Uniform Inlet Velocity Research Articles

Prediction of the flow structure in a turbulent rectangular free jet

A numerical analysis is conducted for turbulent flow of a rectangular free jet with an aspect ratio of 2:1. The computations were performed using two standard two-equation turbulence models (the k–ε and the k–ω models). Two inflow boundary conditions were evaluated with each model: a uniform inlet velocity profile and a profiled inlet velocity fitted to experimental data. The results show that the k–ε model with the profiled inlet velocity succeeded in predicting the main features of the flow, including the vena contracta and the saddle-shaped velocity profiles in the near-field region, and the rate of velocity decay in the far-field region.

Thermal-fluid transport phenomena in an axially rotating flow passage with twin concentric orifices of different radii

SUMMARY This paper investigates the thermal fluid-flow transport phenomena in an axially rotating passage in which twin concentric orifices of different radii are installed. Emphasis is placed on the effects of pipe rotation and orifice configuration on the flow and thermal fields, i.e. both the formation of vena contracta and the heattransfer performance behind each orifice. The governing equations are discretized by means of a finitedifference technique and numerically solved for the distributions of velocity vector and fluid temperature subject to constant wall temperature and uniform inlet velocity and fluid temperature. It is found that: (i) for a laminar flow through twin concentric orifices in a pipe, axial pipe rotation causes the vena contracta in the orifice to stretch, resulting in an amplification of heat-transfer performance in the downstream region behind the rear orifice, (ii) simultaneously the heat transfer rate in the area between twin orifice is intensified by pipe rotation, (iii) the amplification of heat transfer performance is affected by the front and rear orifice heights. Results may find applications in automotive and rotating hydraulic transmission lines and in aircraft gas turbine engines. Copyright # 2005 John Wiley & Sons, Ltd.

Numerical Study of a Separated-Reattached Flow on a Blunt Plate

Large-eddy simulation (LES) of transitional separatingreattaching flow on a flat plate with a blunt leading edge has been performed. The Reynolds number based on the uniform inlet velocity and the plate thickness is 6.5103. A dynamic subgrid-scale model is employed in the transitional flow case. The LES results compare reasonably well with the available experimental data. The entire transition process has been visualized by using the LES data, and large-scale vortical structures have been observed at different stages of transition. It is known that different vortex shedding frequencies exist, especially the so-called low-frequency flapping when there is a separation bubble. It is not clear whether all transitional and turbulent separatingreattaching flows have different vortex shedding frequencies. It is also not clear what the working mechanisms are behind the so-called low-frequency flapping as reported widely. These issues are addressed.

Characterization of the Hydrodynamically Developing Flow in a Microtube Using MTV

Micro-molecular tagging velocimetry (μMTV) has been used to characterize the hydrodynamic developing flow in a microtube inlet with a nominal inner diameter of 180μm. Velocity profile data at 11 axial locations within the hydrodynamic developing region were acquired using the μMTV approach and the results represent the first characterization of hydrodynamically developing pipe flow at the microscale. The uncertainty in measurements of time-averaged velocity profiles ranged from 6% to 7% of the centerline velocity. The uncertainty in instantaneous measurements is in the range 8%–16% of the peak maximum velocity. Data were taken at Reynolds numbers of 60, 100, 140, 290, and 350. The data suggest the formation of a vena contracta with either locally turbulent flow or unsteady laminar flow separation early in the tube for the larger Reynolds (Re) numbers, which is quite different from macroscale experiment or numerical simulation where a vena-contracta is not observed for Re<500. The velocity profiles obtained very near the tube entrance exhibited a uniform velocity core flow surrounded by regions of relatively stagnant fluid in the near wall regions. The size of the inferred recirculation zones, measured velocity rms, and maximum shear rates all exhibit increasing magnitude with increasing Reynolds number. The velocity profiles were observed to evolve in the downstream direction until the classical parabolic distribution existed. The total hydrodynamic entry length agrees well with values published in the literature for laminar flow with a uniform inlet velocity, despite the existence of the observed vena contracta.

Wall effects in flow past a circular cylinder in a plane channel: a numerical study

This paper describes a numerical study on the steady flow of an incompressible Newtonian fluid past a circular cylinder confined in a plane rectangular channel. Using FLUENT (version 6), two-dimensional steady state computations were carried out for an uniform inlet velocity and for different values of the Reynolds numbers in the range between 0.1 and 200 and blockage ratios (ratio of the channel width to the cylinder diameter) in the range between 1.54 and 20. The flow parameters such as drag coefficient, length of the recirculation zone, and the angle of separation are presented as functions of the Reynolds number and blockage ratio. The total drag coefficient ( C D) was found to decrease with an increase in the blockage ratio ( λ) for a fixed value of the Reynolds number ( Re) and to decrease with increasing Reynolds number for a fixed value of λ. Similarly, for a fixed value of λ, both the angle of separation and the length of the recirculation zone increase with the increasing Reynolds number.

Numerical study of the instability mechanism in transitional separating–reattaching flow

Laminar separated flows are known to become unstable at relatively low Reynolds numbers. As a result, both the mean and instantaneous flow patterns are highly influenced by instabilities leading to transition to turbulence. Large-Eddy Simulation (LES) is employed to investigate the primary and secondary instabilities of a separated boundary layer transition on a flat plate with a blunt leading edge. The Reynolds number based on the uniform inlet velocity and the plate thickness is 6500. A dynamic subgrid-scale model is employed to compute the subgrid-scale stresses more accurately in the transitional flow case. Statistics of the LES are found to be in acceptable agreement with the available experimental data. Based on the characteristic frequency from the velocity and pressure spectra, the LES results confirm that transition starts with the primary 2D instability originating from the free shear in the bubble as the free shear layer is inviscidly unstable via the Kelvin–Helmholtz mechanism. The flow visualisation together with the spectral analysis for the velocity components and pressure give strong indication of the dominance of the helical-pairing instability which could be mainly responsible for the breakdown to turbulence.

COMPUTATIONAL VISUALIZATION OF SEPARATED/REATTACHED TRANSITIONAL FLOWS ON A BLUNT PLATE

It is well known that large-scale organized motions, usually called coherent structures, exist in many transitional and turbulent flows (if not all). The topology and range of scales of those large-scale structures change widely from flow to flow such as counter-rotating vortices in wake flows, streaks and hairpin vortices in turbulent boundary layers. However, it is not well established what kind of large-scale structures exists in separated/reattached transitional flows. Large-eddy simulation with a dynamic subgrid-scale model is employed in the current study to investigate the physics of transitional separating/reattaching flows over a blunt plate held normal to a uniform stream. The Reynolds number based on the uniform inlet velocity and the plate thickness is 6500. Statistics of the LES are found to be in acceptable agreement with the available experimental data in the laminar, transitional, and turbulent flow regions. The entire transition process leading to breakdown to turbulence has been shown by flow visualization and large-scale structures have been identified at different stages of the transition process of the separating/reattaching flow. The well-known hairpin vortices commonly associated with boundary-layer transition on a flat plate have been clearly shown at about the mean reattachment point and more large-scale structures are presented as well. The large-scale structures are noticed to persist a considerable distance downstream of reattachment before they eventually break into smaller-scale turbulent structures. In addition, it may shed light on the transition mechanisms and the nature of instabilities involved to understand the formation of these large-scale structures, their spatial and temporal evolution and eventual break-up into smaller structures

Confined Vortex Shedding Past a Square Cylinder with a Planar Jet.

In this investigation, confined vortex shedding past a square cylinder with a planar jet is numerically studied. Flow and scalar-transport simulations are presented for various cases including both laminar and turbulent flow situations. It is shown that the ratio of jet velocity to uniform inlet velocity significantly affects the overall flow structures and thus scalar transport downstream of the cylinder. Especially, when the ratio is large enough, the jet penetrates the main vortices shed from the cylinder, resulting in significant changes in the flow and scalar fields. In the case of laminar flow, regions of intense scalar are formed along the streamlines from the jet exit, and the oscillation of the force on the cylinder eventually disappears as the jet velocity is close to the inlet velocity. Large Eddy Simulation of turbulent flow also reveals complex flow structures and intense mixing depending on the velocity ratio; regions of intense scalar coincide with those of high turbulence intensity. The results obtained exhibit fuel-air mixing characteristics observed in a planar combustor where the square cylinder plays the role of a flame-holder.

An investigation of air inlet velocity in simulating the dispersion of indoor contaminants via computational fluid dynamics.

Computational fluid dynamics (CFD) is potentially a valuable tool for simulating the dispersion of air contaminants in workrooms. However, CFD-estimated airflow and contaminant concentration patterns have not always shown good agreement with experimental results. Thus, understanding the factors affecting the accuracy of such simulations is critical for their successful application in occupational hygiene. The purposes of this study were to validate CFD approaches for simulating the dispersion of gases and vapors in an enclosed space at two air flow rates and to demonstrate the impact of one important determinant of simulation accuracy. The concentration of a tracer gas, isobutylene, was measured at 117 points in a rectangular chamber [1 (L) x 0.3 (H) x 0.7 m (W)] using a photoionization analyzer. Chamber air flow rates were scaled using geometric and kinematic similarity criteria to represent a full-sized room at two Reynolds numbers (Re = 5 x 10(2) and 5 x 10(3)). Also, CFD simulations were conducted to estimate tracer gas concentrations throughout the chamber. The simulation results for two treatments of air inlet velocity (profiled inlet velocity measured in traverses across the air inlet and the assumption that air velocity is uniform across the inlet) were compared with experimental observations. The CFD-simulated 3-dimensional distribution of tracer gas concentration using the profiled inlet velocity showed better agreement qualitatively and quantitatively with measured chamber concentration, while the concentration estimated using the uniform inlet velocity showed poor agreement for both comparisons. For estimating room air contaminant concentrations when inlet velocities can be determined, this study suggests that using the inlet velocity distribution to define inlet boundary conditions for CFD simulations can provide more reliable estimates. When the inlet velocity distribution is not known, for instance for prospective design of dilution ventilation systems, the trials of several velocity profiles with different source, air inlet and air outlet locations may be useful for determining the most efficient workroom layout.

Large‐scale structures at various stages of separated boundary layer transition

AbstractLarge‐eddy simulation is employed to investigate separated boundary layer transition induced by a change of curvature of the surface. The geometry is a flat plate with a semi‐circular leading edge. The Reynolds number based on the uniform inlet velocity and the leading edge diameter is 3450. The simulated mean and turbulence quantities compare well with the available experimental data. The LES data have been comprehensively analysed to elucidate the transition process leading to breakdown to turbulence. Large‐scale structures have been observed at various stage of the transition process. Understanding the formation of these large‐scale structures, their evolution and eventual break‐up into smaller structures may help to shed light on the transition mechanism. Copyright © 2002 John Wiley & Sons, Ltd.

The influence of inlet velocity profile on three-dimensional three-generation bifurcating flows

The influence of inlet velocity profile on the three-dimensional three-generation bifurcating flow has been numerically studied using a CFD code based on finite volume method. The bifurcating airways simulated the branches of human lung. The axial length and cross-sectional diameter of the three-generation airway are taken from the anatomic data of the 5th–7th generation airway of an averaged height man. The curvature and bifurcating angle of each junction are taken as 2.265 diameter of the next generation and 70∘, respectively, from physiological consideration. Computations are carried out for eight Reynolds numbers ranging from 200 to 1600, each under uniform and parabolic inlet velocity profiles, to consider the relations between the entry flow patterns and the overall flow characteristics including mainstream flow pattern, secondary flow vortices, asymmetrical flow partition and pressure drop. The mass flow ratio between the medial and lateral branch, and the total pressure drop are closely related to the entry flow patterns for larger Reynolds numbers.

THE NUSSELT NUMBER AND FRICTION FACTOR AT THE ENTRANCE OF A MICRO-DUCT

Expressions are presented for the Nusselt number and friction factor at the entrance of a micro-duct with uniform inlet velocity and temperature profiles and for the gas flow and heat transfer in the velocity-slip and temperature-jump regime. The entrance Nusselt number and friction factor are shown to be of finite values, and they are dependent on the Knudsen number and gas-wall accommodation factors but independent of the geometrical form of duct section

Numerical Analysis on Flow and Heat Transfer in an Axially Rotating Flow Passage with a Concentric Orifice

This paper deals with numerical analysis on steady-state incompressible laminar flow transfer in an axially rotating passage with a concentric orifice. Consideration is given to rotational effects on secondary flow and heat transfer in rotating flow passages. The governing equations are discretized by means of a finite-difference technique and numerically solved for the distributions of velocity vector and fluid temperature subject to constant wall temperature and uniform inlet velocity and fluid temperature. The Reynolds number and rotation rate are varied to determine their effects on both the formation of vena contracta and the heat-transfer performance behind the orifice. It is disclosed that: (i) for a laminar flow through a concentric orifice in a pipe, axial rotation causes the vena contracta in the orifice to stretch and amplifies the heat-transfer performance in the downstream region, and (ii) the elongation of the vena contracta and heat-transfer performance are enhanced with an increase in both the rotation rate and the Reynolds number. Results may find applications in automotive and rotating hydraulic transmission lines.

The entrance effect of laminar flow over a backward-facing step geometry

The study investigates the entrance effect for flow over a backward-facing step by comparing predictions that set the inlet boundary at various locations upstream of the sudden expansion. Differences are most significant in the sudden expansion region. If the geometry has an inlet channel, then shorter reattachment and separation lengths are predicted. Comparisons with experimental data indicate that better agreement is found using a long inlet channel, but only for low Reynolds numbers where the experimental error is less significant. For certain cases, predictions with a high expansion number are perturbed by the entrance effect more than low-expansion-number predictions; however, the effect is localized in the sudden expansion region. Channels with low expansion numbers always experience a greater entrance effect after some distance upstream and downstream of the sudden expansion. The boundary layer growth in the inlet channel was examined using a uniform inlet velocity profile. © 1997 John Wiley & Sons, Ltd.

On the asymmetry of flow through a bifurcated channel

Results are presented of detailed numerical simulation of spatially developed, three-dimensional laminar flow through a bifurcated channel for different inlet flow conditions. The flow development near the bifurcated part of the channel is observed to be very sensitive to the inlet velocity distribution. For an uniform axial inlet velocity, the flow visualization results indicate the formation of two counter rotating symmetric vortical structures, parallel to the port axis, in the bifurcated part of the channel. When a cross-flow (v-component of velocity) is introduced at the inlet, the flow behaviour near the bifurcated part of the channel changes drastically. For such an inlet flow condition one vortical structure becomes larger at the expense of the other, and the asymmetry prevails in the flow field. The phenomena get intensified with the increase of the Reynolds number. The present numerical prediction of the flow phenomena provides some explanation of the interesting observations reported in the previous experimental investigations.

Inertial Deposition of Particles in the Human Upper Airway Bifurcations

ABSTRACT Particle deposition in symmetric bifurcating airways due to inertial impaction was studied numerically for inspiratory flows. Three-dimensional bifurcation models were constructed. The models had different parent and daughter diameters comparable to the airway generations 3–6 of the human lung. Bifurcation angles of the models were also varied (30®, 45®, and 60®). Airflow fields in the models were obtained by a finite–element method (FIDAP, Fluid Dynamics International, Evanston, IL) for different Reynolds numbers of 100, 265, 530, and 1060 under parabolic and uniform inlet velocity conditions. The calculated flow field data were used to simulate particle trajectory in the airways. Particle deposition efficiency was obtained using the limiting trajectory method. Inlet flow velocity profile, flow Reynolds number, and bifurcation angle were found to have substantial effects on particle deposition. Based on calculated deposition results, empirical equations were derived for particle deposition effic...

Theoretical analysis of collection characteristics of a solar air heater with a built-in net.

The collection characteristics of a solar air heater with a built-in net, in which a metallic net was inserted parallel in the space between the transparent cover plate and the absorber, were theoretically analyzed. The working fluid (air), which entered the solar air heater at a uniform inlet velocity and a constant inlet temperature, was divided into two layers (upper and lower layer) by the metallic net. Solar radiation impinges uniformly from above, passing through the transparent cover plate and being absorbed partly by the net and partly by the absorber. For the present solar air heater, the continuity, momentum and energy equations were derived under several assumptions and numerically solved. The calculations were performed for the cases of eight kinds of metallic nets which had different transmissivities, under different combinations of mass flow rate of air and incident solar radiation. Also, the cases of a conventional solar air heater and a solar air heater without a net were theoretically analyzed. The calculated results were compared with the experimental data reported in a previous paper, and the effects of the net transmissivity, net setting position and solar air heater length on temperature distributions within the air layer and the collection efficiency were examined. The calculated results show fairly well the tendencies of the experimental data, and the several assumptions made and the calculation method used in the theoretical analysis are found to be valid.

Flow dynamics across end-to-end vascular bypass graft anastomoses.

In this article, a numerical simulation of steady flow across an end-to-end vascular bypass graft anastomosis is presented. In vitro experiments were performed to determine the variations in the conduit cross section at the anastomosis. Penrose surgical drainage tubing was used to simulate an artery and was anastomosed with Polytetrafluoroethylene (PTFE) vascular grafts using a continuous suturing technique. Artery to artery anastomosis was simulated by suturing two Penrose tubing segments. The anastomotic specimens were subject to static transmural pressure in the physiologic range to determine the instantaneous diameter and compliance as a function of the distance from the anastomotic site. The experimentally determined geometries were used to simulate steady flow through an end-to-end anastomosis using the finite analytic (FA) numerical solution technique. The results demonstrated a region of flow separation 2 mm distal to the Penrose tubing-Penrose tubing anastomosis (simulating an artery-artery anastomosis) at higher transmural pressures. Moreover, wall shear stresses increased proximal to the anastomosis in flow from the Penrose tubing to the graft. In flow from the graft to the Penrose tubing, low wall shear stresses were observed distal to the anastomosis. Flow separation was observed distal to the anastomosis at higher transmural pressures with uniform inlet velocity condition. The region of low shear stress in flow from PTFE graft to the Penrose tubing was located nearer to the anastomosis with thin wall grafts than that with standard wall thickness grafts. Our steady flow model studies suggest a correlation between regions of low wall shear stress and the development of anastomotic neointimal fibrous hyperplasia in end-to-end anastomoses.

Mixed convection analysis in large baffled rectangular chambers with internal heat sources

An investigation to determine the effects of mixed convection on the flow structure of large baffled chambers with internal heat sources has been completed. Baffles and internal heat sources are placed symmetrically about the vertical axis while maintaining a constant and uniform inlet velocity, inlet temperature, and wall temperature. Time-dependent experimental and two-dimensional numerical results are compared at various chamber locations to demonstrate the transition of the flow structure through a sequence of bifurcations with increasing internal heat source temperatures.

Numerical Solution of Incompressible Flow of Power-Law Fluid through a Pipe with Abrupt Contraction.

A numerical solution for an axisymmetric non-Newtonian flow of an incompressible inelastic power-law fluid using a primitive variable formulation is presented. The present method is based on the method of lines using the rational Runge-Kutta time integration scheme combined with the central finite difference method for the spatial discretization. The Poisson equation is solved by means of the SOR method using the concept of the SMAC method. As a test problem, numerical results for a circular pipe flow with uniform inlet velocity are shown and compared with the analytic fully developed velocity profile to test the validity of the present method. Next, numerical results for the flow through a pipe with abrupt contraction are shown. The difference between Newtonian (n=1) and non-Newtonian fluids (n=0.75 and 1.25) for the Reynolds number (Re) from 0.01 to 100 is discussed concerning the flow pattern. It is evident that the size of the secondary vortex increases with increasing volumes of n. The entry length and the excess pressure drop in the contraction increase with decreasing values of n. It is confirmed that the present method is suitable for the numerical solution of a power-law fluid.