SOME PRELIMINARY-RESULTS ON THE INFLUENCE OF RIBLETS ON THE STRUCTURE OF A TURBULENT BOUNDARY-LAYER

A comparison of hot-wire measurements over flat and ribletted walls is described. Details of the experimental apparatus and procedures are given, together with some preliminary results. The test is validated by the calculation of the parameters of the boundary layer. by the comparison of the usual profiles of the streamwise component of the velocity with the available experimental data, and by a conditional analysis of the instantaneous signal Mean and rms profiles for the streamwise component of the velocity evidence, for the riblet plate, an upward shift in law-of-the-wall plots and an overall, small reduction in fluctuation intensities across the boundary layer. Skewness and flatness factors appear to be unchanged. Time series of velocity samples are analyzed by means of power spectra, correlations, and conditional techniques. In particular, the VITA technique, used as an ejection detector, gives results in agreement with previous studies for the flat-plate case, and suggests a moderate increase of the ejection frequency over the riblet surface. Accordingly, the time/space scales of the events appear to be shorter for the riblet-plate flow.