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

A numerical simulation of the circular cylinder as an obstacle in a double forward-facing (DFFS) step was performed. The size and position of the upstream cylinder (c1) and downstream cylinder (c2) were varied to explore their role in heat transfer in both laminar and turbulent conditions. Comparative results of the upper and lower half of the downstream cylinder were plotted as results to understand the heat transfer and flow characteristics around the downstream cylinder due to the effect of the upstream cylinder’s dimension and position. For Re = 800, when the c1 is placed near the bottom of the wall, it results in a pair of rear-side symmetrical vortices, and the c2 cylinder vortices become larger when the c1 is shifted towards the top wall. Additional flow separation happens adjacent to the steps when c1 is greater than c2. These vortices strongly influence the convection heat transfer from the step. However, when Reynolds number (Re) is increased from 800 to 80,000, these vortices’ size is decreased. When c1 moves from 0.375H to 0.75H, the average Nusselt number is increased significantly. Moreover, a hike in Re results in a higher average Nusselt number irrespective of the position of obstacles. The upstream cylinder significantly enhances the Nusselt number when it is placed near the top wall rather than the bottom wall.

A forward-facing step （FFS） immersed in a subsonic boundary layer is studied through a high-order flux reconstruction （FR） method to highlight the flow transition induced by the step. The step height is a third of the local boundary-layer thickness. The Reynolds number based on the step height is 720. Inlet disturbances are introduced giving rise to streamwise vortices upstream of the step. It is observed that these small-scale streamwise structures interact with the step and hairpin vortices are quickly developed after the step leading to flow transition in the boundary layer.

A numerical investigation of entropy generation in laminar forced convection of gas flow over a recess including two inclined backward and forward facing steps in a horizontal duct under bleeding condition is presented. For calculation of entropy generation from the second law of thermodynamics in a forced convection flow, the velocity and temperature distributions are primary needed. For this purpose, the two-dimensional Cartesian coordinate system is used to solve the governing equations which are conservations of mass, momentum and energy. These equations are solved numerically using the computational fluid dynamic techniques to obtain the temperature and velocity fields, while the blocked region method is employed to simulate the inclined surface. Discretized forms of these equations are obtained by the finite volume method and solved using the SIMPLE algorithm. The numerical results are presented graphically and the effects of bleeding coefficient and recess length as the main parameters on the distributions of entropy generation number and Bejan number are investigated. Also, the effect of Reynolds number and bleeding coefficient on total entropy generation which shows the amount of flow irreversibilities is presented for two recess length. The use of present results in the design process of such thermal system would help the system attain the high performance during exploitation. Comparison of numerical results with the available data published in open literature shows a good consistency. nema

Purpose The purpose of this paper is to numerically study the phenomenon of separation and subsequent reattachment that happens due to a sudden contraction or expansion in flow geometry, in addition, to investigating the effect of nanoparticles suspended in water on heat transfer enhancement and fluid flow characteristics. Design/methodology/approach Turbulent forced convection flow over triple forward facing step (FFS) in a duct is numerically studied by using different types of nanofluids. Finite volume method is employed to carry out the numerical investigations. with nanoparticles volume fraction in the range of 1-4 per cent and nanoparticles diameter in the range 30-75 nm, suspended in water. Several parameters were studied, such as the geometrical specification (different step heights), boundary conditions (different Reynolds [Re] numbers), types of fluids (base fluid with different types of nanoparticles), nanoparticle concentration (different volume fractions) and nanoparticle size. Findings The numerical results indicate that the Nusselt number increases as the volume fraction increases, but it decreases as the diameter of the nanoparticles of nanofluids increases. The turbulent kinetic energy and its dissipation rate increase as Re number increases. The velocity magnitude increases as the density of nanofluids decreases. No significant effect of increasing the three steps heights on Nusselt along the heated wall, except in front of first step where increasing the first step height leads to an increase in the recirculation zone size adjacent to it. Research limitations/implications The phenomenon of separation and subsequent reattachment happened due to a sudden contraction or expansion in flow geometry, such as forward facing and backward facing steps, respectively, can be recognized in many engineering applications where heat transfer enhancement is required. Some examples include cooling systems for electronic equipment, heat exchanger, diffusers and chemical process. Understanding the concept of these devices is very important from the engineering point of view. Originality/value Convective heat transfer can be enhanced passively by changing flow geometry, boundary conditions, the traditional fluids or by enhancing thermal conductivity of the fluid. Great attention has been paid to increase the thermal conductivity of base fluid by suspending nano-, micro- or larger-sized particles in fluid. The products from suspending these particles in the base fluid are called nanofluids. Many studies have been conducted to investigate the heat transfer and fluid flow characteristics over FFS. This study is the first where nanofluids are employed as working fluids for flow over triple FFS.

The research is conducted in order to reduce energy losses caused by the secondary flow in the endwall junction. This phenomenon is caused by the interaction of two adjacent viscous flow (symmetric airfoil and endwall). Reduction of energy loss carried out by addition of Foward Facing Step Turbulator (FFST) in the upstream. Endwall junction area is modeled as a NACA 0015 airfoil and a flat plate. Position of FFST is at a distance L = 2/3 C upstream leading edge and a thickness d = 4% C. Free stream conditions Red = 105 with turbulence intensity (Tu) 5%. Research is conducted by numerical and experiment methods. Pathlines of numerical result methods has an identic structure with \"Oil Flow Visualization\" of the experiment.Result of the research states that the addition of FFST can increase the turbulence intensity in the flow near the wall. So at the same angle of attact (α), the saddle point position on the leading edge has distance nearly the same but a little more towards the lower side and the separation line is wider than without FFST. Because the flow has stronger turbulence intensity, attachment line of the upper and lower sides have a better capability of following the contours of the body. So the point of separation can be delayed and blockage (energy loss) can be reduced as well. Reduction of energy loss is most effective on α=8 ° (4.16%),Keyword : Secondary flow, forward facing step, turbulent intensity.

Recently, microfluidics system has been widely employed in various areas for instance biomedical,pharmaceuticals and cell biological researchdue to its advantages. The flow behavior in microchannels with different cross-sections has been topic in previous studies. In this paper, numerical simulation of fluid flow in Forward Facing Step (FFS) configuration was performed to investigate velocity profile after the step. Reynolds numbers (Re) 100 with different step heights, 1μm and 3μm were used to observe trend occurs in the flow characteristics. The result illustrated an increase of velocity distribution with the increase of the step height.

The flow over a forward-facing step (FFS) at $Ma_\\infty =1.7$ and $Re_{\\delta _0}=1.3718\\times 10^{4}$ is investigated by well-resolved large-eddy simulation. To investigate effects of upstream flow structures and turbulence on the low-frequency dynamics of the shock wave/boundary layer interaction (SWBLI), two cases are considered: one with a laminar inflow and one with a turbulent inflow. The laminar inflow case shows signs of a rapid transition to turbulence upstream of the step, as inferred from the streamwise variation of $\\langle C_f \\rangle$ and the evolution of the coherent vortical structures. Nevertheless, the separation length is more than twice as large for the laminar inflow case, and the coalescence of compression waves into a separation shock is observed only for the fully turbulent inflow case. The dynamics at low and medium frequencies is characterized by a spectral analysis, where the lower frequency range is related to the unsteady separation region, and the intermediate one is associated with the shedding of shear layer vortices. For the turbulent inflow case, we furthermore use a three-dimensional dynamic mode decomposition to analyse the individual contributions of selected modes to the unsteadiness of the SWBLI. The separation shock and Görtler-like vortices, which are induced by the centrifugal forces in the separation region, are strongly correlated with the low-frequency unsteadiness in the current FFS case. Similarly as observed previously for the backward-facing steps, we observe a slightly higher non-dimensional frequency (based on the separation length) of the low-frequency mode than for SWBLI in flat plate and ramp configurations.

The impact of a forward-facing step (FFS) on the development of stationary crossflow instability is investigated on a swept wing model in a low-turbulence wind tunnel at chord Reynolds number of $2.3 \\times 10^{6}$. Infrared thermography and particle image velocimetry measurements are used to quantify the transition location and growth of the crossflow instability under the influence of FFSs with different heights. Forced monochromatic stationary crossflow vortices experience an abrupt change in their trajectory as they interact with the step geometry. As the boundary layer intercepts the step an increase in the vertical velocity component and an amplification of the crossflow vortices is observed. Near the step, the vortices reach maximum amplification, while dampening downstream. The smaller FFS cases, show a local stabilising effect on the primary stationary mode and its harmonics, while in the higher step cases transition occurs. The analysis of the temporal velocity fluctuations shows a reduction in the region associated with the type-III travelling crossflow modes downstream of the step. In contrast, the velocity fluctuations in the region associated with type-I secondary instabilities increase past the FFS edge. Nonetheless, in the shortest FFS cases, these velocity fluctuations eventually decay below the clean configuration (i.e. without an FFS) levels. This behaviour is linked to a novel transition delay effect for the shortest step height investigated. The findings highlight new physical aspects driving the interaction between an amplified stationary crossflow vortex and an FFS and provide insight into possible transition delay mechanisms using such geometries.

The effects of large-scale motion (LSM) on the spatio-temporal dynamics of separated shear layers induced by a forward-facing step submerged in a thick turbulent boundary layer (TBL) are investigated using a time-resolved particle image velocimetry. The Reynolds number based on the free-stream velocity and step height was 13 200. The oncoming TBL was developed over a cube-roughened surface and the thickness was 6.5 times the step height. The step height was chosen to coincide with the elevation where the dominant frequency of streamwise fluctuating velocity in the TBL occurred. At this elevation, the local turbulence intensity was 14.5 %. Distinct regions of elevated Reynolds stresses were observed upstream and downstream of the leading edge of the step. The unsteady dynamics of the separation bubbles upstream and downstream of the step was investigated using the reverse flow area. Both separation bubbles exhibit low-frequency flapping motion, and the dominant frequency of the downstream separation bubble is identical to the dominant frequency of the streamwise fluctuating velocity in the oncoming TBL at the step height. As the low-velocity region of LSM passes over the step, the downstream separation bubble is enlarged and subsequently undergoes a high-frequency oscillation. Turbulence motions were partitioned into low-, medium- and high-frequency regimes based on spectral analysis of the Reynolds stresses. The contributions from these partitioned turbulence motions are used to elucidate the effects of LSM on the elevated Reynolds stresses in the shear layers upstream and downstream of the step.