Short-time measurement techniques

 

The lecture includes some flow visualization techniques as well as quantitative measuring methods. All of these are suitable to detect short time events. Compared to the human reaction time of about 0.1 s many gasdynamic processes take place in even shorter times, for example in some hypersonic wind tunnels the useful measuring time only amounts to a few milliseconds. The duration of special atomic excitation states is in the order of 10^-12 to 10^-15 s.

The first chapter deals with optical methods which for short exposure times are also suitable to visualize processes of high frequency or of short duration. The fundamentals of shadow, black and white as well as colour schlieren methods are described. Furthermore, Mach-Zehnder and Michelson interferometry are presented. It is also shown how these methods can be used for quantitative measurements of the density field.

For heat flux measurements around wind tunnel models very often thin film gauges and special coaxial thermocouples are used. Typical for these gauges is a very short response time in the range of microseconds. The heat conduction theory based on the semi-infinite principle is given in detail. The data reduction is performed according to this theory.

For pressure measurements piezoelectric or piezoresistive transducers are used. Design, working principle and special features are demonstrated for simplified as well as commercial transducers.