Flow field, velocimetry

Fajardo, C.M. and V. Sick, Flow field assessment in a fired spray-guided spark-ignition direct-injection engine based on UV particle image velocimetry with sub crank angle resolution. Proceedings of the Combustion Institute, 31(2) 3023-3031, 2007. 

One characteristic of shear banded flow is the presence of fluctuations in the flow field. Such fluctuations also occur in some glassy colloidal materials at colloid volume fractions close to the glass transition. One such system is the soft gel formed by crowded monodisperse multiarm (122) star 1,4-polybutadienes in decane. Using NMR velocimetry Holmes et al. [23] found evidence for fluctuations in the flow behavior across the gap of a wide gap concentric cylindrical Couette device, in association with a degree of apparent slip at the inner wall. The timescale of these fluctuations appeared to be rapid (with respect to the measurement time per shear rate in the flow curve), in the order of tens to hundreds of milliseconds. As a result, the velocity distributions, measured at different points across the cell, exhibited bimodal behavior, as apparent in Figure 2.8.13. These workers interpreted their data... 

Time- and space-resolved fluctuation data for flame gas temperature and major species densities have been obtained from Raman scattering and from stronger inelastic scattering processes. When combined with information about velocity from laser velocimetry, these data and their correlations provide key new information for flow field and combustion field modeling. 

The near field velocimetry technique has been applied, up to now, to the investigation of the flow behaviour of high molecular weights polydimethylsiloxane (PDMS). The samples are mixtures of labelled (less than 5% by weigth) and unlabelled PDMS, with an index of refraction... 

Intrusive measurement techniques such as a Pitot static tube and hot-wire anemometer [24-26], and nonintrusive techniques such as laser Doppler velocimeter and particle image velocimetry (PIV) have been used to study the flow field. Goh, Kusadomi, and Gollahalli [13-15] mapped the velocity field in the flame using a Pitot static tube with a pressure transducer (Barocel). Details of the techniques and selection guidelines are presented in books on experimental aspects of fluid mechanics. Interested readers are referred to Holman [27], Goldstein [28], and Miller [29], to name a few. 

Detailed three-dimensional measurements of ICEO flows are now possible in microfluidic devices. Using particle-image velocimetry applied to thin optical slices, the ICEO flow field around a platinum cylinder has recently been reconstructed experimentally (Fig. 5b) and found to agree well with the theory, up to a scaling factor which could perhaps be attributable to compact-layer effects [6]. There has also been extensive experimental work on AC electro-osmotic flows in microfluidic devices, as discussed in a separate article. 

Wide Field Particle Sizing and Velocimetry is a new technique developed by Everest and Atreya [11] and Putorti et al. [12] to photographically determine the size and spatial distributions of particles in a large flow field. Here, laser induced fluorescence from a tracer dye in the droplets is imaged by a photographic camera at high resolution and analyzed to determine its size... 

By confining the fluidized bed in one direction and using a translucent waU, visual access is restored so that the bed behavior can be studied fuUy and non-intrusively using optical techniques, such as particle image velocimetry (PIV) or digital image analysis (DIA), which are discussed in detail below. With these techniques, it is possible to obtain information on the instantaneous flow fields, but it remains difficult to translate the 2D results quantitatively to 3D. As a learning tool that allows to see and verify different aspects of the bed behavior (e.g., bubble size distribution, instantaneous particle fluxes) however, such techniques are unrivaled. The main focus of this chapter therefore lies on these optical techniques. 

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 velocimetry (LDV), optical Doppler tomography (ODT) and particle image velocimetry (PIV) are the different methods that have been developed in this regard. However, of these methods microscale PIV ( x-PIV) is the most well-developed and popular microscale flow visualization technique. 

A promising device for measuring nanoscale and microscale channel flows is micro PIV, as mentioned in the previous Sect. How Characteristics in Microchannels. Santiego et al. [62] developed a micro PIV for measuring the instantaneous and mean liquid velocities around a circular cylinder of a diameter of 30 p,m placed in a microchannel. The diameter of the tracer particles ranged from 100 to 300 nm. Visualization of flow field in microchannels [63,64] and Brownian motion of a tracer particle [65] can also provide useful insight. Micro PIV is also used for observing the motion of blood cell in a capillary [66,67]. Micro hot-wire anemometer [68] and Doppler velocimetry [69] have also been recently developed for such studies in which bulk carbon nanotube were used as a wire. 

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