THE EFFECT OF DRAG-REDUCING POLYMER ADDITIVES ON WALL-PRESSURE FLUCTUATIONS IN TURBULENT CHANNEL FLOWS

The present paper is concerned with the experimental study of the infl uence of Toms' phenomenon on the convective part of the hydrodynamic t urbulent wall-pressure field. The experiments were carried out in a wa ter tunnel, at flow velocities of 2.3, 3.3 and 4.3 m/s, where amounts of drag reduction up to 40% were obtained by injecting dilute polymer solutions. An investigation of the spectral feature of the wall-pressu re frequency spectrum and the wall-pressure frequency cross-spectrum w as achieved from data taken using pinhole transducers. The scaling law s for the mid-and high-frequency regions of the spectrum were examined both for the water how and the drag-reducing flow. The use of a noise cancellation procedure allowed observation of the maximum of the spec trum in the mid-frequency range at an approximately reduced frequency w delta/u(tau), of 80 for the water flow. This maximum is shifted towa rds a higher value of the reduced frequency in the presence of Toms' e ffect. From cross-spectral data obtained over separation distances ran ging from 0.46 to 2.86, the effect of drag reduction was estimated on the coherence functions as well as the convection velocity. These data show a correlated shift of the threshold which delineates the low wav e number group identified as the large-scale turbulence contributor in the outer region of the flow and the high wave number group associate d with turbulent sources in the log-region. The effect of drag reducti on on the convection velocity data supports the fact that the contribu tions to the different wave number groups should be attributable to ac tivity in the outer and inner regions, respectively. In addition, a co mputation of the root mean square pressure was achieved from the wall- pressure frequency spectrum. When expressed as functions of the percen tage of drag reduction, the rms pressures related to the wall shear st ress are found to increase monotonicaly. The equivalent ratio obtained by scaling the rms pressures using the dynamic pressure is found to d ecrease in the drag-reducing flow. Finally, an estimation of the proba bility density distribution showed that the skin friction reduction af fects predominantly the occurrence of higher pressure amplitudes as we ll as very low pressure amplitudes. This confirms the important contri bution of the large pressure peak events to the overall features of th e wall-pressure. (C) Elsevier, Paris.