LDV - Laser Doppler Velocimetry

Laser Doppler Velocimetery (LDV) is a technique used to measure the instantaneous velocity of a flow field. This technique, like PIV is non-intrusive and can measure all the three velocity components. The laser Doppler velocimeter sends a monochromatic laser beam toward the target and collects the reflected radiation. According to the Doppler effect, the change in wavelength of the reflected radiation is a function of the targeted object's relative velocity. Thus, the velocity of the object can be obtained by measuring the change in wavelength of the reflected laser light, which is done by forming an interference fringe pattern (i.e. superimpose the original and reflected signals).This is the basis for LDV. A flow is seeded with small, neutrally buoyant particles that scatter light. The particles are illuminated by a known frequency of laser light. The scattered light is detected by a photomultiplie tube (PMT), an instrument that generates a current in proportion to absorbed photon energy, and then amplifies that current. The difference between the incident and scattered light frequencies is called the Doppler shift. By analyzing the Doppler-equivalent frequency of the laser light scattered (intensity modulations within the crossed-beam probe volume) by the seeded particles within the flow, the local velocity of the fluid can be determined.

Laser Doppler Optical System

Basic one-component LDV Equipments:

Laser system (Continuous-Wave-CW, single colour for single channel), transmission optics (e.g. Bragg cell, lenses, beam expanders, beam splitter, mirrors, prisms, fibre cable link with laser beam manipulator), receiving optics (e.g. lenses, pinhole, interference filter, photomultiplier), signal processor units (e.g.. fringe-counting, spectral analysis, photon-correlation), traversing mechanism (manual or automated) for transmitting and receiving optics, oscilloscope, seeding generation (solid or liquid vapour) and computer (large capacity hard disk) with a data acquisition board and data handling software. The more compact and easy to handle type of LDV system has fiber transmission and receiving optics.

Pros :
** Non-contacting measurement.
** Very high frequency response.

Cons:

** Sufficient transparency is required between the laser source, the target surface, and the photodetector (receiver).
** Accuracy is highly dependent on alignment of emitted and reflected beams.
** Expensive; fortunately, prices have dropped as commercial lasers have matured.