Probes for multivariant flow characteristics

Diagnostic methods in unsteady, multivariant flows can aim either at measuring one flow property regardless of variations in others, or at measuring each of two or more properties at the same time. This review primarily addresses Eulerian detectors in a continuum, chemically inert environment undergoing a stationary random process, and assumes that the detector is simultaneously affected by two or more flow variables. An outline of the basic theory is given which examines the quasi-steady detector response and its statistical moments in terms of the statistics of the flow variable fluctuations. This theory reveals the need for reinterpretation of classic detecting techniques and sensors in regimes of current technological interest involving nonlinear and high frequency phenomena. The current status of the most popular detectors is discussed and their potential in isolating the individual history of the flow velocity, density, pressure, temperature and concentration is evaluated.