ABSTRACT
Turbulent flow systems are not like laminar flow systems, and their characteristics cannot be predicted by using generalized mathematical models. In most cases, turbulent characteristics have to be determined experimentally, which is a costly exercise. Turbulence measurements can be made using probes (wire, hot-wire, conductivity, or pressure probes) or optical methods (laser-Doppler, marker nephelometry, or optical). In these experimental techniques, the measured turbulent variable (i.e., velocity, concentration, pressure, or temperature) is detected in the form of an output signal r(t) of an instrument (e.g., voltage output). This signal is processed to get information about the turbulence nature of the flow. A low-pass filter is always recommended to filter out the high frequency component of the signal. In filtering the signal, the cutoff frequency must be selected high enough to cover all components of the turbulence spectrum (i.e., 4 kHz could sufficiently high; however, a power spectrum analysis should be performed to estimate the upper limit of the turbulence frequency). Marking the turbulent flow field with a marker is a common practice for the detection of the turbulent field. If particles are used for marking the flow field, these particles should be small enough to follow the smallest turbulent eddy (Becker. 1977).
