Effects of time scales on velocity bias in LDA measurements using sample and hold processing

Abstract

Sample and hold processors may be used to avoid velocity bias errors in laser Doppler anemometer (LDA) measurements if the ratio of flow to measurement time scale is sufficiently high, typically greater than 5. It has been widely assumed that the flow time scale refers to the Taylor time microscale. This paper shows with experimental verification that the appropriate flow time scale is, in fact, the integral time scale of the flow. Furthermore, it shows that if velocity bias exists with a sample and hold processor, it can in many cases be predicted. It has been shown elsewhere that the integral time scale is also the relevant one for the prediction of velocity bias of a controlled processor.