Acoustic Doppler effect

This aspect is not included here, but is related to optical flow diagnostics. It is based again on the principle of the optical Doppler effect. Multifunctional equipment is available for noncontact measurements of flow-induced vibration on surfaces of structural elements, for acoustic measurements, and for calibration of accelerometers and vibration transducers. 

The classic Doppler effect is the shift produced by the relative motion between transmitter and receiver. In Doppler flowmeters, the transmitting and receiving transducers are fixed with respect to each other, and the relative motion is produced by the particles or bubbles carried in the stream. The particles act as intermediate receivers and retransmitters. Commercially available acoustic... 

Brillouin scattering occurs as a result of an interaction between the propagating optical signal and thermally acoustic waves present in the silica fibre giving rise to frequency-shifted components, similar to a Doppler effect. The acoustic velocity is directly related to the medium density and depends on both temperature and strain. As a result, the so-called Brillouin frequency shift carries information about the local temperature and strain of the fibre. Furthermore, Briflouin-based sensing techniques rely on the measurement of a frequency as opposed to Raman-based techniques that are intensity based. 

Figure 2.21 An acoustic Doppler effect V is the mutual speed of source and detector, w, and cUd are emitted and measured frequencies, d and s are detector and source.

Doppler Flow Meters. Doppler flow meters sense the shift in apparent frequency of an ultrasonic beam as it is reflected from air bubbles or other acoustically reflective particles that ate moving in a Hquid flow. It is essential for operation that at least some particles ate present, but the concentration can be low and the particles as small as ca 40 p.m. CaUbration tends to be influenced by particle concentration because higher concentrations result in mote reflections taking place neat the wall, in the low velocity portion of the flow profile. One method used to minimize this effect is to have separate transmitting and receiving transducers focused to receive reflections from an intercept zone neat the center of the pipe. 

To some extent, progress has been limited by the availability of measurements on exchange processes. Until recently, temperature microstructure measurements were the primary approach to quantify near-surface turbulence. The instruments needed to do this were expensive and difficult to operate. The situation is now considerably improved. Microstructure sensors more suited to field use are commercially available, and are more user-friendly . Alternative methods to observe or infer mixing processes have also been perfected, including free-fall CTDs, acoustic doppler sensors and acoustically monitored floats. As these techniques are refined and deployment, operation, and analysis become more routine, it will become increasingly practical to incorporate a mixing component into field studies of UVR effects. 

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