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 (< 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 & Hanratty (1986) of an interaction of the imposed flow oscillation with the turbulent fluctuations within the viscous sublayer.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112089001424

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

Recent developments in Molecular Tagging Velocimetry (MTV) using the phosphorescence of biacetyl are described for gas-phase flows. With improvements in tagging, detection, and processing schemes, whole-field measurements of two components of the velocity vector are obtained simultaneously, typically at more than 300 points over a plane. Application of this measurement approach is demonstrated in mapping the velocity and vorticity fields of the intake flow into a “steady flow rig” model of an internal combustion engine.

/pdf/molecular-tagging-velocimetry-mtv-measurements-in-gas-phase-g9wac1rx1b.pdf

Molecular Tagging Velocimetry (MTV ) measurements in gas phase flows

Visualization of a two‐dimensional velocity field by means of laser‐induced phosphorescence is demonstrated in a nitrogen flow at room temperature. A pulsed dye laser is used to excite seeded biacetyl molecules along a line in the flow. Two successive exposures of the emitted phosphorescence are recorded with an intensified 100×100 element photodiode array camera. Velocities are determined from the distance traveled in the time interval between exposures. Important factors in connection with the phosphorescence of biacetyl are discussed.

Velocity visualization in gas flows using laser-induced phosphorescence of biacetyl

/pdf/molecular-tagging-velocimetry-aex2n1b0kb.pdf

Molecular Tagging Velocimetry

Although many photochemical structures obtained in irradiated solutions have been investigated recently1–4, the origin of these patterns has remained unclear. Evidence has been presented which suggests that these structures are due to evaporative cooling. Using the Schlieren technique, photographs of prepatterns have been obtained before illumination and we show here that these prepatterns are associated with convective motions in the bulk. As a result, the hydrodynamic motions must be given proper consideration in the interpretation of spatio-temporal chemical patterns. The photochemical reaction also provides a useful way of visualizing slow convective motions.

/pdf/hydrodynamic-instabilities-and-photochemical-reactions-1loadele1g.pdf

Hydrodynamic instabilities and photochemical reactions

Poursuivant la realisation de methodes de caracterisation d'un fluide en ecoulement par diffusion Rayleigh forcee, nous avons mis au point une technique de marquage d'une figure bidimensionnelle quelconque dans le fluide par imagerie optique. L'impression de ce reseau et la visualisation directe de sa deformation permettent d'obtenir une mesure directe des composantes du gradient de vitesse.

https://hal.archives-ouvertes.fr/jpa-00244976/document

A. D'Arco

Papers

Nouvelle technique de marquage d'écoulement par utilisation de molécules photochromes