In many practical flow situations the complexity of the flow and the environmental conditions demand the use of non-invasive optical flow measurement techniques. Accurate, quantitative measurements are needed to obtain important flow details, vital data for design modifications, as well as validate and improve computational models. In many practical applications, qualitative visualization approaches provide general understanding of the flow in a large region of the flow.
This paper will describe the use of qualitative and quantitative approaches to study flow problems. Flow measurements are carried out using laser based systems. For qualitative studies a laser light sheet probe is used. The probe head is very small allowing it to be placed in flow geometry's where access is limited. The system is also made flexible by the use of optical fibers so that the light sheet could be delivered at the desired location. Using such a system, the global nature of the flow field was examined, for example, in the engine compartment of an automobile.
In automotive as well as other practical flow measurement applications past efforts have mostly focused on qualitative visualization processes. In many of the situations, simultaneous measurements at multiple locations in the flow were desired. These measurements have generally been hindered by the size, complexity and the associated operational costs of the instrumentation involved.
A compact system using fiberoptics is used to obtain accurate non-invasive measurements of the flow field. The use of fiber-optics provides a Laser Doppler Velocimeter (LDV) system that allows the measuring head to be small and the laser and other electronics system to be kept away from the actual flow measurement region. Such systems are used to measure velocity fields in both gas and liquid flow. These systems measure the velocity field at one point in the flow. The ability to multiplex the measurements (signals) from various locations has also been developed. Such a system can provide continuous real time flow measurements at the desired probe locations. Finally, a LDV system using diode lasers was used to provide stand alone compact measuring heads that could be multiplexed to measure flow velocities at multiple locations. The signal processing and data analysis packages were also selected with the aim of minimum set up time and user interaction for the proper operation of the system. It is hoped that these approaches will pave the way for automating and quantifying the flow measurement process.