Flow visualization techniques

A review of flow visualization techniques suitable for use in hospital laboratories was carried out several years ago. 

E. F. Matthys 1988, (Measurement of velocity for polymeric fluids by a photo-chromic flow visualization technique the tubeless siphon), J. Rheol. 32 (8), 773-788. 

Early studies of the transition to turbulence relied on flow visualization techniques for liquid flow through arrays of spheres. Jolls and Hanratty (1966) found a transition from steady to unsteady flow in the range 110<7 e< 150 for flow in a dumped bed of spheres at N — 12, and they observed a vigorous eddying motion that they took to indicate turbulence at Re — 300. In regular beds of spheres, Wegner et al. (1971) found completely steady flow with nine regions of reverse flow on the surface of the sphere for Re — 82, and similar flow elements but with different sizes in an unsteady flow at Re — 200. Dybbs and Edwards (1984) used laser anemometry and flow visualization to study flow... 

Performance of a stack of cells is limited by performance of the weakest cell in stack. It is therefore important to achieve high uniformity in performance of the individual cells in the stack, through stack design, mass production techniques, quality control, and automated stack assembly process. Optimum flow field design may be obtained by careful Computational Fluid Mechanic techniques and experimental validation including flow visualization techniques. 

Withey, R.J. Bowker, A.J. Agitation of the solvent in tablet dissolution studies—a flow visualization technique. J. Pharm. Pharmacol. 1972, 24, 345-351. 

Af John Zink Company s COOLflozv Physical Modeling Facilify, a differenf flow visualization fech-nique has been successfully applied to scale model sfud-ies. In fhis technique, small quanfifies of minufe helium bubbles are suspended in fhe airflow and used as fracer spheres. The helium bubbles are neufrally buoyanf wifh a nominal densify close to fhaf of fhe air af ambienf femperafures. Figure 10.4 shows fhe helium bubble flow visualization technique applied to a scale model. 

A comprehensive listing of various tracers and experimental methods is provided by Wen and Fan [2], including flow visualization techniques. 

Flow visualization is a branch of fluid mechanics that provides visual perception of the dynamic behavior of fluids flows. The fundamental principle of any flow visualization technique lies in the detection of fluid transport by altering the fluid properties while leaving the fluid motion unaltered. Microscale flow visualization focuses on imaging microfluidic flows, with the most common techniques broadly classified into particle-based and scalar-based methods. 

Particle-based flow visualization techniques introduce marker particles into the flow field to study the motion of the bulk by determining the velocity vectors associated with these particles. Laser Doppler velocimetiy (LDV), optical Doppler tomography (ODT), and particle image velocimetiy (PIV) are the different methods that have been developed in this regard. However, of these methods, microscale PIV (p-PlV) is the most well-developed and popular microscale flow visualization technique. 

As mentioned earlier, most microscale flow visualization techniques have focused on determination of flow velocity in liquids (or velocimetry). Fluorescence-based techniques, e.g., LIF and PF, have been extensively used for this application. However, in this section, we will look at another microfluidic application for which the development of microscale flow visualization techniques have been critical-micromixing. 

Smits AJ, Lim TT (2000) Flow visualization techniques and examples. Imperial College Press, London... 

Several variations of microflow visualization have been developed for microfluidic applications such as particle-based flow velocimetry and scalar-based flow velocimetry [5]. In terms of the zeta potential measurement, these visualization techniques such as micro-PlV are used to measure the velocity profile and flow rate under electroosmotic flow. Once the velocity is known, the zeta potential can be calculated from Eq. 6. The main advantage of using a flow visualization technique is that the electroosmotic velocity can be measured directly and in real time. In general, the small amount of particles or dye used has a negligible effect on the electroosmotic flow being measured. The oifly significant disadvantage of this technique is that the extent and cost of the hardware may be prohibitive. 

Natural rubber tends to cling to rotors, and most synthetic rubbers are torn into chunks. The addition of carbon black and oil to an internal mixer has been studied by flow visualization techniques for a wide range of elastomers. The tearing of elastomers into crumbs disrupts the mixing of carbon black, which proceeds more surely with natural rubber. The rate of oil absorption depends on available surface area of the elastomer and proceeds more rapidly with torn rubber crumbs and slowly with natural rubber. 

White [23,24] observed the circulatory flow behavior using markers with flow visualization technique in the R-R handed screw and double-flighted rotors mixer (Figure 8). This makes possible measurement of the circulation time as a criteria of mixing. Since the flow visualization experiments have limitations in observing the overall flow motion in the mixer, the following experiment was carried out to determine the distribution of elastomers. 

Gramann, P.J., Noriega, M.P., Rios, A.C., Osswald, T.A., Understanding a Rhomboid Distributive Mixing Head Using Computer Modeling and Flow Visualization Techniques, SPE Technical Conference, Toronto (1997)... 

The intensive mixers mentioned above are illustrated in Figure 9.3, together with others proposed in the literature in general they have been well characterized in terms of internal hydrodynamics and mixing performances, either by flow visualization techniques or using chemical methods, by mixing sensitive competitive reaction schemes computational fluid dynamics also supplied much information. 

G. Kychakoff, R.D. Howe, R.K. Hanson Quantitative flow visualization technique for measurements in combustion gases. Appl. Opt. 23, 704 (1984)... 

G. Kychakoff, R.D. Howe, R.K. Hanson, J.C. McDaniel Quantitative visualization of combustion species in a plane. Appl. Opt. 21, 3225 (1982) G. Kychakoff, R.D. Howe, R.K. Hanson Quantitative flow visualization technique for measurements in combustion gases. Appl. Opt. 23, 704 (1984)... 

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