Reattachment Length Research Articles

Flow investigation in a microchannel with a flow disturbing rib

The purpose of this work is to study the flow around a flow-disturbing rib in a rectangular microchannel and to investigate the effect of key design parameters (i.e. the rib height and length as well as the Reynolds number) on the size of the reattachment length of the recirculation zones and the wall shear stress profiles in the vicinity of the rib. Initially the wall shear rate along the channel as well as the velocity field were experimentally determined using the electrodiffusion technique and the μ-PIV method, respectively. The experimental results are then used for validating a CFD code. Finally, the CFD code was employed for investigating the effect of the design parameters on the flow characteristics by performing a parametric study based on the Design of Experiments (DOE) and the Response Surface Methodology (RSM). It was found that the recirculation length in the laminar regime is affected mainly by the Re value and the rib height, whereas in the turbulent regime it is affected strongly by the rib height and slightly also by the rib length. Based on our results two new correlations, which can predict the length of the bottom recirculation zone with reasonable accuracy, are proposed and can be used for the design of microdevices.

Experimental Investigation of Heat Transfer of Single Jet Impingement on a Aluminium Block

Jet Impingement is used in many applications where extensive heating (or) cooling is necessary to produce high heat transfer rate in a localized region. Those applications include glass production, drying of papers, annealing of metals and cooling of electronic equipments. Present work is involved with the experimental investigation of single jet impingement on Aluminium block. The effect of Reynolds number and the distance between the jet and block (H/d ratio) are considered as the interesting variable parameters. The heat transfer rate and reattachment length are reported in detailed for the various Reynolds number and various jet to block ratio. The flow physics revealed that when the Reynolds number increases the reattachment length also increases. The heat transfer rate increases with increase in Reynolds number up to critical heat flux and then further increase of Reynolds number leads to decrease in heat transfer.

Numerical Simulation of Turbulent Flow in Carbon Steel Pipes Leading to Flow Accelerated Corrosion

In the present study, turbulent flow fields in diverse carbon steel pipe components leading to flow accelerated corrosion (FAC) are analyzed using the Realizable turbulence model. The present study is inspired by the detailed case studies of flow accelerated corrosion in nuclear power plants around the globe. The Numerical study is conducted with the main objective of analyzing the flow around the recirculation regions arising in different piping components, such as sudden expansion, double elbow, orifice and T-bend, since these components are known to be more susceptible to FAC. Employed Realizable k-e turbulence model and the flow simulations are validated with the Experimental and Numerical results available in the literature. The topological consistency is verified by means of a topological invariance relation of the flow based on the Euler number. The calculated reattachment length increased, but the concentration of ferrous ions decreased or remained almost constant at the corner recirculation regions as a function of Reynolds number. As the shear stress distribution is one of the most important factors in predicting the local regions of pipes that are highly susceptible to FAC, this distribution was calculated in the considered geometries. These results would help to select the components that are more susceptible to FAC. In addition, the locations of the maximum wall shear stress are identified in each component. It is observed that the double elbow and orifice are two components that are more susceptible to FAC among all the considered components.

Impact of fuel composition on the recirculation zone structure and its role in lean premixed flame anchoring

We investigate the dependence of the recirculation zone (RZ) size and structure on the fuel composition using high-speed particle image velocimetry (PIV) and chemiluminescence measurements for C3H8/H2/air lean premixed flames stabilized in a backward-facing step combustor. Results show an intricate coupling between the flame anchoring and the RZ structure and length. For a fixed fuel composition, at relatively low equivalence ratios, the time-averaged RZ is comprised of two counter rotating eddies: a primary eddy (PE) between the shear layer and the bottom wall; and a secondary eddy (SE) between the vertical step wall and the PE. The flame stabilizes downstream of the saddle point of the dividing streamline between the two eddies. As equivalence ratio is raised, the flame moves upstream, pushing the saddle point with it and reducing the size of the SE. Higher temperature of the products reduces the velocity gradient in the shear layer and thus the reattachment length. As equivalence ratio approaches a critical value, the saddle point reaches the step and the SE collapses while the flame starts to exhibit periodic flapping motions, suggesting a correlation between the RZ structure and flame anchoring. The overall trend in the flow field is the same as we add hydrogen to the fuel at a fixed equivalence ratio, demonstrating the impact of fuel composition on the flow field. We show that the reattachment lengths (LR), which are shown to encapsulate the mean RZ structure, measured over a range of fuel composition and equivalence ratio collapse if plotted against the strained consumption speed (Sc). Results indicate that for the flame to remain anchored, the RZ structure should satisfy LR,isothermalLR,reacting·ScU∞∼0.1. If this criterion cannot be met, the flame blows off, flashes back or becomes thermoacoustically unstable, suggesting a Damköhler-like criterion for aerodynamic flame stabilization in backward-facing step flows.

Roughness Effects on Turbulent Flow Downstream of a Backward Facing Step

This paper investigates the effects of downstream wall roughness on characteristics of separated and reattached turbulent flow downstream of a backward facing step. Detailed particle image velocimetry (PIV) measurements were conducted over rough walls produced from sandpaper 36 and 24 grits and a reference smooth acrylic wall positioned downstream of a backward facing step, one after another. All the experiments were performed at the same Reynolds number of 7050 based on the step height, h and approach mean velocity, ratio of initial boundary layer thickness to step height of 2.2 and expansion ratio of 1.25. The results showed that wall roughness increased the mean reattachment length over the sandpaper 36 and 24 grits by 5 b% and 7 %, respectively, in comparison with the smooth wall value. However, the other mean flow properties within the recirculation region are nearly independent of wall roughness. Beyond 5 step heights from the reattachment point, wall roughness reduced the streamwise mean velocity in the region adjacent to the rough walls. Wall roughness significantly increased the levels of the Reynolds stresses in the recirculation and redevelopment regions. In spite of the higher Reynolds stresses observed over the rough walls compared with the smooth wall, the spatial coherence of turbulence structures embodied in the streamwise and wall-normal auto-correlation function is significantly reduced over the rough walls.

Numerical Prediction of Thermal Effect on Flow Field around a High-Rise Building Model

Recently, CFD (Computational Fluid Dynamics) is being more and more used in the investigation of the flow and thermal around high-rise buildings. In this study, prediction of thermal effect on flow pattern around high-rise buildings model located in the surface boundary layer was carried out using k-e turbulence model. Investigation of thermal effect on flow pattern behind the building and comparison between isothermal and non-isothermal building using standard k-e showed that the surface temperature of building wall led to a strong upward motion close to heating wall and decreased the reattachment length behind it. The model sittings of validation study was accomplished by comparing the simulation of wind flow around building models with the experimental data of Architectural Institute of Japan (AIJ) that results showed in weak wind region, standard k-e has good agreement with experimental.

Prediction of Wind Flow around High-Rise Buildings Using RANS Models

Recently, CFD (Computational Fluid Dynamics) is being increasingly used to investigation of the flow around high-rise buildings. In this study, prediction of wind flow around two cases high-rise buildings model placed within the surface boundary layer were carried out using various turbulence models. The accuracy of two-equation turbulence models were compared with experimental data of Architectural Institute of Japan (AIJ).as a result, all two-equation models are shown overestimation the reattachment length behind the buildings.

In vitro quantification of time dependent thrombus size using magnetic resonance imaging and computational simulations of thrombus surface shear stresses.

Thrombosis and thromboembolization remain large obstacles in the design of cardiovascular devices. In this study, the temporal behavior of thrombus size within a backward-facing step (BFS) model is investigated, as this geometry can mimic the flow separation which has been found to contribute to thrombosis in cardiac devices. Magnetic resonance imaging (MRI) is used to quantify thrombus size and collect topographic data of thrombi formed by circulating bovine blood through a BFS model for times ranging between 10 and 90 min at a constant upstream Reynolds number of 490. Thrombus height, length, exposed surface area, and volume are measured, and asymptotic behavior is observed for each as the blood circulation time is increased. Velocity patterns near, and wall shear stress (WSS) distributions on, the exposed thrombus surfaces are calculated using computational fluid dynamics (CFD). Both the mean and maximum WSS on the exposed thrombus surfaces are much more dependent on thrombus topography than thrombus size, and the best predictors for asymptotic thrombus length and volume are the reattachment length and volume of reversed flow, respectively, from the region of separated flow downstream of the BFS.

Zonal Detached Eddy Simulation of the Flow Around a Simplified Launcher Afterbody

This paper presents the numerical investigation of the unsteady separated flow around a simplified space-launcher afterbody. A zonal detached eddy simulation is used to deal with the high-Reynolds-number flow. The geometry consists of a cylinder of finite length extended by another cylinder of smaller diameter for the central body, and two cylinders similar to the Ariane 5 launcher boosters on each side. The main purpose was to evidence the impact of the side cylinders on the flow characteristics of the isolated central body. To this end, instantaneous and statistical results are compared to those of the isolated central body. The flow topology with side cylinders is found to be dramatically different from that of the axisymmetric case. The mean and rms values for the pressure and velocity fields appear to be organized along the two planes of symmetry of the geometry. Besides, the levels of the mean wall-pressure coefficients are lower by 10% in the three-body configuration. Conversely, the maximum rms coefficients are higher by approximately 45% and located in four quadrants bounded by the planes of symmetry. In the plane normal to the boosters, the mean solid reattachment length is shorter than that of the reference case by 25%. Finally, a three-dimensional Fourier analysis is performed, which highlights that the spectral content organization strongly depends on the location around the emerging cylinder, leading to very different properties of the dynamic buffet loads. Such significant discrepancies in the instantaneous, statistical, and fluctuating flow features suggest that the expertise gained from the axisymmetric body experiments cannot be straightforwardly extended to three-dimensional geometries.

Numerical study of solid particle erosion in a cavity with different wall heights

Ideal cavities face a high risk of erosion in relation to pneumatic conveying. To reduce erosion, we introduce a height difference in the cavity, with the leading wall higher than the aft wall. To improve the cavity structure, the URANS (SST)-DPM method is adopted to study the effect of height difference h on gas flow, particle distribution, and particle erosion of a horizontal ideal cavity. The simulation procedure is validated by comparing the computational fluid dynamics results with experimental results. The results of the gas flow modeling show that the shear layer impinges on the aft wall when h is relatively small, and re-attaches on the downstream bottom wall as h increases. The re-attachment length is initially identical to the cavity width, then increases uniformly with h. Results of the particle distribution simulations indicate that for a Stokes number (St) of 0.12, the particles are evenly distributed in the cavity, and the distribution is independent of h; for St=1.2, dominated by the centrifugal force, the particles are mainly concentrated along the periphery of the main vortex when h is small. Owing to the variation of the main vortex, as h increases, more particles will fall down into the main vortex. For St=9.1, where gravity is dominant, the particles collide on the aft wall and rebound into the cavity through the main vortex when h is small. Otherwise, particles will move forward and only a few particles appear in the cavity. We explored the erosion distribution on the downstream bottom walls and extracted and analyzed the initial erosion positions. We also investigate the erosion on the aft walls, and the results indicate that lowering the aft wall even slightly will significantly reduce erosion.

Heat Transfer and Turbulent Fluid Flow over Vertical Double Forward-Facing Step

Abstract — Numerical study of heat transfer and fluid flow over vertical double forward facing step were presented. The k-w model with finite volume method was employed to solve continuity, momentum, and energy equations. Different step heights were adopted for range of Reynolds number varied from 10000 to 40000, and range of temperature varied from 310K to 340 K. The straight side of duct is insulated while the side of double forward facing step is heated. The result shows augmentation of heat transfer due to the recirculation region created after and before steps. Effect of step length and Reynolds number observed on increase of local Nusselt number particularly at recirculation regions. Contour of streamline velocity is plotted to show recirculation regions after and before steps. Numerical simulation in this paper done by used ANSYS FLUENT 14. Keywords — Turbulent flow, Double forward, Heat transfer, Separation flow.I. I NTRODUCTION HE crises of energy in global has been encourage researchers to look for new techniques which improve of thermal performance. One of common method to increase efficiency of energy system equipment is by change the design geometry of channel. The fluid flow over forward or backward-facing step found in many practical applications such as heat exchangers, chemical process, turbine blades, and power plants. There are many experimental and numerical study of effect separation flow at forward or backward-facing step on increase of enhancement heat transfer rate. Heat transfer to fluid flow over single and double forward-facing step experimentally studied by Shakouchi and Kajino [1] with using laser Doppler anemometer where the results appeared that the reattachment region by a new heat flux probe, and Oyakawa et al. [2], [3] used jet discharge. Saldana and Anand [4] performed numerical study of laminar fluid flow over a horizontal forward-facing step in three dimensions. They showed that the increase of Reynolds number leads to increase of reattachment length and average Nusselt number.

Effects of Inlet Centre Line Turbulence on the Turbulent Flow through an Axi-symmetric Sudden Expansion

The turbulent fluid flow through sudden expansion passage has both fundamental scientific interest and numerous practical applications. The engineering applications such as the separated flow associated with pipelines, dump combustors, different ducting devices etc. are examples of the turbulent flow through the sudden expansion. In the present analysis the effect of centre line turbulence on reattachment length in an axi symmetric sudden expansion flow has been investigated. The change of size and strength of recirculation bubble has been studied, which is directly involved with reattachment length.

High Reynolds number flow over a backward-facing step: structure of the mean separation bubble

In the present paper, the structure of the mean separation bubble downstream of the backward-facing step is studied at large Reynolds numbers. The flow over the step at these Reynolds numbers is turbulent with the presence of unsteady large-scale structures. There is however a well-defined time-averaged mean separation bubble. We study the effect of Reynolds number and expansion ratio on the structure of this mean separation bubble, the expansion ratio being the primary geometrical parameter in this case. Using PIV measurements within the separation bubble, parameters such as the reattachment length, mean velocity field, and the turbulent stresses are systematically mapped out. These measurements show that there exists a high Reynolds number separation bubble structure that is nearly independent of both Reynolds number and expansion ratio, as long as the Reynolds numbers are large (Re > 36,000 based on step height). Within this large Reynolds number separation bubble, the normalized mean velocity field and the normalized turbulent stresses are found to be similar for all expansion ratio cases studied. Using these measurements, the streamwise force balance of the mean separation bubble is studied. The analysis of the data shows that in this case, the contribution to the streamwise force from both the Reynolds normal and shear stress is significant, although the Reynolds shear stress contribution is larger. Differences in the force contributions from other geometries are highlighted.

Velocity–vorticity RANS turbulence modeling by boundary element method

Turbulent flow over various geometries is studied numerically. Incompressible set of Navier–Stokes equations is considered and solved by boundary domain integral method (BDIM). Governing equations are written in velocity–vorticity form. Turbulence models used are based on eddy-viscosity hypothesis. Integral form of equations, discretization and the solution algorithm are presented. The algorithm is tested with two separate test cases. The first is the turbulent channel flow for two different Reynolds numbers: Re τ = 180 and Re τ = 395 . Results show very good agreement with corresponding DNS data. The second test case is the flow over backward facing step for Reynolds number Re h = 5000 , which shows good agreement with literature data on mean reattachment length.

Numerical investigation on mass transfer enhancement downstream of an orifice

Mass transfer enhancement is an indispensible element in flow accelerated corrosion (FAC). In order to investigate mass transfer enhancement downstream of an orifice, the two-dimensional computational fluid dynamics (CFD) calculation is conducted. First, validation of turbulence model, the ζ–f model, is conducted in the fully developed pipe flow, the flow through an orifice and the flow downstream of a sudden expansion. The validation shows that the model is adequate to predict mass transfer enhancement in all the tested flows. Effects of Reynolds number, orifice thickness and diameter ratio on mass transfer enhancement downstream of an orifice are then investigated based on the numerical calculation. The investigation shows that the mass transfer enhancement ratio decreases with the increasing Reynolds number. However, the locations of reattachment point and the peak transfer rate point are not affected by the Reynolds number. Parametric study on the orifice thickness shows that a thin orifice helps mass transfer enhancement in its downstream. This is attributed to more intense turbulence generation downstream of a thin orifice. Moreover, the peak transfer point appear about 0.4Lr (reattachment length) downstream of the orifice. The results of the parametric study are synthesized as a correlation,StmaxStfd=4.78Re-0.12dD-1.161+0.82(L1/d)31+(L1/d)3which is expected to be valid in the range of 4.2×104⩽Re⩽1.3×105, 0.4⩽d/D⩽0.75, 0.13⩽L1/d⩽2.61.

Flow noise induced by small gaps in low-Mach-number turbulent boundary layers

The flow-noise induced by small gaps underneath low-Mach-number turbulent boundary layers at Reθ = 4755 is studied using large-eddy simulation and Lighthill's theory. The gap leading-edge height is 13% of the boundary-layer thickness, and the gap width and trailing-edge height are varied to investigate their effects on surface-pressure fluctuations and sound generation. The maximum surface pressure fluctuations, which increase with gap width and trailing-edge height, occur at the trailing edge or near the reattachment point if there is separation from the trailing edge. The downstream recovery towards an equilibrium boundary layer is significantly faster for gap flows compared to step flows, and the recovery distance scales with the reattachment length for gaps with trailing-edge separation. The acoustic field is dominated by the forward-facing step in the gap and resembles forward-step sound for wide gaps and/or asymmetric gaps with trailing edge higher than leading edge. In these cases, the dominant acoustic source mechanisms are the impingement of the separated shear layer from the leading edge onto the trailing edge and the unsteady separation from the trailing edge, coupled with edge diffraction. For narrow and symmetric gaps, the destructive interference of sound from the leading and trailing edges causes a significant decline in low-frequency sound and thereby creates a broad spectral peak in the mid-frequency range. The effects of gap acoustic non-compactness and free-stream convection are investigated by comparing solutions based on a compact gap Green's function with those from a boundary-element calculation. They are found to be negligible at the typical hydroacoustc Mach number of 0.01, but become significant at Mach numbers as low as 0.1 and moderately high frequencies.

Numerical Investigation of Three-Dimensional Flow over a Swept Backward-Facing Step Using a Multi-Scale Turbulence Model

Using a multiscale model based on the variable interval time average method and a set of averaged equations for incompressible turbulent flows, we simulate the three-dimensional separated and reattaching flow over a swept backward-facing step. The results show that the reattachment length and vortex structure vary with the swept angles from 15° to 60° through a detailed study about velocity profile and pressure distribution. Generally the reattachment length and span wise vorticity decrease dramatically after =30°. The result fully accord with experimental data. Results show that this model is suitable for separated flows, and accurately predict the flow field, so this model should be useful in engineering application.

Turbulent flow over a rough backward-facing step

This work characterizes the impacts of the realistic roughness due to deposition of foreign materials on the turbulent flows at surface transition from elevated rough-wall to smooth-wall. High resolution PIV measurements were performed in the streamwise-wall-normal (x–y) planes at two different spanwise positions in both smooth and rough backward-facing step flows. The experiment conditions were set at a Reynolds number of 3450 based on the free stream velocity U∞ and the mean step height h, expansion ratio of 1.01, and the ratio of incoming boundary layer thickness to the step height, δ/h, of 8. The mean flow structures are observed to be modified by the roughness and they illustrate three-dimensional features in rough backward-facing step flows. The mean reattachment length Xr is significantly reduced by the roughness at one PIV measurement position while is slightly increased by the different roughness topography at the other measurement position. The mean velocity profiles at the reattachment point indicate that the studied roughness weakens the perturbation of the step to the incoming turbulent flow. Comparisons of Reynolds normal and shear stresses, productions of normal stresses, quadrant analysis of the instantaneous shear-stress contributing events, and mean spanwise vorticity reveal that the turbulence in the separated shear layer is reduced by the studied roughness. The results also indicate an earlier separation of the turbulent boundary layer over the current rough step, probably due to the adverse pressure gradient produced by the roughness topography even before the step.

Numerical simulation of an axisymmetric separated and reattached flow over a longitudinal blunt circular cylinder

This article presents a numerical investigation of turbulent flow in an axisymmetric separated and reattached flow over a longitudinal blunt circular cylinder. The governing equations were discretized by the finite-volume method and SIMPLER method was applied to solve the equations on a staggered grid. The turbulent flow was numerically simulated using the standard k–ε, Abe–Kondoh–Nagano (AKN) and Shear Stress Transport (SST) turbulence models. The comparisons made between numerical results and experimental measurements showed that the SST model is superior to other models in the present calculation.Computations were performed for three different Reynolds numbers of 6000, 10000 and 20000 based on the cylinder diameter. To our knowledge, this study represents the first numerical investigation of the present flow configuration. The computational results were validated with the available experimental data of reattachment length, mean velocity distribution and wall static pressure coefficient in the turbulent blunt circular cylinder flows. Further, other characteristics of the flow, such as turbulent kinetic energy, pressure, streamlines, and the velocity vectors are discussed.The results show that the main characteristics of the turbulence flow in the separation region, such as reattachment length or velocity profiles, are nearly independent of the Reynolds number. The obtained results showed that a secondary separation bubble may appear in the main separation bubble near the leading edge. Furthermore, it was found that the turbulent kinetic energy has a large effect on the formation of the secondary bubble.

Numerical investigation of solid-liquid slurry flow through an upward-facing step

Abstract The flow of a solid-water mixture through an upward-facing step in a channel is numerically investigated. The effect of expansion ratio, mean solids volume fraction and particle diameter on the velocity field, pressure distribution and solid volume fraction field is studied. Expansion ratios of 0.50 and 0.67, particle diameter of 125 μm and 440 μm and mean solid volume fraction between 0.05 and 0.20 are considered. Particle density is 2465 kg m-3. An Eulerian twofluid model is used to simulate the flow. Due to the lack of experimental data, the model was validated by comparison to other numerical investigations and to experimental data about the horizontal pipe case. Afterwards, it is studied the effect of the above mentioned parameters upon the degree of coupling between the phases and the extension of the disturbance region in the pressure and solid volume fraction fields downstream the step. Parameters of engineering interest, such as the reattachment length and the pressure recovery downstream the enlargement, are investigated.