Stable Flow Pattern Research Articles

Flow Structures of a Liquid Film Falling on Horizontal Tubes

Patterns of a liquid film falling across a vertical array of horizontal tubes change from droplet mode at low flow rates to liquid sheet at high flow rates. Between these limits, liquid columns form as a further stable flow pattern. The transition from one flow mode to another occurs via unstable structures consisting simultaneously of droplets and columns or of merging columns. The boundaries of the flow modes can be obtained from relationships expressing the flow rate as a function of physical properties, that is, the Reynolds number as a function of the Kapitza number. Correlations for the pattern boundaries recommended in the literature are compared with each other and found to be in acceptable agreement for practical purposes.

Dynamic control of extracellular environment in in vitro neural recording systems

A technique is presented for rapid fabrication of microfluidic channels on top of multichannel in vitro neural recording electrode arrays. The channels allow dynamic control of both stable and transient flow patterns over localized areas of the array, over biologically relevant timescales. A cellular model consisting of thermally sensitive dorsal root ganglion neurons was integrated into the devices. The device was used to demonstrate precise control of the extracellular microenvironment of individual cells on the array. Since the methods presented here are not specific to a particular cell type or neural recording system, the technique is amenable to a wide range of applications within the neuroscience field.

Investigation of Strouhal frequencies of two staggered bluff bodies and detection of multistable flow by wavelets

The results of a wind tunnel investigation of the Strouhal frequencies of two identical, stationary, parallel circular cylinders arranged in staggered configurations, carried out in a uniform cross-flow at a Reynolds number of 5.5×10 4, are presented in this paper. Results of measurements of the Strouhal frequencies of a square cylinder (prism) and a circular cylinder arranged in tandem and in some selected staggered configurations are also presented. In the case of two circular cylinders, the investigation was performed at staggered angles of α = 10 ° , 25°, 45°, 60° and 75° in the range of T / D = 0.1 – 5.0 , where α is the angle between the free-stream flow and the line connecting the centers of the cylinders, T is the gap width between the cylinders, and D is the diameter of a cylinder. The new findings in this study for two circular cylinders are: (i) three stable flow patterns with regard to Strouhal numbers were identified for α = 25 ° in the range of T / D = 1.1 – 1.8 and (ii) two stable flow patterns with regard to Strouhal numbers were identified for α = 45 ° in the range of T / D = 0.8 – 2.1 . Intermittent mutual lock-in of the two frequencies of two cylinders caused such multistable flow patterns. These multistable flow patterns and the intermittent lock-in phenomenon were elucidated from wavelet analysis results of fluctuating pressures simultaneously stored from the surfaces of the cylinders. Strouhal number distributions for α = 60 ° and 75° were almost the same in nature as those of α = 45 ° . When a square cylinder and a circular cylinder were arranged at α = 25 ° with the square cylinder being used as the upstream cylinder, the downstream cylinder was found to shed vortices always in synchronization with the upstream cylinder. For α = 60 ° , when the square cylinder was used as the upstream cylinder, both cylinders were found to be locked-in to shed vortices with an intermediate Strouhal number for a certain range of values of T/D.

F222 水平二重円管における自然対流 : 3次元螺旋流の熱伝達と流動特性(一般セッション 熱伝達)

Experiments of natural convection for a horizontal cylindrical annulus of the inverse relative gap width Di/L=6.7 were carried out with water in the range of Rayleigh number from 8.23×10^2 to 2.16×10^5, in which the unsteady flow pattern is characterized by the 3-D spiral flow. In order to clarify the heat transfer characteristics, flow patterns were visualized by adding the tracer particles of nylon to a flowing fluid. A stable crescent-shaped flow pattern is observed at low values of the Rayleigh number, but the flow does not develop completely into the upper portion of the annulus. As the Rayleigh number is increased, counter-rotating cells are appeared in the longitudinal direction and the flow pattern becomes a 3-D spiral flow. Then the flow reverts to the stable crescent-shaped pattern developed fully in the upper portion of the annulus at about 3×10^3 of the Rayleigh number. Heat transfer characteristics can be explained well from the flow patterns revealed by the flow visualization.

Stability of the crossflow pattern around a slender and influence of disturbance

Topological structure and stability of a slender cross flow is discussed by the stability theory of dynamic system. The inner boundary of flow field was limiting streamline and it was proved that the topological structure connected saddles by limiting streamline is stable. It is proved that the development of slender vortices leads to the change of topological structure about cross flow. And it is the change from stable and symmetrical vortices flow pattern to unstable and symmetrical vortices flow pattern, and then to stable and asymmetrical vortices flow pattern due to little disturbance which leads to the development of asymmetrical slender vortices. The influence of disturbance to flowfield structure was discussed by unfolding theory too.

Stability analysis of the flow in a cubical cavity heated from below

A numerical study of bifurcations and stability of the steady convective flow of air in a cubical enclosure heated from below was carried out using a Galerkin spectral method. The set of basis functions was chosen so that all boundary conditions and the continuity equation were implicitly satisfied. A parameter continuation method was applied to determine the steady solutions and bifurcations of the nonlinear governing equations as a function of Rayleigh number (Ra) for values of Ra up to 1.5×105. The eigenvalue problem associated with the stability analysis of the steady solutions along the different branches of solutions was solved using the Arnoldi method. The convergence of the method was consistent with the number of modes used and the results were also verified by a numerical solution of the unsteady equations of motion using a finite-difference solver. Present results show that different stable convective flow patterns can coexist for different ranges of the Rayleigh number.

A Study on Flow Around Two Circular Cylinders in Staggered Arrangements (2nd Report, Estimation of Strouhal Number Using in Combination with Wavelet Analysis)

Characteristics of vortex shedding of two identical circular cylinders in staggered configurations were investigated experimentally at a Reynolds number of 5.5×104. Measurements were performed for stagger angles α=10°, 25°, 45°, 60° and 75°, and gap spacing T/D=0.1-5.0, where a is the angle between the free-stream flow and the line connecting the centers of the cylinders, T is the gap width between the cylinders, and D is the diameter of the cylinders. The estimation of vortex shedding frequencies was done by using in combination with wavelet analysis results of fluctuating pressures on surface of two cylinders. The new-dings for vortex shedding frequencies of two cylinders as follows. (i) For α=10°, two Strouhal numbers appears for the range of T/D=0.1-2.1, in which two flow patterns switch from one to the other. (ii) For α=25°, in the range of T/D<1.1, the two cylinders behave like a single bluff body at least with regard to vortex shedding frequency. Also, for the range of T/D=1.1-1.8, two stable flow patterns with regard to vortex shedding frequency persist, (iii) For α=45°, two stable flow patterns with regard to Strouhal numbers persist for T/D =0.8-2.1. (iv) Strouhal number distibutions for α=60° and 75° are almost the same in nature as those for α=45°.

Reversible switching of high-speed air-liquid two-phase flows using electrowetting-assisted flow-pattern change.

This work is the first demonstration of electrical modulation of surface energy to reversibly switch dynamic high-speed gas-liquid two-phase microfluidic flow patterns. Manipulation of dynamic two-phase systems with continuous high-speed flows is complex and interesting due to the multiple types of forces that need to be considered. Here, distinct stable flow patterns are formed through a multipronged approach: both surface tension forces generated by surface chemistry modulation as well as viscous and inertial forces produced by fluid flows are employed. The novel fluidic actuation mechanism provides insights into better understanding microscale two-phase flow dynamics and offers new opportunities for the development of two-phase biochemical microsystems that are mechanically simple and operational at high speeds.

Aerodynamic characteristics of two side-by-side circular cylinders and application of wavelet analysis on the switching phenomenon

The aerodynamic characteristics of two stationary cylinders, both circular and square, in a side-by-side arrangement were investigated experimentally in a uniform flow at a Reynolds number of 5.5×10 4, although the square cylinder results are rather limited. This Reynolds number is within the range in which fluid forces acting on a single cylinder are comparatively insensitive to change in the Reynolds number. The focus of this study was on the determination of the characteristics of steady and fluctuating fluid forces, wake frequencies and switching phenomena in two side-by-side cylinders. For a spacing ratio of T/ D<1.2 ( T, gap spacing between cylinders; D, diameter), the gap flow was biased to one side, resulting in the formation of a narrower wake behind one cylinder and a wider wake behind the other. Steady and fluctuating fluid forces acting on the cylinders were decomposed for the narrower wake and the wider wake flow patterns. For T/ D>0.20, the action of lift forces on both cylinders was in an outward direction (repulsive); however, for T/ D=0.10, the action of lift force on the cylinder associated with the narrower wake was inward and that on the other cylinder was outward. In the bistable flow regime, T/ D=0.2–1.2, when the gap flow switched spontaneously from one side to the other, another short duration stable flow pattern (which can be termed an intermediate flow pattern) persisted in the intermediate time, in which the gap flow was oriented parallel to the free-stream flow and the Strouhal number was almost equal to that of a single cylinder. Flow visualization images and results of modal analysis and wavelet analysis of velocity signals obtained from two hot-wires corroborated the appearance of the intermediate flow pattern. In the case of synchronized vortex shedding and even in the intermediate flow pattern, a predominant antiphase synchronized vortex was found in both the case of two side-by-side circular cylinders and the case of two side-by-side square cylinders.

Observations of hysteresis in flow around two square cylinders in a tandem arrangement

This paper reports an experimental study on the flow characteristics around two square cylinders in a tandem arrangement. The spacing between the centers of the cylinders ranges from 1.5 to 9 widths of the cylinder and the Reynolds number ranges from 2.0×10 3 to 1.6×10 4. Hysteresis with two discontinuous jumps is present for all the Reynolds numbers studied when the spacing is varied in two different ways, one being a progressive increase and the other a progressive decrease. The hysteresis is associated with two different flow patterns, and the discontinuity is attributed to a sudden change of the flow pattern. In the hysteresis regime, only one flow pattern is stable, differing from the bistable phenomenon that involves intermittent switching between two stable flow patterns as reported for higher Reynolds numbers by previous authors. It is found that the hysteresis regime shifts rapidly towards smaller values of spacing as the Reynolds number is increased in the lower range Re=2.0×10 3–5.3×10 3, and then remains nearly unchanged between Re=5.3×10 3–1.6×10 4. The time history and spectra of velocity as well as the flow visualization corresponding to the change of flow pattern in the hysteresis regime are presented and discussed in detail. The effects caused by hysteresis and Reynolds number on drag forces, mean and fluctuating pressures, and Strouhal numbers of the cylinders are examined.

Cyclic alternating pattern and positive airway pressure titration

Objective: To demonstrate that stability of the upper airway during continuous positive airway pressure (CPAP) titration is influenced by the microstructure of sleep as defined by the cyclic alternating pattern (CAP). Methods: Retrospective review of 12 CPAP titration records. The patterns of flow-limitation during CPAP at subtherapeutic pressures were characterized as ‘stable’ (persistent and non-progressive inspiratory flow limitation) or ‘unstable’ (progressive increase in inspiratory flow-limitation terminating in an arousal), and continuous periods of at least 10 min were identified. Sleep stage scoring by both conventional Rechtshaffen and Kales criteria and the CAP were done. The relationship between flow type and CAP was determined. Responses to an increase in applied pressure on flow-limitation were noted. Results: There were a total of 50 periods fulfilling the above criteria, totaling 1113 min of titration time. Thirty periods (757 min, 68% of total) showed a stable flow-limitation pattern. A total of 29/30 periods showing a stable flow pattern during sleep was scored as non-CAP, and only a single 18-min period of stable flow was scored as CAP. A total of 19/20 periods showing an unstable flow pattern was in sleep with CAP characteristics, the exception being a single 14-min period where unstable flow was noted in non-CAP. Flow-limitation was stable and non-progressive or absent during non-CAP, even at less than optimal pressures. This was noted irrespective of the presence or absence of delta sleep as scored by conventional criteria. Pressure increases during non-CAP, when the profile of the inspiratory flow was flattened, never resulted in a discernable change in the flow profile, while at least two-thirds of pressure increments during CAP periods improved flow. Conclusions: The microstructure of sleep as determined by CAP and non-CAP have practical implications for manual pressure titration algorithms and research on upper airway physiology during sleep. The appearance of a period of non-CAP, irrespective of conventionally scored delta sleep, may falsely suggest that the CPAP is optimal or close to it. Large increases in non-CAP that may be seen during a titration night can reduce the window of opportunity for titration. Increases in CPAP should be avoided in non-CAP.

Scale-up of mixer granulators for effective liquid distribution

There is considerable anecdotal evidence from industry that poor wetting and liquid distribution can lead to broad granule size distributions in mixer granulators. Current scale-up scenarios lead to poor liquid distribution and a wider product size distribution. There are two issues to consider when scaling up: the size and nature of the spray zone and the powder flow patterns as a function of granulator scale. Short, nucleation-only experiments in a 25L PMA Fielder mixer using lactose powder with water and HPC solutions demonstrated the existence of different nucleation regimes depending on the spray flux Ψ a—from drop-controlled nucleation to caking. In the drop-controlled regime at low Ψ a values, each drop forms a single nucleus and the nuclei distribution is controlled by the spray droplet size distribution. As Ψ a increases, the distribution broadens rapidly as the droplets overlap and coalesce in the spray zone. The results are in excellent agreement with previous experiments and confirm that for drop-controlled nucleation, Ψ a should be less than 0.1. Granulator flow studies showed that there are two powder flow regimes—bumping and roping. The powder flow goes through a transition from bumping to roping as impeller speed is increased. The roping regime gives good bed turn over and stable flow patterns. This regime is recommended for good liquid distribution and nucleation. Powder surface velocities as a function of impeller speed were measured using high-speed video equipment and MetaMorph image analysis software. Powder surface velocities were 0.2 to 1 ms −1—an order of magnitude lower than the impeller tip speed. Assuming geometrically similar granulators, impeller speed should be set to maintain constant Froude number during scale-up rather than constant tip speed to ensure operation in the roping regime.

Optimising miller cuff dimensions: the influence of geometry on anastomotic flow patterns.

since cuff dimensions are variable, we studied the influence of cuff geometry on flow mechanics, in an attempt to identify the optimum configuration. bench studies involved the manufacture of anatomically accurate models of varying cuff dimensions, perfused in a specifically designed flow rig, simulating physiological conditions. Flow visualisation studies incorporating laser illumination of tracer particles enabled accurate analysis of flow patterns. the vortex created within the proximal cuff of each model during the deceleration phase of the cardiac cycle was strongly influenced by the aspect ratio (AR=cuff height:length). The standard and high cuffs (AR=1.63 and 1.18, respectively) demonstrated cohesive vortices and stable flow patterns. Low and long cuffs (AR=2.6 and 2.25, respectively) created more complex vortices with large areas of flow separation and low velocities. aspect ratio has an important influence on flow within the distal anastomosis, with cuff dimensions of 13 mm long and 8-11 mm high (standard and high cuffs) creating beneficial flow patterns anticipated to optimise wall shear stress and inhibit myointimal hyperplasia.

SE—Structures and Environment: AirFlows and Temperature Patterns induced in a Confined Greenhouse

Two-dimensional airflows and temperature distributions, corresponding to a typical single-cell Rayleigh–Benard convection pattern, were characterized in a reduced scale, mono-span, greenhouse module. The module has floor heating; it is kept airtight and the walls are insulated. It is designed to simulate free convection, generated above a plant canopy heated by solar radiation, in a real multi-span greenhouse.The temperatures were measured with thin thermocouples placed on a static grid; airflow lines were determined by a visualization technique, laser tomography, and by velocity measurements by laser Doppler anemometry. Stationary, weakly turbulent, regimes were observed for Rayleigh numbers ranging from 0·8×1010to 2·3×1010. The only stable flow pattern, which breaks the symmetry of the greenhouse, is a large convective cell which sweeps the heated floor, rises along one insulated side, cools down along the roof and descends along the opposite side. Temperature and velocity gradients are confined to borders and a large static and homogeneous air core stays at the centre without participating in heat transfer. Due to confinement, actual air velocities inside the convective cell are much larger than those that would be obtained vertically from a simple heated plate. Apparent heat exchange coefficients from the floor to the air and from the air to the roof were deduced from this experiment and compared with other values found in the literature for full-scale greenhouses.

Experimental study of the critical flashing flow through a relief line: evidence of the double-choked flow phenomenon

New experimental results are presented for steady-state critical steam–water flows through a horizontal relief line involving an abrupt cross-sectional area change. In the stagnation conditions, the pressure is fixed to 6 bars and the temperature is varied in the range of 110–150°C. Two geometrical singularities are tested: the sudden enlargement and the circular orifice. For both geometrical configurations, the occurrence of the so-called double-choked flow phenomenon is demonstrated on the basis of experiments performed by varying the back-pressure. For a small enough value of the back-pressure, two simultaneous stable locations of critical sections are highlighted: the first one is located close to the singularity and the other is located at the outlet of the relief line. The maximum possible value of the mass flowrate is limited only by the first critical section. The occurrence of the second critical section does not affect the mass flowrate any more. The interpretations of the associated transfer phenomena (the locations of critical sections, the critical mass flowrate and pressure ratio) are supported by the visualisation of three stable flashing flow patterns close to the singularity: the confined free jet, expanded jet and submerged jet patterns.

An Experimental Study on the Kinetics of Fluidized Bed Iron Ore Reduction.

To optimize existing iron ore reduction processes or to develop new ones, it is necessary to know the reduction kinetics of the iron ore of interest under the relevant operating conditions. In this work the reduction kinetics of hematite fine iron ore was studied for industrial-scale processes using the fluidized bed technology. Especially designed batch tests were performed in a laboratory-scale fluidized bed reactor fluidized with H2, H2O, CO, CO2, N2 at atmospheric and elevated pressures to simulate the relevant process conditions. To obtain the reduction rates and the degree of reduction, the concentrations of H2O, CO, and CO2 in the outlet gas were analyzed by FT-IR spectroscopy. Preliminary reduction tests showed a strong effect of the sample weight on the reduction rates, especially in the early stages of reduction. The optimum sample weight was determined by partly replacing the hematite with silica sand. Additionally, the silica sand provided a constant and stable flow pattern throughout the reduction tests. The effects of temperature, gas composition, particle size and pressure on the rates of reduction were tested and discussed. Rate analysis showed the existence of two phases with different rates during the reduction tests. Additional investigations (microscope analysis, SEM) demonstrated that in the first phase the rates were controlled by mass transport in the gas phase and in the second phase by the reduction process within the small grains of the iron ore particles.

Effect of Surface Corrugation on Convection in a Three-Dimensional Finite Box of Fluid Saturated Porous Material

The problem of finite amplitude thermal convection in a three-dimensional finite box of fluid saturated porous material is investigated, when the lower boundary of the fluid is corrugated. The nonlinear problem of three-dimensional convection in the box for the values of the Rayleigh number close to the classical critical value and for small values of the amplitude of the corrugations is solved by a perturbation technique. The preferred mode of convection is determined by stability analysis. In the absence of corrugation three-dimensional modes of convection can be either stable or unstable depending on the values of the aspect ratios of the box, while two-dimensional rolls are always stable, provided that the box aspect ratios allow the existence of such modes of convection. In the presence of boundary corrugation with the appropriate form, different three-dimensional or two-dimensional modes of corrugation can be stable or unstable. For a rough boundary with local roughness sites, the location, size, and number of the roughness elements plus the wave numbers of the convection modes and the box aspect ratios can all play a role leading to either stable or unstable particular three- or two-dimensional flow patterns. For a wavy boundary, resonant wave-vector excitation can lead to the preference of stable two- or three-dimensional flow patterns whose wave vectors are in a subset of those due to the wavy boundary, while nonresonant wave-vector excitation can lead to the preference of stable flow patterns whose wave vectors are not generally in a subset of those due to the wavy boundary. Heat transported by convection can either be enhanced or be reduced by certain proper forms of the corrugations and by appropriate values of the box aspect ratios. Due to the surface corrugation highly subcritical modes of convection are stable, while highly supercritical modes of convection are unstable.

Optimized performance of the Abiomed BVS 5000: adjustment of the pump height based on Doppler control of the flow pattern.

The Abiomed BVS 5000 is an automatic volume-driven paracorporeal pulsatile assist device providing left, right or biventricular support. The paracorporeal position allows optical adjustment of filling volumes of the device, which determines the output of the system. A procedure to adjust for maximal stable flow has not yet been established. In vitro measurements have been performed to assess the flow and pressure characteristics of the Abiomed BVS 5000 by raising the preload in 5 mmHg steps before running the system. Doppler probes were placed at the inflow and outflow lines of the pump. After setting the afterload at 80 mmHg the assist device was started. Two measurements were performed to find optimal flow (based on Doppler control and optical adjustment). (1) By Doppler control a stable flow pattern was found at a preload of 25 mmHg with a mean atrial pressure of 5 mmHg and a mean flow of 5.3 +/- 0.7 l/min (mean +/- standard deviation) at the inflow and outflow sites (the console flow was 4.8 +/- 0.4 l/min with a frequency of 61.8 +/- 2.0 l/min). (2) Optical adjustment of the pump height gave rise to a preload of 35 mmHg where we recorded a maximal atrial pressure of 107 +/- 5.8 mmHg, a maximal retrograde flow of -4.3 +/- 1.2 l/min at the inflow and -1.2 +/- 0.4 l/min at the outflow site. The mean flow at the inflow and outflow sites was 5.1 +/- 0.5 l/min (the console flow was 4.6 +/- 0.3 l/min with a frequency of 59.6 +/- 2.6 Hz). At an initial afterload of 60 and 40 mmHg the system showed the same qualitative behaviour, but the results were less accurate. Optical adjustment of the pump height may result in an atrioventricular valve insufficiency with undetected retrograde flow and high atrial pressures. We conclude that a Doppler flow probe must be placed at the inflow site to guarantee maximal stable flow.

3‐D hydrodynamics generated in a stirred vessel by a multiple‐propeller system

AbstractThe aim of this study is to optimize the position and the number of propellers in a non‐standard tall vessel. Laser sheet flow visualization experiments were carried out for selected geometrical arrangements which produced stable flow patterns and good transport between the propellers. Four double‐propeller arrangements corresponding to frequent industrial cases and a three‐propeller system have been chosen. Comparison of LDA measurements in the r‐z plane, dimensionless global parameters NQp, Ntm, Np, Ep and spatial distribution of local energy dissipation rate ϵ shows that a three‐propeller system is the most efficient.

Extension of the Wall-Driven Enclosure Flow Problem to Toroidally Shaped Geometries of Square Cross-Section

The two-dimensional wall-driven flow in an enclosure has been a numerical paradigm of long-standing interest and value to the fluid mechanics community. In this paradigm the enclosure is infinitely long in the x-coordinate direction and of square cross-section (d × d) in the y-z plane. Fluid motion is induced in all y-z planes by a wall (here the top wall) sliding normal to the x-coordinate direction. This classical numerical paradigm can be extended by taking a length L of the geometry in the x-coordinate direction and joining the resulting end faces at x = 0 and x = L to form a toroid of square cross-section (d × d) and radius of curvature Rc. In the curved geometry, axisymmetric fluid motion (now in the r-z planes) is induced by sliding the top flat wall of the toroid with an imposed radial velocity, ulid, generally directed from the convex wall towards the concave wall of the toroid. Numerical calculations of this flow configuration are performed for values of the Reynolds number (Re = ulidd/ν) equal to 2400, 3200, and 4000 and for values of the curvature ratio (δ = d/Rc) ranging from 5.0 · 10−6 to 1.0. For δ ≤ 0.05 the steady two-dimensional flow pattern typical of the classical (straight) enclosure is faithfully reproduced. This consists of a large primary vortex occupying most of the enclosure and three much smaller secondary eddies located in the two lower corners and the upper upstream (convex wall) corner of the enclosure. As δ increases for a fixed value of Re, a critical value, δcr, is found above which the primary center vortex spontaneously migrates to and concentrates in the upper downstream (concave wall) corner. While the sense of rotation originally present in this vortex is preserved, that of the slower moving fluid below it and now occupying the bulk of the enclosure cross-section is reversed. The relation marking the transition between these two stable steady flow patterns is predicted to be δcr1/4 = 3.58 Re-1/5 (δ ± 0.005).