Doppler technique

The Doppler technique measures the frequency shift of scattered waves with respect to incident sound waves. The technique, therefore, requires the presence of scatterers in the flow that is being monitored. The scatterers could be turbulent eddies or vortex shedding for liquid single-phase flows, and solid particles for solid/fluid mixed-phase flows. The basic geometry of a Doppler 

FI GURE 5-7 Schematic representation of a basic Doppler flowmeter and a 

Consider two successive peaks of the sound wave, which are separated by the time interval of one period t (equal to the reciprocal of the frequency /, i.e., t = Iff). During the time interval (t), the distance between the particle and 

FIGURE 5-8 Block diagram of basic cross-correlation flowmeter. 

The term C/cosd, in Eq. 5.14 can be determined from the sensing geometiy. According to Snell s law, the cosine of the angle 6, of the sound beam in the 

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The laser-Doppler anemometer measures local fluid velocity from the change in frequency of radiation, between a stationary source and a receiver, due to scattering by particles along the wave path. A laser is commonly used as the source of incident illumination. The measurements are essentially independent of local temperature and pressure. This technique can be used in many different flow systems with transparent fluids containing particles whose velocity is actually measured. For a brief review or the laser-Doppler technique see Goldstein, Appl. Mech. Rev., 27, 753-760 (1974). For additional details see Durst, MeUing, and Whitelaw, Principles and Practice of Laser-Doppler Anemometry, Academic, New York, 1976. 

L. E. Drain. The Laser Doppler Technique. London John Wiley Sons, 1988. 

There have been recent studies on the importance of NO in modulating skin blood flow in both normal animals and in inflammatory models. Khan etiU. (1993), using laser-Doppler techniques, showed that the NOS inhibitor L-NAME inhibited rabbit ear blood flow. It was possible to do this chronically for up to 2 weeks using implanted osmotic pumps. Pons et id. (1993) also used laser Doppler to show that the vasodilator eflFect of LPS in rabbit skin, which mimics the efiect of Gram-negative bacteria, was likely to involve both i-NOS and IL-1. We have already discussed the damaging eflPects of neutrophils... 

Methods for analysis of the particle size distribution in the aerosol cloud include techniques such as time of flight measurement (TOE), inertial impaction and laser diffraction. Dynamic light scattering (photon correlation spectroscopy) is confined to particles (in suspension) in the submicron range. In addition to the size distribution, the particle velocity distribution can be measured with the Phase Doppler technique. 

There are many modifications of the basic Doppler technique resulting in systems such as DORAN, EXTRADOP, TRIDOP, UDOP others (Ref 5)... 

Earlier studies (ref. 440-442) with ordinary air microbubbles (without any synthetic surfactant coating) have already shown that echocardiographic contrast produced by microbubbles is useful in the qualitative analysis of blood flow and valvular regurgitation. In addition, quantitative studies (ref. 440) have shown a correlation between individual contrast trajectories on M-mode echocardiography and invasive velocity measurements in human beings. Meltzer et al. (ref. 441) have shown that velocities derived from the slopes of contrast trajectories seen on M-mode echocardiography correlate with simultaneous velocities obtained by Doppler techniques. (This correlation is expected because both measures represent the same projection of the microbubble velocity vector, that is, in the direction of the sound beam.) More detailed studies (ref. 442) confirmed that microbubble velocity obtained from either Doppler echocardiography or M-mode contrast trajectory slope analysis correlates well with actual (Doppler-measured) red blood cell velocity. Thus, these early studies have shown that microbubbles travel with intracardiac velocities similar to those of red blood cells. 

Naqui et. al. [153] describe a phase Doppler technique, for measuring particle velocity and statistcal information about particle size of irregular particles, based on a phase shift signal. The technique works on near back-scatter which leads to a robust set-up under conditions of limited optical access. Preliminary measurements in a crystallizer were presented and good agreement with TSI Aerodynamic particle sizer was found. 

The use of lasers for diagnostic purposes is well established. It includes various techniques like cytofhiorometry 26>88) (e.g. cell sorting and counting), Doppler techniques (e.g. measurement of blood flow), various kinds of spectroscopy, and holography 86). 

Drain, L. E. The Laser Doppler Technique Wiley Chichester, England, 1980 pp 85-118. 

The application of optical sensors to measure the velocity of solids can be realized by incorporating Doppler, cross-correlation, or spatial-filtering methods (Yan, 1996). The Doppler technique measures the frequency shift of the signals reflected by the solids particles. Based on the Doppler-shift principle, the difference frequency Af gives a direct measure of solids velocity Vs via the relationship... 

For ensuring that only in-focus particles are considered for size measurement, the upper and lower threshold levels have to be set appropriately Extensive studies on the required threshold level have been performed by Hardalupas et al. (1994). With appropriate settings the sizing error can be reduced to about 10%. The shadow-Doppler technique is presently being developed further. Recently, first attempts to measure the particle mass flux were also made (Maeda et al. 1996 a)). 

Bauckhage, K., The phase-Doppler-difference-method, a new laser-Doppler technique for simultaneous size and velocity measuremets. Part. Part. Syst. Charact., 5, 16-22 (1988)... 

Hardalupas, Y. and Taylor, A.M.K.P., Phase validation criteria of size measurements for the phase Doppler technique. Exper. in Fluids, 17, 253-258 (1994)... 

Roisman, I. V., and Tropea, C. "Flux Measurements in Sprays Using Phase Doppler Techniques." Atomization and Sprays 11, no. 6 (2001) 667-99. 

The TOE under Wiley-McLaren conditions and the Doppler techniques are similar in that the initial velocity produces a linear response in the measured quantity (T — T and Vobs o> respectively). Likewise the energy analysis of product ions produces a signal... 

Flg.1. Measurements during the cultivation of recombinant E. coli K-12 MF cells in a 60-1 working volume air lift tower loop reactor without gene expression using SE9 antifoam agent (AFA) [50,51]. a Variation in the mean bubble velocity. Bubble velocity measured in situ by an ultrasound Doppler technique. AFA added to the medium, b Variation in the specific interfacial area (m ) measured in situ by an ultrasound technique... 

Shadowgraphs, Mie-scattering, and phase Doppler techniques used to study fuel flux and droplet size distributions. 1 [Pg.681]

A more recent development is based on combination of Doppler techniques with polarisation techniques For a particular frequency of a Doppler broadened line, only species with a well defined velocity component along the direction of the probe laser are excited. The excitation efficiency of the molecules flying in a particular direction depends again on the direction of the absorption dipole relative to the direction of the electric vector of the probe laser. The resulting shape of the Doppler broadened line depends in this case on the relative direction of the fragment velocity to the rotation plane of the probed fragment. An analysis of such experiments yields information about the direction of the vectors for the product angular momentum relative to the velocity vector ("vector correlation ). 

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