BEHAVIOR OF MEAN AND FLUCTUATING SKIN FRICTION IN A CONICAL DIFFUSER WITH INSTANTANEOUS FLOW REVERSALS

Measurements were made of the mean and fluctuating skin friction in an 8 degrees total divergence angle conical diffuser using modern pulsed-wire anemometry. The development of streamwise and spanwise fluctuating skin friction, skewness and flatness factors of streamwise skin friction, and probability density distribution of the wall shear stress show the rapidly changing nature and complexity of the wall layer turbulence structure in an axisymmetric severe adverse pressure gradient flow. The relative intensities of skin friction increase consistently and achieve very high values in the final stages of diffuser flow. The quantitative instantaneous backflow and the mean and fluctuating skin friction distributions were examined to determine their interrelationship. The initiation of instantaneous reversals was found to strongly influence the near-wall flow field and to coincide with the precipitous drop in skewness factor of the streamwise fluctuating wall shear stress, to approximately 10% of its initial value near the diffuser inlet. The flatness factor of the streamwise fluctuating wall shear stress also decreases in the latter stages, indicating a lesser inrush of high-velocity fluid from the central region. Comparison with conventional Preston tube data indicated the appropriateness of pulsed-wire skin friction measurements in severe adverse pressure gradient flows. In this diffuser the minimum streamwise wall shear stress near the diffuser exit is close to 3% of its inlet value, indicating that the flow approaches mean-flow separation. The present results of mean and fluctuating skin friction also differ substantially from those reported by other investigators using hot-film anemometry, both in level and shape of profiles. The differences are attributed to the rectification errors in hot-film anemometry at high turbulence intensities (near-wall turbulence level over 80%) and the appreciable instantaneous backflow (instantaneous reversals up to 30% of the time) present in the final stages of conical diffuser flow.