Secondary Flow Velocity Research Articles

Effects of stent porosity on hemodynamics in a sidewall aneurysm model

Computation and experiment have been complementarily performed to study the fluid flow inside a stented lateral aneurysm anchored on the straight parent vessel. The implicit solver was based on the time-dependent incompressible Navier–Stokes equations of laminar flow. Solutions were generated by a cell-center finite-volume method that used second-order upwind and second-order center flux difference splitting for the convection and diffusion term, respectively. The second-order Crank–Nicolson method was used in the time integration term. Experimental techniques used were flow visualization (FV) and particle tracking velocimetry (PTV). Experimentally, the straight afferent vessel had an inner diameter 10 mm. The diameters of the aneurysmal orifice, neck, and fundus were 14, 10, and 15 mm, respectively, and the distance between the orifice and dome measured 20 mm. A 30 mm long helix-shaped stent was tested. Four stent porosities of 100%, 70%, 50%, and 25% were examined. Volume-flow rate waveform of a cerebral artery was considered with a maximum Reynolds number of 250 and Womersley number of 3.9. Results are presented in terms of the pulsatile main and secondary flow velocity vector fields, the volume inflow rates into the aneurysm, and the wall shear stress (WSS) and wall pressure at the aneurysm dome. Some comparisons of computed results with the present FV and PTV results and with the data available from the literature are also made. The maximum flow velocity inside the aneurysm ostium and the WSS in the dome region at the peak flow can, respectively, be suppressed to less than 5% of the parent vessel bulk velocity (or 20% of the unstented case) and 8% of the unstented case if the stent porosity is smaller than 40% (about the porosity of the two-layer stents). In general, the three-layer stents seem not as effective as the two-layer stents in reducing the magnitude of aneurysm inflow rate and WSS.

In vitro validation of computational fluid dynamic simulation in human proximal airways with hyperpolarized 3He magnetic resonance phase-contrast velocimetry

Computational fluid dynamics (CFD) and magnetic resonance (MR) gas velocimetry were concurrently performed to study airflow in the same model of human proximal airways. Realistic in vivo-based human airway geometry was segmented from thoracic computed tomography. The three-dimensional numerical description of the airways was used for both generation of a physical airway model using rapid prototyping and mesh generation for CFD simulations. Steady laminar inspiratory experiments (Reynolds number Re = 770) were performed and velocity maps down to the fourth airway generation were extracted from a new velocity mapping technique based on MR velocimetry using hyperpolarized (3)He gas. Full two-dimensional maps of the velocity vector were measured within a few seconds. Numerical simulations were carried out with the experimental flow conditions, and the two sets of data were compared between the two modalities. Flow distributions agreed within 3%. Main and secondary flow velocity intensities were similar, as were velocity convective patterns. This work demonstrates that experimental and numerical gas velocity data can be obtained and compared in the same complex airway geometry. Experiments validated the simulation platform that integrates patient-specific airway reconstruction process from in vivo thoracic scans and velocity field calculation with CFD, hence allowing the results of this numerical tool to be used with confidence in potential clinical applications for lung characterization. Finally, this combined numerical and experimental approach of flow assessment in realistic in vivo-based human airway geometries confirmed the strong dependence of airway flow patterns on local and global geometrical factors, which could contribute to gas mixing.

Numerical Simulation of Oscillatory Flow in Realistic Model Human Airways

Computational fluid dynamics (CFD) simulations were carried out for an oscillatory flow in a 3D realistic model of the human central airways to reveal the effect of airway geometry on the oscillatory flow structure. This 3D realistic airway model is a computational model of multi-branching airway constructed based on X-ray CT images of actual human airways in which the airway diameter ranged from approx. 2 to 14 mm, and the flow in this airway model was simulated using CFD software (Fluent). The resultant inspiratory flow patterns in this 3D realistic airway model were relatively similar to patterns observed in a simplified planar multi-branching airway model, whereas the expiratory flow patterns strongly depended on the realistic airway geometry and showed more complicated secondary flow patterns. Secondary flow velocities were higher in the realistic airway model than in the simplified airway model in both the inspiratory and expiratory flows. Langrangian fluid particle tracking clarified the convective dispersion due to asymmetric inspiratory and expiratory velocity profiles.

Hall current effect on MHD mixed convection flow from an inclined continuously stretching surface with blowing/suction and internal heat generation/absorption

Hydromagnetic heat transfer by mixed convection along an inclined continuously stretching surface, with power-law variation in the surface temperature or heat flux, in the presence of Hall current and internal heat generation/absorption has been studied. The surface is considered to be permeable to allow fluid suction or blowing, and stretching with a surface velocity varied according to a power-law. Two cases of the temperature boundary conditions were considered at the surface. The governing equations have been transformed into non-similar partial differential equations which have been integrated by the forth-order Runge–Kutta method. The effect of Hall parameter, magnetic parameter, dimensionless blowing/suction parameter, space and temperature dependent internal heat generation/absorption parameters and buoyancy force parameters on the temperature, primary and secondary flow velocity have been studied parametrically. All parameters involved in the problem affect the flow and thermal distributions except the temperature-dependent internal heat generation/absorption in the case of prescribed heat flux (PHF). Numerical values of the local skin-friction and the local Nusselt numbers for various parametric conditions have been tabulated.

Transport of Particulates in an Internal Cooling Ribbed Duct

A ribbed square duct (P∕e=10, e∕Dh=0.10) subjected to sand ingestion is studied using large-eddy simulations (LES). Particle sizes of 10μm, 50μm, and 100μm with nondimensional response times (normalized by friction velocity and hydraulic diameter) of 0.06875, 1.71875, and 6.875, respectively are considered. The calculations are performed for a nominal bulk Reynolds number of 20,000 under fully-developed conditions. Distributions of impingement density, impingement velocities and angles, together with fractional energy transfer are presented for each surface. It is found that about 40% of the total number of 10micron particles are concentrated in the vicinity (within 0.05 Dh) of the duct surfaces, compared to 25–30% of the 50 and 100micron particles. The 10micron particles are more sensitive to the primary and secondary flow velocities than the larger particles. While the 10micron particles exhibit high energy transfer to the surface near the rib side-wall junction and immediately upstream of the rib, the larger particles exhibit more uniform distributions. The largest fraction of incoming particulate energy is transferred to the front face of the rib and is between one to two orders of magnitude larger than the other surfaces. As particle size increases, substantial particle energy is also transferred to the back face of the rib by particles bouncing off the front face and carrying enough momentum to impinge on the back face of the preceding rib.

Experimental Study of Nanoparticle Penetration Efficiency Through Coils of Circular Cross-Sections

Nanoparticle penetration efficiencies through five different coils of circular cross-sections were measured as a function of particle size, Dean number, and curvature ratio of coil. Silver particles with diameters ranging from 3 to 50 nm were used, and the Dean number was varied from 21 to 1779. According to the equation of critical Reynolds number for the coil (Ito 1959), all the flows in this study were laminar. The penetration efficiency through coils was found to increase with increasing particle size and also with increasing Dean number. The influence of curvature ratio on penetration efficiency seemed to be negligible when the Dean number was greater than approximately 200. Secondary flow developed due to centrifugal force in the coil is known to increase diffusional particle loss. Boundary layer thickness (δ) of the secondary flow can affect the particle loss in coils, because the average flow velocity of secondary flow within the boundary layer is related to δ. The particle loss in coils is assume...

Fundamental Analysis of the Secondary Flows and Jet-Wake in a Torque Converter Pump—Part II: Flow in a Curved Stationary Passage and Combined Flows

Previously, experimental results for the velocity field in a torque converter pump showed strong jet/wake characteristics including backflows and circulatory secondary flows. Navier-Stokes flow models were developed herein to independently analyze the pump pressure-to-suction side jet/wake flow, the core-to-shell side jet/wake flow, and the secondary flows. Two relatively simple models were employed: (i) a rotating two-dimensional straight-walled duct and (ii) a 180 deg flow bend. Parametric studies were undertaken to evaluate the effect that operating conditions and geometry had on the characteristics. Results from the model showed that the core side wake, which was due to flow separation caused by rapid radial flow turning, was primarily a function of the Reynolds number; increasing the Reynolds number increased the core-to-shell side jet/wake flow. The passage length (or curvature) strongly affected the core-to-shell jet/wake. Using the modified equations for the generation of streamwise vorticity and the results from the two-dimensional jet/wake model for the normal and binormal vorticity components, trends for the secondary flows in the torque converter pump were predicted. Predicted secondary flows in the torque converter pump circulated in the counterclockwise direction (positive streamwise vorticity) in the pump midplane and in the clockwise direction (negative streamwise vorticity) in the pump exit plane. These trends agreed with experimental observations. Both the Reynolds number and the modified Rossby number were seen to have a significant influence on the streamwise vorticity and, thus, on the magnitude of the secondary flow velocities. The pump midplane counter-clockwise secondary flow circulation was primarily caused by the interaction of the pressure-to-suction side jet/wake nonuniform flow (and the associated normal vorticity component) with the high radial/axial flow turning angle the flow underwent while passing through blade passage. Similarly, the pump exit plane clockwise secondary flow circulation was caused by the core-to-shell side jet/wake nonuniform flow (and the associated binormal vorticity component) being rotated about a fixed centerline (pump shaft). Thus, the pump streamwise vorticity, which was responsible for the generation circulatory secondary flows, was directly related to the pump jet/wake phenomena.

Turbulent flow in a rectangular duct with a smooth-to-rough step change in surface roughness

Experimental study has been conducted on the response of turbulent flow in a rectangular duct to a smooth-to-rough step change in surface roughness on its short-side walls. The distributions of the primary flow velocity, secondary flow velocities, turbulence intensities, and turbulent shear stresses have been measured in seven cross-sections located from 2.0 d h to 37.5 d h downstream from the smooth-to-rough junction, and the relaxation process of this turbulent flow has been examined. It has been found that the relaxation process of this flow is considerably complex, but the flow characteristics except for the primary flow velocity in the core region as well as near the rough wall reach an almost equilibrium state, i.e., the fully developed rough-duct flow condition, within a distance of 37.5 d h from the change in the surface roughness.

Local Heat (mass) Transfer and Flow Characteristics in Serpentine Channels with an Inclined Partition Wall

Flow and heat/mass transfer characteristics for turbulent flow in serpentine channels with an inclined partition wall have been examined experimentally. Detailed velocity distributions have been measured using Particle Image Velocimetry for three kinds of partition wall's inclination angle with respect to the outer walls (α=+4°, 0°, -4°) at Reynolds number of 3.5×104. The PIV measurements results were compared with the published local mass transfer results for these three cases, and the mechanism of the heat (mass) transfer from the channel wall after turn section were discussed. Based on these data, the inclination angle can bring significant changes to the fluid characteristics and local heat (mess) transfer characteristics. The heat (mass) transfer enhancement after turn is controlled by the value of secondary flow velocity, convective mean velocity, turbulent kinetic energy, and their contribution rates are remarkably changed by the channel configurations.

Laser flow visualization and velocity fields by particle image velocimetry in an electrostatic precipitator model

Although improving electrostatic precipitator (ESP) collection of fine particles (micron and submicron sizes) remains of interest, it is not yet clear whether the turbulent flow patterns caused by the presence of electric field and charge in ESPs advance or deteriorate fine particle precipitation process. In this paper, results of the laser flow visualization and Particle Image Velocimetry (PIV) measurements of the particle flow velocity fields in a wire-to-plate type ESP model with seven wire electrodes are presented. Both experiments were carried out for negative and positive polarity of the wire electrodes. The laser flow visualization and PIV measurements clearly confirmed formation of the secondary flow (velocity of several tens of cm/s) in the ESP model, which interacts with the primary flow. The particle flow pattern changes caused by the strong interaction between the primary and secondary flows are more pronounced for higher operating voltages (higher electrohydrodynamic number NEHD) and lower primary flow velocities (lower Reynolds number Re). The particle flow patterns for the positive voltage polarity of the wire electrodes are more stable and regular than those for the negative voltage polarity due to the nonuniformity of the negative corona along the wire electrodes (tufts).

A study on the unsteady flow in a helical pipe

A study on the unsteady low-frequency oscillatory flow in a helical circular pipe is carried out based upon the blood flow in vessels, using the method of bi-parameter perturbation. The second order perturbation results were obtained and the characteristics were analyzed at different time of the axial velocity, of the secondary flow, and of the wall shearing stress. Also done the analysis of above-mentioned variables that varied along with time and Womersley number. The results indicate that for a helical pipe, the torsion exerts the main influence on the distribution of secondary flow velocity, especially when the absolute value of axial press gradient is rather small. the severe variation of stream function takes place within a very short period, during which time the stream function develops from positive value to negative value and vice versa, while in most cases in a cycle, the variation is smooth. The wall shearing stress changes severely with theta too.

Rotating Effect on Fluid Flow in Two Smooth Ducts Connected by a 180-Degree Bend

Laser Doppler velocimetry (LDV) measurements are presented of turbulent flow in a two-pass square-sectioned smooth duct simulating the coolant passages employed in gas turbine blades under rotating and nonrotating conditions. For all cases studied, the Reynolds number characterized by duct hydraulic diameter and bulk mean velocity was fixed at 1×104. The rotation number Ro was varied from 0 to 0.2. It is found that as Ro is increased, both the skewness (SK) of streamwise mean velocity and magnitude of secondary-flow velocity increase linearly, SK=2.3 Ro and U2+V2¯/Uh=2.3 Ro+0.4, and the magnitude of turbulence intensity level increases exponentially. As Ro is increased, the curvature induced symmetric Dean vortices in the turn for Ro=0 is gradually dominated by a single vortex most of which impinges directly on the outer part of leading wall. The high turbulent kinetic energy is closely related to the dominant vortex prevailing inside the 180-deg sharp turn. The size of separation bubble immediately after the turn is found to diminish to null as Ro is increased from 0 to 0.2. A simple correlation is developed between the bubble size and Ro. A critical range of Ro responsible for the switch of faster moving flow from near the outer wall to the inner wall is identified. For both rotating and nonrotating cases, the direction and strength of the secondary flow with respect to the wall are the most important fluid dynamic factors affecting local the heat transfer distributions inside a 180-deg sharp turn. The role of the turbulent kinetic energy in the overall enhancement of heat transfer is well addressed.

Étude numérique du champ de vitesse dans un échangeur à vortex

Flow through a cylindrical flat chamber, a model of some particular heat exchanger, is investigated numerically using the CFD-ACE code. Turbulence is modelled using the classical k– ε model. A better understanding of the secondary flow is then obtained: the k– ε model shows a strong dependence of the secondary flow velocity field with Reynolds number as was pointed out with precedent experimental results. Variations of the number of vortexes composing the secondary flow, giving a symmetrical or asymmetrical aspect, will influence the fluid particle trajectories and time residence. To cite this article: S. Petitot et al., C. R. Mecanique 330 (2002) 749–756.

Local Heat Transfer and Flow Characteristics in Two-Pass Channels with an Inclined Divider Wall.

Experimental study has been conducted on local heat/mass transfer and flow characteristics in rectangular cross-sectioned two-pass channels with a sharp turn having an inclined divider wall. Detailed distributions of local Sherwood numbers are presented that are obtained on all the walls of the channels by the naphthalene sublimation method, and the influence of the inclination angle of the divider wall on the local heat/mass transfer are examined. Then the flow characteristics in the channels, such as flow separation and reattachment around the turn section, secondary flow velocities in and after the turn, are described based on the time-mean flow vectors obtained by using the PIV. The correlation between the local heat/mass transfer and flow structure is discussed.

Relaxation of Turbulent Flow in a Rectangular Duct with a Step Change in Surface Roughness.

The response of a turbulent flow in a rectangular cross-sectioned duct to a rough-to-smooth step change in the surface roughness of the short-side walls has been investigated experimentally. The distributions of the primary and secondary flow velocities, turbulence intensities, and turbulent shear stresses have been measured by hot-wire anemometers in seven cross-sections set up in the smooth-duct part. In the core region of the duct cross-section, large secondary currents are maintained over a relatively long distance after the rough-to-smooth junction, and they cause large velocity gradient in the primary flow and large turbulent shear stress there. The streamwise changes of the turbulence intensities are rather complex. In the core region they once overshoot the values in the rough duct just after the rough-to-smooth junction, then undershoot those in the smooth duct, and finally approach an equilibrium state, while near the short-side wall they are decreased quite rapidly and monotonously in the streamwise direction.

Spatial and temporal variation of secondary flow during oscillatory flow in model human central airways.

Axial and secondary velocity profiles were measured in a model human central airway to clarify the oscillatory flow structure during high-frequency oscillation. We used a rigid model of human airways consisting of asymmetrical bifurcations up to third generation. Velocities in each branch of the bifurcations were measured by two-color laser-Doppler velocimeter. The secondary velocity magnitudes and the deflection of axial velocity were dependent not only on the branching angle and curvature ratio of each bifurcation, but also strongly depended on the shape of the path generated by the cascade of branches. Secondary flow velocities were higher in the left bronchus than in the right bronchus. This spatial variation of secondary flow was well correlated with differing gas transport rates between the left and right main bronchus.

Fluid flow in a helical pipe

Without simplifying the N-S equations of Germano's[5], we study the flow in a helical circular pipe employing perturbation method. A third perturbation solution is fully presented. The first- second- and third-order effects of curvature κ and torsion τ on the secondary flow and axial velocity are discussed in detail. The first-order effect of curvature is to form two counter-rotating cells of the secondary flow and to push the maximum axial velocity to the outer bend. The two cells are pushed to the outer bend by the pure second-order effect of curvature. The combined higher-order (second-, third-) effects of curvature and torsion, are found to be an enlargement of the lower vortex of the secondary flow at expense of the upper one and a clockwise shift of the centers of the secondary vortices and the location of maximum axial velocity. When the axial pressure gradient is small enough or the torsion is sufficiently larger than the curvature, the location of the maximal axial velocity is near the inner bend. The equation of the volume flux is obtained from integrating the perturbation solutions of axial velocity. From the equation the validity range of the perturbation solutions in this paper can be obtained and the conclusion that the three terms of torsion have no effect on the volume flux can easily be drawn. When the axial pressure gradient is less than 22.67, the volume flux in a helical pipe is larger than that in a straight pipe.

FLOW CHARACTERISTICS OF GRADUALLY SHALLOWED OPEN CHANNEL FLOW

Accelerating flow fields in gradually shallowed open channel flows are measured with a hot-film anemometer. Mean velocity profiles of the inner and outer region are well expressed by the log-wake law. The wake strength parameter II varies slowly according to the sudden decrease of non-dimensional pressure gradient parameter β in the flow direction. The relationship between II and β can be expressed by a biquadratic function. Reynolds stress distributions become slender in comparison with the universal distribution for a 2-D uniform flow. Secondary flow velocities are depressed and their flows pattern can not be confirmed in the accelerating flow. Iso-velocity lines in a cross-section become parallel to walls of the channel. Turbulence intensities decrease toward down stream direction in the accelerating region.

Effects of Turbulence Models on the Phase Distributions for Developing Two-Phase Bubbly Flow in Rotating Square-Sectioned Duct.

A fully three-dimensional numerical procedure based on the two-fluid model is proposed for the prediction of developing turbulent two-phase bubbly flow in a rotating squre-sectioned duct. The effects of turbulence models on phase distributions are investigated by utilizing the standard k-e model and Speziale's nonlinear k-e model with or without additional source terms relating to Coriolis modification or phase-fraction fluctuations. By comparing with existing experimental results, it is found that the additional source terms related to phase-fraction fluctuations tend to underestimate the lateral phase distribution and that Reynolds stress anisotropy has more or less effect on the phase distribution but inconsiderable effect on the pressure, axial and secondary flow velocities.

Effects of Ekman Layer on Flows and Temperature of Viscoelastic Fluid in a Rotating Frame

An experimental analysis of the hydrodynamic flow of a fluid and the temperature distribution in a porous medium is presented. The experiment has been carried out using viscoelastic fluid in a rotating channel, bounded by two impermeable infinite plates at a constant temperature, under the action of a uniform pressure gradient in the direction of the flow. The primary and secondary flow velocities, the temperature distribution and heat transfer in terms of the Nusselt number of the fluid are studied in the model due to the effects of the Ekman layer on the fluid for various values of the viscoelastic parameter (Ve) in the channel height h=-1 to +1.