The Principle of Doppler Global Velocimetry

 

Schematic setup of DGV recording system

Doppler global velocimetry (DGV), also known as planar Doppler velocimetry (PDV), relies on the measurement of the frequency shift of light scattered by moving particles. DGV takes advantage of the fact that the frequency of the scattered light is shifted in frequency due to the Doppler effect:

Dn = n-n0

(1)

with n₀ the laser frequency, n the scattered light frequency frequency and Dn the Doppler shift. This shift depends on the particle velocity V, the incident light direction l and the observation direction o:

Dn = n0/c (o-l) V

(2)

The first basic idea of DGV is to measure the scattered light frequency n by transmitting the scattered light through an iodine vapour cell. Iodine has strong absorption lines which can be used as a frequency-to-transmission converter. The frequency n can be determined by measuring the iodine cell transmission of the scattered light which requires two detectors (cameras): one image viewed through the absorption cell (signal) and one reference image. From this image pair an intensity ratio can be computed which can be directly related to a frequency shift (and velocity) if the cell's transmission profile (shown below) is known.

The first basic idea of DGV is to measure the scattered light frequency n by transmitting the scattered light through an iodine vapour cell. Iodine has strong absorption lines which can be used as a frequency-to-transmission converter. The frequency n can be determined by measuring the iodine cell transmission of the scattered light which requires two detectors (cameras): one image viewed through the absorption cell (signal) and one reference image. From this image pair an intensity ratio can be computed which can be directly related to a frequency shift (and velocity) if the cell's transmission profile (shown below) is known.

With equation (2) one component of the velocity vector V can be calculated. The other velocity components can be measured by choosing different observation directions o. Instantaneous three-component DGV measurements therefore require three synchronized receivers (6 detectors). The alternative for integrating (time-averaging) DGV measurements is to introduce the light sheet from three different directions and keeping the camera position fixed, which limits the application of this approach to stationary flows (see also: DLR DGV system).

The reliable estimation of the Doppler shift Dn requires that the laser frequency n0 must be known and stabilized very precisely during the course of the measurement. With the incident light orthogonal to the observation direction the frequency - velocity dependence is on the order of Dn / Du= 2.7 MHz / m/s, which means that the laser should be stabilized at the +/- 1 MHz range.

Some highlights of DGV

• DGV provides velocity pictures
• Fast data acquistion and straight-forward image post-processing allows for quasi on-line flow measurement.
• There is no need to track individual particles, only the scattered light needs to be recorded. This implies that very small particles (soot, dust, etc.) can be used. Further, the optical access quality is not significant.