Transducers of Finite Size in Turbulent Fluctuations

Abstract

We now begin a detailed examination of one of the basic uncertainties in the measured turbulent fluctuation characteristics, namely, that due to the finite size of the sensitive part of the transducer. We recall that most of the methods available for measuring the statistical parameters of particular physical variables in turbulent flows involve as a necessary step the transformation of these variables into an electrical signal. It is precisely this component of the measuring channel, i.e., the converter, that may substantially distort the measured variable. The reason for this lies in the mismatch between the transducer and the object under examination. The point is that the fluctuation field of any turbulent variable contains a set of spatial inhomogenities, the linear dimensions of which lie in a very broad range. This also applies to time intervals characterizing the variation of the turbulent variable at each point in space occupied by the flow. Measurements are free from distortion when the sensitive part of the transducer is infinitesimal and the transducer itself can react instantaneously to changes in the measured variable.