Photochromic Tracer Research Articles

0129 上昇する気泡の運動と後流渦構造(OS1 噴流,後流および剥離流れ現象の探求と技術革新,オーガナイズドセッション)

Motion and wake vortices of a rising bubble of Re =246 in a still liquid in a vertical pipe are studied experimentally using photochromic tracer and high-speed cinematography. Three velocity components of bubble measured by video image processing show that zig-zag motion of the bubble is dominated by lateral velocities. And the shedding of vortex loops behind a bubble correlates well with lateral motion and orientation of the bubble. A stereo display system was constructed by commercial stereo display and used to examine the complex three-dimensional vortices and dye patters in the bubble wake.

Tracer diffusion properties of core–shell latex films studied by photoinduced grating relaxation

AbstractThis article reports the application of the Photo‐Induced Grating Relaxation technique (also known as Forced Rayleigh Scattering) to investigate the dynamics of films prepared from structured core–shell latex particles via the transport property of the photochromic tracer molecule Aberchrome 540®. The core–shell particles were prepared with a fluoropolymer core (immiscible and impenetrable to the tracer) and a poly(butyl methacrylate) shell. The incompletely dried films (with residual water) manifest their spatial heterogeneity via non‐Fickian behavior (spatial scale‐ dependent apparent diffusion coefficient). The diffusion data was interpreted using the two‐state diffusion model, previously developed to describe the tracer diffusion in latex films without any core–shell structure. In contrast to dry latex films made from homogeneous particles, where one observes Fickian diffusion indicative of a homogeneous polymer film, we find that the lattice of fluoropolymer cores leads to a length scale dependent diffusion coefficient for the tracer. This effect can be interpreted as microscopic evidence for a strain hardening effect due to the presence of a hardened layer of matrix polymer (= shell) surrounding the core, which act as nanofillers. This strain hardening effect could be quantified within the two‐state diffusion model in terms of tracer diffusion coefficients and root mean squared displacements. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2823–2834, 2007

Experimental Study on Local Mass Transfer in a Simplified Bifurcation Model: Potential Role in Atherosclerosis

Local mass transfer coefficients and flow patterns were examined in an idealized human aortic bifurcation model. The objectives of this study are to gain further insights on the convective mass transfer process and its possible role in the localization of atherosclerotic lesions. The laser photochromic tracer method provided velocity and wall shear stress estimates in the plane of symmetry of a UV-transparent Plexiglas bifurcation model. Steady flow data were acquired at Reynolds numbers of 500, 600, and 750. A novel copper electrodeposition technique was used to obtain time-averaged convective local mass transfer coefficients in a model identical to that used in the flow experiments. The laminar flow mass transfer data for the trunk of the bifurcation are in good agreement with the analytical Levesque solution. At the bifurcation, higher mass transfer coefficients along the inner wall and lower ones along the outer wall were observed. Further, mass transfer and wall shear stress follow similar patterns both on the inner and outer walls in that StSc 2/3 and C(f)/2 demonstrate analogous behavior. Lower transfer rates of momentum and mass occurred along the outer wall of the branches where lesions tend to develop.

401 円管内を落下する球の軌跡と後流渦のフォトクロミック染料による可視化

401 円管内を落下する球の軌跡と後流渦のフォトクロミック染料による可視化

Flow characteristics and circulatory motion in wavy falling films with and without counter-current gas flow

A laser-induced photochromic tracer technique was used to investigate the flow structure in a thin, wavy falling liquid film in a vertical tube both with and without interfacial shear induced by a counter-current flow of gas. Instantaneous velocity profile and film thickness were measured simultaneously to better understand the turbulence characteristics and the enhancement effect of interfacial waves and shear on the heat and mass transfer rates. While surface ripples were found to exert almost no influence on the near-wall hydrodynamics, the large wave effects could easily be sensed close to the solid wall, altering the turbulence intensity profiles. Flow visualization experiments revealed the occurrence of circulatory motions with significant velocities normal to the tube wall under the large waves, which may be the dominant mechanism for wall-to-liquid and interfacial transport phenomena for wavy films.

Application of active contours for photochromic tracer flow extraction

This paper addresses the implementation of image processing and computer vision techniques to automate tracer flow extraction in images obtained by the photochromic dye technique. This task is important in modeled arterial blood flow studies. Currently, it is performed via manual application of B-spline curve fitting. However, this is a tedious and error-prone procedure and its results are nonreproducible. In the proposed approach, active contours, snakes, are employed in a new curve-fitting method for tracer flow extraction in photochromic images. An algorithm implementing snakes is introduced to automate extraction. Utilizing correlation matching, the algorithm quickly locates and localizes all flow traces in the images. The feasibility of the method for tracer flow extraction is demonstrated. Moreover, results regarding the automation algorithm are presented showing its accuracy and effectiveness. The proposed approach for tracer flow extraction has potential for real-system application.

The turbulent boundary layer over single V-shaped grooves

The photochromic tracer technique was used to study the turbulent boundary layer over two transverse V-shaped grooves. The mean velocity profiles above the individual grooves were parallel to those seen in the outer regions for smooth surfaces, but displaced downwards. Also, the turbulence intensity just above the grooves was higher than the corresponding smooth surface values. These results suggest that enhancement of heat and mass transfer rates by transverse grooves is due to energetic bursts which effectively replace the liquid within the groove, with a contribution from an apparent increase in burst frequency.

A multi-stage nitrogen laser for the photochromic flow visualization technique

A multi-stage nitrogen laser was developed in an attempt to enhance the potential of the photochromic tracer technique for complex flow analysis. With this system, either single or multiple low-divergence ultraviolet beams can be produced. The resulting photochromic traces are of comparable length but are substantially finer than either the tracers produced by a commercial nitrogen laser or those produced by an excimer laser. A significant advantage of these finer traces is that they were not smeared by high shear rates, as is seen with the traces produced by the other types of laser, and this will lead to more accurate analysis of complex flows.

Hemodynamic assessments with the photochromic tracer technique

Hemodynamic assessments with the photochromic tracer technique

Wall shear stress temporal gradient and anastomotic intimal hyperplasia.

Unusual wall shear stress patterns on the bed of end-to-side arterial anastomoses appear to be the primary factor in the development of intimal hyperplasia that often leads to arterial bypass graft failure. In this in vitro study using the photochromic tracer technique, the shear stress variation on the bed of a 30 degrees anastomosis was examined before and after the development of hyperplasia. With the disease-free model, a rapid downstream shift in the stagnation point was seen on the bed during the systolic phase of the flow cycle, which led to sharp temporal changes in the shear stress from positive to negative values, ie, rapid changes in the direction of the shearing force. The resulting negative temporal gradients were roughly eight times larger than those seen in a straight tube, and it is suggested that this may lead to endothelial deformation or injury and eventually to intimal thickening. With the diseased model, the simulated tissue overgrowth on the bed appeared to act as a flow divider that restricted the motion of the stagnation point, and this drastically reduced the rapid changes in the direction of the shearing forces. Thus, it seems that the development of bed anastomotic intimal hyperplasia may be a response designed to reduce shear-induced endothelial deformation or injury.

Visualization of transitional pipe flow using the photochromic tracer method

Two noninvasive measurement techniques are described for investigating transitional flow in a rigid straight tube. Photochromic trace recordings, representing fluid displacement profiles, were made within the core and at the leading and trailing edges of turbulent slugs generated by a disturbance at the tube inlet. Consistent photochromic trace recordings were made possible through the discovery that Doppler ultrasound can be used to detect the passage of turbulent slugs, thereby allowing precise timing of the photochromic traces. Upstream and downstream Doppler probes enabled the leading and trailing edge velocities of slugs to be measured and also enabled the laminar/turbulent transition regions to be investigated in detail. Trailing edge interface velocities were in excellent agreement with existing data, while leading edge velocities agreed less well. Turbulence generation at the trailing edge appeared to be associated with a localized vortical structure which originated in the near wall region and then grew toward the tube centerline. The generation of this vortex is postulated to be induced by a shear layer instability. Mean velocity profiles obtained from within the core of turbulent slugs were similar to those obtained for fully developed turbulent flow.

Spatial and temporal variations of wall shear stress within an end-to-side arterial anastomosis model.

Wall shear stress has been strongly implicated in the initiation of fibrous intimal hyperplasia that leads to arterial bypass graft failure. In this study, the photochromic tracer technique was used to determine the instantaneous value of the wall shear stress in order to provide a detailed description of its spatial and temporal variations within a 45 degrees end-to-side anastomosis model. At the heel and on the bed across from the heel, the shear stress was close to zero throughout the sinusoidal flow cycle. Flow separation was produced just beyond the toe where the peak phase-averaged shear stress was 6 dyn cm-2, whilst on the bed across from the toe it was 27 dyn cm-2. The stagnation point was seen to fluctuate quite sharply on the bed over a distance of about one tube radius upstream from the site across from the toe. This led to large spatial gradients of the wall shear stress with a peak value of 580 dyn cm-3. In addition, the sudden motion of the stagnation point around peak flow produced sharp temporal gradients of the wall shear stress with a peak absolute value of 3400 dyn cm-2 s-1. When compared to the sites where intimal hyperplasia tends to occur, a strong correlation is seen with low wall shear stress at the heel and toe, and with the sharp temporal variations of the magnitude and spatial gradient of the wall shear stress on the bed across from the junction.

Evidence that turbulence is not the cause of poststenotic dilatation in rabbit carotid arteries.

The purpose of this study was to relate local blood flow conditions to the development of poststenotic dilatation (PSD) in the common carotid arteries of rabbits. We investigated the effects of the geometry of the stenoses on PSD formation to gain insight into the mechanisms of this phenomenon. With short stenoses, the maximum diameter of the PSD after 3 weeks increased from about 25% to 50% above the proximal diameter by increasing the severity of the stenoses from 50% to 60% diameter reduction. By contrast, increasing the length of the constricted region of 60% stenoses did not affect the PSD, and in all cases, the site of maximum dilatation occurred within 3 tube diameters from the stenoses. In vitro studies were conducted with the photochromic tracer technique. The most striking observation was that the transition to turbulence did not occur with a 55% short stenosis. By increasing the severity of this short stenosis to 70%, the transition to very localized turbulence was triggered approximately 6 to 8 tube diameters from the stenosis during the early deceleration phase of the flow cycle. The transition point moved farther downstream by increasing the length of this moderate stenosis. This study demonstrated that PSD can occur under turbulence-free conditions, and even when turbulence was generated, the site of the PSD was remote from that of the localized turbulence zone. The wall shear stress pattern was determined for the long 60% stenosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Hemodynamics of a side-to-end proximal arterial anastomosis model

Purpose: The purpose of this article was to analyze the fluid mechanical effects of a side-to-end proximal anastomosis and to compare the results with those from our earlier study on the end-to-side distal anastomosis.Methods: The photochromic tracer technique was used to determine the instantaneous wall shear stress and to visualize the overall flow field under pulsatile flow conditions. The flow consisted of a sinusoid plus a steady component with mean and modulation Reynolds numbers of 355 and 565, respectively, and a Womersley number of 7.9.Results: At the toe and heel of the junction, very high and positive wall shear stresses were seen together with substantial nonperiodic fluctuations. The peak wall shear stress was about four times higher at the toe and about seven times higher at the heel than the maximum values observed at about four tube diameters upstream from the junction. On the bed of the host vessel, nonperiodic fluctuations were also observed, but the shear stresses were mainly negative with magnitudes comparable to those seen upstream. With leakages of 11% and 28% of the mean flow through the blocked end of the host vessel, the shear stress pattern seemed to be significantly affected only at the toe for the higher leakage. Further, when the mean Reynolds number was reduced to 320, the magnitudes of the variations in the wall shear stress were reduced proportionately, except at the heel, where the reduction was much larger than expected.Conclusions: It appears that the preferential development of intimal hyperplasia at the distal end-to-side anastomosis may be promoted by low wall shear stress at the toe and heel, and probably by high shear stresses or shear stress gradients on the bed.

Translational and rotational diffusion in supercooled orthoterphenyl close to the glass transition

Self diffusion coefficients in supercooled orthoterphenyl (OTP) have been determined down toD t =3·10−14 m2s−1 using a1H-NMR technique applying static field gradients up to 53T m−1 In a range of more than two decades theD t values agree with those of photochromic tracer molecules of the same size determined by forced Rayleigh scattering down to the glass transition temperatureT g . A change of mechanism is found for translational diffusion atT c ≈1.2T g whereD t is proportional to the inverse shear viscosityη −1 atT>T c butD t ∼η ξ with ξ=0.75 atT<T c . Rotational correlation times determined by2H-NMR stimulated echo techniques in deuterated OTP remain proportinal toη −1 down toT g . Our results are discussed in relation with mode coupling theory and with models of cooperative motion at the glass transition.

Steady and pulsatile flow fields in an end-to-side arterial anastomosis model

We investigated the flow field within a rigid-walled in vitro model of an end-to-side 45 degree anastomosis in an attempt to identify possible hemodynamic factors that may contribute to the pathogenesis of distal anastomotic intimal hyperplasia. A high-resolution photochromic tracer technique was used to visualize the flow in orthogonal planes and to determine the axial wall shear stress profiles for both steady and pulsatile flows over a range of physiologically relevant conditions. The flow field showed qualitative similarities to those seen in curved vessel: rapidly moving fluid from the graft section affects the bed of the host vessel, that is, the wall opposite the anastomosis, eventually advancing down the host vessel in a spiraling motion. A small mobile separation zone was noted at the toe of the anastomosis. Comparison of wall shear stress profiles with previously reported preferential sites for the development of intimal hyperplasia supported a low wall shear stress and/or flow separation pathogenesis hypothesis. One notable exception was the bed of the host artery that appeared to be subjected to a complex hemodynamic environment.

Pulsatile flow through constricted tubes: an experimental investigation using photochromic tracer methods

A photochromic tracer method has been used to record pulsatile flow velocity profiles simultaneously at three axial locations along a flow channel. Two major advantages of this multiple-trace method are that it enables velocity data to be acquired in an efficient non-invasive manner and that it provides a detailed description of the spatial relationship of the flow field. The latter is found to be particularly useful in the investigation of transitional type flows; for example, in describing coherent flow structures. Studies of the flow patterns in tubes with mild to moderate degrees of vessel constriction were performed using a 2.9 Hz sinusoidal flow superimposed on a steady flow (frequency parameter of 7.5; mean and modulation Reynolds numbers of 575 and 360, respectively). With mild constrictions (&lt; 50% area reduction), isolated regions of vortical and helical structures were observed primarily during the deceleration phase of the flow cycle and in the vicinity of the reattachment point. As expected, these effects were accentuated when the constriction was asymmetric. For moderate constrictions (50%–80%), transition to turbulence was triggered just before peak flow through the breakdown of waves and streamwise vortices that were shed in the high-shear layer. During this vortex generation phase of the flow cycle, the wall shear stress fluctuated quite intensely, especially in the vicinity of the reattachment point, and its instantaneous value increased by at least a factor of eight. Such detailed descriptions of the transition to turbulence and of the spatial and temporal variation of the wall shear stress, particularly near the reattachment point, have not been previously reported for pulsatile flow through constricted tubes. The observed wall shear stress variations support a proposal by Mao &amp; Hanratty (1986) of an interaction of the imposed flow oscillation with the turbulent fluctuations within the viscous sublayer.

A tracer system based on a photochromic dye and on fibre optics for measuring axial dispersion of organic liquids in pilot-scale packed columns

The usefulness of 6-nitro BIPS as a photochromic tracer in organic liquids has been investigated. The photochromic effect of this dye permits the solution to be decoloured conveniently after each tracer experiment so that the same liquid may be used in a closed loop for a large number of runs, which is essential for large scale experiments. The photochromic behaviour of the dye and the stability of its colourless form was studied in a range of solvents. Of all liquids tested, a number of alcohols including methanol, ethanol and their mixtures with glycerol proved to be fully compatible with the desired photochromic features. 6-Nitro BIPS was successfully used to study the axial dispersion of ethanol-glycerol mixtures of a wide range of viscosities in a 0.3 m diameter packed column. Tracer concentrations were monitored colorimetrically at various locations in the packing. The absorbance measurements of the liquid took place in several small flow-through cells inserted into the packing, which were optically connected to a light source and to photomultipliers by fibre optics.

Surface phenomena and mass transfer interactions in liquid‐liquid systems

AbstractThe surface dynamics of pendant aqueous droplets forming at a submerged nozzle in an immiscible organic solvent have been studied using a novel laser‐activated photochromic tracer technique. This enabled a small element of surface to be labeled so that its subsequent history could be recorded photographically.Results for aqueous droplets forming in toluene in the absence of any mass transfer indicated only a slow interfacial circulation from the front to the rear, and there was no indication of any surface renewal as a result of hydrodynamic flow even at nozzle Reynolds numbers as high as 300. By contrast, the introduction of any one of a number of organic solutes into the toluene phase led to sporadic Marangoni‐type interfacial disturbances, which in turn resulted in rapid replacement of elements of the droplet surface. It is proposed that renewal of the droplet surface is dependent on solute diffusion in the present instance and that the processes of surface renewal and mass transfer are likely to be interactive.

Wake shedding and circulatory flow in bubble and droplet-type contactors

A review of published work on continuous phase circulation in spray columns, and on attached wake volumes for single droplets, revealed an apparent paradox in that reported values of the former were an order of magnitude greater than the latter when expressed as a ratio of the dispersed phase flow. These results were confirmed in experiments using a photochromic tracer to reveal the entrainment and circulatory flow. Theoretical considerations show that droplets translate an amount of continuous phase relative to their own volume of 1.5C D / d per unit distance traversed. Droplets carry with them an attached wake (“cap”) which for Re d < about 200 is effectively permanent, the translated continuous phase passing around droplet and “cap” leaving a wake trail. At Re d > 200 a fraction j of the translated continuous phase accumulates in the “cap”, which becomes unstable and sheds a fraction s av as toroids at regular intervals of height z s . Estimated values are given for j, s av , and z s . This result is extended to the case of a continuous droplet flow, leading to an expression for the rate of increase in circulatory flow of continuous phase in the central core with height above the droplet distributor. This flow is balanced by an increasing momentum loss to the wall until a steady state is reached. The implications of this result on the performance of spray columns is discussed.