THE INFLUENCE OF PHASE-AVERAGING WINDOW SIZE ON THE DETERMINATION OF TURBULENCE QUANTITIES IN UNSTEADY TURBULENT FLOWS

The phase-averaging window size is shown to affect the measurement of phase-averaged turbulence quantities in unsteady turbulent flows. The flow turbulence is usually estimated on the assumption of quasi-consta nt how velocity during the duration of the phase-averaging window. The calculated turbulence level then consists of two parts: one due to th e turbulent velocity fluctuations and the other due to the changes in the mean flow velocity. This second part is shown to be directly propo rtional to the averaging window size. In order to determine the true t urbulence the averaging window size has to be made as small as possibl e, especially if the unsteady how exhibits large temporal gradients an d the flow turbulence itself is small.