Optical velocimetry

The reasons for flow instabilities, such as those causing melt fracture, are the subject of scientific research and theories. In a recent study, optical velocimetry was used to measure flow kinetics. Morphological changes on the surface (sharkskin formation) were monitored by high speed video microscopy. It was confirmed in this study that the velocity of the material stream, after it has just left the exit, determines melt fiacture. Increase in throughput rate increases velocity... 

Adrian, R. J. 1986. Image shifting technique to resolve directional ambiguity in double pulsed velocimetry. Applied Optics 25 3855. 

Campbell and Hanratty (1982) used Lau s (1980) measurements with some special optics on a laser Doppler velocimetry system to calculate /3(f) near a fixed interface, in this case, the inside of a clear pipe. They determined w(z,t) from equation (8.52), and solved equations (8.49) and (8.50) numerically for / l(0- Finally, they applied equation (8.51) to determine Kl, which has been the goal all along. The end results (Kl) may then be related to the other, independent parameters that are important to the transfer process, such as diffusivity, viscosity, and turbulence parameters. Campbell and Hanratty performed this operation and found the following correlation ... 

Examples are Laser Differential Microanemometry (LMA) and Total Reflection Microscopy (TMA) (8). Both LMA and TMA measure the velocity profile of the fluid in tube flow. However, such optical techniques are generally not suitable for opaque and/or heterogeneous substances such as foods. Acoustic velocimetry seems to be more promising for determining the velocity profiles of opaque substances. Such an acoustic technique has been applied by Brunn et al (19) as an on-line viscometer for flow of mayonnaises in pipes. 

Laser Doppler velocimetry is a powerful technique for the in situ measurement of fluid velocities. The basic optical configuration for the measurement is shown in Figure 6.1. The velocity measurement is made at the intersection of two laser beams that are focused to a point in the flow. The use of laser radiation is essential since the light must be monochromatic and coherent. This is required since the intersection of the two beams must create an interference pattern within the fluid. Such a pattern is shown in Figure 6.2, where two plane waves intersect at an angle 2(J). The two waves will have the following form [55] ... 

Figure 5. Current overall optical layout for laser velocimetry and Raman scattering diagnostics, shown here on new fan-induced square-cross-section movable combustion tunnel. Note the co-linear Raman and LV probe laser source axes and the colinear detection optics.

Unambiguous determination of the conditions under which slippage occurs requires a technique able to measure the velocity of the fluid in the immediate vicinity of the solid wall over a thickness comparable to the size of a polymer chain, i.e. a few tens of nanometers. Classical laser Doppler velocimetry does not meet this requirement even if it allows for the determination of velocity profiles which clearly reveal a non-zero velocity within typically a few 10 pm from the wall. We have developed a new optical technique. Near Field Velocimetry (N.F.V.) [14], which combines Evanescent Wave Induced Fluorescence (E.WF.) [27] and Fringe Pattern Fluorescence Recovery After Photobleaching (F.P.F.R.A.P.) [28]. The former technique gives the spatial resolution normal to the solid wall, while the latter one enables the determination of the local velocity of the fluid. A major constraint of the technique is that it needs polymer molecules labelled with an easily photobleachable fluorescent probe. 

Local measurements by using laser velocimetry, marking the flowing polymer or by optical techniques [12]. 

Most colloids are polydisperse in terms of their particle size and charge and thus a single sample exhibits a distribution of mobilities. In principle the above mentioned techniques, in particular optical transients, quasielastic velocimetry in the frequency regime, crossbeam and rotating grating velocimetries, can measure the mobility distribution of a polydisperse colloid. 

Agrawal, Y., Quadrature demodulation in laser Doppler velocimetry. Applied Optics, 23, 1685-1686 (1984)... 

Adrian, R.J., ed. Selected Paper on Laser Doppler Velocimetry. SPIE Vol. MS 78, SPIE Optical Engineering Press, 617 pp (1993). 

Barnhart, D.H., Adrian, R.J., and Papen, G.C., Phase-conjugated holographic system for high-resolution particle image velocimetry, Applied Optics, Vol. 33, No. 30, 7159-7170(1994). 

An attractive feature of fiber sensors is the possibility of performing in vivo tests and monitoring. Numerous fiber-optic sensors have already been described that measure physical parameters of the human body [41]. Pressure, temperature, physiological flow, strain, motion, displacement, or flow velocity can be monitored by optical methods such as variable reflection, laser Doppler velocimetry, optical holography, or diffraction. In this section the application of optosensing methods to the determination of molecular species encountered in clinical and biomedical analysis is described. 

Kertzscher, U., Seeger, A., Affeld, K., Goubergrits, L., and Wellnhofer, E. (2004), X-ray based particle tracking velocimetry - A measurement technique for multi-phase flows and flows without optical access, Flow Measurement and Instrumentation, 15(4) 199-206. 

The stainless steel high-pressure vessel used for the flow measurements, designed by ITTB Heerenveen (The Netherlands), is presented in Fig. 3.2. The vessel has an internal diameter of 6 cm and is designed for pressures up to 15 MPa. The temperature is controlled by pumping water from a thermostatic bath through channels in the vessel wall. Pitched-blade impellers with a diameter of half the vessel diameter are used to stir the vessel content. Two glass windows allow for measurement of velocity components in three directions with laser-Doppler velocimetry. The LDV equipment consists of a 2D fiber optics system... 

Optical-Sensing-Based Flow Sensors Backscatter interferometry, optical coherent tomography, and particle-imaging velocimetry are some of the optical techniques for flow sensing. The backscatter interferometric flow-sensing system [3] may be crmsideied to be in this category. This system is based on synchronous... 

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