Conditional averages associated with the fine structure in a turbulent boundary layer

Abstract

The periods of strong activity in the high-frequency components of velocity and temperature fluctuations in a turbulent boundary layer are detected using digital data acquisition and processing by a method similar to that of Rao et al. [J. Fluid Mech. 48, 339 (1971)]. These periods are identified with the fine structure of the flow. Conditional averages of temperature and especially velocity associated with the fine structure differ sufficiently from those determined using the variable interval time averaging (VITA) technique to question whether these two techniques trigger on the same events. The characteristic frequency of occurrence, duration and ensemble averages of the velocity and temperature fields associated with the fine structure scale on outer variables when the momentum thickness Reynolds number exceeds a value of about 2000.