major interest in this study. It was found that the reattachment point and the peak Nusselt number point moved upstream as Ri* was increased. The secondary recirculation region, which developed at the corner of the step, became larger. A secondary flow was also found in a cross section immediately downstream of the step. Flow directed toward the center of the duct became more intensive as Ri* increased, which possibly resulted from an increase in the level of three-dimensionality of the flow and thermal fields. © 1999 Scripta Technica, Heat Trans Asian Res, 28(1): 58–76, 1999

Numerical Simulation of Buoyancy Assisting Backward-Facing Step Flow and Heat Transfer in a Rectangular Duct.

Two-and three-dimensional numerical simulations have been performed for mixed convective upward flows over a backward-facing step in a duct. The Reynolds number, expansion ratio and aspect ratio (in 3D sim.) were kept constant at Re=125, ER=2 and AR=16, respectively. The heat flux at the wall downstream of the step was uniform, while the straight wall, the step and the side walls (in 3D sim.) were assumed to be adiabatic. The effect of the buoyancy level, Ri*, was the major interest in this study. It was found that the reattachment point and the peak Nusselt number point moved upstream as Ri* was increased, while the secondary recirculation region, which developed at the corner of the step, became larger. It was also found that there existed a secondary flow in a cross section immediately downstream of the step. Flow directed toward the center of the duct becomes more intensive as Ri* increases, which possibly results in an increase in the level of three-dimensionality of the flow and thermal fields.