Fluid Dynamics
Fig. 1: Laser Doppler system with diode-pumped solid-state laser (back) and fiber optic probe (front) for the measurement of flow speeds
Laser measuring processes have become established in many application fields. However, measuring the velocity of fluids is a challenging task in fluid dynamics. The laser Doppler technique makes an easy and precise speed measurement with high time and spatial resolution possible. Application fields for this technique are e.g. the investigation of combustion, blood flow, or air flow around aircraft wings. At the LZH, new laser Doppler systems are being developed, tested, and applied. Figure 1 shows precise flow investigations on a test nozzle using a diode-pumped solid-state laser with a Gaussian mode power of over 10 W for a fiber-assisted laser Doppler probe.
Fig. 2: Microsystem technology: Combination of a double-core fiber (below) and a diffractive microoptic (above) as key elements for the implementation of miniaturized laser Doppler sensors; the rainbow is caused by diffraction effects
Currently, research work on the miniaturization of the laser Doppler technique is required, in order to extend its application spectrum, e.g. for integration into endoscopes, catheters, or wind tunnel models. The LZH is working on the simplification and miniaturization of fiberassisted laser Doppler sensors down to dimensions in the millimeter range, using glass fibers, diffractive optics, and further MOEMS (Micro-optical-electrical-mechanical-systems). Figure 2 shows the key components for miniaturization.
An extension of the measuring process to the registration of variations in velocity has been carried out using a time and spatially resolved determination of the Doppler frequency, using a digital signal processor (DSP). The measurement of spatial speed gradients gives sound information on laminar and turbulent shear flows.
