Near-wall measurements by means of the Lloyd and the Doppler effect

Bearbeiter:Dipl.-Phys. Christoph Skupsch


An interference pattern can be formed by only one light source and a mirror. The Figure 1. Lloyd Lloyd interference is due to the superposition of light from an original source and light reflected by a plane mirror, see figure 1. The fringe spacing contains the information of the distance between source and mirror. Light scattering flow tracers near reflective walls also are expected to form Lloyd interference.

For extended sources like laser diodes the working principle can be shown easily, see figure 2. Laser diodes exhibit a scattering characteristic similar to that of tracer particle,Figure 2. Fringe spacing d_Lloyd vs. the distance between mirror and laser diode. but at higher light intensity.

The aim of the work is the development of a measurement technique allowing simultaneous measurement of the velocity of a particle by a common Doppler technique and the distance to the wall by Lloyd interference. The measurement principle was proposed firstly at the 15th International Symposium on Applications of Laser Techniques to Fluid Mechanics in Lisbon, Portugal (shortened presentation).